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Nov. 5, 1946.
c. w. HANsELL
2,410,732
SENSITIVE PHOTOCELL AND CIRCUIT
Filed March 8,' 1943
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Patented Nov. 5, 1946
w
UNITED STATESW PATENT OFFICE
2,410,732
SENSITIVE PHOTOCELL AND CIRCUIT
Clarence W. Hansell, Rocky Point, N. Y., assignor
to Radio Corporatîonîof America, a corporation
of Delaware
l
Application March 8, 1943, Serial No. 478,423
11 Claims. (Cl. Z50-41.5)
The present invention relates generally to
photocell circuits, and more particularly con
cerns a highly sensitive phototube in the form of
a phototube secondary emission amplifier.
One of the objects of the present invention is
to provide a simplified circuit for electrically re
producing sound waves which have been recorded
on nlm.
Another object is to provide a single unit which
serves the dual purpose of a phototube and high
gain ampliñer.
A further object is to provide a magnetron de
vice which is sensitive to light waves and supplies
an output current whose magnitude is approxi
mately proportional to, and follows modulations
of , said light waves.
2
for rapid multiplication of the space charge due
to the emission of secondary electrons from the
cathode. By suitable adjustments of the alter
hating current potential and frequency, which
can be accomplished automatically, it is possible
to make the growth of secondary emission and
space charge such that nearly saturation value is
reached on each cycle, just as it is in the case of
the low emission magnetron oscillator described
in my copending application supra. A modulated
direct current, carried by the electron emission
from the cathode and supplied from a direct cur
rent potential source in circuit with the anode
or an associated electron collector, provides the
useful output power. This direct current may be
enormously greater than the initial photoemis
A still further object is to provide a phototube
device arranged in a magnetic ñeld, whereinV the
sion current. It should be understood, however,
photoemissive cathode also serves as a secondary
not desired‘to reach the ultimate maximum space
20 charge limited secondary emi‘ssive current in each
electron emitter.
Briefly stated, the invention is based upon my
discovery and appreciation of the fact that elec
that in the practice of the present invention it is
cycle, because such a condition of >maximum sec
ondary emission current would produce limiting
trons emitted from a surface may be made to re
in the phototube and distortion of the modulation
turn to the surface with sufficient energy to cause
to be reproduced.
_
.
secondary emission and a growth in rate of emis 25
A feature of the invention lies in the use of a
sion through a combination of electric and mag
photo surface which is a secondary emitter for the
netic fields, one or both of which is changing in a
photo electrons.
correct direction at a suñîciently rapid rate. This
phenomenon is discussed in detail in my copend
One advantage of the dual purpose phototube
and ampliñer of the invention is that it has an
ing application Serial No. 477,062, ñled February 30 extremely large electron multiplying ratio and
25, 1943, to which reference is made. I employ
is simpler to build and easier to operate than
this phenomenon to make asingle surface serve
prior constructions which employ a multiplicity
the same function in multiplying photoemissive
of electron multiplying stages in series.
electrons by means of secondary emission as
A more detailed description of the invention fol
though a large number of surfaces had been used 35 lows in conjunction with the drawings, wherein:
in cascade.
Figs. 1 and la illustrate diiferent cross-sec
A simple form of the invention comprises a
tional views of a 'simple form of phototube sec
phototube device employing a photoemissive cath
ondary emission ampliñer in accordance With the
ode which is also a good secondary electron emit
invention. Fig. la is a view taken along the line
ter, so arranged in a magnetic field that the mag 40 Ia-Ia of Fig. l; and
netic ñux lines extend parallel to the cathode
Figs. 2, 3 and 4 schematically illustrate three
and at right vangles to the electric ñeld between
different complete circuit embodiments of the
an anode and the cathode. The phototube device
invention.
of the invention is thus built in the manner of a
In Figs. l and la, the phototube amplifier is
magnetron. A modulated light, such as is pro 45 shown as including a cylindrical cathode I0 which
duced by light shining through a sound track on
is treated to be photoemissive and also capable of
a motion picture film, is arranged to shine 0n the
emitting copious secondary electrons when bom
cathode to provide an initial or primary emission.
