WE‐C‐BRB‐10: Acceleration of Protons by High‐Contrast Ultra‐Intense Laser Pulses

Abstract

Purpose: To increase the proton energy generated in a laser‐plasma accelerator by pulse contrast improvement and minimization of phase distortions. Method and Materials: In a redesigned experiment the laser chain has been upgraded to a 150TWlevel and the pulse contrast has been improved by the implementation of double chirped pulse amplification (DCPA) and a cross‐polarized wave (XPW) modulation technique. This novel method of prepulse reduction has been shown to generate contrast levels of the order of 10−10‐10−11. In our new laser system stable XPWis achieved in two‐pass geometry on a single BBO crystal, making the setup compact and versatile. The laser pulse is subsequently stretched and amplified in an additional 4‐pass amplifier. High‐dynamic range third‐harmonic autocorrelator is used for pulse contrast evaluation. The wavefront distortions in the laser pulse are monitored by a 2D micro‐lens array detector. The accelerated protons are registered on a CR‐39 nuclear track detector behind a range filter (for energy measurement). Results: The implementation of the XPWtechnique led to an improvement of the prepulse contrast from 2×10−5 to 5×10−9. The “cleaner” laser pulse allowed us to use thinner targets (<10 ⌈m) and observe more energetic protons (>3.5 MeV). The wavefront distortions due to thermal effects in the amplifier crystals and astigmatism in our imaging systems are monitored and minimized (phase distortion <λ/3) in order to achieve optimum conditions for tight focusing. Conclusion: Proton acceleration in excess of 3.5 MeV has been experimentally demonstrated using a more powerful laser source with better control over the laser pulse parameters. Further experiments to optimize target parameters for higher proton energies are underway using our 150TW laser as a proof that upward scaling of the laser power in a controlled fashion can bring us into the range of therapeutically useful protons.