Title: Enhanced plasma ion heating by lasers in inhomogeneous external magnetic field

Abstract: Recent studies have shown direct ion heating (vashistha2020new,Juneja_2023) by lasers EM (Electromagnetic) wave interacting with a plasma threaded by an external uniform magnetic field. The EM wave frequency was near the lower hybrid (LH) resonance frequency. The LH resonance occurs at the edge of the pass band of the magnetized dispersion relation. The group speed of the wave is negligible at resonance. In these studies, the energy absorption remains essentially confined at the plasma surface. However, to heat the ions in the bulk plasma and at a desired location, a tailored inhomogeneous external magnetic field profile has been chosen here. The strength of the magnetic field at the plasma edge is such that the EM wave frequency lies inside the pass band, where the group velocity has a significant value. It enables the wave to enter the bulk plasma. The external magnetic field is then spatially tailored appropriately to have the LH resonance at a desired spatial location inside the plasma. The Particle-In-Cell (PIC) simulations using the OSIRIS4.0 platform have been carried out, which demonstrates that the EM wave pulse comes to a standstill at the location of the resonance. The wave pulse is observed to break down subsequently, and the energy consequently goes dominantly to the local plasma ions. The absorption is significantly enhanced compared to the case in which the magnetic field profile was homogeneous. The dependence of absorption on the choice of magnetic field profile, the laser intensity, etc., has also been carried out.