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Physics of plasmas 27(5), 053102 (2020). doi:10.1063/5.0002769, A plasma channel created by the combination of a capillary discharge and inverse Bremsstrahlung laser heating enabled the generation of electron bunches with energy up to 7.8 GeV in a laser-driven plasma accelerator. The capillary discharge created an initial plasma channel and was used to tune the plasma temperature, which optimized laser heating. Although optimized colder initial plasma temperatures reduced the ionization degree, subsequent ionization from the heater pulse created a fully ionized plasma on-axis. The heater pulse duration was chosen to be longer than the hydrodynamic timescale of ���1 ns, such that later temporal slices were more efficiently guided by the channel created by the front of the pulse. Simulations are presented which show that this thermal self-guiding of the heater pulse enabled channel formation over 20 cm. The post-heated channel had lower on-axis density and increased focusing strength compared to relying on the discharge alone, which allowed for guiding of relativistically intense laser pulses with a peak power of 0.85 PW and wakefield acceleration over 15 diffraction lengths. Electrons were injected into the wake in multiple buckets and times, leading to several electron bunches with different peak energies. To create single electron bunches with low energy spread, experiments using localized ionization injection inside a capillary discharge waveguide were performed. A single injected bunch with energy 1.6 GeV, charge 38 pC, divergence 1 mrad, and relative energy spread below 2% full-width half-maximum was produced in a 3.3 cm-long capillary discharge waveguide. This development shows promise for mitigation of energy spread and future high efficiency staged acceleration experiments., Published by American Institute of Physics, [S.l.]

Capillary discharges are widely used in many experiments devoted to laser-plasma interaction as a simple tool to create plasma with required parameters. One of the application of these experiments is laser-plasma accelerators (LPA) of charged particles. Such LPAs are able to accelerate electrons bunch to more that a GeV on the centimeters distances [1]. Essential part of these experiments is capillary discharge. It is used to create plasma waveguide in order to channel an accelerating laser pulse. The long (several decimetres) and thin (several microns) capillary is needed to achieve maximum acceleration but its fabrication is laborious and unreasonably expensive for the LPA experiments. Also capillary can be damaged by electric current pulse that is used to create plasma waveguide. Additional heating of the plasma channel by a nanosecond laser pulse is used in order to avoid these limitations [2]. Propagation of a heater laser through the plasma waveguide deepens it further in the vicinity of the capillary axis. Recent experiments show positive effect of such heating on the final acceleration of the electrons. This leads to a problem of choosing optimal parameters to achieve maximal acceleration. Consistent numerical modeling of plasmadynamics and laser pulse propagation in plasma channel is required to maintenance and optimise the future experiments. The magnetohydrodynamic (MHD) code MARPLE [3] previously used for discharge simulations [3-5] was improved by taken into account additional heating due to laser radiation. Results of simulations that were done for the BELLA experimental facility will be presented at the conference. The work was supported in part by the Competitiveness Program of MEPhI No.02.A03.21.0005, basic research program of the Project 3-OMN RAS, U.S. DOE under Contract No.DE-AC02-05CH11231, EU Reg.Dev.Fund Ns.CZ.02.1.01/0.0/0.0/15 008/0000162 and CZ.02.1.01/0.0/0.0/15_003/0000449 and by the MoEYaS of the Czech Republic No.LQ1606. [1] W.Leemans et al. Phys. Rev. Lett. 113, 245002 (2014) [2] N.Bobrova et al. Phys. Plasmas 20, 020703 (2013) [3] G.Bagdasarov et al. Phys. Plasmas 24, 053111 (2017) [4] G.Bagdasarov et al. Phys. Plasmas 24, 083109 (2017) [5] G.Bagdasarov et al. Phys. Plasmas 24, 123120 (2017)

© 2019 American Physical Society. Guiding of relativistically intense laser pulses with peak power of 0.85 PW over 15 diffraction lengths was demonstrated by increasing the focusing strength of a capillary discharge waveguide using laser inverse bremsstrahlung heating. This allowed for the production of electron beams with quasimonoenergetic peaks up to 7.8 GeV, double the energy that was previously demonstrated. Charge was 5 pC at 7.8 GeV and up to 62 pC in 6 GeV peaks, and typical beam divergence was 0.2 mrad.

Our paper concerns with simulation of plasma dynamics in hydrogen-filled capillary discharge with circular and square cross sections under the same conditions. External domain with moderate size which is formed by a passive insulator was taken into account to perform these simulations. The general dynamics of plasma inside these capillaries are found to be quite similar to each other.