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Патент USA US3078398

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Feb, 19, 1963
c. w. HANKS ETAL
3,078,388
METHOD AND APPARATUS FOR CONTROLLING ELECTRICAL DISCHARGES
Filed Oct. 27. 1958
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Feb- 19, 1953
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METHOD AND APPARATUS FOR CONTROLLING ELECTRICAL DISCHARGES
Filed Oct. 27, 1958
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3,078,388
METHOD AND APPARATUS FOR CONTROLLING ELECTRICAL DISCHARGES
Filed Oct. 27, 1958
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United States Patent ()??ce
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1
the other hand, the voltages that are employed are
much greater than can be maintained across an are dis
3,078,388
METHOD AND APPARATUS FOR CONTROLLKNG
ELECTRHCAL DHSQHARGES
charge, and the currents (for equal power dissipated) are
correspondingly less. The use of saturated thermionic
emission discharges of this diffuse, luminous character
Charles W. Hanks and Charles D’A. Hunt, Urinda, and
David A. Vance, Lafayette, Calif, assignors to Stauit'er
Chemical Company, New York, N.Y., a corporation of
therefore permits high-power electric heating without the
employment of excessively high voltages or large cur
rents. Moreover, the high vacuum permits greater puri?
Delaware
Filed Oct. 27, 1958, Ser. No. 769,927
17 Claims. (Cl. 315-107)
3,078,388
Patented Fair. 19, 1963
cation by evaporation of impurities from the melted ma
10 terial than does conventional arc melting, and the dis
charge is diifused over the entire surface of the melt,
This invention relates to the control of electrical dis
instead of being localized as in a hard core arc.
charges used for heating, melting, and otherwise treat
ing materials in a high vacuum by electron bombard
Most
of the electric power supplied to the discharge is utilized
in accelerating the primary electrons to high velocities.
ment; and its chief object is to maintain the power ex~
The gaseous medium is suf?ciently rare?ed that few of
pended in such a discharge and the voltage developed 15 the primary electrons experience collisions en route, so
across it within desired ranges, irrespective of variations
that their kinetic energy is mostly expended in bombard
in operating conditions, such as may result from the
ing the anode, whereby the anode surface is heated with
irregular release of gases or vapors from the treated ma
terial, or other changes that may occur.
good e?iciency and uniformity.
is primarily directed, the discharge to be controlled is
just described is for high-vacuum melting and casting of
metals having a high melting point, a high chemical activ
In the type of treatment to which the present invention 20
One of the important uses for discharges such as that
established within a constantly-pumped vacuum chamber,
ity, or both, whose original reduction from their ores
which is maintained at an average absolute pressure of
leaves them in either powder or sponge form. Examples
the order of one micron of mercury or less. The dis
of such materials are tungsten, titanium, and columbium,
25
charge occurs from a heated, thermionic cathode to an
among many others. Rods and ingots can also be melted
anode, which is usually and preferably the material to be
and recast. The raw materials usually contain impurities
treated but may under certain circumstances be a crucible
which can greatly affect the physical characteristics of
containing the material. A certain amount of gaseous
matter is preferably introduced into the discharge near
the anode, sometimes from an external source but usually
by the evolution of vapors and gases from the heated ma
terial, so that a pressure gradient exists from the dis
the metal as ?nally produced. Many of the impurities
30 commonly present in such materials have vapor pressures
higher than that of the desired material at its melting
point, so that high-vacuum melting can result in a high
degree of puri?cation of the material treated. Successive
charge outward into the body of the vacuum chamber,
remeltings and recastings of the material in a high vacuum
and the gas density is greatest immediately adjacent to
may be employed for greater puri?cation. The very low
the anode. The gas in the immediate vicinity of the elec 35 gas pressure at which the present process is carried out
trical discharge is believed to be highly ionized; probably,
facilitates the evaporation of the impurities, which are
for the most part, by secondary emission of electrons
removed by condensation on cooler surfaces and by the
from the anode, although ionization by primary electrons
vacuum pumps. A greater degree of puri?cation can
and by photons may occur to some extent. The discharge 40 therefore be accomplished, or an equal degree in fewer
is diffuse and luminous; gas focusing concentrates it
upon the molten anode surface, over which it spreads
fairly uniformly.
remeltings, than is possible under the much higher gas
pressures that are necessary to sustain the arcs employed
in ordinary arc-melting, even when such melting is pos
sible.
the anode may become so highly conductive that there
Such diffuse, luminous discharges as those described
is relatively little voltage drop across or through it, and 45 are only metastable at high power levels. If the highly
it forms a plasma which may be considered a virtual
conductive plasma extends itself too near the cathode
anode, much closer to the cathode than the physical anode
structure-—-focusing electrodes, supports, or the cathode
formed by the bombarded material. In the vicinity of
itself—and the voltage gradient near the cathode becomes
the cathode, however, the gas density is much lower,
excessive, a localized discharge of much lower resistance
and the resistivity of the gaseous medium is su?iciently 50 shunts the desired diffuse discharge, and if it persists, the
high that several thousand volts can be maintained be
localized discharge soon degenerates into a self-sustaining
In large-scale, high-power operation, ionized gas near
tween the cathode and the virtual anode when the dis
charge is properly regulated and controlled as herein ex
plained.
Such a discharge tends to break down into a
are that restricts the discharge voltage to a relatively low
value until the arc is extinguished. Breakdown of this
kind may occur as the result of bursts of gaseous matter
low-voltage arc; the present invention inhibits such break 55 released from the melt. A different cause of breakdown
down, so that the desired, moderately high-voltage dis
that becomes increasingly likely in higher-power opera
charge can be maintained and controlled at higher power
tion, where the anode current exceeds 2 or 3 amperes at
levels than heretofore.
several thousand volts, is this: the nominally cold por
Some positive ions escape from the plasma and partly
tions of the cathode structure (such as focusing shields)
neutralize, or may even overneutralize, the negative space 60 may in fact become quite hot, and this reduces their work
charge of the electronic current. Owing to their low
function and renders electron emission easier. They may,
mobility as compared to the electrons, the positive ions
however, be cool enough to condense vapors emanating
contribute little to the discharge current. The discharge
from the melt, and the condensate contaminates their sur
is essentially electronic, of a di?use, luminous type and
face. Areas of such contamination on the cathode struc
the current is approximately equal to the cathode emis 65 ture may further reduce the work function where they
sion.
occur. Particularly if this is in a region of high potential
The proximity of the virtual anode to the cathode per
gradient, considerable electron emission may occur from
mits the establishment of a potential gradient high enough
the nominally cold, contaminated surface, and initiate an
to achieve saturation current, limited by cathode emis
undesired,
low-resistance, local discharge.
sion rather than space charge, at a much lower total volt 70
Many of these low-resistance discharges are self-quench
age than would be required in a gas-free discharge path
ing. The discharges themselves quickly clean the cathode
between the same cathode and the treated material. On
3
1%
surface and may restore its emission to normal value,
whereupon the localized concentration of current stops.
