David H. Dowell

On behalf of the LCLS Commissioning Team

SLAC National Accelerator Laboratory

LCLS Injector Performance

and Impact on Lasing

Workshop on the Physics and Applications of

High Brightness Electron Beams

Maui, Hawaii

November 16-19, 2009

•Description of Injector

•Review of GTF Gun Studies•Description of Enhanced BNL/SLAC/UCLA gun => LCLS gun

•Performance of the LCLS Injector and FEL at 250 pC and 20 pC

•Summary 1

Major Components of the LCLS Injector

Drive LaserLocated in room above gun

S-Band Gun & Solenoid

Dual Feed S-band Linac

+Diagnostics

;1 linac

linac

gund

dEEE

linaclinaclinaclinac

linac

gun Ed

dEE

sin

•Observed 8 degS phase, 30 MeV linac energy gain yields a

•correlated energy of 73 keV/degS for a 3.4 MHz separation.

•LCLS gun mode separation is 15 MHz giving ~20 keV/6degS.

•Energy spread produces chromatic aberration in gun solenoid:

See: D.H. Dowell et al., “The Development of the Linac Coherent Light Source RF Gun”

http://www-bd.fnal.gov/icfabd/Newsletter46.pdf… and references therein.J. Schmerge et al., Proc. High Brightness Electron Beam Workshop, Erice Sicily, Oct. 10-14, 2005.

GTF Studies Showed a Dynamical Unbalancing of the Gun RF Field Due to Beating Between the 0- and p-modes

Linac

Quad1 Quad2

OTR

100 micron thick Yag

Spectrometer

Screen

LTD1

Phosphor

Energy

Spectrometer

YAG1YAG2Linac

Quad1 Quad2

OTR

100 micron thick Yag

Spectrometer

Screen

LTD1

Phosphor

Energy

Spectrometer

YAG1YAG2

Gun &

Solenoid

GTF Beamline GTF Gun

Bead-drop RF field shape meas.

GTF gun RF probe signals

RMS energy spread at 35 MeV vs. linac phase

p-mode only

p0-modes

p

kLkLkLkp

xchromaticn

cossin2

, Solenoid chromatic aberration:

PHOS

0 0.5 1 1.5 20

0.1

0.2

0.3

0.4

0.5

20 keV

6 keV

3 keV

Beam rms size at solenoid (mm)

Chro

mati

c E

mit

tance

(m

icro

ns)

-0.005

-0.004

-0.003

-0.002

-0.001

0

0.001

0.002

0.003

0.004

0.005

-180 -120 -60 0 60 120 180

rf phase

cylindrical cavity

racetrack coupler cell

half-cell with laser ports

γβr

Features of the LCLS Gun Design & Comparison with GTF GunPulsed heating mitigated with longitudinal

coupling and increasing radius of RF aperture

Dual RF feed and racetrack shape in full cell

eliminate the dipole and quadrupole RF fields.e-beam

Z-coupling

Cathode

assembly

on flange

•L. Xiao, R.F. Boyce, D.H. Dowell, Z. Li, C. Limborg-Deprey, J. Schmerge, “Dual Feed

RF Gun Design for LCLS,” Proceedings of 2005 Particle Accelerator Conference.

•C. Limborg et al., “RF Design of the LCLS Gun”, LCLS-TN-05-3, May 2005.

2.0 1.3 - coupling grazing or normal laser incidence

z (longitudinal) theta (azimuth) rf coupling copper copper or Mg cathode

deformation plunger/stub tuners 0.1 mrad /mm 4 mrad /mm peak quadrupole field

120Hz 10Hz repetition rate 15MHz 3.4MHz 0 - p mode separation

racetrack circular cavity shape dual feed single w/compensation port rf feed 140MV/m 120MV/m cathode field

