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Investigation of effects associated
with electrical charging
of fused silica test mass
V. Mitrofanov, L. Prokhorov, K. Tokmakov
Moscow State University
P. Willems
LIGO Project, California Institute of Technology
LIGO-G040097-00-Z
Introduction
Fused silica mirrors — dielectric bodies suspended in high vacuum
They can trap and store electrical charges
These charges interact through electrostatic coupling
with the environment and electrostatic actuators in particular
Result of this interaction may be:
п‚· additional loss пѓћ degradation of mechanical Q пѓћ additional thermal
noise
п‚· accidental variation of electrical charge пЃ¤q пѓћ variation of Coulomb
force пѓћ additional noise
2
Mechanical loss in fused silica oscillators
due to electrical charges or electric field
A number of experiments which have demonstrated the effects of
electrical charge on mechanical loss:
•
•
•
Univ. of Glasgow (Class. Quantum Grav., 14 (1997) 1537
Moscow State Univ. (Phys. Lett. A., 278 (2000) 25
MIT (Rev. Sci. Instrum., 74 (2003) 4840
The mechanism of loss is not clear (only hypotheses were proposed)
Nevertheless it is likely that the additional thermal noise associated
with charges is not dangerous for Adv. LIGO if the electrical
charge on mirrors is not unduly large
3
Experimental setup
Vacuum
p < 10-7 Torr
All fused silica bifilar pendulum:
Mass M = 0.5 kg, Fibers: L = 25 cm, d =200 пЃ­m,
Torsion mode
f п‚» 1.14 Hz,
Quality factor Q п‚» 8п‚ґ107 ,
Relaxation time пЃґ* п‚» 2.2п‚ґ107 sec,
Initial amplitude A п‚» 0.07 rad
Multistrip capacitive probe
(two sets of gold strips sputter-deposited
on fused silica plate) connected with high
impedance amplifier.
Probe voltage U = kп‚ґqп‚ґA
q – electrical charge on the pendulum
(distribution of charge is unknown)
4
Long-term measurements of probe signal
in process of free decay of the cylinder oscillation
T im e d e p e n d e n c e o f v o lta g e f ro m th e p ro b e
About 2 years of observation
a lot of runs ( each run is about
30 days )
п‚· Charge variation corresponds to
negative charging of the cylinder
п‚· Resolution in measurement of
charge is limited by seismic
noise and parasitic effects
sig n a l fro m p ro b e , a rb itra ry u n its
п‚· Averaged rate of charge variation:
п‚» 104 e/cm2 day
(assuming uniform distribution
of charge on the end face
of the cylinder)
1 .9
1 .8
1 .7
1 .6
1 .5
1 .4
0
2
4
6
8 10 12 14 16 18 20 22 24 26 28 30 32 34
tim e , d a y s
5
Why we continue to study behavior
of electrical charges on fused silica test masses?
2)
1)
High energy particle
Conductive plate
mass -
q+пЃ¤q
q=108e
пЃ¤q= 104e
mass
d
D=3 mm
Surface
layer
ОґF el пЂЅ
q п‚ґ Оґq
8ПЂ пЃҐ 0 d
2
п‚» 10 пЂ­ 11 N пЂѕ F gr
-
Surface
charge
•Generation of mobile charges
within surface layer
of the material
•Their separation in electric field
of surface charges
6
Rare big jumps of signal from the probe synchronous
with fast decrease of the cylinder amplitude
4 events in 12 runs
of measurements
T im e d ep en d en ce o f am p litu d e o f th e cylin d er
an d vo ltag e fro m th e p ro b e
multistep structure of jump
After the jump:
of order 108 e/cm2
(assuming uniform surface
free charge density)
Change of amplitude
in the process of jump
corresponds to damping
Q-1 of order of 10-4
Resolution of details is
determined by averaging
time – 70 sec
80
v o lta g e f ro m p ro b e , a rb itra ry u n its
п‚·
3
Initial charge density: of
order of 106 --107 e/cm2
a m p litu d e ,a rb itra ry u n its
п‚·
2.8
2.6
2.4
2.2
2
1.8
70
60
50
40
30
0
10
20
tim e, da ys
30
40
0
10
20
30
tim e, da ys
7
40
Behavior of the system after the first jump
A m p litu d e o f th e c y lin d e r
After the first jump:
Repeated increase of torsion amplitude
by means of electrostatic excitation
results in a new cascade of jumps