Relatively stable operation can continue, even with a
power supplied to heat the cathode directly from the
more-or-less continual succession of such short-duration,
trols the resistance of the discharge path for maintaining
a substantially constant average applied high voltage. As
minor arcs or discharges. If, however, a localized dis
charge of this kind persists for more than about a second,
it can excite copious electron emission from even a clean
power supply is reduced as a non-proportional function of
the voltage between the cathode and anode, which con
the process proceeds and equilibrium conditions are estab
lished, the directly supplied, cathode-heating power is con
tinuously varied as a non-proportional function of the dis
cathode surface, or from adjacent, not-so-clean surfaces,
which builds up the discharge progressively until a hard
charge voltage to maintain the power and voltage of the
core, self-sustaining arc is formed through an ionized path 10 main discharge both within their desired operating ranges.
'kept supplied by vaporization of the cathode material.
More speci?cally, the emission current is regulated at the
The discharge potential drops, usually, from several thou
power supply to prevent substantial ?uctuations in the dis
sand to less than 100 volts. Practicallyr the only limit on
charge current, and the cathode temperature is separately
the current is that imposed by the impedance of the supply
regulated to control the voltage gradient at the cathode
source. Normal operation can be re-established only by
for maintaining the average resistance of the discharge
using external means for breaking the arc and starting
path and the average power dissipated at the anode sub
stantially constant.
From another viewpoint, it may be noted that the re
Any cathode structure has a certain thermal capacity,
sistance of the discharge path tends to fluctuate and there
and there is a consequent time lag between changes in
by to set up oscillatory conditions leading to breakdown.
the power supplied to it, both directly from the power
In particular, the evolution of gaseous matter from the
supply and indirectly from the discharge, and changes in
anew.
.
melt proceeds irregularly, and produces irregular vari
the emission current. Generally, a decrease in the resist
- ations in the gas pressure and density within the discharge.
An increase in gas density tends to increase the supply of
ance of the discharge path, caused, for example, by the
applied high voltage remains constant, expansion of the
cathode, for the purpose of reducing the cathode tempera
ture to increase the voltage gradient at the cathode.
sudden release of a burst of gas from the melt, is coun
ions, and the plasma expands toward the cathode. If the 25 tered by a reduction in the heating current supplied to the
plasma tends to increase the voltage gradient at the
cathode, which tends to increase the emission current,
which tends to further increase the ion supply. Hence,
breakdown is inevitable unless the above sequence of
events is broken.
On the other hand, a mere reduction
Hence, when an arc occurs, the cathode heating current
might be reduced essentially to zero, and if the arc persists
long enough the cathode may cool below the minimum
emission temperature. Then, when the arc breaks the
resistance of the discharge path will be very high, the
of voltage proportional to decreases in the resistance of
-the discharge path results in a ?uctuating power dissipa
. applied high voltage will rise to an excessive value and a
tion at the anode, further ?uctuations, with a time delay,
new are will be struck. According to another feature of
in the evolution of gas, and a high probability, at high 35 this invention, such oscillatory conditions are avoided by
power levels, for the development of uncontrollable oscil
a controlled supply of heating current to the cathode dur
lations leading to breakdown.
ing arcing.
The present invention provides a method and apparatus
It will be seen that the operations thus described can
for so controlling diffuse, luminous discharges of the type
be controlled manually, either in whole or in part; they
described as to maintain them at maximum ef?ciency, 40 have, in fact, been so controlled, and normally are so
which in practice commonly means near instability. More
controlled in establishing optimum conditions for ma
speci?cally, major objects of the invention are to provide
terial of initially unknown quality or characteristics.
a method of control that will maintain both the voltage
Continuous manual control is, however, expensive, and
becomes increasingly dif?cult with increase in size and
across a diffuse, luminous discharge and the power dis
sipated by it constant to within a narrow range of values, 45 capacity of the equipment to be controlled. The present
that will normally extinguish minor, localized discharges
invention therefore contemplates apparatus for perform
ing automatically each of the steps described above.
The detailed description of the invention which follows
is illustrated by the accompanying drawings wherein:
‘extinction immediately re-cstablish the desired, diffuse,
luminous type of discharge, and that will prevent the oc 50
FIG. 1 is a diagram, partly schematic and partly in
before they can develop into self-sustaining arcs, that will
promptly extinguish any arcs that do form and upon their
currence of dangerous excesses in either current or voltage
if an arcing fault does occur. A further object of the
invention is to provide apparatus whereby control can be
block form, of equipment embodying this invention;
rial and by which control can thereupon be transferred to
current and voltage gradient at a hot cathode;
FIG. 4 illustrates the general relation to be established
between cathode or ?lament current and applied high volt
age for stable operation in the treatment of different ma
FIG. 2 is a partial circuit diagram illustrating in more
detail certain equipment symbolized in block form in
FIG. 1;
exercised manually until optimum operating parameters
have been determined for the treatment of a speci?c mate 55
FIG. 3 illustrates the general relation between emission
automatic equipment for maintaining optimum operation.
In accordance with the present invention, operation is
initiated by supplying sufficient electrical power directly
from a power supply to a thermionic cathode to heat it 60
to a temperature at which it will thermionically emit suf
‘ ?cient electrons to carry the current corresponding to the
terials;
’
FIG. 5 illustrates an approximate voltage distribution
etween the cathode and anode.
power desired in the, main discharge at the desired operat
FIG. 1 is a greatly simpli?ed diagram, partly schematic
ing voltage. With the desired emission established, volt
and partly in block form, illustrating the elements of a
agevis supplied to the cathode-anode circuit to establish 65 control system for a high-vacuum, electron-‘bombardment
the discharge. Thereafter the current is maintained sub
furnace, in accordance with the present invention. In
stantially constant at the established value; initially, the . this diagram the furnace is symbolized by a vacuum
voltage across the discharge path will be higher than the
chamber 1, which is evacuated to an absolute pressure
normal operating voltage, but it will fall gradually as
in the order or" one micron of mercury or less through a
the material treated heats and by the evolution of gaseous 70 duct 3 by suitable pumps 5.
