Fast ion absorption of the high harmonic fast wave in the National Spherical Torus Experiment

Abstract

Ion absorption of the high harmonic fast wave in a spherical torus [Y.-K. M. Peng et al., Nucl. Fusion 26, 769 (1986)] is of critical importance to assessing the viability of the wave as a means of heating and driving current. Analysis of recent National Spherical Torus Experiment [M. Ono et al., Nucl. Fusion 40, 557 (2000)] shots has revealed that under some conditions when neutral beam and rf power are injected into the plasma simultaneously, a fast ion population with energy above the beam injection energy is sustained by the wave. In agreement with modeling, these experiments find the rf-induced fast ion tail strength and neutron rate at lower B-fields to be less enhanced, likely due to a larger β profile, which promotes greater off-axis absorption where the fast ion population is small. Ion loss codes find the increased loss fraction with decreased B insufficient to account for the changes in tail strength, providing further evidence that this is a rf interaction effect. Though greater ion absorption is predicted with lower k∥, surprisingly little variation in the tail was observed, along with a neutron rate enhancement with higher k∥. Data from the neutral particle analyzer, neutron detectors, x-ray crystal spectrometer, and Thomson scattering are presented, along with results from the TRANSP [R. J. Hawryluk, Physics of Plasmas Close to Thermonuclear Conditions 1, 19 (1981); J. P. H. E. Ongena et al., Fusion Technol. 33, 181 (1998)] transport analysis code, ray-tracing codes HPRT [J. Menard et al., Phys. Plasmas 6, 2002 (1999)], and CURRAY [T. K. Mau et al., RF Power in Plasmas: 13th Topical Conference (1999), p. 148], full-wave code AORSA [E. F. Jaeger et al., RF Power in Plasmas: 14th Topical Conference, 2001, p. 369], quasilinear code CQL3D [R. W. Harvey et al., in Proceedings of the IAEA TCM on Advances in Simulation and Modeling of Thermonuclear Plasmas, 1992], and ion loss codes EIGOL [D. S. Darrow et al., in Proceedings of the 6th IAEA TCM on Energetic Particles in Magnetic Confinement Systems, 2000, p. 109] and CONBEAM [J. Egedal et al., Phys. Plasmas 10, 2372 (2003)].

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

“…Although the HHFW power couples predominantly to the electrons, the power absorbed by fast ions could take more than 30% of the total input HHFW power [14]. At the usual toroidal field of 0.55 T, the 30 MHz waves resonate with deuterium cyclotron harmonics at several radial locations with the central resonance corresponding to the eighth harmonic and the outer edge resonance corresponding to the eleventh harmonic, as shown in figure 1(b).…”

“…The wave absorption by fast ions at the fundamental cyclotron frequency or low harmonics can form an energetic, anisotropic tail distribution and this wave absorption competes with electron absorption, which reduces the available power for electron heating and current drive by the fast waves. Acceleration of fast ions at the third, fourth and fifth harmonics is now an established technique and has been demonstrated in many recent experiments [4][5][6][7][8][9][10][11][12][13][14][15]. In particular, the k ⊥ ρ f dependence (k ⊥ is the perpendicular wavenumber and ρ f is the fast-ion gyroradius) is well established.…”

“…The acceleration of fast ions by HHFW in NSTX was previously studied by Rosenberg et al with neutron and E||B type neutral particle analyzer (NPA) measurements [14]. Although the acceleration was observed by both diagnostics, little information was obtained on the spatial profile of the absorption.…”

Edge Ion Heating by Launched High Harmonic Fast Waves in NSTX

“…On NSTX, observations have been made of fast-ion tails excited by the application of rf simultaneously with NBI [23,24]. Recently on Alcator C-Mod observations have been made which indicate that IBW's generated from parametric decay of launched HHFW's are responsible for enhanced ion energies in the edge [25,26].…”

Use of Fast Ion D-Alpha diagnostics for understanding ICRF effects

“…As a reference, p/ < 3 cm in DIII-D and p/ < 20 cm on NSTX. Fast ion acceleration by ICRF waves has been already observed on both machine in past years [8] [9]. Since then, a set of Fast-Ion DAlpha (FIDA) spectroscopic diagnostics has become routinely available on both devices, allowing more detailed studies of the response of fast ions to HHFW injection.…”