investigation of effects associated with electrical charging of fused silica test mass v....
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Investigation of effects associated with electrical chargingof 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
2
Introduction
Fused silica mirrors — dielectric bodies suspended in high vacuum
They can trap and store electrical charges
These charges interact through electrostatic couplingwith the environment and electrostatic actuators in particular
Result of this interaction may be:
additional loss degradation of mechanical Q additional thermal noise
accidental variation of electrical charge q variation of Coulomb force additional noise
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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
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Experimental setup Vacuum p < 10-7 Torr
All fused silica bifilar pendulum:
Mass M = 0.5 kg, Fibers: L = 25 cm, d =200 m,
Torsion mode f 1.14 Hz,
Quality factor Q 8107 ,
Relaxation time * 2.2107 sec,
Initial amplitude A 0.07 rad
Multistrip capacitive probe (two sets of gold strips sputter-depositedon fused silica plate) connected with high impedance amplifier.
Probe voltage U = kqA
q – electrical charge on the pendulum(distribution of charge is unknown)
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Long-term measurements of probe signalin process of free decay of the cylinder oscillation
About 2 years of observation a lot of runs ( each run is about 30 days )
Averaged rate of charge variation: 104 e/cm2 day (assuming uniform distributionof charge on the end faceof the cylinder)
Charge variation corresponds to negative charging of the cylinder
Resolution in measurement of charge is limited by seismic noise and parasitic effects
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0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34
tim e, days
sign
al f
rom
pro
be,
arb
itra
ry u
nits
Tim e dependence of voltage from the probe
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- - - - - - - - - -
Why we continue to study behaviorof electrical charges on fused silica test masses?
1) 2)
mass-q+q
d
Conductive plate
q=108e
q= 104e
D=3 mm
mass
High energy particle
Surfacelayer
Surfacecharge
•Generation of mobile charges within surface layer of the material
•Their separation in electric fieldof surface charges
gr11
20
el FN10d8π
δqqδF
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Rare big jumps of signal from the probe synchronous with fast decrease of the cylinder amplitude
4 events in 12 runsof measurements
multistep structure of jump
Initial charge density: of order of 106 --107 e/cm2
After the jump:of order 108 e/cm2
(assuming uniform surface free charge density)
Change of amplitudein the process of jump corresponds to damping Q-1 of order of 10-4
Resolution of details is determined by averaging time – 70 sec
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2
2.2
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2.8
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am
plit
ud
e,a
rbitr
ary
un
its
0 10 20 30 40
tim e, days
3 0
4 0
5 0
6 0
7 0
8 0
volta
ge
fro
m p
rob
e, a
rbitr
ary
uni
ts
0 10 20 30 40
tim e, days
Tim e dependence of amplitude of the cylinderand voltage from the probe
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Behavior of the system after the first jump
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”)
With time elapsed from the first jumpfast changes of the probe’s voltage decrease. Spoiled state transfers to the original state within relaxation time of order of one month
4 0 8 0 1 2 0 1 6 0 2 0 0
tim e, hours
1.8
2
2.2
2.4
2.6
2.8
3
amp
litud
e, a
rbitr
ary
units
4 0 8 0 1 2 0 1 6 0 2 0 0
tim e, hours
1
2
3
4
5
6
pro
be s
igna
l, ar
bitr
ary
uni
ts
First jump
Excitation (increase)
of amplitude
Excitation (increase)
of amplitude
Amplitude of the cylinder
Voltage from the probe
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Possibility that the pendulum modesof the cylinder causes it to touch the electrode
• A contact electrification can take place if the pendulum is swingingfar 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
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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) 2.72
2.76
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2
2.4
2.8
3.2
3.6
4
0 20 40 60
tim e, m in
Change of torsional amplitude in the case of "jump" (black) and of touching (red)
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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
These facts can not be the evidence for the absence of touching in our experiments. It is worth further investigation 0
0.1
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0.5
sign
al f
rom
pro
be,
arb
itra
ry u
nits
0 20 40 60
tim e, m in
Change of probe signal after admission of air
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Search for correlation between signals fromthe probe and cosmic ray detectors
Cosmic ray detector system
11 particle detectors (plastic scintillator paddles 180180.8 cm3 ) suppliedby photomultipliers were installed around the vacuum chamber with the pendulum
Selection of events with maximum summarized voltage from all paddles
Vacuumchamber
Scintillationdetectors
ADC
Threshold setting
Computer
Peak detectors
Wall
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Result of measurements
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
But we can not exclude correlation with:
low energy particles passing locally through the surface layer of the test mass
1 2 1 4 1 6 1 8
tim e, hours
3 2
3 6
4 0
4 4
4 8
5 2
prob
e volta
ge (q)
2
2.2
2.4
2.6
2.8
amp
litud
e (A
)
0
5
10
15
20
25
cosm
ic r
ay d
ete
ctor
sig
nal (
CR
)
A
q
CR
Fragment of record of am plitude and signalsfrom the probe and scintillation detectors
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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