matter and secondary electron emission establishes a zone
The discharge within the furnace takes place from a
of ionization. As this occurs, ionic bombardment of the
thermionically-emissive cathode 7 to an anode 9, which
cathode and other processes, particularly changes in the
may be a molten pool of the material treated contained in
electric ?eld distribution, tend to increase the electron
a conductive crucible it} and electrically grounded through
emission; hence, in accordance with the invention, the 75 the metal walls of the vacuum tank. Electrons emitted
3,078,388
Because of the constant-current, variable voltage char
acteristics of the network 17, it should be obvious that if
by the cathode are accelerated to high velocities by the
cathode-anode voltage, and bombard and heat the molten
surface of the material within crucible 10. Gaseous mat
the output circuit of the recti?er bank is opened the volt
ter evolved from the melt becomes ionized and forms a
network, supplied by a 440 volt input, the output voltage
plasma (a highly conductive ionized body of essentially
neutral charge) extending outward from the melt. The
principal voltage drop occurs between the cathode and
the plasma; and the chief object of the invention is to
control the discharge so as to keep the resistivity of this
age delivered to it can rise to very high values; in one such
on open circuit was computed at 13 kv., with dangerously
high circulating currents in the network. As a- safety
measure if is therefore preferable to provide horn gaps
33 across each phase of the secondary winding 29, adjust
ed to break down at some convenient, predetermined value
high-voltage region suf?ciently high, and to prevent arc 10 of
output voltage. In one apparatus embodying this in
ing, and other forms of breakdown, while operating at
vention these gaps are set to break down at 7.5 kv.; the
high power levels. In the drawing, focusing electrodes,
recti?er network, therefore, supplies a DC. output with
heat shields, crucible-cooling means, provisions for con
substantially. constant current at voltages in the range
tinually supplying and withdrawing the material treated,
and the like, with which the present invention is not di 15 between substantially zero and 10 kv.
It will be realized that no “constant-current” device,
rectly concerned, have been omitted for simplicity and
can provide absolute constancy of current through an un
clarity.
limited range of impedances. Practical constant-current
The cathode 7, in a preferred form of the device, is a
devices, as the term is used in the art, are characterized by
?lament in the form of a single loop of tungsten wire or
rod. Current passed through the loop heats the cathode 20 a dynamic impedance,
dE
to electron-emitting temperature, and leads for supplying
Z17
this current are brought in through the sides of the
chamber by insulating bushings, symbolized at 11. Cath
ode-heating current is supplied through a ?lament trans
former 13 which, in the usual arrangement, derives its 25
power from a 60-cycle commercial source. Heating cur
rent is controlled by means of a saturable reactor 15, con
trolled as will be described hereinafter.
relatively much higher than their effective impedance
E
T
within their operating range. Thus a “constant-current”
source, such as the network 17 as viewed from the dis
High voltage between the cathode 7 and anode 9 is
charge through the recti?er bank, will limit the current
provided by a constant-current D.-C. supply, whereby the 30 through the discharge to a de?nite maximum value even
applied D.-C. voltage is proportional to the resistance of
if the cathode and anode are shorted, so that the im
the discharge path between the cathode and the anode.
pedance of the gap and the voltage across it approach
In the embodiment of the invention illustrated, the power
zero, and will maintain the current within a few percent
used is derived from a three-phase, 60-cycle commercial
of that maximum even if the effective impedance of the
source. Power from that source is ?rst supplied to a 35 gap rises to such a value that the potential required rises
conventional, three~pl1ase constant-current network, pref
to several thousand volts.
erably of the type known as a “Steinmetz constant-current
One side of the recti?er bank is grounded. The other
network,” indicated generally by the reference character
side connects through lead 35 to the secondary of the
17. This network comprises three delta-connected legs,
?lament transformer 13 and thence to the cathode 7. For
each including an inductor 19 in series with a condenser 40 supervisory purposes, and as an aid to manual control, an
21 tuned to series resonance at the supply frequency.
ammeter 37 is preferably provided in the ground lead of
Schematically the inductors 19 and condensers 21 are
the recti?er bank to indicate the discharge current. A
shown as variable, for the sake of simplicity; in practice
voltmeter 39, in series with an external multiplier resistor
the condensers preferably take the form of tapped con
40, connects from lead 35 to ground for like purposes.
denser banks and the inductors are also tapped so that 45
Also connected from lead 35 to ground is a high resist
the legs can be tuned and the output current adjusted by
ance voltage divider consisting of resistor 41 and po
selection and interconnection of the proper taps.
tentiometer 42 in series. Lead 53, connected to the cir
The three-phase input leads 23a, 23b and 230, are
connected to the apices of the delta network, so that the
cuit junction between elements 41 and 42, picks off of
the voltage divider a small voltage, generally about -—200
input phase vector rotates into the inductor of each leg 50 volts, proportional to the voltage across the discharge in
?rst—i.e., counterclockwise in network 17 as illustrated
the furnace, and feeds it to a variable-gain D.C. ampli?er
in FIG. 1. The output leads 25a, 25b and 250 connect
47, which is adjustably biased, as hereinafter explained.
to the junction between the inductor and the condenser of
Output current from the ampli?er 47 is supplied to a
each leg. As is well known, when the circuit illustrated
winding 49 of the saturable-core reactor 15, to supply
is properly tuned, the voltage developed across the out 55 thereto a current directly related to the voltage across the
put leads is very nearly proportional to the effective im
discharge path within the furnace. Increase in this cur
pedances connected across these leads, with the corollary
rent,
increasing the saturation of the core, decreases the
that the current supplied to the output circuit is very
effective reactance of reactor 15 and thereby increases the
nearly constant. For present purposes, no substantial
electrical energy supplied to heat the cathode 7 and thus
error is involved by considering it to be a constant, and 60 increases its temperature. A decrease in voltage has, of
it Will be so treated in what follows.
course, the opposite effect.
Output leads 25 of the constant-current network con
The conditions existing within the discharge path are
nect to the delta-connected, primary 27 of a three-phase,
not only complex but change from moment to moment
step-up transformer. The secondary 29 of this trans
owing, in part at least, to variations in volume and posi
former is star connected, in the example illustrated. The 65 tion of the ionic plasma. Some understanding of the
secondary is connected to a recti?er bank comprising, for
nature of the control exercised by the cathode tempera
example, six mercury-vapor recti?ers, collectively desig
ture can be derived from a consideration of FIGS. 3
nated by the reference character 31. These recti?ers are
through 5.
connected in known manner to lead 35.
FIG. 5 shows the approximate voltage distribution be
Various other types of constant-current supply are avail 70
tween the cathode and the anode. Gaseous matter
able, and the invention is not limited to the use of any
released from the molten anode becomes ionized and
particular type. The arrangement shown is preferred
forms a low-resistance, ionic plasma adjacent to the anode.
because of its simplicity and high e?iciency. The current
Because of its high electrical conductivity, there is little
can be reduced, when so desired, by manually detuning
75 voltage drop across this plasma, and its outer surface,
the network.
she/sees
7
%
which is at nearly the same potential as the molten anode,
geometry. The relatively large changes in voltage with
forms a virtual anode which attracts electrons emitted
small changes in cathode temperature give a powerful
negative feedback with which to hold the average voltage
by the cathode. Between the cathode and the plasma,
the resistivity of the gaseous medium is relatively high,
and it is here that most of the voltage drop across the
discharge appears. In practice, the plasma expands and
substantially constant.
