Majority Ion Heating by Neutral Beam Injection and Confinement of Fast Ions in the Madison Symmetric Torus Reversed Field Pinch

Abstract

A new 1 MW neutral beam injector (START-20F) is in operation on the Madison Symmetric Torus (MST) reversed field pinch. The beam, consisting of two arc discharge plasma generators, an optimized ion optical system and an integrated neutralizer/injector tank, operates at 25kV and up to 40A of neutrals for a 20 msec pulse (compared to a typical MST pulse length of 60 msec). The injected 1 MW of hydrogen neutrals (with approximately 85% in the full energy component) is significant compared to the 3-4 MW of ohmic input power in a typical target discharge. At this beam energy and a background electron density of about 1x10¹⁹ m^–3 and temperature ≤1keV, roughly 90% of the injected power is deposited within the plasma. Initial experiments with the high power NBI show a large heating of the bulk ions: the fit of the width of energy spectrum as measured by Rutherford scattering (which is generally related to core ion temperature) quickly increases from 180eV to 230eV. This apparent significant and rapid heating of bulk ions is difficult to explain by classical collisions only, as modeling predicts 75% of the injected power is deposited on electrons and 15% on ions. The confinement of the fast ions (measured by the persistence in time of fusion neutrons due to a small fraction of deuterium in the beam fuel) is much greater than the canonical 1 msec confinement of particles and energy in the MST. The fast particle confinement is measured to increase with magnetic field strength. Further recent experiments document fast particle confinement time versus direction of injection (parallel or antiparallel to central magnetic field), beam energy, and background plasma properties.