C.Limborg-Deprey

LCLS FAC , April 2005 limborg@slac.stanford.edu

April7-8 2005

Injector Physics

C.Limborg-Deprey, D.Dowell ,Z.Li*, J.Schmerge, L.Xiao*

RF Gun modifications Linac Sections modificationsRisk Mitigation Plans

QE studiesPulse Shaping 3D-ellipsoidR&D Laser (see S.Gilevich Presentation)0.2nC (see P.Emma Presentation)

Summary

(*)

C.Limborg-Deprey

LCLS FAC , April 2005 limborg@slac.stanford.edu

April7-8 2005

RF Gun Modifications

Z-Coupling

15 MHz

= 2

Feedback:

Control signals (Reflected power, metal temperature)

Actuator (water T.)

Push Pull deformable tuners (No Plunger)

November 04 review, Report from J.Wang et al. PIC simulations

Bead-drop procedure ?

Back-plate dynamically movable

Include cell probes

C.Limborg-Deprey

LCLS FAC , April 2005 limborg@slac.stanford.edu

April7-8 2005

Modified RF gun design Gun fabricated at SLAC

RF design completeMechanical model in progress120Hz heat calculations under way

Dual FeedSuppresses the time dependent dipole modeMatching phase for 2 feeds by holding mechanical tolerances on both arms

Z coupling (instead of -coupling)Pulsed heating reduced + easier machiningRacetrack shape compensates for stronger quadrupole mode

15 MHz mode separation adoptedLower cathode voltage for the 0-mode

“Suppresses” two degrees of freedom in parameter spaceLarger radius for coupling cell iris

Reduces RF emittanceEasy to accomodate elliptical curvature to reduce surface field

Shaping of RF pulse for reducing average power4kW -> 1.8 kW ; cooling channels designed for handling 4kWReduce reflected power from gun

LCLS-TN-05-3.pdfhttp://www-ssrl.slac.stanford.edu/lcls/photoinjector/reviews/2004-11-03_rf_review/

Courtesy L.Xiao

C.Limborg-Deprey

LCLS FAC , April 2005 limborg@slac.stanford.edu

April7-8 2005

Linac: Dual Coupler at entrance cell

Kick is reduced by more than 4 times in output coupler

11

2

n

twiss

o

nominal 1nC Ent. L0a Exit L0a Ent. L0b Exit L0b

(/) at 0 single feed in % 1.8 0.4 12 0.6/0.5

With dual feed reduction head-tail kick reduced by 20

(/) at 0 dual feed in % 0.005 0.4 0.04 0.6/0.5

Operating point

rms head-tail trans. kick for 10ps bunch

Dual feed at entrance cell BUT NOT at exit Quadrupole head-tail not a problem at exit cell

Head-tail dipole kick from single feedGenerates emittance growth

C.Limborg-Deprey

LCLS FAC , April 2005 limborg@slac.stanford.edu

April7-8 2005

L0a L0b: New Design With WR284 Waveguide

++

b=35.8785

w=24.2100

d=13.000

+r=1.000

WR284 waveguide

a1 a2 a3 a4 t1 t2 t3 t4

b2 b3a b3b/b4

beampipe

originalnewCoupler cup2 cup3-a cup3-b

R0.5 R0.5 R1.38

Using standard WR284 waveguide – eliminate all tapers (flanges closer to body, to accommodate linac solenoid )Coupler cell lengthened to match height of WR284 waveguide Racetrack parameters readjusted

Courtesy Z.Li

C.Limborg-Deprey

LCLS FAC , April 2005 limborg@slac.stanford.edu

April7-8 2005

Waveguide stiffners

Courtesy J.ChanPumpOut

Linac: New Design With WR284 Waveguide

Enough clearance for solenoidWaveguide curvature adjusted to minimize S11Waveguide cold-tested

2 arms adjusted for identical match

C.Limborg-Deprey

LCLS FAC , April 2005 limborg@slac.stanford.edu

April7-8 2005

Cathode- QE improvement

Courtesy D.Dowell, R.Kirby

LCLS QE Spec. 6x10-5 @ 255nm

After H-beam cleaning 1.2x10-4

Idt for

H-ion beam

%Carbon on surface

initial 30

1H 0.0378 C 11

2H 0.124 12

3H 0.1818 10

4H 0.614 8

H-ion Cleaning ExperimentQE at low voltage (No Shottky Enhancement)

Surface unaltered by H-ion beam cleaning contrary to effect of laser cleaning

C.Limborg-Deprey

LCLS FAC , April 2005 limborg@slac.stanford.edu

April7-8 2005

QE improvement

Schottky Enhancement of the QETheory(*)• Approximations

R = 0.34 (reflection) 1phot -> 1 e No e-scattering (before emission) Fermi-Dirac at 0 K no roughness, no surface features

• Theoretical model can be refined

More tests planned

• More samples (process)• Find Optimal H-ion beam current and integration time (and temperature)

Implement on GTF gun?

