A. Zholents (ANL) and M. Zolotorev (LBNL) 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

“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

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

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

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).

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

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

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

One can cancel unwanted energy gain using two TM010 mode side cavities 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

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

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:

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

Bunch length and energy spread at the end of the scheme 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 Using entire mapping one obtains :

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

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:

… 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 a Deferred 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

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 L2 Spreader BC BC b) or use two BCs, one in usual location and one after the linac

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

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:

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

Illustration: compression by a factor of 15 tcav B2 B1 B2 tcav 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.

Numerical values of beam sizes at key locations QD QF 1 2 3 4 5 6 7 8 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

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

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: HGHG: EEHG: This method: DE/sE z/l Plots of longitudinal phase space at various locations

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

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

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

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

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 B B QD QF QF QD B B B B QF QD x → z emit. exch. QD QF B z →x emit. exch. FODO B

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 B B QD QF QF QD B B B B QF QD QD QF B z →x emit. exch. Telescope x → z emit. exch. B Using demagnification of the beam size before slits and magnification after the slits can help to obtain a real tight spacing of microbunches

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

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.