LCLS Gun 1 BNL/SLAC/UCLA; GTF

2.0 1.3 - coupling grazing or normal laser incidence

z (longitudinal) theta (azimuth) rf coupling copper copper or Mg cathode

deformation plunger/stub 0.1 mrad /mm 4 mrad /mm peak quadrupole field

120Hz 10Hz repetition rate 15MHz 0 -

racetrack circular cavity shape dual feed single w/compensation port rf feed

cathode field LCLS Gun BNL/SLAC/UCLA Gun III

RF

near normal

David H. Dowell

dowell@slac.stanford.edu

0

0.5

1

1.5

2

2.5

3

-0.3 -0.2 -0.1 0 0.1 0.2 0.3

-100

-80

-60

-40

-20

0

20

40

60

80

100uncorrected quad

corrected with pc-quad

uncorrected phase

corrected phase

Characteristics of the Emittance Compensation Solenoid

Relatively strong effect on the beam emittance,

especially at high charge

Max on quad corrector

Expected ~setting

from mag. meas.

Solenoid requires

<0.1% precision

1 nC

Solenoid Int. Field kG-m

0.2%

No

rmal

ized

em

itta

nce

(m

icro

ns)

Distance from solenoid center (m)

Qua

d fie

ld o

ver

prob

e le

ngth

(gau

ss)

Qua

d fie

ld p

hase

(de

g)

Gun Solenoid in SLAC Mag. Meas. Lab

bucking coil rotating coil

Quad Correctors:

long quads on Gun1

long & short on Gun2

long quad wires

short PC quads

Parameter symbol value unit

UV wavelength on cathode l 253 nm

Spot diam. on cath. (edge) 2R 1.3 mm

Rel. energy stability (rms) E/E 1.2 %

Pulse duration (fwhm) tfw 6 ps

Timing jitter w.r.t. RF (rms) t 0.16 ps

x & y centroid jitter (rms) x,y/R 2.5 %

UV energy before transport EUVb 1.5 mJ

UV energy on cathode EUVc 0.3 mJ

Repetition rate f 30-120 Hz

Unsched. downtime/(6 mo.) 0 2 %

Matching to the linac for optimum emittance compensation:

Ferrario working point for LCLS* is1.4 m from cathode to linac.

Parameter sym dsgn meas unit

Final injector e energy mc2 135 135 MeV

Bunch charge Q 1 1 nC

Init. bunch length (fwhm) t0 9 11 ps

Fin. bunch length (fwhm) tf 2.3 0.4-11 ps

Initial peak current Ipk0 100 95 A

Projected norm emittance x,y 1.2 1.2 μm

Slice norm. emittance sx,y 1.0 0.9 μm

Slice energy spread (rms) sx,y <5 <6 keV

Single bunch rep. rate f 120 10-30 Hz

RF gun field at cathode Eg 120 115 MV/m

Laser energy on cathode ul 250 300 μJ

Laser wavelength l 255 253 nm

Laser spot diam. on cath. 2R 2.0 1.3 mm

Cathode quantum eff. QE 6 3.7 105

Commissioning duration - 8 5 mo

Summary of Injector Performance at 1 nCDrive Laser Parameters

Cathode

Linac entrance

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 0.5 1

No

rmal

ized

Em

itta

nce

(m

icro

ns)

Bunch Charge (nC)

Projected Emittance vs. Charge

*M. Ferrario et al., “HOMDYN study for the LCLS RF Photo-injector”,

Proc. of the 2nd ICFA Adv. Acc. Workshop on

“The Physics of High Brightness Beams”,

UCLA, Nov. 1999 & SLAC-PUB-8400. Parameters are for the first cathode as reported in:

Akre et al., PRST-AB, 11,030703(2008)

Undulator Gain Length & Pulse Energy at 1.5 Å:

3.3 m & >2 mJ per x-ray pulse

FEL Performance at 250 pC

0

0.5

1

1.5

2

0

1

2

3

4

5

6

7

8

0 0.5 1 1.5 2

Cen

ter S

lice E

mit

tan

ce

(mic

ro

ns)

Ga

in L

en

gth

(m

)

Projected Emittance

(microns)

Gain Length vs.