Courtesy D.Dowell

180 200 220 240 260 280 3001 10

6

1 105

1 104

1 103

0.01

QE(theory) at 0MV/mQE(theory) at sin(30)*120MV/mQE(expt) at 0MV/m

QE's at 0MV/m and sin(30)*120MV/m

Wavelength (nm)

QE

GTF (measured)

LCLS Specifications

LCLS minimum required

(*) Based on J.Schmerge et al., Proc.FEL04, 205-208

C.Limborg-Deprey

LCLS FAC , April 2005 limborg@slac.stanford.edu

April7-8 2005

Minimum Emittanceperfect machine ~ 0.9 m.rad (for nominal 1nC tuning)

Only ~ 0.1 m.rad margin for emittance growth

Contributions to emittanceLarge cathode emittance

for copper measured 0.6 m.rad per mm of rlaser (theoretical is 0.3 m.rad )

Minimum set by space charge limitMinimum rlaser or electrons cannot leave cathode (for metal cathodes)

rmin. = 0.82 mm at 54 MV/m for a 1nCcathode > 0.5 m.rad

RF emittance small ~0.15 m.rad

space charge can be supressed by appropriate “emittance compensation”

uniform distribution inside an ellipsoid produces linear space charge force

Linear “emittance compensation” corrects for this term

Should we investigate on 3D-ellipsoid pulse shaping ?

2arg

22echspaceRFcathodetot

??22 roughnessthermalcathode spotlasercathode r

sin

2 peakoo

Er

QE

cathodetot ~

C.Limborg-Deprey

LCLS FAC , April 2005 limborg@slac.stanford.edu

April7-8 2005

Ellipsoidal Emission pulse

“Beer Can” is not the optimaldistribution

Electrons uniformly distributed in 3D ellipsoid volume

22

2

2

2

2

2

Ac

z

b

y

a

x

.constdzdydx

N

rmax = 1.2 mm

Pulse length

Radial profile = half-circle

fwhm = 10 ps

Pulse length

Line Density = parabola

C.Limborg-Deprey

LCLS FAC , April 2005 limborg@slac.stanford.edu

April7-8 2005

Standard “Beer can” against “3D ellipsoid”

rmax = 1.2mm

r= 1.2mm

cath.= 0.69mm.mrad per mmcath.= 0.6 mm.mrad per mm

C.Limborg-Deprey

LCLS FAC , April 2005 limborg@slac.stanford.edu

April7-8 2005

“Beer Can” vs “3D ellipsoid”

Best Tunings for ~ 100A at end of injector = 1.02 mm.mrad; 80% = 0.95 mm.mrad

= 0.57 mm.mrad ; 80% = 0.58 mm.mrad

using standard “cathode” = 0.6 mm.mrad per mm radius !!

Simulations with similar numerical meshing parameters and 200k particles

C.Limborg-Deprey

LCLS FAC , April 2005 limborg@slac.stanford.edu

April7-8 2005

Sensitivity and Safety Margin

Solenoid RF rlaser Pulse length

“Beer Can” 1% < 5 ~0.1 mm <1ps

“3D-ellipsoid” >3% >10 > 0.3 mm >4ps

Solenoid 2%

Tuning + Stability of injector are eased; very large margin below 1mm.mrad

Margin for emittance below 1 mm.mrad for the 80%

0.67 mm.mrad for “3D-ellispoid” (projected = 80% )

0.9/1.0 mm.mrad for “beer can” (80%/ projected)

C.Limborg-Deprey

LCLS FAC , April 2005 limborg@slac.stanford.edu

April7-8 2005

3D-Ellispoid Feasibility ?

Two solutions proposedPulse Stacker

With 12 Gaussians of alternating polaritiesToo lossy, uses too much space, unweildyAwkward but easy control on individual components Technically feasible with many $$$$$$$ for

controls, to achieve alignment , timingmeasurement to adjust amplitude coefficient

Spectral Control technique Masking technology for IR exists Probably better for space and money than previous solutionBefore or after amplifier ?

Before = recover lost energy but shape might not be preserved through chain After = difficult power handling (high losses in gratings and masking)

Direct UV might be more appropriate; masking technology needs to be developed (transmissive or reflective scheme)- need to solve high damage threshold issue

zyx X mask

y mask

Chirped input,

temporally

tx

yt

To cathode

(z,y) plane

(z,x) plane

• Fluence < 150 mJ/cm2, E = 50mJ

• BW < 15 nm, Chirp = 4.8.1023 THz/ps

= 2200 groves per mm, = 6.7

Dpencil beam (1m) =11.7 cm

Dy = 2 waist y = 2 = 0.9 cm

• Fluence < 150 mJ/cm2, E = 50mJ

• BW < 15 nm, Chirp = 4.8.1023 THz/ps

= 2200 groves per mm, = 6.7

Dpencil beam (1m) =11.7 cm

Dy = 2 waist y = 2 = 0.9 cm Courtesy of P.Bolton

C.Limborg-Deprey

LCLS FAC , April 2005 limborg@slac.stanford.edu

April7-8 2005

What we would like to have ?Optically Controlled Spatial Filtering

Spatial frequency mask in Fourier Plane with sub-ps dynamics for switching Easy to generate flat disk of fixed radius in image plane by masking in Fourier plane

a(t) controlled by driver pulse

Courtesy of P.Bolton

Object

Driver with temporal shape = half-disk

Mask in

Fourier Plane

ImageTransmissive

or

Reflective Optics

FFT-1

ta

taJta

212

)(ta

rcirc

a(t)

