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LPOL-cavity

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. E g. Compton Scattering: e+ g g e+ g Cross Section:

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LPOL-cavity

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

  2. Principle of the Pe Measurement with a Longitudinal Polarimeter Eg Compton Scattering: e+gg e+g Cross Section: ds/dEg = s0(Eg) - PeSgs1(Eg) s0, s1: known (QED) Pe:Polarization of thee beam to be measured Sg: level of circular polarization of thelaser beam Luminosity (electron-laser): g (k=1.165eV) e (27.5GeV) a Scattered photon Photon detector Pe=0.6 Ie: e beam intensity PL: Laser beam power Large PL & Sg high precision forPe

  3. Gain 8000 Fabry-Perot cavity: principle e beam L Polar. Circ. Polar. Lin. When nLaser =n0c/2Lresonance • But :Dn/nLaser = 10-11 laser/cavity feedback • Done by changing the laser frequency • Laser: Nd:YAG (infrared, l =1064 nm)

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

  5. qwp P-diode laser glan cavity nlaser Data (oscillo) 2Hz & 10V pic-pic Ramp fit V Intensity reflected zoom Intensity transmited Dnlaser=75MHz (nlaser=3.108MHz) t(oscillo)/s t(oscillo)/s 200 ms 100 ms Because of mirror Coatings… gain cavity test  2000 (8000 expected) 

  6. Final cavity Orsay: Oct. 2002/feb.2003 e beam Ellipsometer Hera plane is not Horizontal …

  7. 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

  8. Gain estimated by Christian’s fits: Transmitted Power % time Reflected power % time Good agreement impossible without laser linewidth

  9. Cavity gain via cavity decay time (V. Soskov): • laser pumping diode switched off when cavity is locked • transmitted power measured as function of time Best with test cavity 1rst & 2nd try With final vavity 1rst test cavity The biggest the power inside cavity, the higher the decay time ( formula…)

  10. Ellipsometry (`classic’) : Quarter wave plate cavity degree of circular polarisation after cavity • such :(I1-I2)/(I1+I2) = • Quart wave plate is the most sensitive element … :- • Choice & calibration important for a per mill level measurement … reached after 2 years of efforts …

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

  12. Polar vertical Calibration of the ellipsometer p-diode I1 Polar vertical f 100 mW YAG Laser l/4 Wollaston cube p-diodeI2 Glan Thomson Polar elliptic p-diode I0 Polar horizontal • Performances Wollaston & Glan Thomson : 10-5 (verified) • Measurements of I1/I0 et I2/I0 (2MHz ADCs) as function of f for différent incident angles •  fit  no, ne & thickness

  13. c2 < 50 nm/150 mm c2 De/mm < 0.1% c2 Dno < 0.1% Dne f Results f Pate auto-calibration by Interferometry • Laser polar controlled at 0.1% level for HERA (obtained with p-diode electronics stabilised at 0.05o )

  14. 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

  15. 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 …

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