Transverse Phase Space Optimization in a 1.625-cell SRF Photoinjector Gun Cavity

F. Marhauser, EUROFEL WP5 collaboration meeting on simulation results, 02-03. Juni 2005, BESSY/Berlin

Transverse Phase Space Optimization in a 1.625-cell SRF Photoinjector Gun Cavity

o overview on different focussing methods proposed

o simulation studies for a 1.625-cell photogun I) with solenoidal focussingII) with combined electric RF- and magnetic focussing

o at 2.5 nC (i.e. BESSY-FEL design bunch charge)

o at 1.0 nC („typical“ bunch charge)


f0 = 1.3 GHz

bunch charge 1.0 nC

Eacc (~ 0.5*Emax) 25 MV/m

Ebeam 10.0 MeV

laser pulse length (FWHM)

(superposition of 3 Gaussian

laser pulses with = 3ps)

18 ps

laser spot radius 1.5 mm

thermal emittance th - mm mrad

n,rms @ 1.5m behind cathode2.28/0.98

(100 / 90 %) mm mrad

1 - RF Focussing

„RF FOCUSSING – AN INSTRUMENT FOR BEAM QUALITY IMPROVEMENT IN SUPERCONDUCTING RF GUNS“

V. Volkov (BINP), D. Janssen (FZR)

EPAC 2000

PARMELA simulation results


2 - Magnetic Mode Focussing (+ RF Focusing)

„EMITTANCE COMPENSATION IN A SUPERCONDUCTING RF PHOTOELECTRON GUN BY A MAGNETIC RF FIELD“

D. JANSSEN (FZR), V. VOLKOV (BINP)

EPAC 2004

bunch charge 1.0 nC

Emax (~ 2*Eacc) 50 MV/m

Ebeam 8.82 MeV

TE-mode frequency 3.802 GHz

Bz,max 324 mT

Bs,max (below quench limit) 144 mT

laser pulse length (uniform) 20 ps

rise/fall time 1 ps


thermal emittance th - mm mrad

n,rms @ 4.44 m behind cathode 0.78-0.98 mm mrad

ASTRA simulation results


bunch charge 1.0 nC

Emax 60 MV/m

Ebeam 6.5 MeV

Bz,max @ 0.36 m 300 mT

laser pulse length (uniform) 19.8 ps


working point position 3.3 m

n,rms @ WP ~2.2 mm mrad

Eacc 13 MV/m

thermal emittance th 0.5 mm mrad

n,rms @ 14m (after 8 x TESLA cavities (117 MeV)) ~ 1 mm mrad

3 - Split Photoinjector Concept

„AN ULTRA-HIGH BRIGTHNESS, HIGH DUTY FACTOR, SUPERCONDUCTING RF PHOTOINJECTOR“

M. FERRARIO (INFN-LNF), J.B. ROSENZWEIG, G. TRAVISH (UCLA), J. SEKUTOWICZ, W.D. MÖLLER (DESY)

EPAC 2004

PARMELA/HOMDYN simulations results


w/o 3rd harmonic cavities

with 3rd harmonic cavities

- motivated by proposed BESSY Soft X-ray FEL project:

challenge:microbunch charge = 2.5 nC @ slice emittances = 1.5 π mm mrad

Studies at BESSY - Split Photoinjector Concept

„PHOTOINJECTOR STUDIES FOR THE BESSY SOFT X-RAY FEL“

F. MARHAUSER

EPAC 2004

ASTRA simulation:

Q = 2.5 nC

Emax = 60 MV/m in SRF Gun


ASTRA simulations results

bunch charge 2.5 nC

Emax 60 MV/m

Ebeam after gun 6.25 MeV

Bz,max @ 0.6 m 214.6 mT

laser pulse length (uniform) 54.1 ps

rise/fall time 4 ps


thermal emittance th 0.83 mm mrad

working point position 4.5 m

n,rms @ WP 5.4 mm mrad

Eacc4x6.35 + 12x15.6 MV/m

Edec harmonic cavities 8 x -9.6 MV/m

Ebeam after 2nd linac 203 MeV

n,rms @ 34m (behind 2nd cryomodule)1.9/1.1

(100 / 95 %) mm mrad

drawbacks at optimum transverse emittance

- long pulse needed to compensate for large s.c. forces

- to match beam rather low linac field necessary

Results for a 1.625-cell SRF Photogun

54.1 ps

6.4 MV/m(but only for the first 4 cavities)

L. Serafini and J.B. Rosenzweig, 1997, Phys. Rev. E55 p 7565

Invariant Envelope condition:

.Alv

peak

rms I3

I2~

0


6 Parameter-Optimization:

Lt, R, inj, Bz,max, Eacc, Cpos well optimized to yield minimum

transverse emittance (or close to) at the linac exit

read outtransverse emittance

Further Investigations - Schematic Setup

vary Lt, R

Can we reduce the pulse length ?


projected transverse normalized rms beam emittance (n,rms)

ASTRA NP 4000

H_max(max. time step for Runga Kutta integrator)

0.01

n,rms

ASTRA NP 4000

H_max 0.005


5 Parameter Optimization at constant Lt

Q = 2.5 nC

ASTRA simulations with 20000 particles

- we can reduce the bunch length to ~40 ps at only slight expense of the transverse (slice) emittance

- s2e-simulations (nc gun) have shown: Lt= 40 ps ok

- s2e for SRF gun with Lt = 40 ps is in progress


slice emittances (exit of cavity #4)

Q = 2.5 nC

goal: 1.5 m


combined electric RF & magnetic focussing


w/o rf-focusing2.5 nC, z = 0 – 2 cm

with rf-focusingsame settings

first 2 cm (correlation of z and divergence)

z (mm)

z (mm)

divergence px/pz (mrad)



w/o rf-focusing2.5 nC, z = 0 – 2 cm


z (mm)

z (mm)



first 2 cm (correlation of z and divergence)


transverse trace space (through gun)

w/o rf-focusing,2.5 nC, z = 0 – 30 cm


x (mm)

px/pz (mrad)

x (mm)

px/pz (mrad)


transverse trace space

w/o rf-focusing,2.5 nC, z = 0 – 30 cm


x (mm)

px/pz (mrad)

x (mm)

px/pz (mrad)


beam emittance


results with booster linac

benefits at high bunch charge of 2.5 nC:

- higher Eacc possible in first cavities

- n,rms less sensitive to Eacc

altered WP apart from I.E.

n,rms


w/o rf-focusing

with rf-focusing

slice emittances (exit of cavity #4)

< design goal: 1.5 m


Q = 2.5 nC

w/o RF-focussing

projected transverse normalized rms beam emittance (n,rms)

n,rms

Q = 2.5 nC

with RF-focussing

- optimum found at larger Lt

- but n,rms less sensitive to Lt

- might choose lower Lt at minor expense of n,rms


Results @ 1 nC

Q = 1 nC

w/o RF-focussing

n,rms

Q = 1 nC

with RF-focussing


Position of Magnet

o magnet closer to the gun (40cm from cathode)

o switch on magnetic field after cool downno trapped flux lines principally


Solenoid Center at 40 cm

n,rms

Q = 2.5 nC

w/o RF-focussing

Q = 2.5 nC

with RF-focussing

- optimum now at smaller Lt


n,rms

Q = 1 nC

w/o RF-focussing

Q = 1 nC

with RF-focussing

Results @ 1 nC


Summary of Best Results in n,rms (so far !)

1.625-cell SRF Gun

bunch charge Q nC 1 2.5

laser

Rlaser mm 1.40 1.28 1.08 1.16 1.96 1.91 1.77 1.80

thermal emittance mm mrad

0.52 0.54 0.46 0.49 0.83 0.81 0.75 0.76

rise/fall time ps 4

Llaser (flat top) ps 48 65 42 62 54 69 46 63

gun

rf focussing no yes no yes no yes no yes

Emax,gun MV/m 60

inj,rel ° 0 0 0 2 -2 6 -1 3

Ebeam MeV 6.2x

magnetsolenoid center cm 60 60 40 40 60 60 40 40

Bz,max mT 216 229 257 260 215 234 257 270

linac

4 x cavities

E0,acc linac MV/m 10.5 20 11 9.5 7.5 20 10 20

center of 1st cavity m 5.5 3.5 4.4 3.0 5.0 3.0 3.8 2.6

n,rms (100%) mm mrad 1.0 1.0 0.9 0.8 1.9 1.6 1.5 1.4

n,rms (95%) mm mrad 0.7 0.7 0.6 0.6 1.2 1.1 1.0 1.0

Documents

Transverse Phase Space Optimization in a 1.625-cell SRF Photoinjector Gun Cavity