Velocity bunching from S-band photoinjectors

Julian McKenzie1st July 2011

Ultra Bright Electron Sources WorkshopCockcroft Institute

STFC Daresbury Laboratory, UK

Introduction• Normal conducting S-band RF guns are often the gun

of choice for modern FELs• Have provided very low emittance beams• However, FELs typically require multiple stages of

magnetic compression• Velocity bunching schemes have been proposed for

low bunch charge applications such as electron diffraction

• Can we apply the same techniques to 100 pC bunches to serve as an FEL driver?

S-band RF gun• ALPHA-X / Strathclyde (TU/e + LAL)• 2.5 cell, 2998.5 MHz• Cu photocathode• 266nm laser

Courtesy Bas van der Geer, Marieke de Loos, Pulsar Physics

ASTRA simulations• Take on-axis field-maps, feed

into ASTRA• Assume thermal emittance as

per LCLS measurements:0.9 mm mrad per mm (rms) of laser spot*

• Assume gun can achieve peak on-axis field of 100 MV/m

• Beam energy on exit ~6 MeV

• Start with low charge (10 pC) and scale up to high charge...

* D.Dowell, “Unresolved emittance issues of the LCLS gun”, 5/08/2010, LBNL workshop on “Compact X-Ray FELs using High-Brightness Beams”

Red = Ez field from gunBlue = Bz field of combined bucking and focussing solenoids

Shortest bunch from gun• At the multiple-picosecond level, it is safe to assume that the

bunch length from the gun is equivalent to that of the drive laser

• This assumption breaks down sub-ps due to space charge limitations

Assumed a 0.5mm diameter beam, Gaussian temporal profile, and scanned laser pulse length.Minimum of ~250 fs electron bunch.

Similar figure to results from Osaka University, K. Kan et al at IPAC’10/Linac’10

Shortest bunch from gun• Can reduce bunch length by

increasing laser diameter• However, there is a trade-off with

emittance• Previously assumed linear

correlation between laser spot size and emittance

• This emittance cannot be improved but bunch length can

• Therefore initially use 0.5 mm spot, assume best case laser, 10 fs rms

Add bunching cavity• 2m long S-band cavity• Operating at the

bunching zero-cross phase

• 7.5 MV/m

• Bunch length continues to increase after gun

• Operate gun at -15°to help mitigate this effect

• Place bunching cavity as close to gun as possible

BuncherGun

NB// using different gun/solenoid fieldmaps here

Bunch length comes to a focus ~6m from cathodeMinimum of 27 fs rms

Buncher cavity length

RED = 1mGREEN = 2mBLUE = 3m

Don’t gain anything by increasing to 3m, therefore utilise 2m long buncher

Capture cavity

• Add linac cavity at waist to capture the short bunch length

• For simulations used 2m long S-band cavity operating at 20 MV/m

• Beam energy on exit ~ 50 MeV

Buncher LinacGun

Buncher LinacGun Buncher LinacGun

RED = no solenoidsBLUE = solenoid around buncher and before linacGREEN = small solenoid at end of buncher

Transverse focussing schemes

Optimisation• Utilise genetic/evolutionary optimisation

algorithm• Multi-objective shows trade-off between

transverse emittance and bunch length• Uses non-dominated sorting technique, based

off NSGA-II*• 100 generations of 60 runs each, takes

overnight

*Kalyanmoy Deb et al., IEEE Transactions on Evolutionary Computation 6 (2) 2002, pp183-197

Optimisation parameters• Laser spot size (flat-top)• Laser pulse duration (Gaussian)• Gun field strength• Gun phase• Gun solenoid strength• Buncher field strength• Buncher solenoid strength

Optimisation front @ 10 pC

RED = small solenoid at buncher exit

BLUE = solenoids all around buncher

BuncherGun

BuncherGun

Manual versus genetic optimisation

LinacBuncherGun LinacBuncherGun

Manual optimisation Genetic optimisation

NB// head of bunch to the right

10 pC50 MeV

100 pC optimisationRED = small solenoid at buncher exit

BLUE = solenoids all around buncher

BuncherGun

BuncherGun

Optimisation frontsat various bunch charges

RED = 10 pCGREEN = 100 pCBLUE = 250 pC

Selected bunches (100pC)

Bunch A

Bunch B

Parameter A B units

Emittance 0.71 1.10 mm mrad

Bunch length 153 23 fs

Peak current 331 3340 A

Energy spread 58 187 keV

Energy 48 50 MeV

Bunch A: 300A Bunch B: 3kA100 pC50 MeV

NB// head of bunch to the right

Optimised parameters• 100 pC, 50 MeV

Parameter Bunch A Bunch B unitsLaser radius rms 0.40 0.45 mmLaser length rms 140 50 fsGun peak field 97 71.5 MV/mGun phase -2 -10 °Gun solenoid peak field 0.248 0.246 TBuncher peak field 15.6 14.6 MV/m

Buncher solenoid peak field 0.038 0.039 T

Linac operated on-crest, 20 MV/m

Bunch B to 240 MeV

6 MeV 240 MeV50 MeV

Linac 0BuncherGun Linac 1 Linac 2

Jitter• 500 runs • 10,000 macroparticles per run• Random jitter based on the following sigmas (cut off at 3

sigma)Parameter(s) Sigma Unit

Bunch charge 1 pC

Laser position (x,y) 0.1 mm

Laser timing 100 fs

RF gradients 0.1 %

RF phases 0.1 °

Solenoid strengths 0.1 %

NB// all RF jitter applied individually to each cavity,similarly with solenoids (except bucking and gun solenoid locked)

Jitter

~ 0.2 mm mrad~ 15 fs

~ 600 fs ~ 0.6 MeV

Tolerances: Arrival time

Summary• A velocity bunching scheme was presented based

around an S-band gun and followed by a 2m long S-band buncher and a further S-band capture cavity

• This scheme can provide 100 pC bunches to the sub-ps level, kA peak current and 1 mm mrad emittance

• Simulated beam parameters are capable of delivering beam to an FEL

• However, jitter remains a big issue