Stochastic heating of plasmas in inhomogeneous magnetic fields

The effects of the shear of the magnetic field on the stochastic acceleration of ions due to an electrostatic wave with frequency in the lower-hybrid range, are considered. Using a Hamiltonian formalism to describe such a system, we analyse numerically and theoretically the equations of motion and use the surface of section method to visualize their solutions and for comparison with the theoretical predictions. We conclude that inherently to the stochastic process is a formation of a hot tail in the ion velocity distribution. This tail is a reflection of the existence of an upper adiabatic barrier for the stochastic region. This barrier in the action variables depends significantly on the strength of the electrostatic wave and on the shear of the magnetic field. Increasing shear results in lowering the threshold for onset of stochasticity and in shifting the tail structure towards lower values of velocities