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

2. 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

3. 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

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

5. 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

6. 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

7. 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

8. 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

9. 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

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

11. 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

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

13. 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

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

15. 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

16. 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

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