barded by electrons, a surrounding cylindrical
By means of a suitable alternating current poten
metallic anode Il, and a pair of coils I'2, I2
tial, at a suitable frequency, applied between the 50 for producing a magnetic field having ilux lines
anode and cathode, there is obtained ata par
extending approximately parallel to the cathode.
ticular part in the cycle a condition of rising
The anode, it should be noted, has end portions
potential required for the accumulation of space
or closures I5 and these end portions are closer
charge due to the photoemission, followed about
to the cathode than the cylindrical portion of the
a quarter cycle later by the conditions required 55 anode. The space between the anode and _cath
2,410,732
3
ode is evaçuated in the manner of the customary
vacuum tube or magnetron.
This can be done
by surrounding the anode with an air-tight glass
envelope or, as shown, by making the anode
as the envelope and sealing off the portions
through which the leads extend by means of a
glass seal I3. The metallic anode is apertured
at i4 and the aperture `tightly sealedïby a'trans
parent window i5 to permit light >rays to shine
4
application of a superimposed alternating po
tential on the anode circuit, to a value below cut
off (i. e., below the value at which the electrons
strike the anode on their first outward trip),
then electrons will begin to miss the anode and
return to the cathode. If the reduction or fall
in anode potential is sufficiently rapid, electrons
idrawnfrom the cathode or from the space close
`to it, while the potential is falling, will circu
10 late out from the cathode and return to the
on the cathode i0.
' .cathode with an excess of energy, striking the
Fig. 1 shows one manner of impressing modu
cathode and causing the emission of additional
lated light waves on the cathode. An >illumi
electrons due to secondary emission. Thus, while
nating light source I1, in the> form of a lamp,
the anode-to-cathode potential is decreasing, if
throws light onto the soundtrack of .an .ad
vancing motion picture film I8 through ya lens
IB which focusses the light upon a mask, not
the decrease >is within a suitable range of rates,
there isv obtained a rapid growth in total cath
uponthefñlm lß is elongatedin‘cross-section with
ma ,in a very short time, reach values enormous
ode emission due to secondary emission. Up to
shown, adjacent the film. Light passing through
the point where space charge limiting becomes
the sound track of the film is modulated and
a factor, this increase in emission is an exponen
passes through the window I6 for shining-upon
the cathode i0. The 'beam of light impinging 20 tial function of time. The mam‘mum emission
ly greater than the photoemission but `still may
have values proportional to the photoemission.
If, before substantial space charge limiting sets in,
Fig. 2 shows a .complete circuit arrangement
for using the phototube secondary emissive yde 25 the anode-to-cathode potential reaches low values
then the emission will automatically stop rising
vice'of Figs. -l -and la.. In Fig. 2 there is pro
and will decrease again to nearly Zero. Thus
vided .a radio frequency parallel ltuned `oscil
large emission takes place in pulses, repeated once
latory circuit .2G coupled across the anode tand
each cycle of the radio frequency potential ap
'cathode‘through a radio frequency by-pass con
denser 2 l. A `power source‘2-2 of radio frequency 30 plied between anode and cathode. The value of
the pulses of emission, both instantaneous and
current is magnetically coupled tothe tuned cir
average, is proportional to the photoemission but
cuit 20, as shown. A direct current power source
lmay be enormously greater than the photoemis
23 supplies energizing current for the ñeld coil
sion.
I2 over lleads `24, and supplies a suitable posi
tive polarizing potential between cathode ID and 35 If the device is suitably designed, and peak
emissions are allowed `to become large enough,
vanode Il over a-path which includes the resistor
magnetron oscillations may .take place which may
25, the primary .winding of the audio coupling
add still further to the production of emission.
transformer `26, and the-coil of tuned circuit '20.
