Sustained Spheromak Physics Experiment (SSPX): design and physics results

The Sustained Spheromak Physics Experiment (SSPX) was a high-temperature (Te up to 0.5 keV) spheromak formed by coaxial helicity injection (CHI) and with plasma duration of a few milliseconds following the high-current formation stage. Clean walls and low impurity operation were obtained by a combination of baking, discharge cleaning and titanium deposition on the walls, allowing the generation of high-quality plasmas. Resistive-magnetohydrodynamic simulations, benchmarked to the experiment, were used to elucidate the physics. The detailed characteristics of the nφ = 1 toroidal mode associated with CHI were determined as was the physics of the nonlinear current drive and magnetic reconnection that formed and sustained the spheromak. If the helicity injection rate was reduced following formation the plasma became relatively quiescent and magnetic surfaces formed. The measured thermal diffusivity in the core was as low as ∼1 m2 s−1. However, reconnection events during buildup or sustainment of the plasma current by CHI were found to open magnetic surfaces throughout the plasma allowing rapid energy loss to the walls. As a result, experiments and simulations in SSPX found no path to simultaneous sustainment by CHI and good energy confinement. Additional physics results are also presented in this review.

[1]  C. Barnes,et al.  Energy confinement studies in spheromaks with mesh flux conservers , 1988 .

[2]  J. Taylor,et al.  Steady-state dynamo and current drive in a nonuniform bounded plasma , 1992 .

[3]  D. N. Hill,et al.  New mode of operating a magnetized coaxial plasma gun for injecting magnetic helicity into a spheromak. , 2003, Physical review letters.

[4]  R. M. Mayo,et al.  Time of flight measurement of ion temperatures in spheromaks , 1991 .

[5]  C. Romero-Talamas,et al.  Magnetic Reconnection in the Spheromak: Physics and Consequences , 2007 .

[6]  Naito,et al.  Relaxation oscillations and toroidal-current regeneration in a helicity-driven spheromak. , 1993, Physical review letters.

[7]  R. Jayakumar,et al.  Measurements and phenomenological modeling of magnetic flux buildup in spheromak plasmas , 2007 .

[8]  C. Barnes,et al.  Experimental determination of the conservation of magnetic helicity from the balance between source and spheromak , 1986 .

[9]  A. Boozer.  Magnetic helicity and dynamos , 1993 .

[10]  Thomas R. Jarboe,et al.  Formation and steady-state sustainment of a tokamak by coaxial helicity injection , 1989 .

[11]  Juan C Fernández,et al.  Progress with energy confinement time in the CTX spheromak , 1990 .

[12]  A. C. Smith,et al.  Production of field‐reversed plasma with a magnetized coaxial plasma gun , 1981 .

[13]  P. Beiersdorfer,et al.  Tungsten spectroscopy relevant to the diagnostics of ITER divertor plasmas , 2010 .

[14]  Y. Nogi,et al.  Spheromak Formation by Theta Pinch , 1980 .

[15]  R. H. Bulmer,et al.  MHD equilibria in a spheromak sustained by coaxial helicity injection , 1999 .

[16]  D. N. Hill,et al.  Particle control in the sustained spheromak physics experiment , 2000 .

[17]  D. N. Hill,et al.  Magnetic reconnection during flux conversion in a driven spheromak , 2005 .

[18]  D. N. Hill,et al.  Sustained spheromak coaxial gun operation in the presence of an n=1 magnetic distortion , 2006 .

[19]  Steven J. Plimpton,et al.  Nonlinear magnetohydrodynamics simulation using high-order finite elements , 2004 .

[20]  M. Rosenbluth,et al.  MHD stability of Spheromak , 1979 .

[21]  Cunningham,et al.  Observations of the magnetohydrodynamic dynamo effect in a spheromak plasma. , 1993, Physical review letters.

[22]  M. G. Rusbridge,et al.  Structure of the n = 1 mode responsible for relaxation and current drive during sustainment of the SPHEX spheromak , 1997 .

[23]  D. N. Hill,et al.  A phenomenological model of the current filamentation instability driven by cathode processes in the Livermore spheromak , 2003 .

[24]  J B Taylor,et al.  Relaxation and magnetic reconnection in plasmas , 1986 .

[25]  E. Hooper,et al.  Physics issues of a spheromak refluxing scenario , 2011 .

[26]  Harold P. Furth,et al.  Quasistatic formation of the spheromak plasma configuration , 1981 .

