New type of a bunch compressor and generation of a short wave length coherent radiation

1. New type of a bunch compressor and generation of a short wave length coherent radiation A. Zholents (ANL) and M. Zolotorev (LBNL) LBNL, August 06, 2010

2. “Any fool with four dipoles can compress a bunch” - anonymous OK, but there may be a few details to consider… P. Emma, ICFA Workshop, Sardinia, 2002

3. DE/E DE/E sz0 ‘chirp’ z z sE/E Dz = R56DE/E V = V0sin(wt) RF Accelerating Voltage Path Length-Energy Dependent Beamline Magnetic Bunch Compression Courtesy P. Emma DE/E under-compression z sz Chicane RF

4. DE/E z sz Full compression is not good because of emittance growth due to CSR LCLS Full compression under-compression K. Bane et al., PRST AB, 12, 030704 (2009) Some issues DE/E Needs: de-chirping De-chirping is typically done by using wake fields and/or off-crest acceleration under-compression z sz

5. References Results presented here use the idea of transverse to longitudinal emittance exchange M. Cornacchia, P. Emma, “Transverse to longitudinal emittance exchange”, Phys. Rev. ST Accel. Beams 5, 084001 (2002) P. Emma, Z. Huang, K.-J. Kim, P. Piot, “Transverse-to-Longitudinal Emittance Exchange to Improve Performance of High-Gain Free-Electron Lasers”, Phys. Rev. ST Accel. Beams 9, 100702 (2006). R.P. Fliller, D.A. Edwards, H.T. Edwards, T. Koeth, K.T. Harkay, K.-J. Kim, “Transverse to longitudinal emittance exchange beamline at the A0 Photoinjector”, Part. Acc. Conf., PAC07, (2007).

6. Variant 1 M+ D+ M+ T M- D- M- QD QF QD QF QD QF QD QF QD B B QF QF QD B B B B QF QD QD QF B B TM110 TM010 TM010 Deflecting cavity A schematic of the bunch compressor (manipulate longitudinal phase space with ease of a transverse phase space) Focusing properties of individual sections I I I T I QD QF QD QF QD QF QD QF B B QD QF QF QD B B B B QF QD QD QF z →x emit. exch. x → z emit. exch. Telescope B B

7. M+ D+ M+ B B QD QD QF QD QF QD QF QF QF QD QD B B B QF QF QD QD TM110 TM010 TM010 B B Deflecting cavity Variant 2 M+ D+ M+ T B A schematic of the bunch compressor: alternative variant

8. Deflecting cavity B B Courtesy: Li, Corlett Thin cavity approximation: In agreement with Panofsky-Wentzel theorem

9. Thick deflecting cavity: d1 TM110 One can cancel unwanted energy gain using two TM010 mode side cavities d TM010 TM110 TM010 Required energy gain in each TM010 cavity

10. FNAL: SRF five cell prototype Ebeam = 250 MeV fRF= 3.9 GHz V0 = 4 MV (1m, 3x7 cells: Li, Corlett) V1 = 0.4 MV k = 0.013 cm-1 SLAC: copper X-band LOLA Ebeam = 250 MeV fRF= 11.4 GHz V0 = 22 MV ( 0.5 m) V1 = 6.6 MV k = 0.21 cm-1

11. First leg of emittance exchange scheme B QF QD B Complete emittance exchange scheme QD QF QD B B QF QD B The following constrains were used:

12. T QD QF QD QF QD QF I I I T I QD QF QD QF QD QF QD QF QD B B QF QF QD B B B B QF QD QD QF z →x emit. exch. x → z emit. exch. Telescope B B Telescope Total transformation

13. Define: - bunch length before compression, - uncorrelated relative energy spread before compression, and - energy chirp before compression - bunch length after compression, - uncorrelated relative energy spread after compression Bunch length and energy spread at the end of the scheme Using entire mapping one obtains :

14. No chirp case, e.g., h = 0 Case 1 then Case 2 Then the compression is not yet finished

15. The last element of proposed BC is the chicane/dogleg with its R56 used to cancel the R56 accumulated upstream Then, the entire mapping takes the following form: … and we obtain for the final bunch length and relative energy spread:

16. … or one may prefer to wait until the electron beam gains energy from E1 to E2 and do the final compression at E2 Then R56 for the final step should be: … and we obtain for the final bunch length and relative energy spread: Possible advantage of aDeferred Compression is reduction of the gain of the microbunching instability and impact of other collective forces: a) electron bunch is not yet compressed to the shortest size b) energy spread has already grown up to the final value

