Abstract
Using fully three-dimensional particle-in-cell simulations, we show that readily available femtosecond laser systems can stably generate proton beams with hundred MeV energy and low spread at $\sim1\%$ level by parallel irradiation of a tens of micrometers long plasma plate. As the laser pulse sweeps along the plate, it drags out a huge charge ($\sim$100 nC) of collimated energetic electrons and accelerates them along the plate surface to superponderomotive energies. When this dense electron current arrives at the rear end of the plate, it induces a strong electrostatic field. Due to the excessive space charge of electrons, the longitudinal field becomes bunching while the transverse field is focusing. Together, this leads to a highly monoenergetic energy spectrum and much higher proton energy as compared to simulation results from typical target normal sheath acceleration and radiation pressure acceleration at the same laser parameters.
Highlights
The development of compact laser-plasma ion sources has attracted significant attention in the past two decades [1,2,3,4,5]
When a laser pulse is incident along an overdense plasma surface with a sharp edge, a surface plasma wave (SPW) can be excited and propagate at the plasma-vacuum interface [55,56,57]. This is different from the case where the laser pulse is incident on the target plane at a finite angle, when corrugated surfaces and very special resonance conditions would be required for an efficient SPW excitation [62,63,64,65]
For a focused Gaussian laser pulse in vacuum, its phase velocity near the axis is determined by vLph 1⁄4 −ð∂φ=∂tÞ=ð∂φ=∂xÞ, where φ is the total phase of the laser pulse [66]
Summary
The development of compact laser-plasma ion sources has attracted significant attention in the past two decades [1,2,3,4,5]. Prospective applications include production of warm dense matter [6,7], proton radiography for implosion dynamics and ultrafast science [8,9], nuclear physics [10], tumor therapy [11], etc For many of these applications, quasimonoenergetic ion beams are preferred [6,11,12], especially for tumor therapy, which requires energy spread of only about 1% [2,3,11]. Proton beams with cutoff energy near 100 MeV [15,16] and carbon ions of about 50 MeV=μ [17] have been demonstrated in various laser systems. Most of the experiments that established new records of maximum proton energy at their
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