FACET Collimator Systems for
Longitudinal Bunch Shaping
Joel England
FACET Users Meeting Tues Oct 9, 2012
Collimation for Bunch Shaping
Initial beam
"notch" mask
"jaw" mask
Muggli, P., et al. PRL 101, 054801 (2008).
Collimators have recently been installed in Sector 20 to provide adjustable masks of two types:
 "notch" collimator: movable tantalum blade for two-beam (drive/witness) operation
- could potentially be modified for other mask designs if desired
 "jaw" collimator": pair of transverse scrapers for ramped bunch (high tr. ratio) operation
- can also be used to remove high or low-energy "tails"
Collimator Locations
collimators
W-chicane lattice (cartoon)
E-collimation,
ramped
bunches
for 2-bunch
generation
June 11-15, 2012
3
Collimator Location
collimators
chicane lattice (cartoon)
collimator location
Collimator Location
R56 = -10mm (2-bunch config)
collimators
R56 = 0mm (ramped bunch config)
recently installed
March 3, 2012
3 FACET Configurations
End of W-Chicane
Collimator
"B"
"A"
high-current
single bunch
full compression
W-chicane
compression factor  R56 = 4mm
drive/witness
configuration
overcompressed
R56 = 10mm
location "A" 
undercompressed
R56 = 0 mm
jaws
notch
location "B"
ramped bunch
Notch Collimator
schematic of notch collimator
recently installed
March 3, 2012
notch collimator
jaw collimator
notch collimator
insertable blade
FACET: 2-bunch case
Exploit Position-Time Correlation on e- bunch to create
separate drive and witness bunch
Disperse the beam in energy
dp/p [%]
z [mm]
130 µm
x [mm]
...selectively collimate
8
8
courtesy M. Hogan
x  E/E  t
Adjust final compression
dp/p [%]
Modeled using similar analytic framework
(CSR)
as LCLS as well as tracking/shower codes
Measurement of 2-Bunch Scenario
Slide courtesy of M. Litos
Jaw Collimator
x
adjustable momentum slit
x
x
e-beam axis 
z
y
y
separately moveable
titanium blocks
Note: beam dimensions are exaggerated for
illustrative purposes
Ramped Bunch at FACET
W chicane
Due to upstream compression, need R56 = 0 in chicane
Collimators can remove low-E tail.
Ramped bunch has L = 200 µm ; Ipeak = 4 kA ; nb/n0 = 17
kpL/2 = 10 for plasma n0 = 3x1017 cm-3
However, to avoid hosing instability, require R ≤ 5
n0  3  1017 cm3
nb / n0  17
R  k p L / 2  10
E  37.5 GV/m
  0.5 for I p  4 kA
n0  n0 / 4
E  E0 
n0  0.75  1017 cm3
nb / n0  33
R  kp L / 2  5
E  18.7 GV/m
Ramped Bunch: PWFA
1. Particle phase space generated with ELEGANT simulation of beamline.
2. Focusing of beam at plasma transition (plasma lensing) modeled in Mathematica.
3. Beam parameters used in QUICKPIC to model propagation in 1.2e17 cm-3 plasma.
4. Resultant transformer ratio from longitudinal E-field is R ~ 6.
orange: beam, blue: plasma
beam direction
R = E+/E-W.
= 6An
PIC simulation courtesy W. An
Ramped Bunch: DWA
ID: 200 µm; OD: 330 µm;
glass tube (smallest of E-201 tubes
currently in use)
• ACE3P (Cho Ng)
• Axial beam current with 200µm ramped bunch
• 1.2 nC beam charge
vb
Transformer R ~ 1.5
(vs. 1.2 for back of envelope calc)
E+ = 540 MV/m
(vs. 780 MV/m for back of envelope)
DWA Gradient
Dispersion relation for TM/TE modes at speed-of-light: A.M. Cook, PhD Dissertation, 2009.
solutions where curve crosses x-axis
Note: FACET IP spot size ~ 20 µm
for fiber diameter a = 30µm, b = 300µm
TM01 excitation occurs for k-1 = 16 µm
For expected FACET ramped bunch
length of L =160 µm
This gives TR ~ k L / 2 = 5
14
Summary
•
•
•
•
•
•
1. Collimators have been installed at FACET for generation of 2bunch and ramped bunches.
2. High-transformer ratio PWFA studies require a pre-ionized
plasma.
3. Possibility of doing nearer-term DWA studies using existing
structures from E-201 program.
4. Optimal excitation of the fundamental DWA mode requires
smaller tubes (limited by e-beam bunch size) or longer ramps.
5. Difficult to further reduce R56, but may get longer bunches by
re-phasing.
6. Initial studies indicate possibility of interesting wake amplitudes
and transformer ratios.
Thank You!
SLAC
Mark Hogan
Mike Litos
Joel Frederico
Spencer Gessner
Erik Adli
Selina Li
Dieter Walz
Christine Clarke
C-K. Ng
UCLA
Gerard Andonian
Warren Mori
Chan Joshi
Weiming An
Tsinghua Univ.
Wei Lu
Max Planck Institute
Patric Muggli
Application for DWA
E 
4N b re mc 2
 8

a
 z  a 
  1

[Cook, et al., PAC 2009]
E
R
 kL / 2
E
E  RE
17
Transformer Ratio
For a triangular bunch of length L, the wake function is given by
Transformer ratio is obtained by extremizing the top and bottom lines and
dividing:
This solution is valid for all kL (in linear 1D). For kL > 1, it
can be approximated by R ~ k L / 2
18
DWA Structures for E-201
cutoff wavenumbers for speed-of-light
solution to TM dispersion relation
Assume nominal L = 200 µm
k-1 (µm)
150
200
260
620
925
1130
Tube geometries for E-201 Experiment at FACET, courtesy of G. Andonian
Tube diameters appear large for high-TR with the current nominal
ramped bunch parameters.
19
k L/2
0.6
0.5
0.4
0.16
0.11
0.08
Gradient Estimate
Retarding field (inside bunch)
Accelerating field (behind bunch)
TR ~ 3 for longer bunches
For smallest diameter tube (fused silica).
Variation in L corresponds to linac phase variation for R56 = 0
Assumes 3nC initial bunch + collimation loss of ~ 50%
20