LPOL-cavity• Introduction

• Tests at Orsay

• Optics (laser polarisation)

• Calorimeter DAQ

• Mechanics & installation at DESY Norbert’s talk

Principle of the Pe Measurement witha Longitudinal Polarimeter

Compton Scattering: e+ e+

Cross Section: d/dE = 0(E) - PeS1(E)

0, 1: known (QED) Pe: Polarization of the e beam to be measured S: level of circular polarization of the laser beam

Luminosity (electron-laser): e

2 2e,y ,y

L IL

k

P

e (27.5GeV) (k=1.165eV)

E

Photondetector

Ie: e beam intensityPL: Laser beam power

Scatteredphoton

Pe=0.6

Large PL & Shigh precision forPe

Fabry-Perot cavity: principle

e beam

Polar.Lin.

Polar.Circ.

When Laser =0 c/2L resonance

L

•But : /Laser = 10-11 laser/cavity feedback

•Done by changing the laser frequency

Laser: Nd:YAG (infrared, =1064 nm)

Gain 8000

Photodiode feedback (Saclay)

Laser ND:YAG

Mirrormounts

Motorised mirrors

CCD

Vacuum pump

Optcal roomTemperature:

0.5o

Test cavity at orsaySept. 2001/oct. 2002

V

2Hz & 10V pic-pic Ramp

Intensitytransmited laser=75MHz

t(oscillo)/s

zoom

t(oscillo)/s

Intensity reflected

100 s

laserglancavity

P-diode

200 ms

fit

Data (oscillo) laser

gain cavity test 2000 (8000 expected)

(laser=3.108MHz)

Because of mirrorCoatings…

qwp

Hera plane is notHorizontal …

Final cavity Orsay: Oct. 2002/feb.2003

e beam

Ellipsometer

Results with final cavity at Orsay

•Mirrors movable from outside cavity mirrors quality not homogeneous

cavity gain is now >7000•BUT:

•only 65%-70% on the laser incident power is coupled to the cavity

•under investigation: we suspect the laser linewidth ( 5kHz for 1ms % Cavity bandwith 3kHz for 0.05ms)

Power inside cavity: 65% * 7000* 700mW 3200W

TransmittedPower % time

Reflected power % time

Gain estimated by Christian’s fits:

Good agreement impossible without laser linewidth

•Cavity gain via cavity decay time (V. Soskov):

•laser pumping diode switched off when cavity is locked•transmitted power measured as function of time

The biggest the power inside cavity, the higher the decay time ( formula…)

1rst test cavity

1rst & 2nd tryWith final vavity

Best with testcavity

cavity

•Quart wave plate is the most sensitive element … :-

•Choice & calibration important for a per mill level measurement … reached after 2 years of efforts …

Ellipsometry (`classic’) :

• such : (I1-I2)/(I1+I2)

degree ofcircular polarisation

after cavity=

Quarter wave plate

Temp. controlled p-diodeElectronics (Peletier module)

ccd

QWP

HBSBeam splitter

wollaston3 InGaAs p-diodes

Laser beamafter cavity

Beam shutter(p-diodes pedestals)

100 mW YAG Laser

Wollaston cube

4

p-diode I1

p-diode I2

Glan Thomson

Polar vertical

Calibration of the ellipsometer

p-diode I0

• Performances Wollaston & Glan Thomson : 10-5 (verified)

• Measurements of I1/I0 et I2/I0 (2MHz ADCs) as function of

for différent incident angles

fit no, ne & thickness

Polar horizontal

Polar vertical

Polar elliptic

e/m

2

2

2

ne

no

< 0.1%

< 0.1%

< 50 nm/150 m

Results

Laser polar controlled at 0.1% level for HERA (obtained with p-diode electronics stabilisedat 0.05o )

Pate auto-calibration

by Interferometry

Calorimeter readout

•Same system as H1-Lumi calo readout but

•RIO card with 1 Mb MFCC L2 cache memory (polarisation for all bunches 10MHz)

•MFCC FPGA Programming is done and tested•Histogramming in the L2 cache is being programmed •The slow control part (PVSS+LabView) is also being programmed

Conclusions

• Feedback and cavity gain– Work fine, power inside cavity also fine:

70%*8000*700mW=4000W

• Laser polarisation– Per mill level reached after 2 years of

work…• Calo. DAQ should be ready before

the end of the shutdown• Laser is being aligned and locking …