Working Group 8 Laser Technology for Laser-Plasma Accelerators Co-leaders Bill White & Marcus Babzien

Working Group 8: Overview Relatively small group this year: 8 oral / 3 poster presentations For 2016 consider presenting more laserrelated talks in WG8 Time for detailed group discussion Attendance high during talks Joint sessions with WG1 & WG5

Presentation Summaries Dimitri Kaganovich Pulse contrast improvement Laura Corner Multiple-pulse laser drivers for LWA Mikhail Polyanskiy 100TW CO2 laser upgrade Igor Pogorelsky CO2 laser driven light sources Kei Nakamura Optimizing BELLA temporal structure Han Shin Mao Optimizing BELLA spatial modes Zhaohan He Controlling LWA coherent behavior through spatial profile optimization

Origin of the pedestal is in the limited bandwidth of the laser system laser intensity (a.u.) pedestal! 10-1 10-1 compressor 10-3 10-3 10-5 10-5 frequency bandwidth ω/ω laser pulse duration (fs) Dimitri Kaganovich

laser intensity (a.u.) Simple solution for significant reduction of the pedestal: split and recombine with small time delay e.g. Michelson interferometer A(ω) 2 = (A 02 /4)τ L2 exp ( ω 2 τ L2 /2){1 + cos [ ω + ω 0 τ D ]} τ D τ D = 2πn/ω 0 n 2ω 0 / ω 0.5 constructive interference laser pulse duration (fs) frequency bandwidth ω/ω *D. Kaganovich, J. R. Penano, M. H. Helle, D, F. Gordon, B. Hafizi, and A. Ting, Optics Letters 38, 3635 (2013)

laser intensity (a.u.) Spectral modulation reduces the pedestal τ D = 0 τ D = 4π/ ω pedestal reduction 10-1 10-3 10-5 laser pulse duration (fs) The concept is analogous to the nearly diffraction-free propagation of a Bessel beam through a matched aperture

Experimental demonstration of the pedestal reduction FROG retrieval convergence limit FROG trace retrieval for different experimental parameters and for the transform limited Gaussian beam that was also processed through the same retrieval algorithm. The intensity was normalized by the total area under each curve, while small contribution from under the black dotted line was neglected.

Enhanced Pedestal Laser Pulse Accelerates Electrons to higher Energy Two beams 0 delay Ionization of gas (He) starts 300-400 fs before the main pulse Two beams 20 fs delay No ionization outside of 150 fs laser window MeV 100 60 45 30 22

Initial interest in easier laser construction for high efficiency & rep. rate. Higher efficiency generation of wake? Lower total intensity as energy spread across pulse train. Less damage, smaller optics. Shorter incoupling distance for staged acceleration. Advantages of multi-pulse excitation Umstadter et. al. PR E 51, 3484 (1994) Laura Corner

Single crystal easier to align, fewer optics, lossy, non-adjustable. Beatwave approach Stacked Michelson interferometer Low energy phase shaping Laura Corner

100-TW CO 2 laser upgrade path Now 2016 Solid-state (OPA) seed-pulse generator In operation Chirped-pulse amplification In progress New (isotopic) main amplifier Design Nonlinear pulse compressor R&D 2-3 TW - 2 ps 5-10 TW 25 TW 100 TW - 100 fs 13

CPA Simulations First application of chirped pulse amplification in CO 2 laser No unexpected gain dynamics disrupt classical CPA Recently observed increased energy extraction in regenerative amplifier first step to full application Chirped Spectrum Compressed 25TW Peak 2 ps <2% post-pulse

Larger Final Amplifier - Based on the design of the current main amplifier - Three 10x10x100 cm 3 discharge sections - Isotopic active medium - Triple vessel ~2000 L 15

Nonlinear pulse compressor Ti:Al2O3 OPA 35 µj 350 fs Stretcher 10 µj 100 ps Amplifier 1 100 mj Amplifier 2c Amplifier 2b Amplifier 2a 70 J 50 J 2 ps 25 TW Non-linear compressor 10 J 100 fs 100 TW 16 16

All-optical Compton source Benefits from using CO 2 laser: 100 TW 100 fs plasma X10 gain in e-beam charge and X10 higher photon number per Joule together give X100 higher Compton yield 1 TW 25 ps Tunable 50 kev - 50 MeV Total photon yield 10 11 Total photon flux 10 24 /s LWFA Compton source dipole e-beam 18

Concept of all-optical FEL Using low-emittance linacs, instead of synchrotrons, resulted in inception of 4 th generation coherent lightsources FELs. CO 2 Bubble Regime LWFA Ti:sapph CO 2 Example Plasma linac electrons: 30 MeV, 0.3 nc, 50fs, DE/E=10-2, e n =0.03 mm CO 2 laser: 1TW, a 0 =0.5, 10 ps FEL: 7.6Å, L s =3 mm, 10 9 photons, 30 MW, B pk =10 27 We may be on a verge of a similar development that will add temporal coherency to other attractive features of Compton sources. Trojan Horse PWFA (low e) Compton FEL 19

