Open field equilibrium current and cross-field passing electrons as an initiator of a closed flux surface in EC-heated toroidal plasmas

A model for the non-inductive initiation of a closed flux surface observed in electron cyclotron (EC) heated toroidal plasmas is presented. First, a pressure-driven equilibrium toroidal current develops under a weak external vertical field so as to counter balance the pressure-ballooning and current-hoop forces. When the self-field from the current almost cancels out the external vertical field, a forward energetic part of electrons in the velocity space begins to make cross-field passing (CFP) orbits. The CFP electrons are generated by the EC heating of bulk electrons and subsequent pitch-angle scattering, which is analyzed using the Fokker–Planck equation. They provide an additional current that closes the field lines. The model is examined for experiments in the small low aspect ratio device of LATE and in the large conventional device of JT-60U with a search for appropriate modes of EC heating. Simultaneous coincidence of the model with these two experiments is obtained in terms of microwave power and driven current. The results predict that initiation of a closed flux surface requires more and more EC power as the plasma major radius increases. In particular, careful injection of high N‖ EC waves is needed for large devices, both for initiation of a closed flux surface and for subsequent enlargement of the flux surface by the usual EC current drive onto the closed flux area.

[1]  J. Byun,et al.  Atmospheric-pressure plasma sources for biomedical applications , 2012 .

[2]  T. Luce,et al.  Noninductive plasma initiation and startup in the DIII-D tokamak , 2011 .

[3]  S. Ide,et al.  Generation of initial closed flux surfaces by ECH at a conventional aspect ratio of R/a ∼ 3: experiments on the LATE device and JT-60U tokamak , 2011 .

[4]  Liang Wang,et al.  Transient process of a spherical tokamak plasma startup by electron cyclotron waves , 2011 .

[5]  Hitoshi Tanaka,et al.  Study of Electron Orbits for Formation of Toroidal Closed Flux Surface by ECH , 2011 .

[6]  M. Uchida,et al.  Current circulation and equilibrium in toroidal electron cyclotron resonance (ECR) plasmas in the LATE device , 2010 .

[7]  M. Uchida,et al.  Observation of vertical charge-separation current in an electron-cyclotron-heated toroidal plasma , 2010 .

[8]  T. Maekawa,et al.  Rapid current ramp-up by cyclotron-driving electrons beyond runaway velocity. , 2010, Physical review letters.

[9]  K. Ikeda.  Progress in the ITER Physics Basis , 2007 .

[10]  David Allan Humphreys,et al.  Chapter 8: Plasma operation and control , 2007 .

[11]  M. Uchida,et al.  A current profile model for magnetic analysis of the start-up phase of toroidal plasmas driven by electron cyclotron heating and current drive , 2007 .

[12]  A. Ejiri,et al.  RF start-up and sustainment experiments on the TST-2@K spherical tokamak , 2006 .

[13]  T. Maekawa,et al.  Spontaneous formation of closed-field torus equilibrium via current jump observed in an electron-cyclotron-heated plasma. , 2006, Physical review letters.

[14]  Y. Nakamura,et al.  Design study of fusion DEMO plant at JAERI , 2006 .

[15]  Hitoshi Tanaka,et al.  Formation of spherical tokamak equilibria by ECH in the LATE device , 2005 .

[16]  Hitoshi Tanaka,et al.  Polarization adjustment of incident electromagnetic waves for optimal mode-conversion to electron Bernstein waves , 2004 .

[17]  T. Osborne,et al.  Investigation of the formation of a fully pressure‐driven tokamak* , 1994 .

[18]  Maekawa,et al.  Up-shifted frequency electron-cyclotron current drive in a lower hybrid current drive plasma. , 1993, Physical review letters.

[19]  Forest,et al.  Internally generated currents in a small-aspect-ratio tokamak geometry. , 1992, Physical review letters.

[20]  Charles F. F. Karney.  Fokker-Planck and Quasilinear Codes , 1986, physics/0501066.

[21]  George H. Neilson,et al.  External inductance of an axisymmetric plasma , 1986 .

[22]  Hitoshi Tanaka,et al.  Physical Interpretation of Current Start-Up by rf in WT-2 Tokamak , 1985 .

[23]  Takashi Shimozuma,et al.  Toroidal Plasma Current Startup and Sustainment by rf in the WT-2 Tokamak , 1983 .

[24]  M. Ono,et al.  Current generation by unidirectional lower hybrid waves in the ACT-1 toroidal device , 1980 .

[25]  Y. Terumichi,et al.  Numerical Studies of Wave Trajectories and Electron Cyclotron Heating in Toroidal Plasmas , 1980 .

[26]  Terry Kammash,et al.  Microwave start-up of tokamak plasmas near electron cyclotron and upper hybrid resonances , 1977 .

[27]  A. W. Trivelpiece,et al.  Introduction to Plasma Physics , 1976 .

[28]  W. L. Harries,et al.  Equilibrium of a Toroidal Plasma with a Conducting Aperture Limiter , 1963 .