When, due to the growth of total emission from
‘The audio frequency output modulations 4are
amplified in audio amplifier 2l and then utilized 4.0 secondary emission, the total space charge has
the majoraxis of the elongated beam lying trans
verse of Athe beam.
in a suitable circuit such as a loudspeaker or
become great enough, high frequency oscillation
headphones, not shown. The radio frequency by
pass condenser 2i »prevents the radio I)frequency
in the anode circuit will start, due to the condi
tion of negative resistance. Oscillations will con
energy in tuned circuit 2i? from entering the
audio transformer 28.
A relatively long time constant'volume-control
circuit comprising a vacuum tube ‘28 serves to
maintain the average direct current through ythe
tinue until the anode-to-cathode potential drops
“ to a Value too low to maintain oscillation.
The
presence of oscillations adds to electron bombard
ment of the cathode in a manner to increase and
prolong the production of secondary emission.
It has also been pointed out in my copending
by varying a load on the tuned circuit 20 by 50 application that the same result, in producing
growth `of electron emission, obtained by a rapid
means of coil 29 which _is coupled to the tuned
ly decreasing electric ñeld can alternatively be
circuit "2% and connected between the anode and
achieved by a rising magnetic field. That is, an
cathode of the tube .28., The control electrode
alternating magnetic field, in place of, Yor in ad
>or grid of tube 28 is connected to one terminal
dition to, the high frequency electric ñelcl may
of resistor 25 whose other terminal Vis connected
be used to cause the growth in electron emission
to the cathodethrough a resistor-condenser ar
from the cathode due to secondary emission.
rangement 30. A condenser 3| in shunt to re
Likewise, decreasing electric ñeld and increasing
sistor 25 Vby-passes all alternating currents and
magnetic field may be used together to cause the
gives only a small response to currents of mod
growth of cathode emission due to secondary
ulation frequency and higher. )In this way the
phototube substantially constant,»and this is done
phototube sensitivity is controlled, and limiting
in the phototube amplifier is prevented.
emission.
. _Abrief >exposition of the principles underlying
as illustrated in Figs. 1 and la, and as used in Fig.
2, is so designed that electrons are pulled out of
4the 'operation of the'lpresent invention, and which
'is described'in my copending application, Serial
No. 477,062, will now begiven. .If the anode-to
cathode potential of the phototube is _somewhat
`greater than the cut-off potential corresponding
The phototube device of the present invention,
the space charge longitudinally, by components
of motion parallel to the magnetic ñeld and
caused to strike the lanode end portions i5. This
is shown very roughly by the dotted lines of Fig.
1u.. Thus, during the growth of space charge, Aa
to 'a particular intensity of magnetic field chosen,
then >nearly all :the electrons which leave the 70 portion of the electrons move oiî the end of the
,cathode to the end closure i 5, even though at »this
»cathode 'l0 will strike the anode H on the ñrst
particular time the anode-to-cathode Vpotential
outward trip. In other words, the direct current
may be below cut-01T potential for electrons which
potential is >adjusted high enough to prevent the
would have to move at right angles to the‘mag
accumulation of space charge. Now,’^if the anode
.potential is reduced rapidly, as by pulsing or the
netic field. It is only the components ‘of `motion
2,410,732
5
6
ñux lines which are bent or curved in direction
cipauy by tne'strength' of the magnetic neld and.
may be determined approximately by magnetron
of the electrons which are at right angles to the
to produce a rotating space charge. v As a conse
formulas to be
quence of the longitudinally moving electrons to
the end portion l5, there is an ave-rage direct cur
rent flowing to the end closure l5 which may be
F=2.29X (10) 6H
where H is the magnetic field strength in gausses.
modulated by light intensity. This modulated
direct current, supplied from the direct current
to the invention, magnetic ñeld strengths up to
-
»
In practical magnetron phototubes, according
say 2200 gausses are practical which would corre
potential source 23 in series with the primary
winding of transformer 26 constituting an output 10 spond to doubling the cathode emission at a rate
of 5X (l0)9 times per second. Therefore, the
coupling impedance and in series with the mag
time for each period of falling potential and
netron phototube, then provides useful audio out
growth of emission may be very short even when
put power. This modulated direct current is
enormously greater than the initial photoemis
sion current.
