Controlled transition to different proton acceleration regimes:Near-critical-density plasmas driven by circularly polarized few-cycle pulses

A controlled transition between two different ion acceleration mechanisms would pave the way to achieving different ion energies and spectral features within the same experimental set up,depending on the region of operation.Based on numerical simulations conducted over a wide range of experimentally achievable parameter space,reported here is a comprehensive investigation of the different facets of ion acceleration by relativistically intense circularly polarized laser pulses interacting with thin near-critical-density plasma targets.The results show that the plasma thickness,exponential density gradient,and laser frequency chirp can be controlled to switch the interaction from the transparent operating regime to the opaque one,thereby enabling the choice of a Maxwellian-like ion energy distribution with a cutoff energy in the relativistically transparent regime or a quasi-monoenergetic spectrum in the opaque regime.Next,it is established that a multispecies target configuration can be used effectively for optimal generation of quasi-monoenergetic ion bunches of a desired species.Finally,the feasibility is demonstrated for generating monoenergetic proton beams with energy peak atℰ≈20–40 MeV and a narrow energy spread ofΔℰ/ℰ≈18%–28.6%confined within a divergence angle of∼175 mrad at a reasonable laser peak intensity of I0≃5.4×10^(20)W/cm^(2).

Parametric amplification as a single-shot time-resolved off-harmonic probe for laser–matter interactions

An optical probing of laser–plasma interactions can provide time-resolved measurements of plasma density;however,single-shot and multi-frame probing capabilities generally rely on complex setups with limited flexibility.We have demonstrated a new method for temporal resolution of the rapid dynamics(∼170 fs)of plasma evolution within a single laser shot based on the generation of several consecutive probe pulses from a single beta barium borate-based optical parametric amplifier using a fraction of the driver pulse with the possibility to adjust the central wavelengths and delays of particular pulses by optical delay lines.The flexibility and scalability of the proposed experimental technique are presented and discussed.