Cecile Limborg-Deprey Theory Club: The LCLS [email protected] December 3rd 2004 The LCLS Injector C.Limborg-Deprey Emittance compensation

Cecile Limborg-Deprey

Theory Club: The LCLS Injector [email protected]

December 3rd 2004

The LCLS Injector C.Limborg-Deprey

•Emittance compensation– linear emittance compensation for ideal laser beams– limits of emittance?

• thermal emittance

•Nominal and alternate tunings– Beamline layout– 1nC, – 0.2 nC– last year modifications

• laser heater• RF structures

•How much can we believe PARMELA– GTF, DUVFEL PARMELA vs experiment– Code comparison– What could we be missing?

• Commissioning measurements – Spectrometers– Emittance measurement– 6D measurements



December 3rd 2004

Gun SolenoidLinac

Emittance Compensation

Photocathode RF gun

adequate to generate coldest electron beam

photoemission produces some transverse momentum px

“thermal emittance” ~ x px

also called “intrinsic emittance” or “minimum” emittance

We want to preserve at best the beam emittance along the transport line

(space charge, wakefield, CSR …)

Space charge very strong at low energy generates large energy spread

• Appropriate choice and tuning of components allow to compensate for variation in transverse dimension (size, divergence) due to chromatic effects

= Compensate for the mismatch between slices



December 3rd 2004

Single Particle Dynamics

defocusingfocusing

defocusing

focusing

Single particle dynamics in gun

Gun



December 3rd 2004


defocusingfocusing

defocusing

focusing

Electric field effects

RF effects are non linear

RF Kicks are time dependent: so vary along the bunch

Are not be compensated for

Very small contribution to total

~ 0.1 mm.mrad in our S-Band Gun

Magnetic field effects



December 3rd 2004

Gun Solenoid


Solenoid focusing

focal length energy dependent

Gun SolenoidLinac

Focusing kick at entrance of Linac

Time dependent

Used in emittance Compensation process



December 3rd 2004

Gun SolenoidLinac

Simulations

Diverging: Space charge

RF kick at exit cell

Converging: Solenoid

RF kick at entrance cell



December 3rd 2004

Emittance Compensation



December 3rd 2004

Linac

Gun S1 S2

Movie 1 Movie 2

Movie 3Movie 4

Movies 1,2,3 : thermal = 0.72 mm.mrad

Movie 4 : thermal = 0 mm.mrad

3D Ellipsoid

Space Charge linear with r ,

optimal shape for perfect emittance

compensation



December 3rd 2004

Movie 1 Movie 2 Movie 3Movie 4



December 3rd 2004

Preinjector:

PORCU

PINE CATHODE

HOLDE

R

UHV A

LL METAL

GATE VALVES

SPOOLS FRO

M VALVE

SEAL

LONG B

ELLOWS

ASSEM

BLY

TREATMEN

T CHAM

BER

VACUU

M PUMPS

SLAC Main Linac Beamline

SECTOR 20

VAULT



December 3rd 2004

Parameter Value

Peak Current 100 A

Charge 1 nC

Normalized Transverse Emittance: Projected/Slice

< 1.2 / 1.0 micron (rms)

Repetition Rate 120 Hz

Energy 135 MeV

Energy Spread@135 MeV:Projected/Slice

0.1 / 0.01 % (rms)

Gun Laser Stability 0.50 ps (rms)

Booster Mean Phase Stability

0.1 deg (rms)

Charge Stability 2 % (rms)

Bunch Length Stability 5 % (rms)

Goal parameters



December 3rd 2004

Gun S1 S2 L0-119.8MV/m

L0-224 MV/m

‘Laser Heater’

‘RF Deflecting cavity’ TCAV1

3 screen emittance measurement

6 MeV = 1.6 m ,un. = 3keV

63 MeV = 1.08 m ,un. = 3keV

135 MeV = 1.07 m ,un. = 3keV

DL1

135 MeV = 1.07 m ,un. = 40keV

Spectrometer

Lina

c tu

nnel

UV Laser 200 J, = 255 nm, 10ps, r = 1.2 mm

Spec

trom

eter



December 3rd 2004

~19 parameters to optimize

GunE (MV/m)Balance

SolenoidPositionLengthField

Solenoid 2PositionLengthField

Linac0-aPositionE(MV/m)