C.Limborg-Deprey

LCLS FAC , April 2005 limborg@slac.stanford.edu

April7-8 2005

Summary on 3D pulse shapingIdeal emission pulse = “3d-Ellipsoid” not “Beer Can”

Perfect emittance compensation in high charge regime Impressively less sensitive to tuning parameters

Much larger tolerances than those defined for “beer can” pulse Much easier to tune

projected as low as 0.6 mm.mrad

Ellipsoidal Laser Pulse is a Technical challenge maybe only slightly more challenging than “beer can” generation if direct UV shaping is considered for “beer can”, the “ellipsoid generation” shares many of the same difficultiesSolution has (by construction) adaptive correction

Of ShottkyOf non-uniformity on cathode

Solution should be corrected on e-beam measurement (using Genetic Algorithm as suggested at ERL05)

C.Limborg-Deprey

LCLS FAC , April 2005 limborg@slac.stanford.edu

April7-8 2005

ConclusionGun

RF design completedMechanical design under way (thermal analysis on-going)

LinacRF design completedMechanical design in progress

Risk Mitigation Plans for 1nCCathode studies: H-ion cleaning for higher QE 3D-pulse shaping Tuning of 0.2nC completed (see P.Emma)

On-Going studiesLaser steering stabilization Feedback for Laser Energy/ RF gun (P,) Commissioning PlansBeam BA strategy

C.Limborg-Deprey

LCLS FAC , April 2005 limborg@slac.stanford.edu

April7-8 2005

BACK-UP

C.Limborg-Deprey

LCLS FAC , April 2005 limborg@slac.stanford.edu

April7-8 2005

Early thought : Stacking pulses

6+6 beamlets of different radii

Gaussians Wash out discrete steps of rms value

Interferences

C.Limborg-Deprey

LCLS FAC , April 2005 limborg@slac.stanford.edu

April7-8 2005

Fighting interferences in Stacker

Alternating polarization + appropriate choice of , interference effect is minimized

No interference Interferences random phases

~<15 %

for all draws

i

i

i

i

tt

ip eeAE 242

2

22

i

i

i

i

tt

is eeAE 2412

2

212

sp III

*. ppp EEI *. sss EEI

C.Limborg-Deprey

LCLS FAC , April 2005 limborg@slac.stanford.edu

April7-8 2005

PARMELA simulations using stacker distributions

Beer Can

Direct beer can

Ellipsoid ideal

50 Beamlets no interferenceStacker

12 Beamlets and random phase

= 1.02 mm.mrad; 80% = 0.95 mm.mrad (with standard “cathode” =0.6)

= 0.71 mm.mrad ; 80% = 0.71 mm.mrad (with overestimated “cathode” =0.7)

= 0.80 mm.mrad; 80% = 0.80 mm.mrad (with overestimated “cathode” =0.7)

IDEAL

C.Limborg-Deprey

LCLS FAC , April 2005 limborg@slac.stanford.edu

April7-8 2005

Stacker Layout

Profile shaper

pulse energy control

delayline

imaging optics

collimator-magnifier

delayslide

gratinggrating

spectralfilters

photocathode

halfwaveplate

polarizing cube

launch mirror

Courtesy of P.Bolton

“what to try to avoid…” from P.Bolton

C.Limborg-Deprey

LCLS FAC , April 2005 limborg@slac.stanford.edu

April7-8 2005

Spectral Control Principle

• Create a time-space correlation

• Ideally better in UV but masking technology does not exists yet

• Constraints 1- Fluence < Damage threshold

2- BW not too large (for THG)

3- Space

4- if possible after amplifiers

• Create a time-space correlation

• Ideally better in UV but masking technology does not exists yet

• Constraints 1- Fluence < Damage threshold

2- BW not too large (for THG)

3- Space

4- if possible after amplifiers

Chirped input,

temporally

In z,x plane

Vertical mask

z

yx

• Fluence F < 150 mJ/cm2

• pulse energy E = 50mJ

• BW < 15 nm

• Lgratings->mask < 2m

•

Chirp = 4.8.1023 THz/ps

= 2200 groves per mm

= 6.7

Dpencil beam (1m) =11.7 cm

Dy = 2 waist y = 2 = 0.9 cm

Beam dispersed enough to have beam size negligible

• Fluence F < 150 mJ/cm2

• pulse energy E = 50mJ

• BW < 15 nm

• Lgratings->mask < 2m

•

Chirp = 4.8.1023 THz/ps

= 2200 groves per mm

= 6.7

Dpencil beam (1m) =11.7 cm

Dy = 2 waist y = 2 = 0.9 cm

Beam dispersed enough to have beam size negligible

In z,y plane

Courtesy P.Bolton

FE

2

2