In .the practice of the present invention, it is
not essential that oscillations occur, although the
the multiplication of photoemission by secondary
15 emission in each period is very large.
-
The foregoing formula has been given without
considering the effect of accumulation of Space
charge due to photoemission while the anode-to
cathode potential is rising, assuming it does not
during oscillation, the out-of-phase electrons 20 rise all the way to the cut-olf potential for the
strength of magnetic field used. If this is taken
strike the cathode with sufficient veloci-ty to cause
into account, the initial impacting current ,to
secondary emission even when the anode poten
start secondary emission, when the potentialbe
tial is not falling rapidly enough to cause growth
gins to decrease, may be greater than the current
of emission by itself. An advantage of allowing
due to photoemission.
oscillations is that it permits a reduction in the
As for the operation of the volume control cir
applied alternating anode-to-cathode potential.
cuit of Fig. 2, as current flows in the anode l I the
To explain the operation in greater detail, as
growth of secondary emission will be aided by the
onset of magnetron oscillations. This is because,
sume that light causes emission of an electron
from the cathode at a time when the anode-to
cathode potential is somewhat less than the cut
grid of vacuum tube 28 becomes more positive
relative to the cathode of tube 28„ as a result of
which the effective resistance of tube 28 is low
ered and additional loading is thrown on the
off potential and is decreasing rapidly due to the
radio frequency tuned circuit 20, thus tending to
high frequency excitation. Because of the elec
keep the direct current through the magnetron
tric field ‘between anode and cathode, the electron
phototube substantially constant. Any other well
will be accelerated away from the cathode toward
the anode but the magnetic ñeld will bend its 35 known arrangement for controlling a high fre
quency potential in response to a direct current
path and cause it to return to the cathode after
potential may be used. The arrangement is sim
an excursion out toward the anode. Because of
ply a slow acting automatic volume control to
the falling potential and falling electric ñeld, the
prevent overloading of the phototube amplifier
electron is given more energy in moving away
from the vcathode than is given to it while re
turning to the cathode. It therefore returns to
the cathode with considerable energl7 and velocity
and, if the energy is great enough, each returning
electron can, on the average, cause the impact
emission of more lthan one secondary electron.
tube.
Fig. 3 shows an alternative arrangement to
that of Fig. 2. In Fig. 3, the arrangement of the
anode lll’A is different from that of I0 of Fig. 2, in
that Fig. 3 employs an auxiliary electron collector
» output anode 32 to which are joined cooling fins
34. Auxiliary anode 32, in effect, serves the same
purpose as the end closure I5 of Figs. 1 and 2 but
may be operated at a lower potential to reduce
power loss. -A separate glass envelope 33 serves
k low Value up to 10 or more, depending upon the
character of the cathode surface. A ratio of 2 50 to maintain an evacuated space within the pho
totube. The anode lil’ is apertured at I4’ to per
appears to be reasonable for surfaces capable of
mit modulated light to shine on the cathode.
good life and stability. In practice, the surfaces
The radio frequency source 22 is coupled to tuned
used are unlikely >to be uniform in manufacture
circuit 29 for varying the potential of anode to
and will vary with age but the variations may
cathode at the radio frequency of source 22. The
generally be compensated for by operating ad
audio output transformer 26 is here shown cou
justments.
The maximum possible ratio of secondary emis
sion electrons to impacting electrons, for opti
mum impact potentials, may vary from some very-
In using oxide cathodes, made by reducing
barium and strontium carbonates to oxides in
vacuum, which type of cathode is photoemissive,
I have been able, with magnetrons, to demon
strate growth in emission current from cold cath
odes, due to falling anode-to-cathode potential
followed by radio frequency magnetron oscilla
tion, from values too small to measure up to 20
or 30 amperes peak Value. It appears that a
single electron, released from the cathode at the
pled between the auxiliary electron collector an
ode 32 and the anode I û’.
