CER SEMINAR

Investigation of particle acceleration from ultraintense lasers using electrostatic collisionless shockwaves and positron beam dynamics

Shaun Kerr, University of Alberta
August 18, 2017, 11:00am - 12:00pm, SERF 329

ABSTRACT:

Ultraintense lasers can produce beams of protons with many potential applications, but controlling the beam spectrum and understanding the dynamics of the acceleration processes remains a challenge¹. The work I performed experimentally investigated electrostatic collisionless shockwave acceleration (CSA) from a 1 μm laser, which can generate quasi-monoenergetic proton beams. I also explored the target normal sheath acceleration (TNSA) mechanism computationally, by probing the time-evolution of the sheath field using positron beams.

The CSA experiment was performed using the 1 μm Titan laser and near critical density targets. Monoenergetic proton beams from shock acceleration have been observed previously using a 10-μm CO₂ laser with a H₂ gas jet target,² but scaling these results to petawatt lasers with wavelengths of 1 μm is challenging due to the lack of critical density targets. Careful shaping of the target density profile is necessary to create both the density discontinuity required for shock formation and to inhibit the strength of TNSA fields that would broaden the CSA spectrum. This shaping was achieved experimentally by ablating a 0.5-μm thick CH foil target with a ~4x10¹⁰ W/cm², 10ns laser pulse, then using delayed irradiation with the ultraintense short pulse laser. Narrow energy spread proton and ion beams were observed using an Imaging Proton Spectrometer (IPS). I will present these results and compare them to particle-in-cell (PIC) simulations.

Positrons produced in ultraintense laser-matter interactions are accelerated by the TNSA field, and their low mass makes them a possible probe of the time-dependent behavior of this field. Experimental results from OMEGA EP show higher order features developing in the positron spectra when the laser energy exceeds one kilojoule³, which was hypothesized to be due to sheath field dynamics. I performed 2D PIC simulations using the LSP code to give insight into these spectral features. They suggest that for high laser energies multiple, distinct phases of positron acceleration can occur due to time-dependent TNSA. I will discuss the detailed dynamics of positron acceleration and sheath field evolution.

¹Macchi et al., RMP., 85, 2751–793 (2013).
²Haberberger et al., Nat. Phys. 8, 95 (2012).
³Chen et al., PoP 22, 056705 (2015).