The confinement of dilute populations of beam ions in the national spherical torus experiment

Abstract

Short ∼3 ms pulses of 80 keV deuterium neutrals are injected at three different tangency radii into the national spherical torus experiment. The confinement is studied as a function of tangency radius, plasma current (between 0.4 and 1.0 MA), and toroidal field (between 2.5 and 5.0 kG). The jump in neutron emission during the pulse is used to infer prompt losses of beam ions. In the absence of MHD, the neutron data show the expected dependences on beam angle and plasma current; the average jump in the neutron signal is 88 ± 39% of the expected jump. The decay of the neutron and neutral particle signals following the blip are compared to the expected classical deceleration to detect losses on a 10 ms timescale. The temporal evolution of these signals are consistent with Coulomb scattering rates, implying an effective beam-ion confinement time 100 ms. The confinement is insensitive to the toroidal field despite large values of ρ∇B/B ( 0.25), so any effects of non-conservation of the adiabatic invariant µ are smaller than the experimental error.

Profiles of fast ions that are accelerated by high harmonic fast waves in the National Spherical Torus Experiment

“…Because the neutron rate is dominated by beam-plasma reactions, this indicates that a significant fraction of high-energy fast ions are accelerated by the RF heating. The pulses in the neutron rate between 0.17 and 0. analyzed using the technique described in [29]. For each pulse, the neutron emission R n rises nearly linearly during the beam injection, then decays approximately exponentially following the pulse.…”

Measurements of classical transport of fast ions

“…[9][10][11][12][13][14] Also the slowing-down of a barium ion beam with energy 0-40 eV was measured in a Q-machine in which the quiescent plasma ͑T e Ϸ 0.2 eV͒ is confined by a uniform magnetic field. 15 The changes in velocity ͑due to beam slowing-down͒ agree with the predictions of the Fokker-Planck equation within error bars for different ions at different densities.…”

Calculation of the Non-Inductive Current Profile in High-Performance NSTX Plasmas

“…Before examining those calculations directly, it is useful to compare measurements and modeling of the neutron evolution before and after a step change in the neutral beam power. Previous experiments have studied the behavior of "dilute" populations of fast ions [32,49,50,74], including studies in NSTX L--mode plasmas [74]. The latter study showed that the prompt loss and slowing down of fast ions in those plasmas were consistent with classical behavior.…”

“…The initial rise of the neutron signal is indicative of the prompt loss rate [74]. The decay of the neutron emission when a beam is turned off is fit to…”