Within the above-saturation range, the voltage gradi
ent near the cathode approaches the total voltage ap
contracts, and if the discharge voltage, and hence power,
is to be kept constant, it is evident that the voltage gradi
ent near the cathode must change accordingly.
plied across the discharge gap divided by the distance
between the cathode and the virtual anode. It will be
into the current-saturation range, and are necessarily only
rough approximations in view of the very complex nature
fore the power) constant, since the current is substantial
apparent from the curves of
3 that a change in cath
The curves illustrated in FIG. 3 are somewhat related 10 ode temperature can compensate for a change in position
to the familiar current~voltage curves of a diode, carried
of the virtual anode to maintain the voltage (and there
of the phenomena under consideration. The currents are
plotted as ordinates; because, however, the size and posi
tion of the ionic plasma, constituting a virtual anode, is
variable, and in varying changes the con?guration as well
as the actual length of the discharge path through the
relatively high-resistance region, the abscissas represent
the voltage gradient in the immediate vicinity of the cath
ode and not the total voltage across the discharge. The
gradient varies as a direct, non-linear function of the total
voltage and an inverse, non-linear function of the length
of the high-resistance zone between the cathode and the
plasma.
Subject to this understanding of the meeting of the
curves, the cathode emission current varies with the volt
age gradient at the cathode in approximately the manner
shown.
ly constant by postulate.
The position of the virtual anode, however, depends
15 (among other factors) on the rate at which gaseous mat
ter is evolved from the anode, either from evaporation of
the treated material or liberation of impurities. It is
therefore a function of the power dissipated in the dis
charge (which mostly goes into electron bombardment
of the anode) among several other factors, including the
structure of the furnace and the composition of the raw
iaterial, its melting point, vapor pressure, and purity.
The nominal current and voltage ratings are design
parameters of the particular furnace to be controlled.
25 To establish stable operating conditions for a specific
melt, the cathode temperature may ?rst be set to give a
higher emission than required to carry the value of cur
rent delivered by the constant-current network 17. Ini
In the region of space-charge limitation, the
tially, the discharge will be space-charge limited, the re
gradient at the cathode is substantially zero, or even slight
30 sistance of the discharge path will be high, and the oath
ly negative. As the current is increased, the point com
monly called saturation is reached, where nearly all of the
electrons emitted by the cathode are drawn over to the
anode. By continuing to increase the applied voltage, or
ode-anode voltage will be correspondingly high. As the
bombarded and heated anode releases gaseous matter,
ions form which lower the resistance of the discharge
path, and the voltage drops.
The cathode-heating cur
to decrease the spacing between the cathode and the 35 rent can then be gradually reduced to keep the voltage
virtual anode, a positive voltage gradient can be obtained
and power at the desired values. Whenever the voltage
fairly close to the cathode, and thereby, due to many,
(and hence the power) drops below the desired value,
complexly interrelated factors, not requiring discussion
the cathode temperature should be lowered to increase
here, a small but signi?cant increase in current over the
the voltage gradient and the resistance of the discharge;
“saturation” value can be obtained. According to the 40 conversely, whenever the voltage rises above the desired
present invention, the cathode is operated in this beyond
value, the cathode temperature should be raised. The
saturation region. Of course, if this is pushed too far and
operation can now be switched over to automatic con
the voltage gradient becomes too large, breakdown and
trol, which will maintain the voltage and power at sub
arcing will occur.
stantially constant value.
Emission current also varies as a function of cathode
With materials of known characteristics the operating
temperature. In FIG. 3, curve A applies to one'cathode
points can be pre-set and transfer to automatic control
temperature, while curve B applies to another, somewhat
effected promptly. To achieve stable operation at the
higher cathode temperature. If the cathode becomes too
highest possible power level, with materials of unknown
cool, no substantial emission occurs, in the high vacuum
characteristics, particularly as to their impurity content,
under consideration, until the voltage gradient becomes 50 ordinarily requires further adjustment, because of the ef
so high as to cause almost immediate breakdown. Hence,
fect of the material treated on the discharge itself.
the cathode must be kept above the minimum temperature
The quantities that aifect the discharge are’so inter
for thermionic emission, which depends on the cathode
related'that operating parameters cannot be set arbitrari
material.
ly. The density and pressure of gaseous matter at the
The dotted line C of FIG. 3 indicates the current sup 55 anode depends on the temperature of the melt, the ma
plied by the constant-current network 1'7; it has a slightly
terial melted, and particularly on its purity. The rate
negative slope, indicating the slight decrease in current as
at which the cathode loses energy by radiation also de
the effective impedance across the discharge gap increases
pends, in part, on the anode temperature, and this, in
from zero to a relatively high value. In the process under
turn, affects the direct heating energy that must be sup
consideration, ionic currents are very small in comparison 60 plied to it to maintain its temperature. Too much power
to the electronic currents; therefore, the supply current
in the discharge heats the melt too fast, speeds up the
and the emission current must be approximately equal.
evolution of gaseous matter, increases the volume of the
Hence, if the cathode temperature corresponds to curve
ionic plasma, and decreases the spacing between the ionic
A, the operating point is the intersection of curves A and
plasma and the cathode, whereby the discharge becomes
C, and the voltage-gradient will be that corresponding to 65 unstable and forms a lowevoltage arc. Too little power
the abscissa of point X. An increase of cathode tempera
has opposite effects, and the ion density may become so
ture to that corresponding to curve B will drop the volt
low that electronic space charge limits the current, and
age-gradient cordinate to point Y. Hence, other fac
the voltage rises to excessive values. Thus, for each dif
tors remaining the same, the cathode temperature deter
ferent material there is a different set of operating param
mines the voltage gradient. In practice, other factors do 70 eters for achieving the type of operation desired. These
not remain the same for long, and the cathode tempera
parameters must be determined experimentally.
ture is varied to control and stabilize the discharge. The
Furthermore, the characteristics of the melt, the rate
total cathode-anode voltage automatically adjusts itself,
of evolution of gaseous matter, and the size of the ionic
through the constant-current supply to the value deter
plasma may change during the course of a melting op
mined by the gradient established and the discharge 75 eration. To maintain the desired power level, at constant
3,078,888
9
current, the average resistance of the discharge path must
be kept substantially constant. According to this in
vention, the resistance of the discharge is regulated by
controlling the cathode temperature responsive to the
cathode-anode voltage. When the voltage drops, the
heating current to the cathode is reduced; and when the
voltage rises, the heating current is increased. However,
the heating current is not proportional to the voltage; but
may be proportional to the algebraic sum of the voltage
and a negative constant of approximately the same mag
'10
one sense, apparatus whereby the automatic control is
calibrated.
Such parts of the apparatus illustrated in FIG. 1 as are
necessary to the complete description of the second ?g
ure are designated by the same reference characters as
in FIG. 1. The equipment comprised within the block 47
of FIG. 1 is shown enclosed within broken lines 47 of
FIG. 2; similarly the equipment within block 51 of FIG.
10
1 is enclosed within broken lines 51 in FIG. 2.
