Near Critical Density Laser Beat-Wave Acceleration for Radiotherapy Applications

Abstract

This thesis examines 2D Particle-in-Cell (PIC) simulations of a beat-wave interacting with a Near-Critical Density (NCD) plasma to generate an electron beam suitable for radiotherapy. The beat-wave is generated from the co-propagation of an 800nm laser, the primary laser, and a 400nm laser, the secondary laser. The lasers are simulated with a pulse duration of 35 fs and a respective energy of 100 mJ each. A beat-wave is used due to the desirability of a compact setup appropriate in a medical environment. Beat-wave accelerators have historically been used when the typical high-intensity lasers used to reach the bubble-regime were not accessible, as the beat-wave is able to accelerate electrons in the plasma with relatively low intensity. Additionally, NCD plasmas are examined to enhance the charge generated from the interaction while limiting the maximum energy gain of the electrons. The lasers are examined with both linear and circular polarization to examine the polarization effect on overall charge and electron energy gain. The beat-wave is compared to the propagation of the primary laser alone and is found to better propagate through NCD plasmas. The single laser is only able to form cavitons in the first half of the plasma and does not lead to a peak of MeV electrons. Additionally, the limiting of the energy differs from typical, high-energy laser acceleration setups due to the different energy range used in radiotherapy. Linear accelerators currently used for electron radiotherapy generate electron beams with energies from 5-25 MeV and a dose rate of 1-10 Gy/min. The NCD plasma interactions lead to MeV order electrons which meet the energy and dose requirements set by linear accelerators and show aptitude for high-dose rate electron radiotherapy applications.