Fast ion confinement and stability in a neutral beam injected reversed field pinch

Abstract

The behavior of energetic ions is fundamentally important in the study of fusion plasmas. While well-studied in tokamak, spherical torus, and stellarator plasmas, relatively little is known in reversed field pinch plasmas about the dynamics of fast ions and the effects they cause as a large population. These studies are now underway in the Madison Symmetric Torus with an intense 25 keV, 1 MW hydrogen neutral beam injector (NBI). Measurements of the time-resolved fast ion distribution via a high energy neutral particle analyzer, as well as beam-target neutron flux (when NBI fuel is doped with 3-5% D 2 ) both demonstrate that at low concentration the fast ion population is consistent with classical slowing of the fast ions, negligible cross-field transport, and charge exchange as the dominant ion loss mechanism. A significant population of fast ions develops; simulations predict a super-Alfv enic ion density of up to 25% of the electron density with both a significant velocity space gradient and a sharp radial density gradient. There are several effects on the background plasma including enhanced toroidal rotation, electron heating, and an altered current density profile. The abundant fast particles affect the plasma stability. Fast ions at the island of the core-most resonant tearing mode have a stabilizing effect, and up to 60% reduction in the magnetic fluctuation amplitude is observed during NBI. The sharp reduction in amplitude, however, has little effect on the underlying magnetic island structure. Simultaneously, beam driven instabilities are observed as repetitive $50 ls bursts which coincide with fast particle redistribution; data indicate a saturated core fast ion density well below purely classical predictions. V C 2013 AIP Publishing LLC [http://dx. INTRODUCTIONThe envisioned burning plasma experiment, regardless of magnetic concept, relies on sufficient confinement of the charged fusion product for plasma self heating. As such, the confinement of fast ions and their impact on the bulk plasma are crucial issues.A tremendous body of work demonstrates that fast ions in a tokamak plasma (born from fusion reactions, ICRF, or NBI) are generally well confined and thermalize via classical Coulomb collisions. However, a sufficiently intense fast ion population can excite collective instabilities that can lead to resonant fast ion transport. 1 A new body of work on the effects of a large fast ion population in the reversed field pinch (RFP) configuration has recently been opened. Despite the RFP's weak toroidal field and multiple resonant tearing modes which could diminish fast ion confinement, 2,3 NBI-born fast ions in low concentration are observed to slow classically and have a confinement time much larger than thermal particles. 4 The dearth of transport within the modestly stochastic magnetic field is understood to result from the decoupling of the fast ion orbits from the magnetic perturbations. The ions are routinely confined for up to a classical slowing time. 5 In this work, we investigate the effect of...

Identification of island-induced Alfvén eigenmodes in a reversed field pinch

“…A 1 MW neutral beam injector is installed on MST for energetic particle studies in the strongly sheared, high-beta RFP environment [9]. The NBI can fire hydrogen or deuterium beams at energies up to 25 keV, corresponding to beam velocities of × 2.2 10 6 m s −1 for hydrogen and × 1.6 10 6 m s −1 for deuterium.…”

Overview of results from the MST reversed field pinch experiment

“…A 1 MW neutral beam injector installed on MST allows investigation of energetic ion confinement and stability in the RFP [3]. This is unique capability within RFP research, and provides an important new experimental platform to validate critical EP physics.…”

Runaway of energetic test ions in a toroidal plasma

“…While the previously mentioned experiments have largely focused on thermal ion heating, a recently installed tangential neutral beam injector allows the reconnection process to be probed with a well-known population of energetic test ions. 34,35 The beam can inject a neutral flux equivalent to 40 A of ion current at energies ranging from 10 to 25 keV (much greater than the typical ion temperature of 500-1000 eV). The beam is capable of injecting either hydrogen or deuterium; all experiments presented in this work use hydrogen as the primary injected species.…”