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The Fermilab Photo-Injector

The Fermilab Photo-Injector. Jean-Paul Carneiro (Fermilab & Université Paris XI) For the A0 group (N. Barov, M. Champion, D. Edwards, H. Edwards, J. Fuerst, W. Hartung, M. Kuchnir, J. Santucci) Accelerator Physics and Technology Seminars Fermilab, March 23, 2001.

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The Fermilab Photo-Injector

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  1. The Fermilab Photo-Injector Jean-Paul Carneiro (Fermilab & Université Paris XI) For the A0 group (N. Barov, M. Champion, D. Edwards, H. Edwards, J. Fuerst, W. Hartung, M. Kuchnir, J. Santucci) Accelerator Physics and Technology Seminars Fermilab, March 23, 2001

  2. R&D on linear colliders e+/e- at Fermilab

  3. THE TESLA ACCELERATOR • 9-cells superconducting cavities • Must achieve 40 MV/m to get 0.8 TeV COM. • Today ~ 33 MV/m. • To develop the technology of TESLA: installation at DESY (Hamburg) of a TESLA TEST FACILITY accelerator.

  4. THE TESLA TEST FACILITY ACCELERATOR ~ 100 meters • Fermilab contribution to TTF : - design, fabrication and commissioning of the TTF injector (Nov 98). - design and prototyping of RF couplers for the cavities. - design and prototyping of long-pulse modulators for the klystrons.

  5. THE TESLA TEST FACILITY ACCELERATOR

  6. THE TESLA TEST FACILITY PHOTO-INJECTOR

  7. • Self-Amplified Spontaneous Emission observed at 209 nm in February 2000.

  8. Concept of Photo-Injector gun: Photo-cathode

  9. TTF INJECTOR BEAM PARAMETERS Quantity TTF spec. Charge per bunch 1-8 nC Bunch spacing 1 µs Bunches per RF pulse 800 10 Hz Repetition rate Quantity TTF spec. Energy 20 MeV Transverse emittance at 1 nC 2-3 mm-mrad Transverse emittance at 8 nC 15 mm-mrad

  10. A0 PHOTO-INJECTOR LAYOUT (First beam the 3rd of March 1999)

  11. LASER (University of Rochester) 12 nJ/pulse 60 ps 1054 nm 2.5 nJ/pulse 400 ps 800 pulses 2 nJ/pulse 400 ps 100 µJ/pulse 400 ps Oscillator Nd:YLF 81.25 MHz 2 km optic fiber Pockels Cell 1 MHz Multi-pass amplifier Nd-glass Double-pass amplifier Nd-glass STACKED UNSTACKED 10 µJ/pulse 10.8 ps 263 nm 20 µJ/pulse 4.2 ps 263 nm 100 µJ/pulse 4.2 ps 532 nm 400 µJ/pulse 4.2 ps 600 µJ/pulse 400 ps 0.8 mJ/pulse 400 ps Pulse stacker BBO Crystals Compressor Spatial filter

  12. The two regimes of the A0 laser system : UNSTACKED LASER PULSE 4.2 ps FWHM / 20 µJ STACKED LASER PULSE 10.8 ps FWHM / 10 µJ

  13. THE PHOTO-CATHODE PREPARATION CHAMBER (INFN-Milano) • Coat Mo cathodes with a layer of Cs2Te, a material of high quantum efficiency (QE). • Use manipulator arms to transfer the cathode from the preparation chamber into the RF gun while remaining in UHV. • Cathodes must remain in ultra-high vacuum (UHV) for its entire useful life, because residual gases degrade the QE. • Contamination can be reversed by rejuvenation: heat cathode to ~230 C for some minutes. • The same cathode has been used in the RF gun for ~2 years without degradation of its QE (~0.5-3%)

  14. THE RF GUN AND SOLENOIDS (Fermilab & UCLA) RF GUN • RF gun and solenoids developed by Fermilab and UCLA. BUCKING SOLENOID PRIMARY SOLENOID SECONDARY SOLENOID Gun parameters • 1.5-cell copper cavity designed for a high duty cycle (0.8%). TM010,π Mode 1.3 GHz Resonant frequency 24000 Q 35 MV/m Peak field 4.5 MeV Total energy 2.2 MW Peak power dissipation 800 µs Pulse length 10 Hz Repetition rate 28 kW Average power dissipation 4 L/s Cooling water flow rate Solenoids parameters • Bucking & Primary max. Bz --> 2059 G (385A) • Secondary max. Bz --> 806 G (312 A)

