LPOL-cavity Introduction Tests at Orsay Optics (laser polarisation) Calorimeter DAQ Mechanics & installation at DESY  Norbert’s talk

Principle of the P e Measurement with a Longitudinal Polarimeter Compton Scattering: e+   e+  Cross Section: d  /dE  =  0 (E  ) - P e S   1 (E  )  0,  1 : known (QED) P e : Polarization of the e beam to be measured S  : level of circular polarization of the laser beam Luminosity (electron-laser): e (27.5GeV)  (k=1.165eV)  EE Photon detector I e : e beam intensity P L : Laser beam power Scattered photon P e =0.6 Large P L & S   high precision for  P e

Fabry-Perot cavity: principle e beam Polar. Lin. Polar. Circ. When Laser = 0  c/2L  r e sonance L But :  / Laser =  laser/cavity feedback Done by changing the laser frequency  Laser: Nd:YAG (infrared, =1064 nm) Gain  8000

Photodiode  feedback (Saclay) Laser ND:YAG Mirror mounts Motorised mirrors CCD Vacuum pump Optcal room Temperature:  0.5 o Test cavity at orsay Sept. 2001/oct. 2002

V 2Hz & 10V pic-pic Ramp Intensity transmited  laser =75MHz t(oscillo)/s zoom t(oscillo)/s Intensity reflected 100  s laser glan cavity P-diode 200 ms fit Data (oscillo) laser gain cavity test  2000 (8000 expected)  ( laser = MHz) Because of mirror Coatings… qwp

Hera plane is not Horizontal … 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

Transmitted Power % 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 try With final vavity Best with test 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 of circular polarisation after cavity = Quarter wave plate

Temp. controlled p-diode Electronics ( Peletier module ) ccd QWP HBS Beam splitter wollaston 3 InGaAs p-diodes Laser beam after cavity Beam shutter (p-diodes pedestals)

100 mW YAG Laser Wollaston cube 4 4 p-diode I1 p-diode I2  Glan Thomson Polar vertical Calibration of the ellipsometer p-diode I0 Performances Wollaston & Glan Thomson :  ( 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2 22 nene nono < 0.1% < 50 nm/150  m Results  Laser polar controlled at 0.1% level for HERA (obtained with p-diode electronics stabilised at 0.05 o ) 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 …