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

D Liu,R E Bell,E D Fredrickson,E Ruskov,S S Medley,R W Harvey,M Podestà,W W Heidbrink

doi:10.1088/0741-3335/52/2/025006

Abstract

Combined neutral beam injection and high-harmonic fast-wave (HHFW) heating accelerate deuterium fast ions in the National Spherical Torus Experiment (NSTX). With 1.1 MW of HHFW power, the neutron emission rate is about three times larger than in the comparison discharge without HHFW heating. Acceleration of fast ions above the beam injection energy is evident on an E||B type neutral particle analyzer (NPA), a 4-chord solid state neutral particle analyzer (SSNPA) array and a 16-channel fast-ion D-alpha (FIDA) diagnostic. The accelerated fast ions observed by the NPA and SSNPA diagnostics mainly come from passive charge exchange reactions at the edge due to the NPA/SSNPA localization in phase space. The spatial profile of accelerated fast ions that is measured by the FIDA diagnostic is much broader than in conventional tokamaks because of the multiple resonance layers and large orbits in NSTX. The fast-ion distribution function calculated by the CQL3D Fokker–Planck code differs from the measured spatial profile, presumably because the current version of CQL3D uses a zero-banana-width model. In addition, compressional Alfven eigenmode activity is stronger during the HHFW heating and it may affect the fast-ion spatial profile.

Highlights

Introduction and motivationNeutral beam injection (NBI) and high harmonic fast wave (HHFW) are the two major auxiliary heating systems in the National Spherical Torus Experiment (NSTX) [1]
The acceleration of fast ions due to HHFW heating in NSTX is clearly observed with the neutron, E||B type neutral particle analyzer (NPA), solid state neutral particle analyzer (SSNPA) and fast-ion D-alpha (FIDA) diagnostics
The fast-ion tail above the injection energy observed by the NPA and SSNPA diagnostics is mainly from the passive CX reactions at the edge, not from the active CX reactions in the intersectional area

Summary

Introduction

Neutral beam injection (NBI) and high harmonic fast wave (HHFW) are the two major auxiliary heating systems in the National Spherical Torus Experiment (NSTX) [1]. The discharges in NSTX provide a unique opportunity for studying the interactions between HHFW and fast ions at high β and dielectric constant. Because of the relatively low magnetic field, fast ions produced during NBI in NSTX are super-Alfvenic and the ratio of the gyroradius to the perpendicular wavelength is very large. Significant absorption of HHFW power at multiple cyclotron harmonics is expected. These differences of NSTX from conventional tokamaks motivate this careful comparison between theory and experiment in this unusual regime

Results

Discussion

Conclusion