  15. THE CAPTURE CAVITY (DESY & SACLAY/ORSAY) & THE CHICANE (Fermilab) CAPTURE CAVITY CHICANE Capture cavity parameters • 9-cell L-band superconducting cavity of TTF type. • Operated daily at 12 MV/m on axis. Chicane parameters • 4 dipoles of equal strengths, 2 with trapezoid poles and 2 with parallelogram poles. • Operated @ 2A, ~700 Gauss. • Bend in the vertical plane • Compression ratio ~5 - 6 (theory and measurements)

  16. THE LOW BETA SECTION THE WHOLE BEAMLINE EXPERIMENT PLASMA WAKEFIELD ACCELERATION SPECTROMETER

  17. DARK CURRENT STUDIES •Dark current measurement principle : Using a Faraday Cup at X2 (z~0.6 m). Bucking Ib Primary Ip Secondary Is

  18. Comparison of Dark current : March 99 / November 00

  19. Where does the dark current come from? •Probably the surface of the photo-cathode. Dark current spots & photo-current in X6 (z=6.5 m) Photo-cathode & back of the RF gun Edge of the photo-cathode Edge of the photo-cathode

  20. Effect of the solenoids settings on the dark current Round beam Flat beam

  21. Effect of the vacuum on the dark current

  22. QUANTUM EFFICIENCY STUDIES

  23. Effect of the solenoids settings on the Quantum Efficiency Round beam Flat beam

  24. Charge Vs. Laser Energy for 2 longitudinal sizes of the laser beam on the photo-cathode.

  25. Charge Vs. Laser Energy for 3 different transverse sizes of the laser beam on the photo-cathode.

  26. Charge Vs. Laser Energy for  = 0.8 mm on the photo-cathode. (Hartman, NIM A340, p.219-230, 1994)

  27. TRANSVERSE EMITTANCE MEASUREMENTS • The photo-injector is a set of 8 parameters: Laser RF Gun Capture Cavity • Goal: find for a charge Q, the set of parameters that gives the min. transverse emittance. • Remark: for all the emittance measurements, the chicane was OFF and DEGAUSSED.

  28. • How do we measure the transverse emittance at A0: using slits • Slits width: 50 µm • Slits spacing: 1mm

  29. Location of the emittance slits ~ 3.8 m ~ 6.5 m ~ 9.5 m

  30. Example: emittance measurement of 8 nC in X3 (z~3.8 m), beamlets in X4 (∆z = 384 mm) BEAM X3 BEAMLETSX4 Intensity [a. u.] Intensity [a. u.] Position [mm] Position [mm]

  31. How did we proceed with the emittance measurements ? FIXED PARAMETERS

  32. Q=0.4 nC Q=0.5 nC Q=0.8 nC

  33. Min Emit @ 205 A

  34. Min Emit @ 0.5 mm, 260 A

  35. Min Emit @ 1.2 mm, 260 A

  36. Min Emit @ 1.4 mm, 255 A

  37. Min Emit @ 1.6 mm, 245 A

  38. Min Emit @ 2.1 mm, 225 A

  39. Predicts a decrease of 50% using a 20 ps FWHM laser pulse.  [mm] Ib=Ip=Is [A] 0.4 0.5 1.2 1.4 1.6 2.1 205 260 260 255 245 225

  40. 9.4 m 6.5 m

  41. 9.4 m 6.5 m

  42. Transverse Emittance at different locations in the beamline. CASE Q = 1 nC Z [m] Measurement HOMDYN PARMELA 4.1 ± 0.3 Norm. Emit. Y 3.8 9.2 1.7 5.0 ± 0.2 Norm. Emit. X 1.7 6.5 9.1 6.5 1.4 9.2 Norm. Emit. Y 5.1 ± 0.2 6.8 ± 0.2 Norm. Emit. X 9.4 1.6 9.6 5.8 ± 0.2 9.4 9.6 Norm. Emit. Y 0.9 CASE Q = 8 nC Measurement Z [m] HOMDYN PARMELA 10.0 ± 0.1 3.8 11 40.7 Norm. Emit. Y Norm. Emit. X 11.6 ± 0.5 6.5 12.5 39.1 Norm. Emit. Y 8.9 ± 0.7 6.5 9.7 40.5 Norm. Emit. X 14.4 ± 0.5 9.4 8.5 39.3 Norm. Emit. Y 18.3 ± 0.9 9.4 16.4 41.2

  43. BUNCH LENGTH MEASUREMENTS • Principle: - Using a Hamamatsu Streak Camera of 1.8 ps resolution - OTR light at X6 (z=6.5 m) Streak camera OTR screen X6

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