Transcript Injector Physics

Injector Physics
C.Limborg-Deprey, D.Dowell ,Z.Li*,
J.Schmerge, L.Xiao*
(*)
RF Gun modifications
Linac Sections modifications
Risk Mitigation Plans
QE studies
Pulse Shaping 3D-ellipsoid
R&D Laser (see S.Gilevich Presentation)
0.2nC (see P.Emma Presentation)
Summary
April7-8 2005
LCLS FAC , April 2005
C.Limborg-Deprey
[email protected]
RF Gun Modifications
November 04 review, Report from J.Wang et al.
PIC simulations
Bead-drop procedure ?
Z-Coupling
15 MHz
 = 2
Feedback:
Control signals (Reflected
power, metal temperature)
Actuator (water T.)
Push Pull deformable tuners (No Plunger)
April7-8 2005
Include cell probes
C.Limborg-Deprey
Back-plate dynamically movable
LCLS FAC , April 2005
[email protected]
Modified RF gun design
Gun fabricated at SLAC
RF design complete
Mechanical model in progress
120Hz heat calculations under way
Dual Feed
Suppresses the time dependent dipole mode
Matching phase for 2 feeds by holding mechanical tolerances on both arms
Z coupling (instead of -coupling)
Pulsed heating reduced + easier machining
Racetrack shape compensates for stronger quadrupole mode
15 MHz mode separation adopted
Lower cathode voltage for the 0-mode
“Suppresses” two degrees of freedom in parameter space
Larger radius for coupling cell iris
Reduces RF emittance
Easy to accomodate elliptical curvature to reduce surface field
Shaping of RF pulse for reducing average power
4kW -> 1.8 kW ; cooling channels designed for handling 4kW
Reduce reflected power from gun
Courtesy
L.Xiao
LCLS-TN-05-3.pdf
http://www-ssrl.slac.stanford.edu/lcls/photoinjector/reviews/2004-11-03_rf_review/
April7-8 2005
LCLS FAC , April 2005
C.Limborg-Deprey
[email protected]
Linac: Dual Coupler at entrance cell
Head-tail dipole kick from single feed
Generates emittance growth

o
 1
 2   twiss

 n
rms head-tail trans. kick for 10ps bunch
Operating
point
1
Kick is reduced by more than 4 times in output coupler
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
Dual feed at entrance cell BUT NOT at exit
Quadrupole head-tail not a problem at exit cell
April7-8 2005
LCLS FAC , April 2005
C.Limborg-Deprey
[email protected]
L0a L0b: New Design With WR284 Waveguide
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
b2
w=24.2100
R0.5
b3a
b3b/b4
R0.5
R1.38
beampipe
b=35.8785
new
Coupler
+
original
cup2
cup3-a cup3-b
+
d=13.000
+r=1.000
a1
t1
a2
t2
a3
t3
WR284 waveguide
Courtesy Z.Li
April7-8 2005
LCLS FAC , April 2005
C.Limborg-Deprey
[email protected]
a4
t4
Linac: New Design With WR284 Waveguide
Enough clearance for solenoid
Waveguide curvature adjusted to minimize S11
Waveguide cold-tested
Waveguide
stiffners
2 arms adjusted for identical match
PumpOut
Courtesy J.Chan
April7-8 2005
LCLS FAC , April 2005
C.Limborg-Deprey
[email protected]
Cathode- QE improvement
H-ion Cleaning Experiment
QE at low voltage (No Shottky Enhancement)
After H-beam
cleaning
1.2x10-4
Idt for
H-ion
beam
initial
LCLS QE Spec.
6x10-5 @ 255nm
%Carbon
on
surface
30
1H
0.0378 C
11
2H
0.124
12
3H
0.1818
10
4H
0.614
8
Surface unaltered by H-ion beam cleaning contrary to effect
of laser cleaning
Courtesy D.Dowell, R.Kirby
April7-8 2005
LCLS FAC , April 2005
C.Limborg-Deprey
[email protected]
QE improvement
Theory(*)
• Approximations
Schottky Enhancement of the QE
R = 0.34 (reflection)
1phot -> 1 e
No e-scattering (before emission)
Fermi-Dirac at 0 K
no roughness, no surface features
QE's at 0MV/m and sin(30)*120MV/m
QE
0.01
1 10
3
1 10
4
• Theoretical model can be refined
More tests planned
1 10
5
1 10
6
180
200
220
240
Wavelength (nm)
QE(theory) at 0MV/m
QE(theory) at sin(30)*120MV/m
QE(expt) at 0MV/m
GTF (measured)
LCLS Specifications
LCLS minimum required
Courtesy D.Dowell
April7-8 2005
LCLS FAC , April 2005
260
280
300
• More samples (process)
• Find Optimal H-ion beam current and
integration time (and temperature)
 Implement on GTF gun?
(*) Based on J.Schmerge et al., Proc.FEL04, 205-208
C.Limborg-Deprey
[email protected]
Minimum Emittance
perfect machine ~ 0.9 m.rad (for nominal 1nC tuning)
Only ~ 0.1 m.rad margin for emittance growth
Contributions to emittance
Large cathode emittance
2
2
2
 tot   cathode
  RF
  space
ch arg e
for copper measured 0.6 m.rad per mm of rlaser (theoretical is 0.3 m.rad )
2
2
 cathode   thermal
  roughness
 ??
 cathode rlaser spot
Minimum set by space charge limit

Q
 2  E peak sin 
Minimum rlaser or electrons cannot leave cathode (for metal cathodes) E 
 o r  o
rmin. = 0.82 mm at 54 MV/m for a 1nC
cathode > 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 ?
 tot ~  cathode
April7-8 2005
LCLS FAC , April 2005
C.Limborg-Deprey
[email protected]
Ellipsoidal Emission pulse
“Beer Can” is not the optimal
distribution
Electrons uniformly distributed in 3D ellipsoid volume
x2 y2 z 2
2