Injector Emittancex,y 0.4 mm (slice)

Ipk 3.0 kA

E/E 0.01% (slice)

(25 of 33 undulators installed)D. Ratner et al., FEL09

Conf. Proc.

•P. Emma, FEL09 Conf. Proc.

•Z. Huang et al., FEL09 Conf. Proc.

•D. Ratner et al., FEL09 Conf. Proc.

•D. H. Dowell et al., FEL09 Conf. Proc.

•Emittance increased by changing

laser transverse shape on cathode

New Second Mode of FEL Operation:

Low Bunch Charge for Extremely Short FEL PulsesLow Charge Performance of Injector

Low charge slice emittance meas. at 20 pC

* K-J. Kim, NIM A275(1989)2001-218

0 0.1 0.2 0.3 0.4 0.5 0.60

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

RMS Laser Spot Size (mm)

No

rmali

zed

Em

itta

nce (

mic

ron

s)

Constant Charge Density Meas.

Constant 20 pC Charge Meas.

Constant 20pC Charge Fit

20 pC Charge Fit + Space Charge Calc.

Theoretical Thermal Emittance

Emittance vs. laser size

at constant charge (20 pC, blue) and

constant charge density (red)

2

22222

,sin

2)(/9.0

x

xx

A

x

laserrfcathode

xscthermaltotalnLI

j

E

mcrmsmmm

m

pm

2,3mc

eff

xtheorythermaln

)(/9.0exp, rmsmmmicronsthermaln parameterized

space charge form factor

z

x

z

xx

LL

m 0934.00653.0

Space charge emittance*:

z

xx

A

x

laserrfcathode

scLI

j

E

mc m

p

222

sin2

j : the current surface density

I A : the Alfven current, 17000 amps

mx : space charge form factor

x : rms transverse beam size

L z : bunch length (full width)

Y. Ding

Z. Huang

Simulation at 1.5 Å based on measured injector &

linac beam & Elegant tracking, with CSR, at 20 pC.

1.5 Å,3.61011 photonsIpk = 4.8 kA 0.4 µm

SIMULATED FEL PULSES

Y. Ding

Z. Huang15 Å,2.41011 photons,Ipk = 2.6 kA, 0.4 µm 1.2 fs

Simulation at 15 Å based on measured injector &

linac beam & Elegant tracking, with CSR & 20 pC.

Measurements and Simulations for 20-pC Bunch at 14 GeV

MEASURED SLICE EMITTANCE

time-slicing at 20 pC

x = 0.14 µm

135 MeV20 pC

Y. Ding et al., PRL 102, 254801(2009).

20 pC tested

J. Frisch, Conf. Proc. of PAC09.

•General features of the gun and injector (Some of the reasons why the LCLS Gun & Injector work so well):

•Matching to the linac for optimum emittance compensation, Ferrario working point

•Increased RF mode separation to minimize RF mode beating > reduces energy spread, etc.

•Symmetric RF fields in gun and s-band linacs

•Z-coupling to minimize pulsed heating for long gun life

•Improved cooling for 120 Hz operation

•Full wakefield mitigation in gun-to-linac beamline

•Emittance compensation solenoid field includes quadrupole correctors

•Stable and reliable diode-pumped drive laser

•Cathode surface roughness less than 40 nm peak-to-peak

Summary

•Achieved LCLS 1 nC emittance requirement:

•1.2 micron (projected), 0.9 micron (slice) at 95 amps

•Performance of Injector and FEL better at 250 pC:

•Gain Length 3.3 m, >2mJ per x-ray pulse at 1.5 angstroms

•0.4 micron slice emittance, 3 kA at undulator

•Low charge (20 pC) produces ultra-short (~few fs) x-ray pulses

•Slice emittance 0.14 microns at 20 pC

•Injector reliably produces great beam from 20 pC to 1 nC

•Allows for a wide range of FEL operational options

Y. Ding et al., “Measurements and Simulations of Ultralow Emittance and Ultrashort

Electron Beams in the Linac Coherent Light Source”, PRL 102,254801(2009)