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).

High-Flux 1ookHz Attosecond Pulse Source Driven by a High-Average Power Annular Laser Beam

High-repetition rate attosecond pulse sources are indispensable tools for time-resolved studies of electron dynamics,such as coincidence spectroscopy and experiments with high demands on statistics or signal-to-noise ratio,especilly in the case of solid and big molecule samples in chemistry and biology.Although with the high-repetition rate lasers,such attosecond pulses in a pump-probe configuration are possible to achieve,until now,only a few such light sources have been demonstrated.Here,by shaping the driving laser to an annular beam,a 100 kHz attosecond pulse train(APT)is reported with the highest energy so far(51 pl/shot)on target(269 pJat generation)among the high-repetition rate systems(>10 kHz)in which the attosecond pulses were temporally characterized.The on-target pulse energy is maximized by reducing the losses from the reflections and filtering of the high harmonics,and an unprecedented 19%transmission rate from the generation point to the target position is achieved.At the same time,the probe beam is also annular and low loss of this beam is reached by using another holey mirror to combine with the APT.The advantages of using an annular beam to generate attosecond pulses with a high-average power laser are demonstrated experimentally and theoretically,The effect of nonlinear propagation in the generation medium on the annular-beam generation concept is also analyzed in detail.