17. Possible advantages • Because there is no need in energy chirp for compression: • one can use BC even after the linac and obtain final compression in a “spreader”/ “dogleg” part of the lattice leading to FEL Gun L1 Spreader L2 BC BC b) or use two BCs, one in usual location and one after the linac

18. Possible advantages (2) Accelerate a relatively long bunch in the re-circulating linac without a chirp and compress it in the final arc. FEL New type of a bunch compressor Possible cost saving

19. Jitter studies No adverse jitter effects are found: Timing jitter is compressed by a compression factor and energy jitter is increased by a compression factor:

20. Illustration In the following numerical example we use: fRF = 2.85 GHz k=0.05 cm-1 cavity length = 1 m At Eb = 250 MeV this corresponds to: V0 = 21 MV V1 = -8.7 MV

21. Illustration: compression by a factor of 15 -I -I -I -I T B1 tcav tcav B2 B2 B1 B2 B2 Magnets: jB1= 0.10 jB2= 0.0445 Length=0.3 m bx=0.25 m bx=56 m Lattice functions for a bunch compressor with a telescopic factor m=15. Note, matching of the vertical beta-function was not pursued.

22. QD 5 7 1 8 3 4 6 2 QF QD QF QD QF QD QF QD B B QF QF QD B B B B QF QD QD QF B B Numerical values of beam sizes at key locations Deferred compression 42 mm -> 10 mm m=15 Input parameters Eb=250 MeV ex=0.5 mm sd0=10-4 sE0=25 keV sz0= 160 mm

23. Same as before, but with a reduced beam energy to 100 MeV This may eliminate a need in the laser heater m=15 Eb=100 MeV ex=0.5 mm sd0=5x10-5 sE0=5 keV sz0= 160 mm

24. Compression of the laser induced energy modulation for microbunching at a shorter wave length Compression factor, m=20 laser BC Laser e-beam interaction Initial modulation DE/sE DE/sE Compressed modulation z/l z/l Bunching efficiency at m-th harmonic of modulating frequency: DE/sE HGHG: EEHG: This method: z/l Plots of longitudinal phase space at various locations

25. DE/sE … or be deferred to a “spreader”/“dogleg” z/l BC Laser e-beam interaction Linac Spreader Compression of the laser induced energy modulation can also be made at the end of the linac BC Laser e-beam interaction Linac

26. Compression of the Echo induced microbunching BC Compression factor, m=5 DE/sE z/l z/l

27. zoom on one peak z/l Compression of the Echo induced microbunching (2) Peak current after compression, kA 1 0.8 0.6 0.4 0.2 z/l

28. Producing 1 nm seeding beginning from 200 nm laser modulation Echo: harmonic number = 20 new type BC, m=10 traditional BC Gun L1 L2 Echo Spreader BC BC Ipeak=100 A Ipeak=1 kA Total harmonic number = 20x10 = 200 !

29. Other uses It is often desirable to get rid off the tails in longitudinal distribution and proposed scheme can be used as efficient tail cutter collimator QD QF QD QF QD QF QD QF QD B B QF QF QD B B B B QF QD QD QF x → z emit. exch. z →x emit. exch. FODO B B

30. Other uses (2) It is possible to create a sequence of a tightly spaced microbunches using a sequence of slits Collimator with slits QD QF QD QF QD QF QD QF QD B B QF QF QD B B B B QF QD QD QF x → z emit. exch. z →x emit. exch. Telescope B B Using demagnification of the beam size before slits and magnification after the slits can help to obtain a real tight spacing of microbunches

31. Other uses (3) Longitudinal phase space tomography x- px tomography here is d- ztomography there QD QF QD QF QD QF QD QF QD B B QF QF QD B B B B QF QD QD QF x → z emit. exch. z →x emit. exch. Telescope B B

32. Summary • Efficient electron bunch manipulation in the longitudinal phase space can be accomplished by first exchanging longitudinal and transverse emittances, manipulating electrons in the transverse phase space and finally exchanging emittances back to their original state. • One application is bunch compressor that does not need energy chirp • This can also be used for a compression of any features introduced to the electron bunch, like, for example energy modulation produced in interaction with the laser. • Proposed techniques for a bunch compression allows deferred compression that might be useful to mitigate possible adverse effects caused by collective forces.