Proton Acceleration to Therapeutic Energies with Ultra-Intense Ultra-Clean and Ultra-Short Laser Pulses

Abstract

The acceleration of protons to therapeutic energies of over 200 MeV by short‐pulse, high‐intensity lasers requires very high temporal intensity contrast. We describe improvements to the contrast ratio of the laser pulse produced by a multi‐terawatt chirped pulsed amplification (CPA) Ti:sapphire laser for the application of proton acceleration. The modified cross‐polarized wave generation (XPW) technique has been implemented on the Hercules laser at the University of Michigan to reject the low‐intensity amplified spontaneous emission (ASE) preceding the main laser pulse. We demonstrate that by using two BaF2 crystals, the XPW technique yields a 10−11 contrast ratio between the main peak and the ASE for a 50 TW laser system which can be maintained up to 500 TW. Such contrast may be sufficient for a preplasma‐free interaction of 225 TW laser pulses with sub‐micron thick foils at an intensity of ∼1022 W/cm2. Particle‐in‐cell (PIC) simulations were conducted under the anticipated experimental conditions: 6.75 J, 30 fs laser pulse without a prepulse, focused to a spot size of 1.2 microns (FWHM) on thin foils of varying thickness. The performed PIC simulations show that for a 0.2 μm thick hydrogen foil protons with energy up to 200 MeV can be produced. In the case of the two‐layer aluminum‐hydrogen foil, the maximum energy of accelerated protons is about 150 MeV, but the flux‐energy spectrum of the accelerated protons has a narrow peak at high energies, which may be more advantageous for medical applications.