Pulse energy measurement for the CTF lasers. Stéphan Del Burgo CERN PS/LP 30 January 1998 DBS 01/98-201

Pulse energy measurement for the CTF lasers. I. Introduction - Where ? What for ? II. Technical data III. One proposal : joulemeter filters telescope the whole system IV. Test and validation V. Conclusion - Next developments DBS 01/98-202

I. Introduction - Where ? What for ? Electron beam production in CTF by illuminating photocathodes with laser beam. Need to measure the pulse energy of the laser beam. Problems :- Wide range of variation for the energy. - High energy and power densities beyond usual damage thresholds. Difficult to evaluate the performances of the photocathodes and the RF gun. DBS 01/98-203

II. Technical Data Probe Beam : Drive Beam : Pulse duration 8 ps Wavelength 262 nm Beam diameter 3 - 10 mm Sample 2 - 5 % DBS 01/98-204

III. One proposal - Joulemeter No commercial detector available with these characteristics : Limitations : Maximum power density ~ 1 MW.cm-2 Noise equivalent energy ~ 35 nJ Best candidate : Molectron P1-45 / P1-65 pyroelectric detector Detector from Molectron Electronics made by Jacques DURAND Same characteristics except sensitivity : P1-65 : sensitivity ~1 V/µJ P1-45 : sensitivity ~7 V/µJ ( no detector head yet ) Sensitive area diameter : 5 mm Measurements made with P1-65 ( ‘Jacques’ detector’ ) : sensitivity : 2 V/µJ NEJ : 8 nJ ( Molectron’s doc : ~ 2 nJ ) DBS 01/98-205

III. One proposal - Joulemeter Power density limitation : ( diameter : 4 mm ) Molectron 25 µJ 2,5 µJ 1 MW.cm-2 1 µJ 80 nJ 75 nJ 50 nJ DBS 01/98-206

III. One proposal - Joulemeter Energy density limitation : ( diameter : 4 mm ) Molectron 25 µJ 1 MW.cm-2 1 µJ 75 nJ 50 nJ DBS 01/98-207

III. One proposal - Filters To attenuate the beam to be below the damage threshold for the detector. High power dielectric UV attenuators : - non-absorbing attenuators T+R=1 - 266 nm , OK for 262 nm Optical Density : OD=-log(T) , T=10-OD 0,3% 1% 5% 3% 32% 10% Drive 100% 20% 2,5 µJ Probe 2,5 2,0 0,5 µJ 1,5 1,3 50 nJ 1,0 0,7 0,5 DBS 01/98-208

III. One proposal - Telescope To adjust the beam size to the detector size. Range of variation for the magnification : Object and image with small divergence  telescope Zoom telescope with 4 lenses : easier to achieve  same translation for 2 elements to change the magnification. f ’=48 mm f ’=147 mm f ’=-32 mm f ’=-74 mm DBS 01/98-209

III. One proposal - Telescope DBS 01/98-210

III. One proposal - The whole system Translation motor Beam size detector Joulemeter Beam splitter Iris diaphragm Telescope Wheel with filters Shutter DBS 01/98-211

III. One proposal - The whole system DBS 01/98-212

III. One proposal - The whole system Beam size detector : Beam diameter > 4 mm  OK Beam diameter < 4 mm  Danger ! Energy on the detector Danger ! Damage ! OK Error ! 4 5 Beam diameter ( mm ) DBS 01/98-213

Joulemeter : P1-65 - Calibration 2 V/µJ  OK ! IV. Test and validation Joulemeter : P1-65 - Calibration 2 V/µJ  OK ! P1-65 Error mean = 2.10-4 Error dev = 2.10-2 Reference Output Voltage DBS 01/98-214

Telescope : Magnification  OK Transmission : Tmean = ( 69,0 ±1,5 )% IV. Test and validation Beam size detector  OK Telescope : Magnification  OK Transmission : Tmean = ( 69,0 ±1,5 )% DBS 01/98-215

V. Conclusion - Next developments Tests in the laboratory : Prototype OK ! Joulemeter : OK ! Still waiting for P1-45 ( Good luck, Jacques ! ) Damage threshold : OK ( ? ) Beam size detector : new electronic card for test Telescope : OK ! All the elements are remotely controlled. Compatibility with the existing equipment in the CTF and the control room. Installation in the CTF : Mechanical part achieved : 2 models Protection from the radiation : Electronics lifetime ? Detector lifetime ? Optical alignment : delicate ( telescope, sensitive area ) Electronics installation : inputs, outputs, DAQ… Hardware security : external shutter control… DBS 01/98-216

V. Conclusion - Next developments Software : Control structure plan : DBS 01/98-217