High gradient, high average power structure development at UCLA and Univ. Rome in X-X. band

Transcription

1 High gradient, high average power structure development at UCLA and Univ. Rome in X-X and S-S band May 23-25, 25, 2007 US High Gradient Research Collaboration Workshop Atsushi Fukasawa, James Rosenzweig, Brendan O Shea, O UCLA, Dept. of Physics & Astronomy, Luca Ficcadenti, Andrea Mostacci, Lugi Palumbo, Rome University La Sapienza, David Alesini,, Massimo Ferrario,, and Bruno Spataro, INFN/LNF

2 Outline - Recent activities of high field guns at UCLA. -What is hybrid photoinjector? - Beam dynamics of the hybrid photoinjector. - Measurement of the cold test model. -Summary -Future work

3 Recent activities of high field guns at UCLA BNL/SLAC/UCLA 1.6-cell S-band RF Gun MHz Mode Separation -5 S 11 (db) RF gun for LLNL - We are going to redesign this type of gun for 100-Hz operation Frequency (MHz) (S. Anderson, LLNL) Based on these experience of the gun, we are developing a novel photoinjector at S- and X-band.

4 A compact photoinjector Conventional photoinjector Circulator Phase shifter Load RF Gun TW structure Chicane Load Hybrid photoinjector SW/TW hybrid structure Injector becomes much simple. - No circulator. (This is important for X-band case.) - No magnetic bunch compression because of velocity bunching.

5 SW/TW Hybrid Photoinjector Laser RF in No Reflection RF out RF Gun π mode IC OC - Very week coupling - 90 deg phase difference between SW and IC. SW TW 2 π /3 mode Set for the beam to go on proper phase in TW Velocity Bunching 3 m S-band (UCLA) X-band (Rome Univ.) SW 60 MV/m (peak) 240 MV/m (peak) TW 13.5 MV/m (Average) 54 MV/m (Average)

6 Gun solenoid for the S-band S Gun 1 st solenoid Waveguide B z along the axis Backing solenoid Laser port 2 nd solenoid - Due to the existence of the laser port, it is very difficult to make a strong field in the first solenoid.

7 Beam dynamics by LANL PARMELA Strategy to tune - Choose incident phase to obtain shortest bunch length at 4 m. - Make the solenoid field for minimum emittance at 4 m. - Here I do not put downstream solenoid for simplicity. Incident particles Charge Shape Radius 1 nc Square, uniform 1 mm Length 10 ps

8 1.5-cell, x and rms εn,rms,x ε nrmsx [mm.mrad] ΔE-Δφ x rms [mm] Emittance 3.7 mm.mrad Injection phase: 48 deg.

9 1.5-cell, zrms Bunch form y - x Bunch length ΔE-Δφ Energy Spectrum Bunch length (rms) 95 μm Kinetic energy 20.8 MeV Energy spread (rms) 1.3 %

10 Scaled to 1 pc in 1.5 cell case All dimensions of x, y, z are scaled down by 1/10. To keep the charge density as the same before, it is scaled down by 1/1000. In this case, the transverse emittance can be 1/100. The incident particles Charge Shape Radius Length 1 pc Square, uniform 0.1 mm 1 ps Thermal emittance was not included.

11 Beam dynamics of 1 pc,, 1.5-cell x and ε nx Bunch length E - φ Emittance mm.mrad Bunch length 4.6 μm Energy spread 0.17 %

12 Scaled to X-band X frequency with an ideal solenoid field - For the solenoid field, we used S-band one just by scaling it down. According to the scaling law, Length: x 1/4 => 75 cm Field: x 4 => 240 MV/m Charge: x 1/4 => 250 pc The incident particles Charge 250 pc 0.25 pc Shape Radius Square, uniform 0.25 mm Square, uniform mm Length 2.5 ps 0.25 ps Thermal emittance was not included.

13 X-band, 250 pc Bunch length x and ε nx E - φ Emittance 2.4 mm.mrad Bunch length 13 μm Kinetic Energy 20.5 MeV Energy spread 1.1 %

14 X-band, 0.25 pc ε nx Bunch length E - φ Emittance mm.mrad Bunch length 0.9 μm Kinetic Energy 20.6 MeV Energy spread 0.16 %

15 Field measurement of the cold test model S-band Hybrid Structure, which is made of Al. HFSS model Beads

16 S11 Measurement S11 [db] HFSS Experiment Frequency [GHz] Experiment agrees with HFSS results.

17 Fields along the axis Amp [a.u.] Amplitude HFSS Experiment z [mm] Phase [deg] Phase HFSS Experiment z [mm] - Field is good except for the amplitude in the standing wave cells. Peak in SW2 HFSS 2.41 (1) Experiment 0.84 (0.349) (Tuners are out in IC, OC, and the TW cells.)

18 Effect of the Bead size St andard Small 0.80 Amp z - 7 % higher in SW2 in the case of the small bead. The measurement around the peak could be unreliable as the perturbation of the field becomes large there.

19 Q factor of the SW cavity Amp at SW2 [a.u.] HFSS Experiment df/ f HFSS (Cu) Experiment (Al) Loaded Q Peak in SW2 13,337 (1) 2.41 (1) 4,600 (0.345) 0.84 (0.349) The RF contact in the SW could be the problem. - Redesigned how to clamp the structure. - Make the SW part of copper. Under testing now

20 Summary of beam dynamics Calculated 1.5-cell S- and X-band hybrid photoinjector. - Hybrid photoinjector gave short bunch with low emittance. - In pico Coulomb case, the beam quality became extremely good. - Thermal emittance could limit the emittance minimum. Charge Normalized Emittance (rms) Bunch length (rms) Kinetic energy Energy spread (rms) S-band 1 nc 1 pc 250 pc 3.7 mm.mrad mm.mrad 2.4 mm.mrad X-band 95 μm 4.6 μm 13 μm 0.9 μm 20.8 MeV 1.3 % 20.6 MeV 0.17 % 20.5 MeV 1.1 % 0.25 pc mm.mrad 20.6 MeV 0.16 % (Thermal emittance was not included.)

21 Summary of Cavity Design - TW section in the Cold test model agrees well with HFSS. - S11 is almost the same. - Amplitude and phase are basically good in TW section. - Field in the SW section is smaller than HFSS. - The bead size were effective to the field amplitude measurement in SW section. - Q value was 0.34 of HFSS result. - The deviation from the simulation could comes from bad RF contact at the SW structure. - We are testing copper model.

22 Future work Beam dynamics - Take into account of the thermal emittance effect. Structure design and tests - Test the copper model. - Review Steele method if it is applicable to a hybrid structure. - Upgrade to 100 Hz gun. - Molybdenum iris for X-band structure. Others - Complete the design of a permanent magnet solenoid.