Speaker
Description
The Novatron magnetic mirror is a novel concept for a fusion reactor [J. Scheffel et al., Nucl. Fusion 65 (2025) 066011]. The first experimental device, N1, is already operating at the Royal Institute of Technology, Stockholm, Sweden, while the next generation device is being designed. To attain good performance the particle- and energy confinement along the magnetic field lines is crucial in open systems. An option to improve the confinement is to induce a ponderomotive force by applying a radiofrequency wave in the plasma, called “RF plugging”.
For effective RF plugging it is desirable to utilize slowly propagating waves with large electric energy densities, e.g. electrostatic waves, because the strength of the ponderomotive force depends on the magnitude of the oscillating electric field. Moreover, energy dissipation through Coulomb collisions implies that plugging by ponderomotive forces acting on ions is likely to require less energy than if the forces act on electrons. With these aspects in mind, we have identified the ion-Bernstein wave (IBW) as a possible candidate for efficient plugging. IBWs exist in separate branches at frequencies above multiples of the ion cyclotron frequency and typically at wavelengths with $k_{\perp}\rho_i \sim 1$. Starting from the single-particle Lagrangian, we have derived an analytical expression for the ponderomotive potential experienced by a charged particle in a static uniform magnetic field and under the influence of an electric field from a generalized plane wave. In our calculations we have retained Finite Larmor radius (FLR) corrections, which are important when $k_{\perp}\rho_i \sim 1$ or larger. Our results indicate that the ponderomotive plugging will be most efficient using a plasma wave with a frequency above, but close to, multiples of the ion cyclotron frequency. The N1 device is currently being equipped with electrodes operating in the ion cyclotron range of frequencies, to give the possibility to compare theoretical predictions of ponderomotive effects to experimental observations.