Magnetic quadrupole simulations for focusing the electron beams emitted by a plasma focus device

A Plasma Focus (PF) device allows for production of electron beams that, through a suitable target interaction, can be converted in X-ray pulses that have been considered for radiobiology or medical applications such as imaging or radiotherapy, in dependence from the working parameters and setup. The Plasma Focus Device for Medical Applications #3 (PFMA-3), hosted at the Montecuccolino Laboratory of the University of Bologna, has been designed for these purposes. In the device, electron pulses are generated during the pinch phase in the order of 1.0 E+15 particles in few tens of ns. One of the main advantages in dealing with the beams emitted by a PF is their self-collimated behavior at the emission time. Unfortunately, during the traveling distance from pinch to target, that collimation can be partially lost due to the repulsive interactions. One solution is to implement a magnetic device based on a quadrupoles triplet able to confine the beam in spots with a few mm diameter. This kind of focusing allows for using the PF as a source for generating extremely short X-ray pulses that could be more easily further managed for specific applications. A computational model of the PFMA-3 has been set using COMSOL© Multiphysics and the Monte Carlo MCNP6 code. The electron spectra used as source for simulations were acquired experimentally using a magnetic spectrometer, while the beam shape entering the magnetic system to be designed has been detected using Gafchromic© HDV2 film dosimeters and used as a benchmark for the numerical models. The magnetic field generated by the quadrupoles has been carefully designed through a parametric study with COMSOL© Multiphysics and the focusing effectiveness verified. The designed geometry has been then modeled in MCNP6 to perform coupled electron-photon transport simulations for estimating electron fluxes, spectra and X-ray doses as modified by the quadrupoles triplet application.