Linac0-bPositionE(MV/m)

1- Analytic formula emittance compensation

2- Envelope equation code (Homdyn , Trace3D)

define components

3- Fine tuning + sensitivity studies (multiparticle tracking code: PARMELA, ASTRA …)

Laser ParametersLongitudinal (length, rise time, flatness)Transverse(r, uniformity, pointing spot) Energy charge



December 3rd 2004

Nominal tuning

Rise/fall 0.7 ps 1.0ps 1.5 ps

projected [mm.mrad] 0.954 1.028 1.141

80% [mm.mrad] 0.894 0.935 0.986

<slice >10..90[mm.mrad] 0.849 0.877 0.901

<slice >1..100[mm.mrad] 0.906 0.953 1.004

proj = 0.954 , 80 = 0.89 mm.mrad



December 3rd 2004

Tolerance as a function of single parameter variation

Solenoid 1 0.3% gun 2.5

Solenoid 2 20% Linac Field 12 %

(EFinal = 150 MeV )

Egun 0.5%

Balance~ 3% is ok



December 3rd 2004

Param. Nom. Units Stability Requirements

Sol1 2.7235 kG 0.02 %

Sol2 0.748 kG 1 %

Gun Phase 27.25 /0-X 0.1

Gun Field 120 MV/m 0.5%

Charge 1 nC 5%

L01Field 18 MV/m 2.5%

Defined after combining errors

Small margin left for laser parameters variation

Stability RequirementsStability requirementsStability requirements



December 3rd 2004

Tolerances – Alignment and Laser UniformityTolerances – Alignment and Laser Uniformity

(*) combined with uniformity of QE

Param. Type Tolerance UnitsSolenoid 1 Transverse Position 500 m

Angular Position 1.5 mrad

Laser Transverse Position 100 m

Laser Uniformity Transverse (Slope) 10 % NA

Transverse (Cross) 10 % NA

Longitudinal 30% ptp Freq.> 1THz (1ps)

Longitudinal 20% ptp Freq.< 1THz

Linac 1 Transverse Position 150 m

Angular Position 120 rad

Linac 2 Same as Linac 1

Solenoid 2 Same as Solenoid 1



December 3rd 2004

Requirements on Laser Pulse - SummaryRequirements on Laser Pulse - Summary

Transverse

10 % ptp maximum on emission uniformity

Longitudinal

=480 m

=240 m

=120 m

5% ok for emittance

But too much for LSC



December 3rd 2004

1nC, long pulse Alternate tunings for cylindrical bunch

th = 0.6 mm.mrad per mm laser spot sizereduce rlaser to 0.85 mm BUT to keep charge density same order lengthen bunch Start with th = 0.51 mm.mrad



December 3rd 2004

Alternate tunings for improving Name Q

(nC)

Laser pulse (ps)

r

(mm)

th

(m.rad)

80

(m.rad)

RF

()

80 5%

Nominal 1 10 1.2 0.72 0.9 32 2.5

1 nC, 17.5 ps 1 17.5 0.85 0.5 0.75 33 1.5

0.2nC,10ps 0.2 10 0.39 0.234 0.38 37 2.5

0.2nC,5ps 0.2 5 0.42 0.25 0.37 32 5

• th = 0.6 mm.mrad per mm laser spot size

• minimum r best , BUT limit on minimum radius = space charge limit (ignoring Shottky)

Esc = Q / ( r2 o)

example:

for 1nC, r = 1.2mm, Esc = 25 MV/m ( 12)

for 1nC, r = 0.85 mm, Esc = 50 MV/m ( 25)

for 0.2 nC, r = 0.3mm, Esc = 80 MV/m ( 42)

for 0.2 nC, r = 0.42mm, Esc = 40 MV/m ( 20)



