Steady state scenario development with elevated minimum safety factor on DIII-D

On DIII-D (Luxon 2005 Fusion Sci. Technol. 48 828), a high β scenario with minimum safety factor (qmin) near 1.4 has been optimized with new tools and shown to be a favourable candidate for long pulse or steady state operation in future devices. The new capability to redirect up to 5 MW of neutral beam injection (NBI) from on- to off-axis improves the ability to sustain elevated qmin with a less peaked pressure profile. These changes increase the ideal magnetohydrodynamics (MHD) n = 1 mode βN limit thus providing a path forward for increasing the noninductive current drive fraction by operating at high βN. Quasi-stationary discharges free of tearing modes have been sustained at βN = 3.5 and βT = 3.6% for two current profile diffusion timescales (about 3 s) limited by neutral beam duration. The discharge performance has normalized fusion performance expected to give fusion gain Q ≈ 5 in a device the size of ITER. Analysis of the poloidal flux evolution and current drive balance show that the loop voltage profile is almost relaxed even with 25% of the current driven inductively, and qmin remains elevated near 1.4. These observations increase confidence that the current profile will not evolve to one unstable to a tearing mode. In preliminary tests a divertor heat flux reduction technique based on producing a radiating mantle with neon injection appears compatible with this operating scenario. 0D model extrapolations suggest it may be possible to push this scenario up to 100% noninductive current drive by raising βN. Similar discharges with qmin = 1.5–2 were susceptible to tearing modes and off-axis fishbones, and with qmin > 2 lower normalized global energy confinement time is observed.

[1]  T. L. Rhodes,et al.  Optimization of the safety factor profile for high noninductive current fraction discharges in DIII-D , 2011 .

[2]  W. Heidbrink,et al.  Particle distribution modification by low amplitude modes , 2010 .

[3]  Lao,et al.  Determination of the noninductive current profile in tokamak plasmas. , 1994, Physical review letters.

[4]  Jet Efda Contributors,et al.  Non-inductive current drive and transport in high βN plasmas in JET , 2009 .

[5]  J. Scoville,et al.  Measurement of the resistive-wall-mode stability in a rotating plasma using active MHD spectroscopy. , 2004, Physical review letters.

[6]  L. L. LoDestro,et al.  CORSICA: A comprehensive simulation of toroidal magnetic-fusion devices. Final report to the LDRD Program , 1997 .

[7]  L. Lao,et al.  MHD Equilibrium Reconstruction in the DIII-D Tokamak , 2005 .

[8]  R. Budny,et al.  Excitation of Alfvén eigenmodes by low energy beam ions in the DIII-D and JET tokamaks , 2008 .

[9]  R. White,et al.  Anomalous flattening of the fast-ion profile during Alfvén-Eigenmode activity. , 2007, Physical review letters.

[10]  J. Kinsey,et al.  Access to sustained high-beta with internal transport barrier and negative central magnetic shear in DIII-D , 2006, Physics of Plasmas.

[11]  Turnbull,et al.  High Beta and Enhanced Confinement in a Second Stable Core VH-Mode Advanced Tokamak. , 1995, Physical review letters.

[12]  C. Holcomb,et al.  Measurements, modelling and electron cyclotron heating modification of Alfvén eigenmode activity in DIII-D , 2009 .

[13]  Martin Jakobi,et al.  Steady state advanced scenarios at ASDEX Upgrade , 2002 .

[14]  Manickam,et al.  Improved plasma performance in tokamaks with negative magnetic shear. , 1994, Physical review letters.

[15]  David Mikkelsen,et al.  Current relaxation time scales in toroidal plasmas , 1989 .

[16]  O. Sauter,et al.  Neoclassical conductivity and bootstrap current formulas for general axisymmetric equilibria and arbitrary collisionality regime , 1999 .

[18]  O. Sauter,et al.  Erratum: “Neoclassical conductivity and bootstrap current formulas for general axisymmetric equilibria and arbitrary collisionality regime” [Phys. Plasmas 6, 2834 (1999)] , 2002 .

[19]  T. Fujita,et al.  Development of advanced operation scenarios in weak magnetic-shear regime on JT-60U , 2009 .

[20]  F. Meo,et al.  Fast-ion transport induced by Alfvén eigenmodes in the ASDEX Upgrade tokamak , 2011 .

[21]  L. L. Lao,et al.  Progress toward fully noninductive, high beta conditions in DIII-D , 2005 .

[22]  John D Galambos,et al.  Commercial tokamak reactor potential with advanced tokamak operation , 1995 .

[23]  L. L. Lao,et al.  The ARIES-AT advanced tokamak, Advanced technology fusion power plant , 2006 .

[24]  T. C. Luce,et al.  Electron cyclotron current drive efficiency in general tokamak geometry , 2003 .

[25]  W. W. Heidbrink,et al.  Basic physics of Alfvén instabilities driven by energetic particles in toroidally confined plasmas , 2008 .

[26]  R. W. Schleicher,et al.  Limitations of power conversion systems under transient loads and impact on the pulsed tokamak power reactor , 1993, 15th IEEE/NPSS Symposium. Fusion Engineering.

[27]  P. Sardain,et al.  Power plant conceptual studies in Europe , 2007 .

[28]  T. Petrie,et al.  Progress toward fully noninductive discharge operation in DIII-D using off-axis neutral beam injection , 2012 .

[29]  A. D. Turnbull,et al.  Optimizing stability, transport, and divertor operation through plasma shaping for steady-state scenario development in DIII-D , 2009 .

[30]  T. C. Luce,et al.  Development of Steady-State Advanced Tokamak Research in the DIII-D Tokamak , 2005 .

[31]  J. Contributors,et al.  Kink instabilities in high-beta JET advanced scenarios , 2012 .

[32]  P. B. Snyder,et al.  Integrated modelling of steady-state scenarios and heating and current drive mixes for ITER , 2011 .

[33]  J. Manickam,et al.  Control of the resistive wall mode with internal coils in the DIII–D tokamak , 2005 .

[34]  J. Schweinzer,et al.  Optimized tokamak power exhaust with double radiative feedback in ASDEX Upgrade , 2012 .

[35]  T. Luce,et al.  Sensitivity of Transport and Stability to the Current Profile in Steady-state Scenario Plasmas in DIII-D , 2012 .

[36]  H. E. St. John,et al.  Off-axis neutral beam current drive for advanced scenario development in DIII-D , 2009 .

[37]  J. Park,et al.  Initial measurements of the DIII-D off-axis neutral beams , 2012 .

[38]  Neville C. Luhmann,et al.  Measurements and modeling of Alfvén eigenmode induced fast ion transport and loss in DIII-D and ASDEX Upgrade , 2011 .

[39]  T. W. Petrie,et al.  Comparison of radiating divertor behaviour in single-null and double-null plasmas in DIII-D , 2008 .

[40]  C. M. Greenfield,et al.  Optimization of DIII-D advanced tokamak discharges with respect to the β limita) , 2005 .

[41]  P. Barabaschi,et al.  ITER: opportunity of burning plasma studies , 2001 .

[42]  G. Bateman,et al.  The tokamak Monte Carlo fast ion module NUBEAM in the National Transport Code Collaboration library , 2004 .

[43]  R. H. Bulmer,et al.  Sustained Spheromak Physics Experiment (SSPX): design and physics results , 2012 .