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FACET Collimator Systems for Longitudinal Bunch Shaping Joel England FACET Users Meeting Tues Oct 9, 2012

FACET Collimator Systems for Longitudinal Bunch Shaping Joel England FACET Users Meeting Tues Oct 9, 2012

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FACET Collimator Systems for Longitudinal Bunch Shaping

Joel England

FACET Users Meeting Tues Oct 9, 2012

Collimation for Bunch Shaping

Muggli, P., et al. PRL 101, 054801 (2008).

Initial beam "notch" mask "jaw" mask

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

June 11-15, 20123

W-chicane lattice (cartoon)

collimators

for 2-bunch generation

E-collimation, ramped bunches

Collimator Location

chicane lattice (cartoon)

collimators

collimator location

Collimator Location

collimators

R56 = -10mm (2-bunch config) R56 = 0mm (ramped bunch config)

recently installedMarch 3, 2012

3 FACET ConfigurationsCollimator End of W-Chicane

"A""B"

location "A"

location "B"

R56 = 4mm R56 = 10mm R56 = 0 mm

full compression overcompressed undercompressedW-chicanecompression factor

high-current single bunch

drive/witnessconfiguration

ramped bunch

notch

jaws

Notch Collimator

recently installedMarch 3, 2012

beam axis

notch collimatorinsertable blade

schematic of notch collimator

notch collimator

jaw collimator

FACET: 2-bunch case

8

8

x ∝ ΔE/E ∝ tDisperse the beam in energy

Adjust final compression

...selectively collimate

x [mm]

dp

/p [

%]

dp

/p [

%]

z [mm]

Exploit Position-Time Correlation on eExploit Position-Time Correlation on e-- bunch to create bunch to create separate drive and witness bunchseparate drive and witness bunch

Exploit Position-Time Correlation on eExploit Position-Time Correlation on e-- bunch to create bunch to create separate drive and witness bunchseparate drive and witness bunch

Modeled using similar analytic framework (CSR) as LCLS as well as tracking/shower codes

Modeled using similar analytic framework (CSR) as LCLS as well as tracking/shower codes

130 µm

courtesy M. H

ogan

Measurement of 2-Bunch Scenario

Slide courtesy of M. Litos

Jaw Collimator

e-beam axis

y

x

y

xadjustable momentum slit

separately moveable titanium blocks

Note: beam dimensions are exaggerated forillustrative purposes

z

x

Ramped Bunch at FACET

Due to upstream compression, need R56 = 0 in chicaneCollimators can remove low-E tail.

Ramped bunch has L = 200 µm ; Ipeak = 4 kA ; nb/n0 = 17kpL/2 = 10 for plasma n0 = 3x1017 cm-3

Λ =0.5 for Ip = 4 kAnb / n0 =17 nb / n0 =33

n0 =3×1017cm−3 n0 =0.75 ×1017cm−3

R =kpL / 2 =10 R =kpL / 2 =5n0 → n0 / 4

E+ =18.7 GV/mE+ =37.5 GV/m E+ =E0 Λ

However, to avoid hosing instability, require R ≤ 5

W chicane

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.

R = E+/E- = 6

orange: beam, blue: plasma

beam direction

W. An

PIC simulation courtesy W. An

Ramped Bunch: DWA

• ACE3P (Cho Ng)• Axial beam current with 200µm ramped bunch• 1.2 nC beam charge

Transformer R ~ 1.5 (vs. 1.2 for back of envelope calc)

E+ = 540 MV/m(vs. 780 MV/m for back of envelope)

vb

ID: 200 µm; OD: 330 µm;glass tube (smallest of E-201 tubes currently in use)

DWA Gradient

14

Dispersion relation for TM/TE modes at speed-of-light: A.M. Cook, PhD Dissertation, 2009.

solutions where curve crosses x-axis

for fiber diameter a = 30µm, b = 300µmTM01 excitation occurs for k-1 = 16 µm

For expected FACET ramped bunch length of L =160 µm

This gives TR ~ k L / 2 = 5

Note: FACET IP spot size ~ 20 µm

Summary

• 1. Collimators have been installed at FACET for generation of 2-bunch 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!

SLACMark HoganMike LitosJoel FredericoSpencer GessnerErik AdliSelina LiDieter WalzChristine ClarkeC-K. Ng

UCLAGerard AndonianWarren MoriChan JoshiWeiming An

Tsinghua Univ.Wei Lu

Max Planck InstitutePatric Muggli

Application for DWA

17

E− =4Nbremc

2

a8πε −1

εσ z + a⎛

⎝⎜⎞

⎠⎟

R =E+

E−

=kL / 2

E+ =RE−

[Cook, et al., PAC 2009]

Transformer Ratio

18

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

DWA Structures for E-201

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1502002606209251130

k-1 (µm) k L/2

0.60.50.40.160.110.08

Tube diameters appear large for high-TR with the current nominalramped bunch parameters.

cutoff wavenumbers for speed-of-lightsolution to TM dispersion relation

Assume nominal L = 200 µm

Tube geometries for E-201 Experiment at FACET, courtesy of G. Andonian

Gradient Estimate

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For smallest diameter tube (fused silica).Variation in L corresponds to linac phase variation for R56 = 0Assumes 3nC initial bunch + collimation loss of ~ 50%

Retarding field (inside bunch) Accelerating field (behind bunch)

TR ~ 3 for longer bunches