Of course, means to
provide the required magnetic field through the
pliototube would be used with the device of Fig. 3,
though it is not shown therein. ,
Fig. 4 illustrates .another vembodiment of the l
invention which. uses an alternating current
magnetic field insteadof a variable direct current
_f anode to cathode voltage for causing a growth of
secondary emission. The two field coils l2, I2
proper time, is suflicient to cause a growth to
are excited from an alternating current. power
taken by electrons to make their excursion _from
the cathode out toward the anode and back. The
interest of simplification of the drawings, and in
rate or frequency of doubling is determined prin
practice would be impressed through the' glass
source 35 through field and generator tuning con
space charge limiting in the device of the inven
densers 36, 36. The anode is shown in dotted.
tion, under suitable conditions of operation.
Assuming a ratio of secondary electrons to -_ lines, since in practice it will preferably be sec
tionalized to prevent short-circuiting eddy cur
impacting electrons of 2, vthen the total cathode
rents due to the alternating current magnetic
emission will be doubled in each time interval
field;
The source of light is not shown in the
2,410,732
7
Aenvelope and through the'aperture between the
anode sectionsupon the cathode.
What is claimed is:~
8
secondary electrons during a portion of the cycle
of said alternating current.
1. An amplifier having within an envelope a
- -
„
5, A phototube secondary emission amplifier
having within an evacuated envelope a. photo
photo-emissive cathode which emits secondary 5 emissive cathode which emits secondary electrons
electrons due to electron bombardment at a ratio
upon bombardment by primary electrons, an
greater than unity, and an anode surrounding
anode structure surrounding said cathode, said
said cathode, said anode having an aperture and
said envelope having a transparent portion
registering with said aperture to enable light from
an external source to strike said cathode, and
means vto produce a periodically changing mag
netic field having flux lines extending parallel
to said cathode.`
`
ì
2. An ampliñer having within an envelope a
anode having an aperture and said envelope hav
ing a transparent portion vregistering with said
aperture to enable light from an external source
to strikesaid cathode, an electron collecting sur
face adjacent one end of said cathode andv more
closely spaced relative to said cathode than said
anode structure, said electron collecting surface
and said anode structure being integral with one
photo-emissive cathode which emits secondary
another,` a source of unidirectional potential con
electrons at a ratio greater than unity, and an
nected between said anode and cathode for main
anode surrounding said cathode, said anode hav
ing an aperture and said envelope having a trans
parent portionregistering with said aperture to
enable light from an external source to strike
taining said anode and said electron collecting
surface at a positive potential relative tov said
cathode, a tuned radio frequency circuit in shunt
to said anode and cathode, a source of radio fre
quency current coupled to said tuned circuit for
varying the electric ñeld between said anode and
cathode at a radio frequency rate, and means for
said cathode, said anode having at least one closed
end constituting an electron collecting surface in
a plane at right angles to the axis of said cathode
for collecting electrons which pass thereto by 25 producing a magnetic field having fluxk lines
parallel to said cathode.
virtue of motions parallel to the magnetic iield,
6. An amplifier having within an envelope a
and means to produce a periodically changing
phGto-emissive cathode which emits secondary
electric field between said anode and cathode.
electrons at a ratio greater than unity, and an
3. A photocell secondary emission ampliiier
having within an evacuated envelope a photo 30 anode surrounding said cathode, said anode hav
ing an aperture and said envelope having a trans
emissive cathode which emits copious secondary
parent portion registering with said aperture to
electrons upon bombardment by primary elec
trons, an anode structure coaxial with and sur
enable light from an external source to .strike said
cathode, said anode having at least one closed end
rounding said cathode, said anode having an
aperture and said envelope having a transparent 35 adjacent one end of said cathode and constituting
an electron collecting surface to collect electrons
portion registering with said aperture to enable
which pass thereto by virtue of motions parallel
light» from an external source to strike said
to the magnetic field, means to produce an electric
cathode, an electron collecting electrode with a
ñeld between said anode and cathode, means to
surface adjacent one end of said cathode and
more closely spaced relative to said cathode than 40 produce a magnetic field having flux lines extend
ing parallel to said cathode, and a source of
said anode structure, said electron collecting
electrode and said anode structure being directly
cyclically varying current coupled to said am
pliñer for causing at least one of said fields to
periodically change at a radio frequency rate,
directional potential connected to said anode and
said cathode for producing an electric field there 45 whereby photo-electrons are caused to return to
said cathode to produce secondary electrons.