December 3rd 2004

0.2nC

A 5ps laser pulse improves dramatically the peak current compared to the 10ps laser pulse case

without damaging too much the slice emittance



December 3rd 2004

Ellipsoid emission bunch



December 3rd 2004

Ellipsoid emission bunch



December 3rd 2004

Ellipsoid emission bunchsquare

ellipsoid

Exit gun Entrance L01

Exit L01 Exit L02

Longitudinal Phase Space

Ek [MeV] vs T [ps]



December 3rd 2004


L0-224 MV/m

‘Laser Heater’



6 MeV = 1.6 m ,un. = 3keV

63 MeV = 1.08 m ,un. = 3keV

135 MeV = 1.07 m ,un. = 3keV

DL1

135 MeV = 1.07 m ,un. = 40keV

Spectrometer

Lina

c tu

nnel

UV Laser 200 J, = 255 nm, 10ps, r = 1.2 mm

Spec

trom

eter



December 3rd 2004

LSC observed at the DUVFEL

Courtesy of Timur Shaftan

Also observed at TTF

Longitudinal Space Charge Instability

Simulations and theoretical studies

Z.Huang et al. PhysRev. SLAC-PUB-10334

J.Wu et al. LCLS Tech Note , SLAC-PUB-10430

G.Geloni. Et al.

DESY 04-112

The self-consistent solution is the space charge oscillation

Current Density Energy



December 3rd 2004

ENERGY

CURRENT

GUN EXIT

6 MeV

ASTRA/ PARMELA Simulations , Amplitude = +/- 5%, = 100 m



December 3rd 2004

ENERGY

CURRENT

End L02

135 MeV

Microstructure at the end of the injector

Laser Heater provide enough energy spread (40keV) for “Landau damping” preventing

-further amplification of the microbunching

- the increase an energy spread (as it needs to remain < the FEL parameter)



December 3rd 2004

Can we believe PARMELA?

• Sensitivity studies fine since relative evolution

• Meshing : by hand in PARMELA , automated in ASTRA

criteria well understood

• Benchmarks

- w.r.t experiences

Proved importance of data on initial distribution

Fitted the slice parameters such as , , projected , slice

- w.r.t other codes

Seems that extraction agree with PIC codes (experiment to be revisited for low accelerating voltage)

Still need to compute fields for lossy copper



December 3rd 2004

DUVFEL measurements200 pC

Good Agreement Slice Emittance and Twiss Parameters for the various solenoid fields

After including thermal emittance, gun field balance between the two cells, transverse non-uniformity and longitudinal profile

DUVFEL measurements200 pC

Good Agreement Slice Emittance and Twiss Parameters for the various solenoid fields

After including thermal emittance, gun field balance between the two cells, transverse non-uniformity and longitudinal profile

Solenoid = 104 ASolenoid = 98

A



December 3rd 2004

DUVFEL MeasurementsDUVFEL Measurements

Thermal emittance experimentConfirms the 0.6 mm.mrad

per mm radius of laser spot size



December 3rd 2004

R&D Status: GTF MeasurementsR&D Status: GTF Measurements

longitudinal emittance

GTF measurements - 1.5 mm.mrad for 130A

Pea

k C

urre

nt (

A)

Instantaneous Peak Current

Spectrometer Imageof Slice Quad Scan Data

Slice Emittances

head tail

-1.5 -1 -0.5 0 0.5 10

50

100

150

Time (ps)

n (

mm

mra

d)

5 100

1

2

slice = 1.5 mm.mrad

for 130 A

~ close to LCLS requirements

Similar measurements at the DUVFEL facility

(Spring 2002)

Slice number

300pC



December 3rd 2004

‘Laser Heater’



Gun Spectrometer

Lina

c tu

nnel

Straight Ahead Spectrometer

Uniformity + Thermal emittance

1

2

43

Commissioning DiagnosticsCommissioning Diagnostics

YAG1 YAG2



December 3rd 2004

Above: Laser cathode image of air force mask in laser room.

Below: Resulting electron beam at pop 2.

Above: Laser cathode image with mask removed showing smooth profile.

Below: Resulting electron beam showing hot spot of emission.