of amplitude and voltage from the
probe (spoiled state of the system
“fused silica pendulum  nearby
electrodes”)
a m p litu d e , a rb itra ry u n its
3
F irs t j u m p
2 .8
2 .6
E x c ita tio n
(in c re a s e )
o f a m p litu d e
2 .4
2 .2
2
1 .8
40
80
120
160
200
tim e , h o u rs
V o lta g e f ro m th e p ro b e
6
p ro b e sig n a l, a rb itra ry u n its
With time elapsed from the first jump
fast changes of the probe’s voltage
decrease. Spoiled state transfers to
the original state within relaxation
time of order of one month
5
E x c ita tio n
(in c re a s e )
o f a m p litu d e
4
3
2
1
40
80
120
160
200
tim e , h o u rs
8
Possibility that the pendulum modes
of the cylinder causes it to touch the electrode
•
•
A contact electrification can take place if the pendulum is swinging
far enough to touch the electrode plate due to seismic excitation
Only torsional amplitude is well measured by the optical sensor
The electrometer signal on the pendulum frequencies give information
about pendulum amplitudes with large uncertainty
So we can not control the touching or close approach of the pendulum to
the electrode plate in this set up
9
Changes of torsional amplitude and
probe signal in the case of provoked touching
For the provoked touching
behavior of the system is
much the same as it is in the
case of our “jumps”: decrease
of amplitude and increase of
probe signal as well as
transfer to the “spoiled” state
There are at least two distinctive
property of provoked
touching:
•
In the case of provoked
touching change of amplitude
and probe signal is fast (within
the time resolution)
C h a n g e o f to rs io n a l a m p litu d e in th e c a s e
o f "j u m p " (b la c k ) a n d o f to u c h in g (re d )
4
2 .8 8
3 .6
2 .8 4
3 .2
2 .8
2 .8
2 .7 6
2 .4
2
2 .7 2
0
20
40
60
tim e , m in
10
Changes of torsional amplitude and
probe signal in the case of provoked touching
Admission of small portion of
air into the chamber results in
significant drop of probe signal
(supposedly due to gas
breakdown). After the “jump”
the probe signal can be
reduced only by electrical
discharge in rough vacuum
пѓј These facts can not be the
evidence for the absence of
touching in our experiments. It
is worth further investigation
0 .5
s ig n a l fro m p ro b e , a rb itra ry u n its
•
Change of probe signal after admission of air
0 .4
0 .3
0 .2
0 .1
0
0
20
40
60
tim e , m in
11
Search for correlation between signals from
the probe and cosmic ray detectors
Cosmic ray detector system
11 particle detectors (plastic
scintillator paddles
180п‚ґ18п‚ґ0.8 cm3 ) supplied
by photomultipliers were
installed around the vacuum
chamber with the pendulum
Selection of events with
maximum summarized
voltage from all paddles
Peak detectors
Scintillation
detectors
ADC
Threshold
setting
Vacuum
chamber
Computer
Wall
12
Result of measurements
low energy particles
passing locally through
the surface layer of the
test mass
2 .8
52
20
48
A
2 .6
CR
15
10
a m p litu d e (A )
c o s m ic ra y d e te c to r s ig n a l (C R )
But we can not exclude
correlation with:
25
44
2 .4
q
40
2 .2
5
0
36
2
12
32
14
16
18
tim e , h o u rs
13
p ro b e v o lta g e (q )
We have not found
statistically significant
time coincidences
between large signals
from detectors and jumps
of charge and amplitude
in the “spoiled” state of
the test mass
F ra g m e n t o f re c o rd o f a m p litu d e a n d s ig n a ls
fro m th e p ro b e a n d s c in tilla tio n d e te c to rs
Conclusion
• We have a number of empirical facts but we can not
unambiguously interpret these facts yet
• We suppose that the electronic properties of the surface of
dielectric mirror can play the important role but they are poorly
studied till now
• We do not yet ready to say that electrical charges are not
dangerous for LIGO
14
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