FACET Collimator Systems for Longitudinal Bunch Shaping Joel

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 R 56 = -10 mm (2 -bunch config) collimators R 56 = 0 mm (ramped bunch config) recently installed March 3, 2012

3 FACET Configurations End of W-Chicane Collimator "A" "B" high-current single bunch full compression W-chicane compression factor R 56 = 4 mm drive/witness configuration overcompressed R 56 = 10 mm undercompressed R 56 = 0 mm jaws location "A" notch location "B" ramped bunch

Notch Collimator schematic of notch collimator recently installed March 3, 2012 notch collimator jaw collimator notch collimator insertable blade beam axis

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 ∝ΔE/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 R 56 = 0 in chicane Collimators can remove low-E tail. Ramped bunch has L = 200 µm ; Ipeak = 4 k. A ; nb/n 0 = 17 kp. L/2 = 10 for plasma n 0 = 3 x 1017 cm-3 However, to avoid hosing instability, require R ≤ 5

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. 2 e 17 cm-3 plasma. 4. Resultant transformer ratio from longitudinal E-field is R ~ 6. orange: beam, blue: plasma beam direction R = E+/E-W. = 6 An PIC simulation courtesy W. An

Ramped Bunch: DWA ID: 200 µm; OD: 330 µm; glass tube (smallest of E-201 tubes currently in use) • ACE 3 P (Cho Ng) • Axial beam current with 200µm ramped bunch • 1. 2 n. C 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, Ph. D Dissertation, 2009. solutions where curve crosses x-axis Note: FACET IP spot size ~ 20 µm for fiber diameter a = 30µm, b = 300µm TM 01 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 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 R 56, but may get longer bunches by rephasing. 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 [Cook, et al. , PAC 2009] 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 k. L (in linear 1 D). For k. L > 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 R 56 = 0 Assumes 3 n. C initial bunch + collimation loss of ~ 50% 20