Abstract
The production of energetic protons using a polyimide tape of 7.5μm 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 2μm (full width at half maximum) using the former value, a precise compensation using the latter value was essential to accelerate protons whose flux was 106/MeV/shot to a maximum energy of 1.1±0.3MeV 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.
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