Neutral beam heating in the START spherical tokamak

Abstract

The tight aspect ratios (typically A≈1.4) and low magnetic field of spherical tokamak (ST) plasmas, when combined with densities approaching the Greenwald limit, provide a significant challenge for all currently available auxiliary heating and current drive schemes. NBI heating and current drive are difficult to interpret in sub-megampere machines, as in order to achieve suitable penetration into the plasma core, fast ions have to be highly suprathermal and, as a result of the low magnetic field, can be non-adiabatic (i.e. non-conserving of magnetic moment µ0). The physics of NBI heating in START is discussed. The neutral beam injector deployed on START was clearly successful, having been instrumental in producing a world record tokamak toroidal beta of ≈40%. A fast ion Monte Carlo code (LOCUST) is described that was developed to model non-adiabatic fast ion topologies together with a high level of charge exchange loss and cross-field transport (present in START due to an envelope of high density gas surrounding the plasma). Model predictions compare well with experimental data, collected using a scanning neutral particle analyser, bolometric instruments and equilibrium reconstruction using EFIT. In particular, beta calculations based upon reconstruction of the pressure profile (by combining measurements from Thomson scattering, charge exchange recombination spectroscopy and model predictions for the fast ion distribution function) agree well with beta values calculated using EFIT alone (the routine method for calculation of START beta). These results thus provide increased confidence in the ability of STs to sustain high beta high confinement H mode plasmas and in addition indicate that the injected fast ions in collisional START plasmas evolve mainly due to collisional and charge exchange processes, without driving any significant performance degrading fast particle MHD activity.

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

“…Recent theoretical studies [5][6][7][8][9] predict reductions that are smaller than the experimental uncertainty so, although this result is consistent with theory, it does not constitute a rigorous test of the predictions.…”

Reconciliation of Measured and TRANSP-calculated Neutron Emission Rates in the National Spherical Torus Experiment: Circa 2002-2005

“…TRANSP analysis using this approach compares favorably with full Monte-Carlo orbit tracking codes such as LOCUST [9]. On MAST, good agreement is observed between measured and TRANSP-calculated neutron rates [10].…”

“…One of the most comprehensive of such tools is the 1 1/2-D time dependent transport code TRANSP [5] that is routinely employed worldwide for analysis of tokamak experiments [6 -8] including the MAST [9,10] and NSTX [11] spherical tokamaks. TRANSP analysis of fast ion transport in MHD-quiescent TFTR plasmas heated with different fractions and powers of deuterium and tritium neutral beams established that the spatially constant diffusion coefficient for energetic ions is low: D f ≤ 0.2 ± 0.2 m 2 /s [6].…”

Investigation Of A Transient Energetic Charge Exchange Fux Enhancement ('spike-on-tail') Observed In Neutral-beam-heated H-mode Discharges In The National Spherical Torus Experiment

“…Resonant interaction of high-energy particles with magnetic perturbations in toroidal devices can produce large-scale modification of the particle distribution, sometimes leading to particle loss. Spherical tokamaks, including NSTX [20 -23], START [24] and MAST [25,26], are particularly susceptible to fast ion driven instabilities due primarily to their relatively low toroidal field but also in some instances due to the direct effect of the low aspect ratio.…”