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, (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, 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 installed March 3, 2012

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

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

FACET: 2-bunch case 8 8 x ∝ Δ E/E ∝ t Disperse the beam in energy Adjust final compression...selectively collimate x [mm] dp/p [%] z [mm] Exploit Position-Time Correlation on e - bunch to create separate 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. Hogan

Measurement of 2-Bunch Scenario Slide courtesy of M. Litos

Jaw Collimator e-beam axis  y x y x adjustable momentum slit separately moveable titanium blocks Note: beam dimensions are exaggerated for illustrative purposes z x

Ramped Bunch at FACET Due to upstream compression, need R56 = 0 in chicane Collimators can remove low-E tail. Ramped bunch has L = 200 µm ; Ipeak = 4 kA ; n b /n 0 = 17 k p L/2 = 10 for plasma n 0 = 3x10 17 cm -3 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) vbvb 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, solutions where curve crosses x-axis 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 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! 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 17 [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 k -1 (µm) k L/ Tube diameters appear large for high-TR with the current nominal ramped bunch parameters. cutoff wavenumbers for speed-of-light solution to TM dispersion relation Assume nominal L = 200 µm Tube geometries for E-201 Experiment at FACET, courtesy of G. Andonian

Gradient Estimate 20 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% Retarding field (inside bunch) Accelerating field (behind bunch) TR ~ 3 for longer bunches