Kei Nakamura

H.-S. Mao Iterative profile optimization by a wavefront sensor driving a deformable mirror through a measured transfer function Excellent pointing stability of 1.2urad:

Coherent control of a laser generated plasma wakefield using a deformable mirror r0 ~ 50 shots exposure (100 ms) Far-field focal spot profile single shot exposure (1.5 ms) electron beam profiles Coherent manipulation of laser wavefront was achieved using deformable mirror with a genetic algorithm (GA) directly using electron as feedback. Good stability and high repetition rate (0.5 khz) of the system are crucial. GA optimization leads to orders of magnitudes improvement in beam charge and divergence. The best electron beams are produced with laser spot having complex mode structures, while the laser focus with the highest intensity generates a much inferior electron beam. PIC simulations confirm mode structures of the laser pulse can affect plasma wave dynamics and electron generation.

Discussion, conclusions & recommendations The DOE accelerator stewardship program has jumpstarted the effort to create a unifying, strategic organizational structure for coordinating & initiating key laser research that has been lacking in US DOD funding mostly targets a different a different set of technologies than short pulse lasers Industry involvement should continue to be addressed but commercial interest is difficult to generate until a concrete large scale facility is funded. Challenges include short term profit focus & small accelerator market size. Industry should not be relied on for needed progress.

Radial position (mm) Radial position (mm) Radial position (mm) Radial position (mm) Radial position (mm) Fluence (F/F 0 ) Fluence (F/F 0 ) Radial position Fluence (F/F Discussion, conclusions 150 & recommendations 1.7 100 Required power & energy increases cannot be de-emphasized. 0.0 However the scale of effort 50 required to simultaneously achieve beam quality & pulse shape improvements 0 must be highlighted 250 200 150 100 50 0 250 200 250 250 (a) Gaussian, n e = 4x10 17 cm -3 (c) (a) (b) Top Gaussian, Top hat, hat, n e n= 7x10 17-3 e 17 cm -3 e = 4x10 17 cm -3 (d) (c) Top hat, nn e = 17 cm -3 e = 7x10 17 cm -3, 200 200 no preformed channel 3.4 150 150 1.7 100 100 50 50 0.0 0 0 0 2 4 6 8 0 2 4 6 8 250 Propagation distance (cm) Propagation distance (cm) (b) Top hat, n e = 4x10 17 cm -3 (d) (b) Top hat, n e 7x10 17-3 e = 4x10 17 cm -3, (d) Top hat, n e = 7x10 17 cm -3, 200 no preformed channel mid-field focus mid-field no preformed channel 3.4 1.7 0.0 150 150 100 50 Texas PW courtesy M. Downer 0 0 2 4 6 8 Propagation distance (cm) 100 50 (10-12 cm) 0 0 2 4 6 actual 8 0 path: 2 ~ 3 cm 4 46 cm 8 Propagation distance (cm) Propagation distance (cm) Clearly, the next generation of accelerator drive lasers must produce high quality, flexible spatial and temporal profiles in addition to order of magnitude increases of peak power, average power, and rep. rate

Discussion, conclusions & recommendations DOE workshop on Laser Technology for Accelerators in Napa was commissioned specifically to identify laser-driven accelerator R&D needed to advance laser performance Full report available at: http://www.acceleratorsamerica.org/workshops/index.html Long history of support for laser research e.g. US dominance in short pulse CO 2 lasers Good progress since Napa workshop, e.g. BELLA, ATF II & 100TW laser upgrade, Napa report recommendations still timely, broadly applicable, and closely represent the needs of lasers for future accelerators Consider plenary fiber laser talk by Almantas Galvanauskas...

Discussion, conclusions & recommendations Table 5.1 Summary of R&D impact of various capabilities and technologies to applications. Capabilities (blue) are dependent upon advances in technologies. Some prioritization for various funding scenarios like P5 report possible

Discussion, conclusions & recommendations Modeling and simulation are essential elements of an R&D path to clearly define materials properties that would enable new levels of laser performance. A peer reviewed theoretical study that considers material properties and trade-offs to optimize a chirped pulse amplifier design for beam quality, pulse quality, and system efficiency would provide important guidance for future materials development. - Napa Report section 5.1.2 Big strides in laser technology depend on materials breakthroughs Laser & AAC communities could both benefit from detailed materials study that quantifies requirements and identifies winners & losers for further development (as was done to with S- FAP for fusion laser) cite Napa report end of section 5.1.3 First theoretical studies can be relatively cost effective Working group members have committed to publish a paper defining the material properties necessary Contact Jay Dawson for contributing to paper - dawson17@llnl.gov

To be continued In 2016