AWAKE Run 2 Laser beamlines For the AWAKE

AWAKE Run 2 Laser beamlines For the AWAKE Collaboration meeting – April 2019 Presented by Eduardo Granados In collaboration with: Mathias Hüther, Joshua Moody, Harsha Panuganti, Patric Muggli, Valentin Fedosseev

Agenda • Requirements laser beam lines AWAKE Run 2 • IR Laser beam line to plasma cell(s) – Multi-cell scheme – Long cell scheme • UV Laser beam line to electron gun – Beam instability mitigation – Multi-bunch generation • Diagnostics update 2

Laser lines in AWAKE • Laser beam line to plasma cell – l = 780 nm – t pulse = 100 -120 fs – E = (<<450 m. J) • Diagnostic beam line(s) (“virtual plasma”) - l = 780 nm - t pulse = 100 -120 fs, - E ≈ 5 m. J • Laser beam line to electron gun – l = 260 nm – t pulse = 0. 3 -10 ps – (E = 0. 5 m. J) 3

Laser lines in AWAKE Run 2 • Laser beam line to plasma cell(s) – l = 780 nm – t pulse = 100 -120 fs – E = 450 m. J – Reflective telescope after compressor – Counter-propagating beam delivery // Long cell – Tape systems • Laser beam line to electron gun – l = 260 nm – t pulse = 0. 3 -10 ps – E = 0. 5 m. J – Delivery via image relay – Synchronization? – Multi-bunch capability – Magnification capability • Diagnostic beam line(s) – l = 780 & 260 nm Option I Up to 20 m Option II <5 m 10 m - X-FROG or X-correlator 4

Option I Advantages: - Reusable parts of laser beam delivery system. - “Easy” synchronization between laser lines. - Potentially lower cost. Disadvantages: - Diffractive effects, inhomogeneous ionization. - Beam pointing stability. - Potential cumulative non-linear optical side effects. - Lack of flexibility. Not possible to optimize both plasma cells independently (if this is desired). Development: - Beam shaping -> elongating confocal parameter (nondiffractive beams) 5

Beam shaping / Non-diffracting beams • • • Off the self item, could be test soon “Well” known, SLAC… Easy to install / align. Allows collimated beam delivery (possibly no need for reflective telescope) Non-Gaussian focusing, different radial distribution E. Adli et al, New J. Phys. 18 (2016) 103013 6

Option II <5 m 10 m Advantages: - More flexible. Independent beam optimization in terms of power, focus and/or timing. - Potentially less laser pointing issues than in a longer single cell. Disadvantages: - Many more components/cost and real estate. - Synchronization between 2 x IR beams and UV beam slightly more challenging. - Long delay line in UV beam may translate into pointing problems. Development: - Beam delivery systems, potentially relocation of compressor. - Relay image systems. - Timing strategy? 7

Current main laser lines ME 1 LSSE 2 VE 1 ME 2 Photoinjector LSSE 1 THG ME 4 ME 3 Delay line 8

Modification of main laser lines Delay to match IR beam path + relay ME 1 Introduce UV relay LSSE 2 VE 1 ME 2 Photoinjector LSSE 1 THG ME 4 ME 3 Delay line Delay Relay 4 f telescope (stretched pulse) Beam path +20 m - Use compressor space to accommodate for UV beam delay in the IR. - Use reflective telescope in compressor for loosely focusing beam or Bessel beams. 9

Modification of main laser lines Delay to match IR beam path + relay ME 1 Introduce UV relay LSSE 2 VE 1 ME 2 Photoinjector LSSE 1 THG ME 4 ME 3 Delay line Delay Beam path +20 m - Use tape to inject laser beam between plasma cells. - Check focusing of second beam, two tapes? - In principle compatible with Bessel focusing. 10

On the use of tapes for staging - Kapton, Mylar, VHS (15 um thick) tapes commonly used. - Ionization intensity for VHS 6. 4 x 1013 W/cm 2 - AWAKE: ~ 1014 W/cm 2 Steinke, S. et al. Nature 530, 190– 193 (2016) 11

On the use of tapes for staging Courtesy Brian Shaw, LBNL 12

Comparison Option IIb Synchronization multiple IR beams Beam delivery Major modifications in beam delivery Beam split and delivery Cost & complexity Re-adjustment UV beam path Diffractive effects (to be checked) New focusing strategy Flexible Lack of flexibility Construct multiple new laser diagnostics Implementation of tape-like system Very flexible - Trade-off: Follow Option I as baseline, build Option IIb in parallel 13

UV variable pulse duration - Optimize THG conversion efficiency, variable pulse duration with constant pulse energy Transmission UV diffraction gratings 50 x 50 mm 1200 grooves / mm, 26. 7 blaze angle Gratings available from EDMUND / OPTOMETRICS / SHIMADZU … 100 fs -> 10 ps L = 350 cm !!! 7+ meters delay Estimated stretcher price ~ 5000 EUR UV stretcher layout L, gratings base Extracted from M. Martyanov AWAKE Col. Meeting 2014

Variable UV spot size ü ü To e- gun 1: 1 4 f telescope From laserlab - Proposed as a substitute for current variable iris + demagnifying telescope. - Highly efficient. - Ability to focus to tighter spot in the cathode. - Currently commissioned at CLEAR with good performance. Very high accuracy (thousands of steps to go through focus) Very large range of spot sizes Easy to change spot from few hundred um to few mm without impacting alignment Standa compatible 15

Multi-bunch generation To e- gun - For delays between e- bunches of up to ~ 2 ns From laser PBS 2 HWP 1 PBS 1 Delay HWP 2 Main ü Very precise delay possible (~30 fs level, stepper motor) Secondary ü Total range is 0 - 2 ns ü Variable amplitudes for both UV beams ü Small footprint, possible to install in laser room instead of near gun ü Standa compatible 16

UV pulse diagnostics and control Parameter Diagnostic Control Next step Pulse energy Sampler + energy meter Motorized filterwheel Motorized waveplate Beam position Virtual cathode camera Motorized mirror (picomotor) Motorized mirror (stepper) Timing Sampler + photodiode Delay stage Adapt beam paths to IR timing? Beam size Virtual cathode camera Motorized iris Variable magnification UV pulse duration X-FROG Motorization of compressor or build UV stretcher Purchasing and commissioning Double pulse setup Photodiode + virtual cathode camera Delay stage, motorized mirror Decision on delay range, purchase 17

Conclusion - Decision on which option to follow or study alternative solutions. - Most UV beam improvements can be performed separately. Set priorities and specification. - Long focusing and Bessel focusing should be tested prior to making decisions. - New diagnostics include UV pulse duration characterization ondemand. Thanks for your attention! 18