David H. Dowell and FriendsSLAC National Accelerator Laboratory

David H. Dowell and FriendsSLAC National Accelerator Laboratory

The LCLS GunThe LCLS Gun

ICFA Beam Dynamics Workshop on Future Light SourcesICFA Beam Dynamics Workshop on Future Light SourcesMarch 1-5, 2010March 1-5, 2010

•The LCLS InjectorThe LCLS Injector•Design features of the LCLS GunDesign features of the LCLS Gun•LCLS gun performanceLCLS gun performance•Cathode contaminationCathode contamination•Brightest possible beamBrightest possible beam•Summary and ConclusionsSummary and Conclusions

Major Components of the LCLS InjectorMajor Components of the LCLS InjectorDrive LaserDrive Laser

Located in room above gunLocated in room above gun

S-Band Gun & SolenoidS-Band Gun & Solenoid

Dual Feed S-band LinacDual Feed S-band Linac

+Diagnostics+Diagnostics

Design Features of the LCLS Gun & InjectorDesign Features of the LCLS Gun & Injector Pulsed heating mitigated with longitudinal coupling and increasing radius of RF aperture

e-beam

Z-coupling

Cathode assembly on flange

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rf phase

cylindrical cavity

racetrack coupler cell

half-cell with laser ports

γβr

Dual RF feed and racetrack shape in full cell eliminate dipole and quadrupole RF fields.

•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.•D. H. Dowell et al., “Development of the LCLS RF Gun”, ICFA Dynamics Newsletter, No. 46, August 2008. http://www-bd.fnal.gov/icfabd/Newsletter46.pdf

•Optimum emittance compensation, Ferrario working point•Increased RF mode separation to minimize RF mode beating•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 at up to 140 MV/m•Full wakefield mitigation in gun-to-linac beamline•Emittance compensation solenoid field with quadrupole correctors•Stable and reliable diode-pumped drive laser•Cathode surface roughness <40 nm peak-to-peak, low dark current

David H. Dowelldowell@slac.stanford.edu

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The Emittance Compensation SolenoidThe 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

Solenoid requires<0.1% precision

1 nC1 nC

Solenoid Int. Field kG-m

0.2%

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mal

ized

em

ittan

ce (m

icro

ns)

Distance from solenoid center (m)Distance from solenoid center (m)

Qua

d fie

ld o

ver p

robe

leng

th (g

auss

)Q

uad

field

ove

r pro

be le

ngth

(gau

ss)

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d fie

ld p

hase

(deg

)Q

uad

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pha

se (d

eg)

Gun Solenoid in SLAC Mag. Meas. LabGun Solenoid in SLAC Mag. Meas. Lab

bucking coil rotating coil

Quad Correctors:long quads on Gun1long & short on Gun2

long quad wires

short PC quads

Low Charge (20 pC) Measurements and AnalysisLow Charge (20 pC) Measurements and Analysis

Low charge slice emittance meas. at 20 pCLow charge slice emittance meas. at 20 pC

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

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RMS Laser Spot Size (mm)

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ized

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ittan

ce (

mic

rons

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Constant Charge Density Meas.Constant 20 pC Charge Meas.Constant 20pC Charge Fit20 pC Charge Fit + Space Charge Calc.Theoretical Thermal Emittance

Emittance vs. laser sizeat constant charge (20 pC, blue) and

constant charge density (red)

Emittance vs. laser sizeat constant charge (20 pC, blue) and

constant charge density (red)

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, sin

2)(/9.0

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laserrfcathodexscthermaltotaln LI

j

E

mcrmsmmm

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xtheorythermaln

)(/9.0exp, rmsmmmicronsthermaln parameterized

space charge form factor

z

x

z

xx LL

0934.00653.0

Space charge emittance*:

z

xx

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laserrfcathodesc LI

j

E

mc

22

2

sin2

j : the current surface densityI A : the Alfven current, 17000 amps

x : space charge form factor

x : rms transverse beam sizeL z : bunch length (full width)

Summary of LCLS Injector PerformanceSummary of LCLS Injector Performance

R. Akre et al., PRST-AB (2007)Y. Ding et al., PRL 102, 254801(2009)

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Projected Emittance vs. ChargeProjected Emittance vs. Charge

Cathode ContaminationCathode Contamination

Three sources of cathode contamination

•Residual contaminants left by fabrication, handling and storage•Contamination by the gun vacuum

•Ambient vacuum•Operating vacuum

•Contamination during operation due to molecular cracking:• By the laser•By the electron beam

For LCLS contamination by molecular cracking is the most problematic.