The resistors 41 and 42, forming a voltage divider for
providing a convenient voltage of about-200 volts propor
nitude. In mathematical terms, if I represents the cath
tional to the much higher cathode-anode voltage, are
ode heating or ?lament current, and V represents the volt
shown at the extreme right of FIG. 2, connected between
age between the cathode and the melt, stable operation
ground and the lead 35 that connects the high-voltage
may be achieved by controlling the ?lament current so
power supply to the center of the secondary of ?lament
15
that
transformer 13 which feeds the cathode 7. Lead 53, ex
tending from the circuit junction between resistors 41 and
42, connects through one pair of contacts 551 of a ganged,
where S and K are experimentally determined constants
automatic-manual changeover switch, to one end of a p0‘
which have different values for di?erent materials, but
tentiometer 43. The lower end of this potentiometer con
have never been found to be zero. In general, S is usual
nects to the moving contact of a second potentiometer 57,
ly only slightly smaller-—~say, 5% less—than the average
one end of which goes to ground and the other to a source
value of V.
of negative potential, preferably a negative tap on a con
It might be supposed that the more rapidly the cathode
ventional power-pack.
heating current responded to changes in discharge volt
The setting of potentiometer 57 determines the bias set
age the more stable would be the control. This is not 25
ting of the ampli?er, and thus determines the approximate
the case in fact. The thermal capacity of the cathode
average voltage between cathode 7 and the melt during
introduces delays in its response to changes in heating
automatic operation. This bias may be set at any value
current, which are tantamount to a phase delay around
between ground potential and —275 volts; for the particu
the feedback loop, which varies with anode temperature
lar apparatus illustrated it will usually be somewhere in
and the other factors dependent upon it, so that too
the neighborhood of -—190 volts, so that the grid of the
rapid a response to voltage changes can result in insta
?rst ampli?er tube is about ten volts more negative than
bility, with violent oscillatory changes in discharge volt
its cathode with normal voltage across the discharge in
age. Also, short-duration voltage ?uctuations actually
the furnace and lead 53 at about ——2OO volts. In plotting
help to stabilize the discharge. For example, frequent,
short-duration, localized discharges may develop between 35 cathode-heating current as abscissas against discharge volt
age as ordinates, as shown in FIG. 4, the setting of poten
the ionic plasma and the cathode or other parts of the
tiometer 57 effectively determines the average voltage
apparatus. These localized discharges have relatively
across the discharge during automatically controlled op
high current densities and low resistances; they cause a
eration. On this same plot the setting of the potentiom
sudden drop in the cathode-anode voltage, which helps
eter contact 45 determines the slope of the curve, i.e., the
to extinguish the local discharge before it can develop
rate at which the ?lament current is increased with in
into a self-sustaining arc. Hence, it is not desired to
creasing voltage across the discharge, during automatic
eliminate all voltage ?uctuations; it is desired to keep the
average voltage approximately constant.
operation.
The contact of potentiometer 57 connects directly to
In addition to the above factors, there is a threshold
temperature below which the cathode will not emit any 45 one cathode 531 of a dual triode 59, while contact 45 con
considerable number of electrons, but secondary effects
nects directly to the grid 601 for controlling the current
may be sufficient to maintain the cathode at full emis
sion even though the heating current supplied to it may
be insu?icient of itself to raise it above the threshold
in this triode. The. anode of the same tube section con
nects through a load resistor 61 to the adjustable tap of a
ode may vary non-linearly with the control current sup
plied to the saturable reactor 15.
In practice all of these inter-related factors can be
tubes, which is usually done during factory calibration of
the ampli?er.
the setting of a bias or off-set voltage in ampli?er 47, and
tective resistor 65. The cathode 582 of the second section
of the tube is connected back to potentiometer 57 through
a cathode resistor 67, whereby this section of the tube 59
potentiometer 63 connected from a +250 volt tap on the
temperature; the resistance of the cathode varies with 50 power supply to ground. This arrangement makes it pos
sible to adjust the average anode voltage of the vacuum
temperature, and the heating current supplied to the cath
The drop across resistor 61 is applied directly to the grid
resolved by the adjustment of two operating parameters: 55 692 of the second section of tube 5% through the usual pro
the adjustment of the ampli?er gain. These adjustments
will be discussed in more detail in the description of the
acts as a cathode follower.
circuits illustrated in FIG. 2.
The voltage developed between the cathode 582 and
The effects of these adjustments are illustrated in FIG. 60
ground is applied to the four control grids of two dual
4, wherein ?lament current is plotted against cathode
tubes 659 in parallel. The cathodes of these tubes are
anode voltage. Curves D and E of this ?gure show typi
grounded directly; their anodes connect, also in parallel,
cal characteristics for stable operation in treating two
through small protective resistors 71 to the control wind
different materials. The important facts to observe
about these curves are their different slopes and their 65 ing 49 of the saturable reactor 15. The current controlled
by these tubes is supplied from a +200 volt tap on the
different intercepts on the zero current axis; these inter
same power supply as is used to provide the other operat
cepts are never at the origin if stable operation is to be
ing voltages for the ampli?er circuit.
maintained. Stated otherwise, neither the cathode heat
Following through the connections described, it will be
ing power, current nor voltage is directly proportional to
70 seen that increased voltage drop across the discharge path
the discharge voltage.
within the furnace drives the grid of the ?rst section of
Equipment whereby all of the necessary adjustments
tube 59 more negative, thus decreases the drop across re
may initially be made manually and then be transferred
sistor 61 and drives the second grid toward positive. The
to automatic control is illustrated in FIG. 2. It is more
second tube section being connected as a cathode follower,
convenient, however, to describe ?rst the automatic con
trol equipment, as the manual control elements are, in 75 it in turn drives the grids of all of the tubes 69 toward
aorases
positive, thus increases the current through these tubes
and through the control winding 4-9 of the reactor, and
increases the cathode heating current, which tends to raise
permits the heating power supplied to the cathode under
the cathode temperature and to lower the resistance of
the discharge path. At constant current, a drop in dis
charge resistance lowers the voltage, and thus a negative
feedback stabilizing action is achieved which tends to hold
so that when the are in the furnace does break the shoe
tive resistance of the cathode-anode gap will be reason
the discharge voltage and power substantially constant, on
such conditions to be set at a point which will bring it up
to any desired temperature within the operating range,
ably close to the normal operating value and the voltage
across it will not rise to so high, a recovery Value that it
would either re-establish the arc in the furnace or cause
the average. In a sense, adjustment of tap 45 varies the
breakdown at the horn gaps in the power supply and
gain of the feedback loop, which because of the substan 10 possibly require that the main power supply be cut oil‘ to
tial time delays and other complex factors involved, must
re-establish operating conditions.
be neither too high nor too low if stable operation is to be
If a self-sustaining arc is established which carries the
maintained.
