FACET Collimator Systems for Longitudinal Bunch Shaping

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

2. 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"

3. Collimator Locations collimators W-chicane lattice (cartoon) E-collimation, ramped bunches for 2-bunch generation

4. Collimator Location collimators chicane lattice (cartoon) collimator location

5. Collimator Location R56 = 0mm (ramped bunch config) R56 = -10mm (2-bunch config) collimators recently installed March 3, 2012

6. 3 FACET Configurations End of W-Chicane Collimator "B" "A" high-current single bunch drive/witness configuration ramped bunch W-chicane compression factor  full compression overcompressed undercompressed R56 = 4mm R56 = 10mm R56 = 0 mm location "A"  jaws notch location "B"

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

8. Adjust final compression Disperse the beam in energy dp/p [%] x∝ΔE/E ∝t z [mm] dp/p [%] x [mm] ...selectively collimate FACET: 2-bunch case 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 courtesy M. Hogan 130 µm 8

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

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

11. 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

12. 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 W. An R = E+/E-= 6 PIC simulation courtesy W. An

13. Ramped Bunch: DWA ID: 200 µm; OD: 330 µm; glass tube (smallest of E-201 tubes currently in use) vb • 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)

14. 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

15. 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.

16. Thank You! UCLA Gerard Andonian Warren Mori Chan Joshi Weiming An SLAC Mark Hogan Mike Litos Joel Frederico Spencer Gessner Erik Adli Selina Li Dieter Walz Christine Clarke C-K. Ng Tsinghua Univ. Wei Lu Max Planck Institute Patric Muggli

17. Application for DWA [Cook, et al., PAC 2009]

18. 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

19. DWA Structures for E-201 Assume nominal L = 200 µm cutoff wavenumbers for speed-of-light solution to TM dispersion relation k-1 (µm) k L/2 150 200 260 620 925 1130 0.6 0.5 0.4 0.16 0.11 0.08 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.

20. 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%