Proton acceleration using doped Argon plasma density gradient interacting with relativistic CO2-laser pulse

We investigate proton and light-ion acceleration driven by the interaction of relativistic $CO_2$ laser pulses with overdense Argon or other heavy-ion gas targets doped with lighter-ion species. Optically shaping the gas targets allows tuning of the pre-plasma scale-length from a few to several laser wavelengths, allowing the laser to efficiently drive a propagating snowplow through the bunching in the electron density. Preliminary PIC-based modeling shows that the lighter-ion species is accelerated even without any significant motion of the heavier ions which is a signature of the Relativistically Induced Transparency Acceleration mechanism. Some outlines of possible experiments at the TW $CO_2$ laser at the Accelerator Test Facility at Brookhaven National Laboratory are presented.