The voltage at the cathode 582 of the output section of
normal operating current of the network 17, it is clear
that additional means must be invoked to break it. This
tube 59 is also applied through a second set of contacts
can, of course, be done by opening the circuits to the
552 on the automatic-manual changeover switch to a lead
constant-current network 17. It is preferred, however,
73, connected through a variable resistor 75 to the cathode
to avoid undesirable transients following re-closure of
of a thyratron 77 and also, through a condenser 79, to
the circuit. It is desirable, too, that normal operating
ground. The control electrode of the thyratron is con
conditions be reestablished as soon as possible following
nected through a current-limiting resistor 81, to the mov 20 the breaking of an are. One way of accomplishing this
able tap of a potentiometer 33, incorporated in a voltage
is illustrated at the right of FIG. 2.
A potentiometer tap 85 is taken off from the resistor 42
~divider~string connected between the —-275 volt tap of the
at the low potential end of the voltage divider string,
power pack to ground. The anode of thyratron 77 is con
nected directly to ground.
which connects to the control grid of a thyratron 87. The
In practice potentiometer 33 can adjust the voltage on 25 cathode of this thyratron connects to ground through a
variable resistor 88; its anode connects to the winding of
the'control electrode of thyratron 77 through a range of
a relay 89 and thence, through a condenser 91, to the
50 or 60 volts, from approximately 110 volts negative to
+250 volt tap of the ampli?er power supply. Condenser
somewhere in the neighborhood of 160 volts negative. In
91 is bridged by a high resistance 92, of a value such that
normal operation, the positive voltage drop across cathode
resistor 67 and the negative voltage of approximately 30 the time constant of the condenser-resistor combination
is in the order of one second.
—l90 volts from potentiometer 57 add algebraically to
Potentiometer contact 85 is set to a point that will hold
make the cathode of thyratron ‘77 sufficiently positive rela~
thyratron 87 nonconductive until the voltage across the
tive to its control grid that the thyratron remains noncon
cathode-anode gap of the furnace drops to the low value
ductive. However, if the voltage across the discharge in
the furnace drops, the current through resistor 67 also 35 indicative of an arc. At this point thyratron 87 ?res and
charges condenser 91 through the winding of relay 89,
drops, and at some pre-set minimum value of this current,
tube 87, and resistor 83, in series. The last-mentioned
after a time delay determined by the values of capacitor 79
resistance is much smaller than resistor 92, and it is ad
and resistor 75, thyratron 77 strikes and effectively con
justed so that the time constant of the series circuit which
nects resistor 75 between ground and resistor 67. This
includes the relay winding and condenser 91 is of the
forms a voltage divider which holds cathode 532 and the
order of one-sixtieth second. Upon ?ring of the tube 87,
grids of tubes 69 at a su?iciently small negative potential
relay 89 closes and actuates the magnetic contactor 93.
for the conduction of considerable current through tubes
The latter is supplied by the main AC. power source
'69 and control winding 49, whereby heating current is re
stored to ?lament 7 before it can cool to less than mini
mum emission temperature.
_ Striking of the thyratron 77 also reduces the resistance
effective across the condenser 79 substantially to zero, dis
to close the contacts 95 and short each leg of the con
stant-current network by closing the circuit interconnecting
each pair or" the output leads 25a, 25b and 256 which
are shown fragmentarily in the ?gure.
Current ?ows
in the coil of relay 89 only until condenser 91 is charged
charges tlie condenser, brings cathode and anode of the
substantially to the 250 volt supply voltage, at which time
thyratron substantially to the same potential, and thus
breaks the discharge in the thyratron. If the arc in the 50 the discharge through tube 87 breaks and relay 89 re
opens. This, in turn, permits magnetic contactor 93 to
furnace has not broken by this time, the condenser re
de-energize and contacts 95 to open, breaking the short
charges through resistor 75, the thyratron strikes again,
circuits across the legs of the constant-current network.
and continues to make and break the circuit across con
The contacts 95 carry only the current normally sup
denser 79 as long ‘as arcing in the furnace continues.
Tube 77 therefore acts as an approximately sawtooth os 55 plied to transformer primary 27, because of the character
cillator. The frequency and breakdown point can be
varied, respectively, by shifting the contacts on the resistor
75 and the potentiometer 83. Between these two adjust
ments the time during which reactor 15 is saturated, and
istics of the constant-current network, but the voltage
across the recti?ers 31 immediately drops substantially to
zero and causes the are within the furnace to break. This
usually happens within a half-cycle of the 60-cycle input
hence its average impedance, can be adjusted so as to pass 60 power.
enough current to maintain the temperature of cathode 7
in the emitting range. Furthermore, it should be noted
that there is a time delay, adjustable through variable re
sistor 75, between a drop of voltage across the discharge
in the furnace and the firing of thyratron 77. Hence, the
thyratron circuit does. not operate responsive to localized
discharges of such short duration that its action is not
By adjusting the two time-constants associated
with condenser 91, the time during which relay 89 re—
mains closed can be adjusted so that it is long enough
to insure the breaking of the are within the furnace, but
no longer. The capacity of condenser 91 must be large
enough to store su?icient energy to hold relay 89 closed
for the required interval. This will, of course, depend
on the sensitivity of the relay. The values of resistors 88
and 92 are chosen accordingly.
it will, of course, be recognized that electronic switches,
needed.
The core of the saturable reactor 15 is never fully sat
urated while thyratron 77 is oscillating as described above, 70 such as ignitrons or thyratrons, can be substituted for the
electro-mechanical relays and contactors here illustrated,
and control-winding 49 has a considerable inductance;
and that the shorting connections may be made across the
hence the current passed by it is an inverse function of
frequency. Therefore the degree of saturation of the re
secondary 29 of the transformer, or between lead 35 and
ground.
actor core can be controlled by adjusting the frequency
of the thyratron oscillations by adjusting resistor 75. This
To switch the apparatus to manual control the ganged
8,078,388
13
switches 511 and 552 are thrown to the opposite position
from that shown in the drawing. This disconnects po
tained constant, to within +5 percent or less, over long
periods of operation where the materials melted are rea
sonably pure or are of constant composition. This con
stancy is to be expected where the discharge is used to
tentiometer 43 from the ampli?er input lead 53, and con
nects it instead to a manually-operated, variable resistor
97, which connects to the —275 volt tap on the source.
remelt and further purify metals that have previously been
The cathode of tube 59 remains connected to the poteni
ometer 57, however, so that current liows from the nega
tive source through resistor 97 and potentiometer 43 back
to potentiometer 57 and thence to ground. The heating
current supplied to ?lament 7 can now be controlled man
1d
at the molten surface of the material treated can be main
melted and cast in a vacuum.
Where the material of the melt is extremely gassy, as,
for example, some of the metallic sponges as supplied by
10 the primary producers of such materials, the term “main
ually by adjusting the position of tap 45 on potentiom
eter 43.
