  The relativistically induced transparency acceleration (RITA) scheme of
proton and ion acceleration using laser-plasma interactions is introduced,
modeled, and compared to the existing schemes. Protons are accelerated with
femtosecond relativistic pulses to produce quasimonoenergetic bunches with
controllable peak energy. The RITA scheme works by a relativistic laser
inducing transparency to densities higher than the cold-electron critical
density, while the background heavy ions are stationary. The rising laser pulse
creates a traveling acceleration structure at the relativistic critical density
by ponderomotively driving a local electron density inflation, creating an
electron snowplow and a co-propagating electrostatic potential. The snowplow
advances with a velocity determined by the rate of the rise of the laser's
intensity envelope and the heavy-ion-plasma density gradient scale length. The
rising laser is incrementally rendered transparent to higher densities such
that the relativistic-electron plasma frequency is resonant with the laser
frequency. In the snowplow frame, trace density protons reflect off the
electrostatic potential and get snowplowed, while the heavier background ions
are relatively unperturbed. Quasimonoenergetic bunches of velocity equal to
twice the snowplow velocity can be obtained and tuned by controlling the
snowplow velocity using laser-plasma parameters. An analytical model for the
proton energy as a function of laser intensity, rise time, and plasma density
gradient is developed and compared to 1D and 2D PIC OSIRIS simulations. We
model the acceleration of protons to GeV energies with tens-of-femtoseconds
laser pulses of a few petawatts. The scaling of proton energy with laser power
compares favorably to other mechanisms for ultrashort pulses.
