A scheme is proposed to produce high-quality quasi-monoenergetic attosecond electron bunches based on laser ponderomotive-force acceleration along the surface of wire or slice targets. Two- and three-dimensional particle-in-cell simulations demonstrate that the electron energy depends weakly on the target density. A simple analytical model shows that the electron energy scales linearly with the laser field amplitude, in good agreement with the simulation results. Electron bunches produced by this scheme are suitable for applications such as coherent x-ray radiation, radiography, and injectors in accelerators, etc.
When two ultrashort and ultrahigh laser pulses spacing an appropriate distance propagate through tenuous plasma, the second laser pulse will be compressed and shift to higher frequency because the anterior part of the pulse experiences the higher plasma density and propagates with lower group velocity while the rear part of the pulse experiences the lower plasma density and propagates with higher group velocity. The frequency shift of the second pulse in our PIC simulation is in good agreement with the theoretical predication. Furthermore, the optimum intervals between the two pulses is larger than that estimated with linear theory.
A scheme for producing high-intensity collimated MeV protons from laser interaction with an umbrellalike (cone cavity with an axial filament stemming from the apex) target back side is investigated by two- and three-dimensional particle-in-cell simulations. The process is based on target-normal sheath acceleration. The characteristics of the proton beam are analyzed and compared to that from the recently proposed cone-shaped back side target. It is found that instead of diverging after first focusing, as in the cone-shaped target, the protons produced from the umbrellalike target are well collimated. The large transverse sheath electric field at TV/m level and the self-generated quasistatic magnetic field at hundreds of megagauss level around the filament play important roles in the collimation of the protons.
It is demonstrated by particle simulations that a relativistic electron beam with 1.25 MeV energy, 1.5 kA current, and 0.5% momentum spread can convert 25% of its energy into electromagnetic radiation based on the Doppler-shift dominated cyclotron maser mechanism. However, the increase in beam thermal spread reduces the available free energy Δγ=γ0−γR by increasing the beam resonant energy γR where γ0 is the initial beam energy and substantially decreases the conversion efficiency. Further increase in thermal spread eventually changes the saturation mechanism from phase trapping to energy depletion.
The processes of resonant absorption, vacuum heating, and anomalous skin effect in plasmas produced by a laser pulse are studied using the two-dimensional multi-time-scale, fully electromagnetic relativistic particle simulation code with mobile ions. The mechanisms of electron heating are expounded and compared.
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