Proton acceleration from overdense plasma target interacting with shaped laser pulses in the presence of preplasmas

Abstract

We report two-dimensional particle-in-cell simulations to study the interaction of shaped laser pulses with overdense plasma targets. Preplasma is introduced as a linearly increasing plasma density ramp in front of the overdense plasma target and the shaping of laser pulse is introduced by considering different durations of leading and trailing pulse edge. The preplasma electrons are pushed towards the target due to the direct action of the laser ponderomotive forces on these electrons. The ponderomotive force associated with the sharp edge asymmetric pulses is significantly enhanced due to change in velocity gradient, hence, the electrons get stronger push inside the target. Thus, it is expected to reach more energetic electrons at the rear side of the target to accelerate the proton by the charge separation field. The combined effect of pulse asymmetry in the presence of preplasma enhances the peak energy of the protons for the case of fast leading edge pulse. We demonstrate the significant role played by the asymmetric pulse interaction with an overdense target to accelerate the proton more effectively from the rear side of the target. The asymmetric pulse with fast leading edge together with preplasma generates a proton beam of energy 12.5 MeV in comparison with the slow leading edge pulse (which generates proton beam of 6.0 MeV energy). This study suggests that the pulse asymmetry can be used as a tuning parameter in the presence of preplasma to improve the proton beam quality in TNSA scheme.

Influence of Pre-Plasma on Accelerated Protons in Ultraintense Laser Interaction with Not-So-Thin Foils

“…As anticipated, the role of a pre-plasma in the acceleration of protons was highlighted in previous simulation studies to identify possible mechanisms of enhancement of fast electron heating. For example, a comparison with Kumar et al, 12 shows that the measured values of the cut-off energy for the two cases with and w/o the pre-plasma are in a qualitative agreement with the calculated ones, although the enhancement factor (3x) observed in our study is significantly higher than the one (1.7x) found in 12 . Assuming that the enhancement factor is entirely due to the presence of the pre-plasma, the reason for this discrepancy should be explored in the interaction with the pre-plasma.…”

“…In Ferri et al 13 simulations at a higher intensity of 7 × 10 19 W/cm 2 at 0.8 µm wavelength (a 0 = 5.7) leads approximately to a three-fold increase of the proton cut-off for single pulse irradiation of a 3 µm thick Al target and for a density scale-length of 0.6 µm. In Kumar et al 12 simulations at ultra-relativistic intensities of 10 20 W/cm 2 and a 0 = 10.6, very similar to the value of our experiment, were investigated. In this intensity regime PIC simulations show that in the presence of a pre-plasma consisting of a 10 µm linear density ramp, the proton cut-off energy is expected to be approximately 8.5 MeV, while without the pre-plasma, the cut-off energy is 5 MeV, showing again an enhancement of approximately a factor 1.7.…”

“…The complex laser dynamics in the plasma section across the critical density for the case L g = 5 µm leads to the formation of a standing wave. The latter is due to a superposition between the incident and reflected electromagnetic wave 11,12 . Here the role of the angle of incidence (15 • in our case) has been discussed recently 18 showing effective contribution of stochastic heating for relatively long scale-length and even larger angles of incidence.…”

Proton beam quality enhancement by spectral phase control of a PW-class laser system

“…The demonstrated stability of this effect during a long period of operation and over a wide range of parameters like target thickness and material as well as laser energy and temporal intensity contrast implies that this method could be easily transferred to other laser systems operating in the PW range. Existing literature simulating asymmetric pulse shapes 35 , 36 show gain values from 50 to 65%, but is not conclusive about the required type of asymmetry. Moreover, they do not cover the experimental parameters of this study as they were performed in a different regime.…”

Enhancement of Target Normal Sheath Acceleration via Improved Fast Electron Heating in a Controlled Pre-Plasma Driven by a Femtosecond Laser Pre-Pulse

“…The complex laser dynamics in the plasma section across the critical density for the case L g = 5 µm leads to the formation of a standing wave due to a superposition between the incident and reflected electromagnetic wave 11,12 . Here the role of the angle of incidence (15 • in our case) has been discussed recently 26 showing effective contribution of stochastic heating for relatively long scale-length and even larger angles of incidence.…”