Electron beam emission image of the cathode after >1 year of operation. The UV laser beam has left a QE hole at its location.

This will be discussed in more detail in the Cathodes Overview talk on Thursday

The Brightest Beam PossibleThe Brightest Beam Possible- How much can the LCLS gun emittance be lowered? -- How much can the LCLS gun emittance be lowered? -

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Ultimate Emittance Vs. Field & Charge

Cathode Field (MV/m)

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eo

reti

cal

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itta

nc

e (

mic

ron

s/m

m-r

ms

) 20 pC Meas. in LCLS Gun at 50 MV/m

Assume all linear and non-linear space charge effects can be corrected/compensated for,assume the cathode is perfectly flat and the cathode physics is correct. Then the lower limit on the emittance depends on the thermal emittance for the divergence and the space charge limit for the beam size:

rmsmmmicronsEx

thermalcathodeSCLxsmallest /,

23mcW

x

thermal

02

0

density charge surface

cathode

bunchcathode R

QE

2

012 mcE

Q

cathode

Wbunchsmallest

cathode

bunchSCLx E

Q

0, 4

Summary and ConclusionsSummary and Conclusions

•Design Features of the LCLS gun and injector:•Optimum emittance compensation, Ferrario working point•Increased RF mode separation to minimize RF mode beating

•Restores field balance between cells•Reduces correlated energy spread => chromatic aberration in solenoid•Gun RF tuning and field balance is more tolerant to geometry and temperature

•Symmetric RF fields in gun and s-band linacs: elimination of dipole and quadrupole field errors•Emittance compensation solenoid field with quadrupole correctors: Mostly effective at high charge•Z-coupling to minimize pulsed heating for long gun life•Improved cooling for 120 Hz operation•Full wakefield mitigation in gun-to-linac beamline•Stable and reliable diode-pumped drive laser•Cathode surface roughness less than 40 nm peak-to-peak, low dark current, low thermal emittance

•The ultimate emittance based on thermal emission and the space charge limit•Improving the LCLS gun performance:

•“Three issues: cathodes, cathodes, cathodes!!“, G. Neil •QE robustness and uniformity. (see Cathode Overview talk)

•Large (apparent) low charge emittance in gun•High thermal emittance? Poor theory? Poor measurement?•Growth due to surface roughness, aberrations etc.?

•Understand role of solenoid quadrupole in emittance compensation•Correcting for quadrupole space charge fields?•Correcting spherical aberration of solenoid?

•Good diagnostics are essential!And did I mention cathodes?

Emission fromLCLS cathode

AcknowledgementsAcknowledgements

LCLS Injector Commissioning TeamLCLS Injector Commissioning Team::R. Akre

A. Brachmann J. CastroY. Ding

D. DowellP. EmmaJ. Frisch

S. GilevichG. Hays

Ph. HeringZ. HuangR. Iverson

C. Limborg-DepreyH. Loos

A. MiahnahriJ. Schmerge

J. TurnerJ. WelchW. White

J. Wu

Special Thanks to the LCLS Gun Group who built the gun and the Commissioning Team who allowed me to show their results.

LCLS Gun Group:LCLS Gun Group:Erik Jongewaard & Klystron Dept

Cecile Limborg-DepreyJohn Schmerge

Bob KirbyC. RivettaZenghai LiLiling Xiao

Juwen WangJim Lewandowski

Arnold VlieksValery Dolgashev