The generation of energetic protons using a polyimide tape of 7.5 μm thickness was carried out with laser pulses of 30 mJ energy and 80 fs duration. A deformable mirror system with a genetic algorithm (GA) was developed to optimize the laser-focusing spot. The fitness values used in the GA were measured from the focusing intensities under the low-gain condition of the power amplifier, or from x-ray in situ signals emitted from the target. Although we obtained a diffraction-limited size of 2 μm (full width at half maximum) using the former value, a precise optimization using the latter value was essential to accelerate protons whose flux was 106/MeV/shot to a maximum energy of 1.1±0.3 MeV with laser pulses of only 30 mJ energy since a laser spot that is too tight may be sensitive to wave-front distortion caused by residual thermal lenses of the power amplifier.
The dependency of the maximum ion energy emitted to rear side of a thin film on laser pulse duration was described by
isothermal plasma expansion model, and the maximum proton energy was increased with pulse duration. 1 MeV
protons were obtained with laser energy of 30 mJ by optimizing focusing using adaptive optics.