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Register a new userProton Acceleration in a Laser-induced Relativistic Electron Vortex
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We show that when a solid plasma foil with a density gradient on the front surface is irradiated by an intense laser pulse at a grazing angle, around 80 degrees, a relativistic electron vortex is excited in the near-critical-density layer after the laser pulse depletion. The vortex structure and dynamics are studied using particle-in-cell simulations. Due to the asymmetry introduced by nonuniform background density, the vortex drifts at a constant velocity, typically 0.2 to 0.3 times the speed of light. The strong magnetic field inside the vortex leads to significant charge separation; in the corresponding electric field initially stationary protons can be captured and accelerated to twice the velocity of the vortex (100-200 MeV). A representative scenario - with laser intensity of 10^21 W/cm^2 -is discussed: two dimensional simulations suggest that a quasi-monoenergetic proton beam can be obtained with a mean energy 140 MeV and an energy spread of about 10%. We derive an analytical estimate for the vortex velocity in terms of laser and plasma parameters, demonstrating that the maximum proton energy can be controlled by the incidence angle of the laser and the plasma density gradient.
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The interaction of ultraintense laser pulses with solids is largely affected by the plasma gradient at the vacuum-solid interface, which modifies the absorption and ultimately, controls the energy distribution function of heated electrons. A micrometer scale-length plasma has been predicted to yield a significant enhancement of the energy and weight of the fast electron population and to play a major role in laser-driven proton acceleration with thin foils. We report on recent experimental results on proton acceleration from laser interaction with foil targets at ultra-relativistic intensities. We show a three-fold increase of the proton cut-off energy when a micrometer scale-length pre-plasma is introduced by irradiation with a low energy femtosecond pre-pulse. Our realistic numerical simulations agree with the observed gain of the proton cut-off energy and confirm the role of stochastic heating of fast electrons in the enhancement of the accelerating sheath field.
We report experimental evidence that multi-MeV protons accelerated in relativistic laser-plasma interactions are modulated by strong filamentary electromagnetic fields. Modulations are observed when a preplasma is developed on the rear side of a $mu$m-scale solid-density hydrogen target. Under such conditions, electromagnetic fields are amplified by the relativistic electron Weibel instability and are maximized at the critical density region of the target. The analysis of the spatial profile of the protons indicates the generation of $B>$10 MG and $E>$0.1 MV/$mu$m fields with a $mu$m-scale wavelength. These results are in good agreement with three-dimensional particle-in-cell simulations and analytical estimates, which further confirm that this process is dominant for different target materials provided that a preplasma is formed on the rear side with scale length $gtrsim 0.13 lambda_0 sqrt{a_0}$. These findings impose important constraints on the preplasma levels required for high-quality proton acceleration for multi-purpose applications.
Magnetic Vortex Acceleration (MVA) from near critical density targets is one of the promising schemes of laser-driven ion acceleration. 3D particle-in-cell simulations are used to explore a more extensive laser-target parameter space than previously reported on in the literature as well as to study the laser pulse coupling to the target, the structure of the fields, and the properties of the accelerated ion beam in the MVA scheme. The efficiency of acceleration depends on the coupling of the laser energy to the self-generated channel in the target. The accelerated proton beams demonstrate high level of collimation with achromatic angular divergence, and carry a significant amount of charge. For PW-class lasers, this acceleration regime provides favorable scaling of maximum ion energy with laser power for optimized interaction parameters. The mega Tesla-level magnetic fields generated by the laser-driven co-axial plasma structure in the target are prerequisite for accelerating protons to the energy of several hundred MeV.
A remarkable ion energy increase is demonstrated by several-stage post-acceleration in a laser plasma interaction. Intense short-pulse laser generates a strong current by high-energy electrons accelerated, when an intense short-pulse laser illuminates a plasma target. The strong electric current creates a strong magnetic field along the high-energy electron current in plasma. During the increase phase of the magnetic field, the longitudinal inductive electric field is induced for the forward ion acceleration by the Faraday law. The inductive acceleration and the target-normal sheath acceleration in the multi stages provide a unique controllability of the ion energy. By the four-stage successive acceleration, our 2.5-dimensional particle-in-cell simulations demonstrate a remarkable increase in ion energy by a few hundreds of MeV; the maximum proton energy reaches 254MeV.
The interaction of ultrashort, high intensity laser pulses with thin foil targets leads to ion acceleration on the target rear surface. To make this ion source useful for applications, it is important to optimize the transfer of energy from the laser into the accelerated ions. One of the most promising ways to achieve this consists in engineering the target front by introducing periodic nanostructures. In this paper, the effect of these structures on ion acceleration is studied analytically and with multi-dimensional particle-in-cell simulations. We assessed the role of the structure shape, size, and the angle of laser incidence for obtaining the efficient energy transfer. Local control of electron trajectories is exploited to maximise the energy delivered into the target. Based on our numerical simulations, we propose a precise range of parameters for fabrication of nanostructured targets, which can increase the energy of the accelerated ions without requiring a higher laser intensity.
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