for proof-masses in space charging & charge...

Charging & Charge Controlfor Proof-Masses in Space�Effects of free charge

• Lorentz force

• Electrostatic forces from mirror charges

• Spring Constants

• Forces from applied voltages

�Charging Estimates

• Rates

• Timelines

�Charge Management

• Measurement procedures

• Discharge procedures

LISA Symposium – PSU 24/07/2002

Lorentz force noise


BvQFl ∧= HallEQ−

MetallicEnclosure(Blaser)

BvQFl ∧=η

( ) ( ) ( )222 vBQBQva nnn ηδδη +=

Lorentz force noise


)Hz(10 C 104 HzT/ 105.2 4-15

11−

−

×≤⇒×

= çQf

Bnδ

antinsignific term2 HzC/ 2

25.2 nd⇒≈

f

QeQn πδ

&

See later!

( ) ( ) ( )222 vBQBQva nnn ηδδη +=

Electrostatic forces


FE

k

C

kV

Q

C

C

k

Q

CV

C

kki

i ii

i

n

= − = + −∑ ∑=

∂∂

∂∂

∂∂

∂∂

1

2 22

2

21

−∆≈

∂

∂+

∂

∂==

232

2

2

2

2

2 4

222 d

Ad

d

A

C

Q

k

C

k

C

C

Q

k

C

C

QF lrk

εδε∂∂

( ) spaceparameter - sconstraint 4

2 32

22

dTdd

A

mC

tQta d ∆⇒

∆≈⇒ε&

( ) ( )2

32

2

2

332

22 442

14

2

∆+

∆+≈⇒ dd

A

mC

QQdd

d

A

Cd

A

mC

Qa n

nn

εδδ

εε

( )

∆+≈⇒ 2

332

2 421

4

2d

d

A

Cd

A

C

Qk

εε

Electrostatic forces


FE

k

C

kV

Q

C

C

k

Q

CV

C

kki

i ii

i

n

= − = + −∑ ∑=

∂∂

∂∂

∂∂

∂∂

1

2 22

2

21

( )

2

2

2

2

2

222

nr

cmr

rn

rnn

kk

CV

mC

Q

k

CV

k

C

mC

Q

k

CV

mC

Q

k

CV

mC

Qa

δ

δδ

∂

∂+

∂

∂∆

∂

∂+

∂

∂+

∂

∂∆≈⇒

Common-mode voltage effects disappear to first order in force

Differential-mode voltages used for charge measurement – see later

2

2

2 k

CV

C

Q

k

CV

k

C

C

Qk r

cmr

∂

∂+

∂

∂∆

∂

∂≈⇒

Summary of Charge Limits


10 µV/√HzδVn

10µm∆d

Assumptions!!

3x10-11Potential (noise) (1Vcm)

10-24Potential (noise)

4x10-13Stiffness (1Vcm)

10-8Stiffness (∆Vdm)

3x10-12Stiffness (assymetry)

1.4x10-11 (10-4 Hz)Charge Noise (∆d)

4x10-11Displacement Noise

4x10-11 (10-4 Hz)Lorentz Noise

Limit (C)Effect

Charging Rates

Jafry, Sumner & Buchman - 96

Jafry & Sumner - 97 } GEANT3

~16 charges/sec

Modeling of Charging (LTP)


•Compare rates using GEANT4 with low-E extension

•Get charging timelines

Modeling of Charging (LTP)


Modeling of Charging (LISA)


Modeling ofCharging(LISA)


‘…the hadron-inducedfluorescence isvery badlyimplemented.So we switchedit off …’

Modeling of Charging (LISA)


Charge Management System


�Charge Measurement using applied dither force intransverse direction with capacitive sensing of test-mass response.

�Discharge technique using differential illuminationof surfaces with UV illumination, with bias voltageenhancement if needed.

Cd

AQEFd

ε2=

Charge Measurement

Dither Technique•Different gaps in eachdirection give differentmeasurement authority

•Need to see ditherabove residual drag-freeposition noise

•Assume transversedither with 1nm/√Hzposition noise

2514

292/1 109.2

107.1

101dx

dxx w

wE ××≈

×

××≈

−

−

τLISA Symposium – PSU 24/07/2002

1.E+01

1.E+02

1.E+03

1.E+04

1.E+05

1.E+06

1.E-03 1.E-02 1.E-01

Dither Frequency (Hz)

Res

po

nse

Tim

e (s

)

Charge Neutralisation

tpevcvcflfi TTLTR ηηηη=

LTiηlfTfηvc2Tvc ~ 3x1010 photons/s

Oxford Instruments HPSUV1000 fibre

Caburn FFTUV1000 (Special) secondper ronsphotoelect 103

later) (see surfaces real''for measuring

needsbut target is A 2537at 10

4

o6

×⇒

≈ −peη


•Dual Surface Illumination with dc bias voltage tomodify ballistic trajectories

—2.1 V/m/eV gives δQ/δt ~ 15 charges/s

—80 V/m/eV gives δQ/δt ~ 6x103 charges/s

—500 V/m/eV gives δQ/δt ~ 3x104 charges/s

•Differential Surface Illumination usingindividual lamp currents to modify electron fluxes

—L(φ) and L(I)

Charge Transport - ηt


•Polar Ouput

Charge Transport

Polar Output of 1mm Diameter HPSUV Fibre Optic Cable

0

5

10

15

20

25

0 5 10 15 20 25

Polar Angle (Degrees)

Ou

tpu

t (P

ho

ton

s/s/

sr

[Rel

ativ

e])


•Lamp Ouput Against Power

Charge Transport

Output of UV Lamp As Function Of Power

y = 3.0778x - 1.8681

0

0.5

1

1.5

2

2.5

3

3.5

0.9 1.1 1.3 1.5 1.7

Power Into Lamp (W)

Ph

oto

n O

utp

ut

(Rel

ativ

e)

DC ROSAT Supply

Commercial Supply


•Differential Surface Illumination

Charge Transport

Electrode:Housing:TM

1 : 1.3 : 0.1

1 : 1.1 : 13.1

-3.9x103< <+3.5x103 /sQ&

Add dc bias to driveharder when required

⇒ x5


Charge TransportUV Lamp


Photo-emissive surfaces•Tests have been done on sample surfaces made of Au and Au/MoS2

within LTP development programme.

Bias voltage on TM (V) vs time (s)

-12

-10

-8

-6

-4

-2

0

2

4

-500 0 500 1000

Time (s)

Bia

s vo

ltag

e on

TM

(V

)

19/06/02

25/06/02

MoS2/Au (CF2/02/9

Bias voltage on TM (V) vs time (s)

-10

-8

-6

-4

-2

0

2

4

-100 100 300 500

Time (s)B

ias

volt

age

on T

M (

V)

18/06/02

24/06/02

Au

η=0.2x10-5


η=~0.7x10-5

LISA discharge in100secs!!

Team at ICSTM

• Henrique Araujo • Chris Lee

• Trevor Beek • Igor Liubarsky

• Arthur Bewick • Andy Rochester

• Chris Carr • Geoff Rochester

• David Davidge • Diana Shaul

• Alex Howard • Tim Sumner

• Manu Joshi • Ray Swain


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