Laser masking of cathode image at DUVFEL

Courtesy W.Graves

Point-to-point imaging of cathode on YAG1

Emission uniformityEmission uniformity1



December 3rd 2004

YAG2

==

Image of

divergence of sourceAssumes th = 0.6 mm.mradAssumes th = 0.6 mm.mrad

Very good resolution of divergence

Infinite-to-point imagingwhat type of momentum

distribution?

Thermal Emittance1



December 3rd 2004

Gun Spectrometer

Energy Absolute energy

Alignment using laser Spectrometer field calibration

Correlated Energy Spread for all chargesUncorrelated energy spread for low charges

Introducing a time-energy correlation (varying injection phase)

Slice thermal emittanceRelay imaging system from YAG1 to spectrometer screens

Point-to-point imaging in both planes

Uniformity of line density

Energy Absolute energy

Alignment using laser Spectrometer field calibration

Correlated Energy Spread for all chargesUncorrelated energy spread for low charges

Introducing a time-energy correlation (varying injection phase)

Slice thermal emittanceRelay imaging system from YAG1 to spectrometer screens

Point-to-point imaging in both planes

Uniformity of line density

YAG01

Spectrometer

YAGG1

YAGG2

Quadrupoles

2



December 3rd 2004

High Charge Operation : 1nC Nominal tuning – no quadrupole on –

Very good linearity

Longitudinal at YAG1

YAGG1YAGG1



December 3rd 2004

Resolves line density uniformity at high charge

YAG1

RF + 25 / nominal

Quadrupoles on for manageable image size

Resolves modulation

+/- 8% modulation on laser beam



December 3rd 2004

Laser Heater

Transverse RF Cavity

OTR Emittance Screens

DL1 Bend

Straight Ahead Spectrometer

135MeV Diagnostics

Point-to-point imaging of the 75 m waist (OTR5)

Horizontal slice emittanceVertical deflecting cavity + 3screen

Vertical slice emittanceQuad scan + spectrometerQuad Scan + Dogleg bend

Verification of thermal emittance

Longitudinal Phase space Vertical deflecting cavity + spectrometerEfficiency of laser heater

(spectrometer has 10 keV resolution)

Horizontal slice emittanceVertical deflecting cavity + 3screen

Vertical slice emittanceQuad scan + spectrometerQuad Scan + Dogleg bend

Verification of thermal emittance

Longitudinal Phase space Vertical deflecting cavity + spectrometerEfficiency of laser heater

(spectrometer has 10 keV resolution)

6D beam measurements



December 3rd 2004

Longitudinal Phase Space at waist • Transverse deflecting cavity

y / time correlation

(1mrad over 10ps )

• Spectrometer

x / energy correlation

From PARMELA simulations (assuming 1m emittance), resolution of less than 10 keV

rms fwhm

Spectrometer + Vertical deflecting cavity

Direct longitudinal Phase Space representation



December 3rd 2004

RF Gun – Racetrack in full cell 2d-: no port = benchmark omega3p/sf

3d-cylin: with coupling ports- cell cylindrical

3d-rtrack: with coupling ports- cell racetrack

Full : with laser ports + racetrack

Full retuned: with laser ports + racetrack+ retuned

-6-4-202468

-180 -130 -80 -30 20 70 120 170

rf phase (degree)

cylindrical cavity (lc=2.475cm)

racetrack cavity (lc=2.413cm)with d=0.315cmracetrack cavity (lc=2.413cm)with d=0.356cm

Qu

adru

po

le

r(1

/m)

-8-6-4-202468

-180 -130 -80 -30 20 70 120 170-180 -130 -80 -30 20 70 120 170

rf phase (degree)

cylindrical cavity (lc=2.475cm)

racetrack cavity (lc=2.413cm)with d=0.315cmracetrack cavity (lc=2.413cm)with d=0.356cm

Qu

adru

po

le

r(1

/m)

-8

From L.Xiao, ACD/SLAC

b b

d

x = y =0.88

x = 0.96 y =1.01

x = y = 0.90

x = 0.97 , y = 0.99

x = 0.91, y = 0.915



December 3rd 2004

RF Studies- L01 coupler

Dipole moment

Quadrupole moment

From Z.Li, L.Xiao,

ACD/SLAC

0.0200.20Cross Dual

0.0040.04Race-track dual

0.0630.63Symmetric dual

0.0780.78SLAC Single feed

Head-tail angle (rad/m)

