  Laser-plasma proton acceleration was investigated in the Target Normal Sheath
Acceleration (TNSA) regime using a novel gas-foil target. The target is
designed for reaching higher laser intensity at the foil plane owing to
relativistic self-focusing and self compression of the pulse in the gas layer.
Numerical 3D particle-in-cell (PIC) simulations were used to study pulse
propagation in the gas, showing a nearly seven-fold increase in peak intensity.
In the experiment, maximum proton energies showed high dependence on the energy
transmission of the laser through the gas and a lesser dependence on the size
and shape of the pulse. At high gas densities, laser energy depletion and pulse
distortion suppressed proton energies. At low densities, self-focusing was
observed and comparable or higher proton energies were measured with the gas.
