1. Bunch Shaping for Future Dielectric Wakefield Accelerators W. Gai Mini-Workshop on Deflecting/Crabbing RF Cavity Research and application in Accelerators Lanzhou, China, 18-20 July 2012

2. COLLINEAR WAKEFIELD ACCELERATION 2

3. Collinear Wakefield Acceleration (I) : High Gradient Q (lose energy) q (gain energy) Accel. Voltage: Wake function: structure related; favors high frequency; Drive bunch current: bunch length related; favors shorter bunch; Examples: 100MV/m for MW structure @ AWA v.s. GV/m for THz structure @ FFTB

4. Collinear Wakefield Acceleration (II) : High Efficiency Q (lose less energy) q (gain energy) Transformer Ratio: (Trailing bunch) (driving bunch) Directly related to efficiency; >>2 is preferred; temporal shape of the bunch determined; favors ramped charge profile and its variations;

5. How to generate R>2 with asymmetric distributions Symmetric Bunch Ramped Bunch Train Microbunch Shaping GaussianRBT TriangleDT

6. TRANSFORMER RATIO 6

7. 7 Why is high R good? R=2 1 nC R=5 5 nC R=10 10 nC R=20 20 nC 10 GeV Beam Witness Beam 5 GeV 2 GeV 1 GeV 0.5 GeV High transformer ratio makes us use limited beam energy efficiently. Only 0.5 GeV is needed to get 10 GeV for the transformer ratio of 20.

8. 8 Why is high R good? High TR Application: High Gain SASE XFEL

9. DEMANDS OF THE COLLINEAR WAKEFIELD ACCELERATION SCHEME High Frequency (0.1THz ~ 1THz) wakefield structures: Dielectric wakefield Accelerator (DWA) is a very good candidate. Robust Bunch shaping techniques: (EEX-based; Dual frequency Linacs; and other nonlinear beam optics based techniques, etc.) High stability of trailing bunch in time. 9

10. Linearly ramped bunch (train)  Slits in a dispersive region  Sextupoles in a dogleg From Dao Xiang, “Overview of Phase Space Manipulations” AAC2012, Austin  Harmonic cavity Piot et al., PRL, 108, 034801 (2012)  Laser shaping Jing et al., PRL, 98, 144801 (2007) England et al., PRL, 100, 214802 (2008) Muggli et al., PRL, 101, 054801 (2008)  EEX …… ………

11. Emittance exchange (EEX) Beam line Cornacchia and Emma PRST-AB, 5, 084001 (2002)  First EEX proposal  Easy to implement  EEX is not complete Cornacchia and Emma beam line Emma et al., PRST-AB, 9, 100702 (2006)  EEX is complete  Dogleg introduces offsets in beam orbit Kim beam line Xiang and Chao, PRST-AB, 14, 114001 (2011)  -I section to reverse η  EEX is complete  NO offset in beam orbit Chicane-type beam line 0 0 0 0 0 0 0 0 From Dao Xiang, “Overview of Phase Space Manipulations” AAC2012, Austin

12. Double dogleg Emittance Exchange Beam line Beam Dipole Deflecting Cavity Emittance Exchanger - Configuration Emittance Exchanger – Transfer matrix Emittance exchanger exchanges transverse emittance for longitudinal emittance, vice versa. Longitudinal governs transverse Transverse governs longitudinal

13. EEX applications  Shaping beam’s longitudinal distribution Properly design a lattice to make x i dominate x i ‘ term limits the accuracy of manipulation Sub-ps bunch train Sun and Piot, Linac08; Sun et al., PRL, 105, 234801 (2010); Ruan et al., PRL, 106, 244801, (2011); Piot et al., PRST-AB, 14, 022801 (2011) Separated energy bands Jiang et al., PRL, 106, 114801 (2011) From Dao Xiang, “Overview of Phase Space Manipulations” AAC2012, Austin

14. EEX-based Bunch Shaper (aiming for the future FEL) 14 y x I t Beam Mask Dipole Deflecting cavity drive witness (a) (b) (c) (d) Works well for a few nC charge after mask; flexible with mask shape witness bunch can be produced with the same mask to avoid the timing jitter; Simulation by P. Piot.

15. Beam pipe OD, 2b1.14 mm Dielectric tube OD, 2a1.24 mm Waveguide cutoff298 GHz Charge of the drive bunch5 nC Length of the drive bunch2.127 ps Charge of the witness bunch250 pC Length of the witness bunch75 fs Time between the bunches9.4 ps Transformer ratio3.16 ΔG/G1.5*10 -5 Minimization of the energy spread in a witness bunch From Evgenya I. Simakov “Possibility for Ultra-bright Electron Beam Acceleration in Dielectric Wakefield Accelerators”, AAC2012, Austin ” AAC2012, Austin By additionally customizing the shape of the main (witness) bunch we designed the configuration which minimizes the wakefield- induced energy spread in the main bunch. The energy spread may be made as low as 0.001%.

16. 16  Reduce construction and operational costs of a high bunch rep. rate FEL facility: –accelerating gradient > 100 MV/m, -- peak current > 1KA, –bunch rep. rate of the order of 1MHz, -- electron beam energy of a few GeV A Schematic of a FEL facility based on a 2.4 GeV DWA

17. PLANNED EXPERIMENT AT AWA 17

18. AWA Beam Line Collinear DWA Section Diagnostics Section Deflecting Cavity Dielectric Tube Spectrometer Witness beam line (2014) Photocathode Gun L-band Linac #1 ~ #6 EEX Quads Mask Dielectric Tube Drive beam line (2013) Measure the current profile (2013) Measure the transformer ratio (2014)

19. S2E PARMELA Simulation ParameterValueUnitMisc. Beam Energy70MeV Initial Charge10nC Charge Loss~60% Bending27.5Degree Bend to Bend60cmProjected value Bend to Cavity50cm -0.56 & 0.24m Bend to Tube0.8mTube Length: 10 cm a & b0.75 & 0.9mmInner & Outer radius 3.75- TM01 Frequency198GHz TM01 Loss Factor8.3kV/pC m Gun Linac #1 ~ #6 EEX Quads Mask Dielectric Tube

20. Vertical size is small enough. Horizontally -1.3~0 mm region will be used. Others will be eliminated by collimator. S2E PARMELA Simulation Tube parameters are chosen for single mode. First four modes are considered. Transformer ratio is 13.3 which is 75% of ideal one. Low loss factor and current make low wakefield of 7 MV/m. We confirms that EEX shaping scheme works well.

21. Limited values for experiment ParameterLimited ValueUnitMisc. Beam Energy52 / 13MeVDrive / Witness Drive Beam Charge~20nCAfter the mask Dielectric Material Quartz (3.75) Alumina (10) Material (permittivity) Dielectric Tube Length30 / 10cmQuartz / Alumina Bunch Length of Witness Beam0.5mmRMS TM01 Frequency< 39GHz Energy Gain of Witness Beam1MeV Energy Loss of Drive Beam0.5MeV Transformer Ratio> 3 Witness beam line (2014) Photocathode Gun L-band Linac #1 ~ #6 EEX Quads Mask Dielectric Tube Drive beam line (2013)

22. Possible dielectric tube parameters and current profile

23. Summary  Collinear dielectric wakefield acceleration requires high transformer ratio.  Emittance exchanger with mask can shape a current profile.  We confirm current profile shaping using EEX and obtain high transformer ratio in the PARMELA simulation.  There are few current profile and dielectric tube parameter sets for experiment.  Double triangular current profile measurement will be done in 2013.  Transformer ratio measurement will be done in 2014.