A
a2 b2 c2
Line Density = parabola
fwhm = 10 ps
Pulse length
April7-8 2005
LCLS FAC , April 2005
N
 const .
dx dy dz
Radial profile = half-circle
rmax = 1.2 mm
Pulse length
C.Limborg-Deprey
[email protected]
Standard “Beer can” against “3D ellipsoid”
rmax = 1.2mm
cath.= 0.6 mm.mrad
per mm
cath.= 0.69mm.mrad per mm
r= 1.2mm
April7-8 2005
LCLS FAC , April 2005
C.Limborg-Deprey
[email protected]
“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
April7-8 2005
LCLS FAC , April 2005
C.Limborg-Deprey
[email protected]
Sensitivity and Safety Margin
Solenoid 2%
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)
Solenoid
RF
rlaser
Pulse length
“Beer Can”
1%
< 5
~0.1 mm
<1ps
“3D-ellipsoid”
>3%
>10
> 0.3 mm
>4ps
Tuning + Stability of injector are eased; very large margin below 1mm.mrad
April7-8 2005
LCLS FAC , April 2005
C.Limborg-Deprey
[email protected]
3D-Ellispoid Feasibility ?
Two solutions proposed
Pulse Stacker
With 12 Gaussians of alternating polarities
Too lossy, uses too much space, unweildy
Awkward but easy control on individual components
Technically feasible with many $$$$$$$ for
controls, to achieve alignment , timing
measurement to adjust amplitude coefficient
Spectral Control technique
Masking technology for IR exists
Probably better for space and money than previous solution
Before 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
To cathode
(z,y) plane
(z,x) plane
Chirped
input,
temporally
Courtesy of P.Bolton
April7-8 2005
LCLS FAC , April 2005
xyz
• Fluence < 150 mJ/cm 2, E = 50mJ
x
t
X mask
• BW < 15 nm, Chirp = 4.8.1023 THz/ps
t
y mask
y
 = 2200 groves per mm,  = 6.7
Dpencil beam (1m) =11.7 cm
Dy = 2 waist y = 2 = 0.9 cm
C.Limborg-Deprey
[email protected]
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
 r 
2 J 2 a t 

circ 
a t  1
a
(t
)
a t 


-1
FFT
a(t) controlled by driver pulse
Object
Driver with temporal
shape = half-disk
a(t)
April7-8 2005
LCLS FAC , April 2005
Mask in
Transmissive
Fourier Plane
or
Reflective
Optics
Image
Courtesy of P.Bolton
C.Limborg-Deprey
[email protected]
Summary on 3D pulse shaping
Ideal 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 difficulties
Solution has (by construction) adaptive correction
Of Shottky
Of non-uniformity on cathode
Solution should be corrected on e-beam measurement
(using Genetic Algorithm as suggested at ERL05)
April7-8 2005
LCLS FAC , April 2005
C.Limborg-Deprey
[email protected]
Conclusion
Gun
RF design completed
Mechanical design under way (thermal analysis on-going)
Linac
RF design completed
Mechanical design in progress
Risk Mitigation Plans for 1nC
Cathode studies: H-ion cleaning for higher QE
3D-pulse shaping
Tuning of 0.2nC completed (see P.Emma)
On-Going studies
Laser steering stabilization
Feedback for Laser Energy/ RF gun (P,)
Commissioning Plans
Beam BA strategy
April7-8 2005
LCLS FAC , April 2005
C.Limborg-Deprey
[email protected]
BACK-UP
April7-8 2005
LCLS FAC , April 2005
C.Limborg-Deprey
[email protected]
Early thought : Stacking pulses
6+6 beamlets of different radii
Interferences
Gaussians Wash out discrete steps of rms value
April7-8 2005
LCLS FAC , April 2005
C.Limborg-Deprey
[email protected]
Fighting interferences in Stacker
Alternating polarization + appropriate choice of , interference effect is minimized
No interference
E p   A2i e

t  t 2 i  2
i
Es   A2i 1 e
4 2

Interferences random phases
ei 2 i
t  t 2 i 1 2
4 2
ei 2 i
i
I  I p  Is
I p  E p .E p *
I s  Es .Es *
April7-8 2005
LCLS FAC , April 2005
~<15 %
for all
draws
C.Limborg-Deprey
[email protected]
PARMELA simulations
using stacker distributions
IDEAL
Beer Can
Direct beer can
Ellipsoid ideal
50 Beamlets no interference
Stacker
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)
April7-8 2005
LCLS FAC , April 2005
C.Limborg-Deprey
[email protected]
Stacker Layout
launch mirror
photocathode
imaging
optics
polarizing
cube
delay
slide
half
waveplate
delay
line
pulse
energy
control
collimatormagnifier
Profile
shape
r
grating
grating
April7-8 2005
LCLS FAC , April 2005
spectral
filters
Courtesy of P.Bolton
C.Limborg-Deprey
[email protected]
“what to try to avoid…” from
P.Bolton
Spectral Control Principle
• Fluence F < 150 mJ/cm2
In z,y plane
x
z
y
Chirped
input,
temporally
• pulse energy E = 50mJ
Vertical mask
• Create a time-space correlation
• Ideally better in UV but masking
technology does not exists yet
• Constraints
1- Fluence < Damage threshold
 2
F
2
• BW < 15 nm
In z,x plane
E
• 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
2- BW not too large (for THG)
3- Space
4- if possible after amplifiers
Beam dispersed enough to have beam
size negligible
Courtesy P.Bolton
April7-8 2005
LCLS FAC , April 2005
C.Limborg-Deprey
[email protected]