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C.Limborg-Deprey
LCLS FAC , April 2005 [email protected]
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 [email protected]
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 [email protected]
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 [email protected]
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 [email protected]
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 [email protected]
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 [email protected]
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 [email protected]
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 [email protected]
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 [email protected]
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 [email protected]
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 [email protected]
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 [email protected]
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 [email protected]
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 [email protected]
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 [email protected]
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 [email protected]
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 [email protected]
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 [email protected]
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 [email protected]
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 [email protected]
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 [email protected]
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