between, means for producing a magnetic field
connectedY together electrically, a source of uni
7. A photocell secondary emission amplifier
having within an evacuated envelope, a photo
emissive cathode which emits copious secondary
coupled to said amplifier for periodically chang
50 electrons upon bombardment by primary elec
ing one of said fields.
trons, an anode structure surrounding said
4. An electron discharge device having within
cathode, said anode having an aperture and said
an evacuated envelope a photo-emissive cathode
envelope having a transparent portion register
which emits secondary electrons at a ratio greater
ing with said aperture to enable light from anex
than unity, and an anode surrounding said
cathode, said anode having an aperture and said 55 ternal source to strike said cathode,- an electron
collecting surface adjacent one end of said
envelope having a transparent portion registering
cathode and more closely spaced relative to said
with said aperture to enable light from an ex
cathode than said anode structure, said electron
ternal source to strike said cathode, said anode
collecting surface and said anode structure being
having at least one closed end constituting an
electron collecting electrode with a surface in a 60 integral with one another, a source of unidirec
tional potential connected between said anode
plane at right angles to the axis of said cathode
and cathode for maintaining said anode and said
for collecting electrons which pass thereto by
electron collecting surface at a positive potential
virtue of motions parallel to the magnetic
field, means for producing an electric field 65 relative to said cathode, a tuned radio frequency
circuit in shunt to said anode and cathode, a
between said anode and cathode means for
source of radio frequency current coupled to said
producing a magnetic field having flux lines
tuned circuit for varying the electric field be'
extending parallel to said cathode, and at
tween said anode and ycathode at a radio fre
right angles to the magnetic ñeld, a source of
quency rate, means for producing a magnetic
alternating current potential coupled to said de
70 field having flux lines parallel to said cathode, a
vice for causing said electric lield to periodically
volume control circuit coupled to said tuned cir
change. at a rapid rate, through values less than
cuit and responsive to the average direct current
the cut-oli" value for the selected intensity of
through said ampliñer to maintain this current
having flux lines extending parallel to said
cathode, and a source of alternating current
magnetic field;`> whereby photo-electrons are
_caused toy return to said cathode to produce 75
nearly constant. _
8. An electron discharge device system corn
2,410,732
10
prising a photo-emissive cathode and means in
circuit therewith for causing the photo-electrons
emitted by said cathode to return to and bombard
said cathode to produce secondary electrons, said
means comprising a source of unidirectional cur
rent for producing an electric field and a source
of variable current for producing an increasing
magnetic ñeld.
magnetic ñeld in the space between said cathode
and anode with the lines of force substantially
parallel to said cathode, and means coupled to
said system and producing an electric ñeld be
tween said anode and cathode, the values of said
fields being such that photo-electrons emitted
by said cathode return to and bombard said
cathode to produce secondary electrons.
9. An electron discharge device system com
11. In an electron discharge device system,
prising a photo-emissive cathode and means in 10 means for multiplying an electron current, pro
circuit therewith for causing the photo-electrons
duced by photo-emission from a cathode, com
emitted by said cathode to return to and bombard
prising an anode, a magnetic field, combined
said cathode to produce secondary electrons, said
direct and alternating high frequency electric
means comprising a source of unidirectional cur
ñelds at right angles to the magnetic field, means
rent for producing a decreasing electric ñeld and 15 for collecting and utilizing a portion of the
a coil having coupled thereto a source of variable
electrons emitted in the form of an anode to
current for producing and increasing magnetic
cathode current, and space discharge means in
field.
circuit with said system and responsive to the
10. An electron discharge device system in»
flow of electron current in said system for auto
cluding a photo-emissive cathode, and a sur 20 matically controlling the average value of said
rounding anode, a source of illumination having
anode to cathode current.
light rays impinging on said cathode, means
coupled to said system and producing a changing
CLARENCE W. HANSELI...
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