Neutral beam heating in the START spherical tokamak

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 ?0). 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.

[1]  Tadashi Sekiguchi,et al.  Plasma Physics and Controlled Nuclear Fusion Research , 1987 .

[2]  N. Conway,et al.  The unfolding of multichord Doppler spectrometry , 1999 .

[3]  G. F. Counsell,et al.  Enhanced performance in the START tokamak , 1998 .

[4]  K. Okano,et al.  Neoclassical formula for neutral beam current drive , 1990 .

[5]  Dennis J Strickler,et al.  Features of spherical torus plasmas , 1986 .

[6]  M. Gryaznevich,et al.  Achievement of Record β in the START Spherical Tokamak , 1998 .

[7]  C. Roach,et al.  Physics of energetic particle-driven instabilities in the START spherical tokamak , 1999 .

[8]  C. Roach,et al.  Magneto-hydro-dynamic limits in spherical tokamaks , 1999 .

[9]  T. J. Martin,et al.  Steady state operation of spherical tokamaks , 2000 .

[10]  D. E. Schechter,et al.  Properties of an intense 50‐kV neutral‐beam injection system , 1982 .

[11]  T. H. Stix,et al.  Heating of toroidal plasmas by neutral injection , 1972 .

[12]  F. Hinton,et al.  Trapped electron correction to beam driven current in general tokamak equilibria , 1997 .

[13]  Conway,et al.  H-Mode operation in the START spherical tokamak , 2000, Physical review letters.

[14]  A. Field,et al.  Poloidal rotation and associated edge behaviour in START plasmas , 2001 .

[15]  P. I. John,et al.  Multiple life times in the nonadiabatic leakage of particles from a magnetic mirror trap , 1980 .

[16]  D. McCune,et al.  Energetic particle orbits in the National Spherical Tokamak Experiment , 1997 .

[17]  C. Roach,et al.  High-performance discharges in the Small Tight Aspect Ratio Tokamak (START) , 1998 .

[18]  A. Sykes.  The spherical tokamak programme at Culham , 1999 .

[19]  C. Roach,et al.  Confinement in START beam heated discharges , 2001 .

[20]  M. Walsh,et al.  Interactive optical design and realization of an optimized charge coupled device Thomson scattering system for the spherical tokamak START , 1999 .

[21]  S. J. Manhood,et al.  First results from the START experiment , 1992 .

[22]  J. Manickam,et al.  Anisotropic pressure tokamak equilibrium and stability considerations , 1987 .

[23]  S. E. Lysenko,et al.  Improved confinement regimes within the transport model of canonical profiles , 1995 .

[24]  D. McCune,et al.  New techniques for calculating heat and particle source rates due to neutral beam injection in axisymmetric tokamaks , 1981 .

[25]  H. H. Haselton,et al.  Analysis of particle species evolution in neutral beam injection lines , 1979 .

[26]  L. C. Appel,et al.  High- performance of the START spherical tokamak , 1997 .

[27]  A. Sykes.  High β produced by neutral beam injection in the START (Small Tight Aspect Ratio Tokamak) spherical tokamak , 1997 .

[28]  L. Lao,et al.  Reconstruction of current profile parameters and plasma shapes in tokamaks , 1985 .