Resistor 97 is preferably calibrated in terms of the gap
voltage indicated by voltmeter 39, so that the current
through potentiometer 43 at a given setting is the same as
at the indicated voltage when on automatic control. The
taining the power in the discharge constant” is applicable
only in connection with average, rather than instantane
ous, power. It is frequently a characteristic of such ma
terials that they contain gas inclusions which are re
leased into the discharge in sudden, violent bursts. When
such a burst occurs the volume of gaseous material that
is released may be so great as to raise the pressure within
current through the cathode ?lament 7 can be read on
the vacuum chamber to a point where the entire volume
an ammeter 99 in the primary circuit of transformer 13,
is ?lled momentarily with a luminous discharge, even
and when proper operating conditions have been estab
lished this current can be matched by the automatic set 20 though the pumps used to evacuate the chamber may
have capacity su?icient to keep the pressure outside
ting. Obviously an entirely separate control could be
of the discharge path down to a fraction of a micron of
used for regulating the cathode heating current, but al
mercury were the gas liberated at a constant rate.
though such separate control mechanism would be much
Using conventional types of power supplies to treat such
simpler per se than operating through the amplifier 47 it
would actually add to the complexity of the system, and 25 gassy materials with metastable discharges of the type
here considered is impossible. Once a localized discharge
make transfer from manual to automatic operation more
is established with such a system it is followed .up by a
di?icult.
rush of current that would quickly establish a self-sus
As has been stated, the biasing point for tube 59, which
taining arc and throw the apparatus out of operation until
determines the average voltage across the discharge in
the furnace during automatic control, is set by potentiom 30 circuit breakers could be reclosed and the whole operation
started afresh.
eter 57 and will usually di?’er least of all of the operating
With the current in the diffuse discharge limited as here
parameters as between different materials treated. This
in described, no such current rush can occur and in the
usual case the pumps quickly scavenge the released gas
potentiometer therefore seldom needs readjustment.
Much more critical is the slope of the ?lament current vs.
discharge voltage curve. This is set by means of the 35 and any momentary, localized discharge breaks of itself,
movable tap 45 on potentiometer 43.
but during the persistence of the low-resistance discharge
In the present
the voltage across it will drop, from say ?ve to six thou
sand volts to something of the order of a hundred volts
case this is done by means of a small, reversible, electric
motor 1M, which may be operated to adjust the position
of contact 45 by operating one or the other of push but
tons 103.
or even less. Less violent bursts of gas-emission cause
40 less violent but still substantial ?uctuations in discharge
resistance.
It will be seen that the operation of contact 552 to the
The low-resistance discharge following a burst of gas
manual position disconnects the circuits of tube 77 from
emission may persist for time intervals varying from a
direct connection with resistor 67, and therefore disables
small fraction of a second to several seconds, and in spite
the equipment for overriding the automatic cathode-cur
rent control when arcing occurs. Connection of the cir 45 of the thermal capacity of the cathode, its temperature
in comparison to its immediate surroundings is so high
cuits of tube 77 can be re-established by operating push
that it is quite possible for it to drop below minimum
button 195. The preferred procedure is to set resistor 97
emission temperature in less than a second. if any of
to correspond to a value of gap voltage that would indi
the usual types of constant-current supply were used and
cate arcing. Push button 165 is then depressed and po
the discharge broken, with the cathode either non~emit—
tentiometer 33 adjusted until tube 77 ?res. This in indi
ting or emitting fewer electrons than required to carry
cated by a sudden increase in the reading of ammeter 99,
from say 2 or 2.5 amperes to 5 or 6 amperes.
the constant current, the result would be a voltage of a
value that could be even more destructive than the short
Adjust
meut of resistor '75 then establishes the cathode current
at the desired value.
circuit currents that would develop were the supply from
Similarly, it may be desirable in establishing working 55 the more usual prior-art constant-voltage system. In the
practice of the present invention this is prevented by over
conditions to open switch 107 and disable the shorting
riding the negative feedback type of control which ren
circuit 51 until a stable operation is established. If arcing
ders the discharge operationally stable except when more
does occur, the arc discharge can be broken by simply
than normal volumes of gas are liberated. Hence where
closing switch 167.
In practice it has been found that one of the major 60 the material treated is of such nature that continuous
stability within very narrow limits becomes impossible,
the present invention operates to prevent instantaneous
advantages provided by this invention lies in the fact that
the various interrelated factors that maintain stability of
the discharge can be adjusted manually during experimen
tal operations until an optimum operating point is found
for each speci?c parameter, and the control of the vari 65
ous operating parameters can be transferred to the auto
matic control equipment, one~by-one, as these points are
determined. Furthermore, because of the order in which
the steps are taken, the danger of destructive voltages or
currents that would normally be inherent in supplying
large amounts of power to a load of unpredictably vary
ing resistance from a constant current source, or from
a constant voltage source as in common prior practice,
is avoided.
Through the procedure described the power dissipated
interruptions of the desired process from developing into
complete breakdown and permits immediate re-establish
ment of substantially the desired operating conditions, and
by so doing permits large-scale commercial melting and
casting operations at higher power levels than were prac
ticable heretofore.
In treating gassy materials, where the most probable
type disturbance to the discharge is from gas bursts, it
may be desirable to disable the shorting circuit 51 by
opening switch M7, and to operate this latter equipment
manually and only when arcing is not cleared by the
operation of the pumps. It may be added that is upon
75 operations on such gassy materials that the effective gain
3,078,388
‘
16
15
of the ampli?er controlling the ?lament current should
be reduced to compensate for the extremely violent varia
tions in discharge voltage that are characteristic with ma
terials of this class.
The speci?c apparatus that has been illustrated offers
a convenient means of practicing the method automatically
and of transferring manual to automatic control and vice
versa. t is not, however, intended that the scope of the
invention be considered as limited by the speci?c appara
discharge in a metastable state wherein a low-resistance
ionized zone is established adjacent to said anode
a
high-resistance zone adjacent to said cathode, which com
prises the steps of continuously evacuating the space be
tween said cathode and anode, supplying electrical energy
directly to heat said cathode to a temperature at least
sufficient to cause emission of an electron current equal
to said constant value, continuously passing a current of
said constant value through said discharge thereby heat
tus illustrated, all intended limitations being speci?ed in 10 ing said ano ‘e and causing the evolution of gaseous mat
the claims.
ter therefrom, adjusting the electrical energy supplied to
What is claimed is as follows:
heat said cathode to bring the voltage across said dis
1. The method of controlling an electric discharge in
charge within said limits while maintaining the electron
a vacuum between a hot cathode and a bombardment
current emitted from said cathode substantially at said
heated anode, which comprises supplying heating current 15 constant value, and thereafter varying the electrical en
to said cathode su'f?cient to produce electron emission
ergy supplied directly to heat said cathode as an inverse
and maintain the discharge between said cathode and said
function of the voltage across said discharge to maintain
anode, the electrons so emitted bombarding and heating
said voltage within said limits irrespective of other agen
the anode thereby causing the evolution of gaseous mat
cies heating said cathode.
ter therefrom and producing an ionic plasma between 20
9. The method set forth in claim 8, additionally com
the anode and the cathode, supplying a substantially
prising the steps of increasing the electrical energy sup
constant direct current to the discharge between said
plied directly to heat said cathode to a ?xed value suiti
cathode and said anode, and continually adjusting said
heating current as an inverse function of the voltage
across said cathode and anode and thereby maintaining
a substantially constant average. voltage between said
cathode and said anode.
