Analysis and modeling of laser-driven ion-beam trace probe diagnostics of poloidal magnetic fields in field-reversed configurations

The field-reversed configuration (FRC) is a promising magnetic confinement fusion concept [M. Tuszewski, Nucl. Fusion 28, 2033 (1988)] and is often chosen as the target plasma for magneto inertial fusion [S. A. Slutz and M. R. Gomez, Phys. Plasmas 28, 042707 (2021)]. In FRCs, the toroidal magnetic field is essentially zero, and the poloidal magnetic field (B p) pressure is comparable with the plasma pressure. Applying the traditional B p diagnostics to FRCs is a major challenge because B p is small, and reversal occurs across the core region of FRCs. The laser-driven ion-beam trace probe (LITP) is a newly developing diagnostic method to measure B p and the radial electric field (E r) in tokamak. Here, the principles of using LITP to diagnose B p in FRCs are proposed, verified, and numerically implemented using an iterative method to reconstruct the B p profile. Least square tomography employing a dissipative term is used to solve the nonlinear tomography problem, which arises when applying LITP to the unique FRC magnetic topology. Numerical modeling results show that the relative errors of the reconstruction are mostly below 10%, verifying the feasibility of LITP diagnostics for FRC internal magnetic field measurements. Ion beam orbits and detector arrangements are optimized to meet the experimental requirements of FRCs. LITP can still be applied to diagnose B p in FRCs when there is 5% measurement errors.