Relativistically Induced Transparency Acceleration (RITA) of Protons and Light-ions with Ultrashort Laser Interaction with Heavy-ion Plasma Density Gradient

摘要

The relativistically induced transparency acceleration (RITA) scheme ofproton and ion acceleration using laser-plasma interactions is introduced,modeled, and compared to the existing schemes. Protons are accelerated withfemtosecond relativistic pulses to produce quasimonoenergetic bunches withcontrollable peak energy. The RITA scheme works by a relativistic laserinducing transparency to densities higher than the cold-electron criticaldensity, while the background heavy ions are stationary. The rising laser pulsecreates a traveling acceleration structure at the relativistic critical densityby ponderomotively driving a local electron density inflation, creating anelectron snowplow and a co-propagating electrostatic potential. The snowplowadvances with a velocity determined by the rate of the rise of the laser'sintensity envelope and the heavy-ion-plasma density gradient scale length. Therising laser is incrementally rendered transparent to higher densities suchthat the relativistic-electron plasma frequency is resonant with the laserfrequency. In the snowplow frame, trace density protons reflect off theelectrostatic potential and get snowplowed, while the heavier background ionsare relatively unperturbed. Quasimonoenergetic bunches of velocity equal totwice the snowplow velocity can be obtained and tuned by controlling thesnowplow velocity using laser-plasma parameters. An analytical model for theproton energy as a function of laser intensity, rise time, and plasma densitygradient is developed and compared to 1D and 2D PIC OSIRIS simulations. Wemodel the acceleration of protons to GeV energies with tens-of-femtosecondslaser pulses of a few petawatts. The scaling of proton energy with laser powercompares favorably to other mechanisms for ultrashort pulses.