'
ient of itself to produce an electron-emission current
equal to said constant current and reducing the current in
said discharge below said constant value during intervals
when the voltage. across said discharge falls materially
below said limits.
2. The method set forth in claim 1, wherein the heating
10. Apparatus for controlling an electric discharge in
current is continually adjusted to a value proportional
a vacuum wherein evolved gaseous matter may form
to the difference between the voltage across the discharge 30 an ionic plasma, comprising a thermionically emissive,
and a constant in the order of 5% smaller than said
?lamentary cathode, a bombardment-heated anode, a
voltage.
constant- urrent D.C. power supply connected across said
3. The method set forth in claim 1, which additionally
comprises adjusting said heating current to a ?xed value
upon the occurrence of low-resistance discharge shunting
anode and cathode for supplying thereto a substantially
the desired discharge.
4. The method set forth in claim 1, which additionally
comprises interrupting said constant current upon the oc~
currence of an arc shunting the desired discharge.
5. in the operation of apparatus for heating materials
in vacuo by bombardment with an electron discharge
through a discharge path from an electrically heated
thermionically emissive cathode to an anode structure
comprising the material to be treated, said discharge being
supplied from a substantially constant-current source,
the method of controlling said discharge to maintain
constant direct current at a voltage substantially propor
tional to the resistance of the discharge, ?lament-current
supply means for supplying electric power to heat said
cathode to electron-emitting temperature, connections pro
viding a ?rst electric signal that varies with changes in
the voltage across the discharge, means providing a bias
in bucking relation to said ?rst signal, amplifying means
connected to amplify the difference between said ?rst sig
nal and said bias to provide a control signal, and control
means responsive to said control signal for automatically
adjusting the power supplied to heat said cathode, where
45 by the average voltage across the discharge is auto
matically kept substantially constant.
therein a metastable state wherein a portion only of said
ll. Apparatus as in claim 10, wherein said control
discharge path is occupied by a plasma or" ions which com
prises the steps of supplying the electrical energy directly
means is a saturable reactor having a variable-impedance
winding connected between said ?lamenucurrent supply
to heat said cathode to a temperature whereat its total 50 and said ?lamenatry cathode, and having a control wind
electron emission is substantially equal to that required
to carry the constant current supplied by said source,
passing current from said source through said discharge
path, and varying thhe electrical energy supplied directly
to heat said cathode as an inverse function of the voltage
- cross said discharge path to maintain said voltage con
stant to within a limited range.
ing connected to said amplifying means for receiving said
control signal.
12. Apparatus as in claim 10, wherein said anode is
grounded, the connections for providing said first signal
comprise a voltage divider connected between said cathode
and ground, the means providing a bias comprises a source
of negative potential and a potentiometer connected be
6. The method set forth in claim 5 additionally com
tween said source and ground, and said amplifying means
prising the step of increasing the electrical energy supplied
has two inputs respectively connected to said voltage di
directly to heat said cathode to a ?xed value when the 60 vider and said potentiometer.
voltage across said discharge path falls below said limited
13. Apparatus for controlling an electric discharge in
range.
vacuo, wherein the path of said discharge includes a low
7. The method set forth in claim 5 additionally com
resistance ionized zone and a high~resistance Zone, com
prising the step of increasing the electrical energy sup
prising a thermionically emissive cathode, an anode heat
plied directly to heat said cathode to a ?xed value when
ed by said discharge, means connected across said anode
the voltage across said discharge path falls to a value
and cathode for supplying thereto a substantially constant
indicative of the degeneration of said discharge into an
direct current at a voltage substantially proportional to
are.
the resistance of the discharge, cathode-heating means
8. The method of controlling an electrical discharge
for supplying electric heating power to heat said cathode
in vacuo from a thermionically emissive, electrically 70 to electron—emitting temperature, a saturable reactor con
heated cathode to an anode to be heated by said discharge
nected to said cathode effectively in series with said
and which anode evolves gaseous matter when heated
so as to keep the average voltage across said discharge
constant to within relatively narrow limits at a substan
cathode-heating means, means connected across said anode
and cathode for deriving a voltage proportional to the
voltage across said discharge, and a direct current ampli?
tially constant current therethrough while maintaining said 75 er connected to respond to the so-derived voltage and to
3,078,388
17
provide to said saturable reactor a control current varying
with the so-derived voltage from a maximum producing
substantial saturation of said reactor to a minimum at
cutoff of said ampli?er.
14. Apparatus as in claim 13, additionally comprising
means for varying the rate-of-change of said saturating
18
bring said oscillator into operation responsive to abnormal
drops in the voltage between said cathode and said anode,
and means for maintaining the supply of heating current to
said ?lamentary cathode while said oscillator is in opera
tion, whereby said cathode is kept at emitting temperature
until normal operation is restored after arcing.
17. Apparatus for controlling an electric discharge in
current with variation of said control voltage.
a vacuum wherein evolved gaseous matter may form an
15. Apparatus as in claim 13, additionally comprising
ionic plasma, comprising a thermionically emissive cath
adjustable biasing means for varying the value of said
10 ode, a bombardment-heated anode, a constant-current
control voltage at which said ampli?er cuts 011.
DC. power supply connected between said cathode and
16. Apparatus for controlling an electric discharge in
said anode for supplying a substantially constant direct
a vacuum wherein evolved gaseous matter may form an
current to the discharge, means for automatically control
ionic plasma, comprising a thermionically emissive, ?la—
ling the temperature of said cathode to keep the average
mentary cathode, a bombardment-heated anode, a cons
tant-current DC. power supply connected between said 15 voltage between said cathode and said anode substantially
constant, and means for automatically shorting said power
cathode and said anode for supplying a substantially
supply upon a decrease of the voltage between said
constant direct current to the discharge, ?lament-current
cathode and said anode to a low value relative to said
supply means connected to supply heating current to said
average voltage.
?lamentary cathode, an ampli?er connected to provide
an electric control signal responsive to voltage changes 20
between said cathode and said anode, control means re
sponsive to said control signal for varying the cathode
heating curent to keep the average value of the cathode
anode voltage substantially constant, a relaxation oscil
lator including a thyratron, means normally biasing said 25
thyratron beyond cuto?f to keep said oscillator out of
operation, connections for overriding said biasing means to
References Cited in the ?le of this patent
UNITED STATES PATENTS
2,159,767
2,310,286
2,408,091
2,850,676
Liebich ____________ _._ May 23,
Hansell _____________ __ Feb. 9,
Olesen ______________ __ Sept. 24,
Kan ________________ __ Sept. 2,
1939
1943
1946
1958
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