()/m

0.0200.20Cross Dual

0.0040.04Race-track dual

0.0630.63Symmetric dual

0.0780.78SLAC Single feed

Head-tail angle (rad/m)

()/m

for 10 ps

Single feedDual feed

Dual feedDual feed +rtrack



December 3rd 2004

Injector Schedule



December 3rd 2004

Conclusion

Gained confidence in PARMELA/ASTRA vs experiment

vs other codes

Injector computations based on large thermal emittance (Twice the theoretical one for copper)

Discrepancy remains to be understood

Mitigation : running at 0.2 nC

Laser Pulse shaping and uniformity is critical to reach parameter goals



December 3rd 2004

Acknowledgements

Many thanks to S.Gierman, J.Schmerge, J.Lewellen, D.Dowell, W.Graves, T.Shaftan, Z.Huang, J.Wu, P.Emma, S.Lydia, J.Qi, M.Ferrarrio, K.Floetmann, L.Serafini, P.Bolton, M.Cornacchia, J.Galayda



December 3rd 2004

Slice-Emittance Measurement SimulationSlice-Emittance Measurement SimulationRF-deflector at 1 MVRF-deflector at 1 MV

slice OTR 10 timesslice OTR 10 times

yy bunch length bunch length

quad scannedquad scanned

4



December 3rd 2004

Slice-Emittance Measurement SimulationSlice-Emittance Measurement Simulation

slice-5slice-5

Injector at 135 MeV with Injector at 135 MeV with S-band RF-deflector at 1 MVS-band RF-deflector at 1 MV

= meas. sim.= calc.= y distribution= actual(same SLAC slice-(same SLAC slice- code used at BNL/SDL)code used at BNL/SDL)

(slice-y-emittance also simulated in BC1-center)



December 3rd 2004

RF Gun – Mode 0 studies

dF 3.4 MHz 8 MHz

3s, Vcath. in 0 mode 11.77 MV/m 4.96 MV/m

0.82s, Vcath. in 0mode 10 MV/m 5.7 MV/m

3.4MHz mode separation 8MHz mode separation

From Z.Li, ACD/SLAC

Solution : Klystron Pulse shaping

Study of 12 MHz mode separation

120MV/m Non-negligeable effect Study suggested

by T.Smith



December 3rd 2004

Scale2" 3"0 1"

FullCell

“Half” Cell

ElectronBeamExit

Photocathode Laser Port

Currently usinga single crystal(100) Cu cathode

GTF 1.6 cell S-band gun



December 3rd 2004

Scale2" 3"0 1"

LCLS Modifications:Dual rf feedCathode plate with brazed cathode plugLoad lock120 Hz coolingFull and ½ cell power monitorsand remote tuners

GTF 1.6 cell S-band RF gunWaveguide Feed

Full Cell Power Monitor



December 3rd 2004

Gun Solenoid


defocusingfocusing

defocusingSolenoid focusing



December 3rd 2004

Search for better tuning for the 2.8 FHWM case

With 1ps rise/fall time, assuming r = 0.42 mm & Retuning



December 3rd 2004

Egun 0.5%


L0-224 MV/m

Solenoid 1 0.3%

Egun 0.5%gun 2.5

…

Tolerance and stability as a function of single parameter variation



December 3rd 2004

Solenoid = 98 A

Solenoid = 108 A

Solenoid = 104 A

Slice emittance vs solenoid strength. Charge = 200 pC.

Solenoid

Eyn

Alpha

Beta

98 A

3.7 um (3.2)

0.4 (1.0)

1.3 m (1.3)

104 A

2.1 um (2.8)

-6.9 (-3.6)

9.8 m (6.8)

108 A

2.7 um (2.7)

-9.0 (-9.6)

45 m (36)

Projected Values

(parmela in parentheses)

Data

Parmela

Documents

Cecile Limborg-Deprey Theory Club: The LCLS [email protected] December 3rd 2004 The LCLS Injector C.Limborg-Deprey Emittance compensation