[27]  R. Gruber,et al.  Numerical study of the ideal-MHD stability limits in oblate spheromaks , 1981 .

[28]  Wysocki,et al.  Evidence for a pressure-driven instability in the CTX spheromak. , 1988, Physical review letters.

[29]  Dean A. Buchenauer,et al.  Plasma diagnostics for the sustained spheromak physics experiment , 2001 .

[30]  Kenji Watanabe,et al.  Compact Toroidal Plasma with Toroidal and Poloidal Magnetic Fields , 1981 .

[31]  D. N. Hill,et al.  Observation of a pressure limit in a gun-driven spheromak , 2006 .

[32]  R. Moses,et al.  Transport implications of current drive by magnetic helicity injection , 2001 .

[33]  D. N. Hill,et al.  Spheromak formation and sustainment studies at the sustained spheromak physics experiment using high-speed imaging and magnetic diagnostics , 2006 .

[34]  B. Cohen,et al.  The role of the nϕ=1 column mode in spheromak formation , 2009 .

[35]  L. Michalski,et al.  Temperature Measurement , 1985 .

[36]  D. Ryutov,et al.  Wall conditioning and power balance for spheromak plasmas in SSPX , 2003 .

[37]  E. Hooper,et al.  Hyper-resistive modeling in the spheromak , 2002 .

[38]  J. H. Irby,et al.  Formation of a spheromak plasma configuration , 1980 .

[39]  C. Sovinec,et al.  Numerical investigation of transients in the SSPX Spheromak. , 2005, Physical review letters.

[40]  M. Rosenbluth,et al.  Electron heat transport in a tokamak with destroyed magnetic surfaces , 1978 .

[41]  E. Hooper,et al.  Aspect-ratio effects in the driven, flux-core spheromak , 2009 .

[42]  Wright,et al.  Observations of spheromak equilibria which differ from the minimum-energy state and have internal kink distortions. , 1986, Physical review letters.

[43]  Thomas R. Jarboe,et al.  Review of spheromak research , 1994 .

[44]  D. N. Hill,et al.  Improved operation of the SSPX spheromak , 2005 .

[45]  M. G. Rusbridge,et al.  The design and operation of the SPHEX spheromak , 1997 .

[46]  C. Sovinec,et al.  NIMROD Resistive Magnetohydrodynamic Simulations of Spheromak Physics , 2007 .

[47]  K. Kawai,et al.  Generation of poloidally rotating spheromaks by the conical theta pinch , 1987 .

[48]  D. N. Hill,et al.  Transport and Fluctuations in High Temperature Spheromak Plasmas , 2005 .

[49]  F L Tabarés,et al.  Tritium inventory control during ITER operation under carbon plasma-facing components by nitrogen-based plasma chemistry: a review , 2013 .

[50]  P. Browning,et al.  Partially relaxed magnetic field equilibria in a gun-injected spheromak , 1990 .

[51]  C. Barnes,et al.  Slow formation and sustainment of spheromaks by a coaxial magnetized plasma source , 1983 .

[52]  R. M. Mayo,et al.  Numerical calculation of Mercier beta limits in spheromaks , 1988 .

[53]  D. N. Hill,et al.  Magnetic helicity balance in the Sustained Spheromak Plasma Experiment , 2003 .

[54]  A. Bhattacharjee,et al.  Onset of Fast Reconnection in Hall Magnetohydrodynamics Mediated by the Plasmoid Instability , 2010, 1010.5284.

[55]  D. N. Hill,et al.  Suppression of MHD fluctuations leading to improved confinement in a gun-driven spheromak. , 2002, Physical review letters.

[56]  R. Talamás,et al.  Investigations of Spheromak plasma dynamics: High-speed imaging at the Sustained Spheromak Physics Experiment and magnetic diagnostics at the Caltech Spheromak experiment , 2005 .

[57]  D. N. Hill,et al.  Increasing the magnetic helicity content of a plasma by pulsing a magnetized source. , 2004, Physical review letters.

[58]  Diego del-Castillo-Negrete,et al.  Formation and sustainment of electrostatically driven spheromaks in the resistive magnetohydrodynamic model , 2001 .

[59]  J. Moller,et al.  Improved magnetic field generation efficiency and higher temperature spheromak plasmas , 2009 .

[60]  D. N. Hill,et al.  Achieving high flux amplification in a gun-driven, flux-core spheromak , 2007 .

[61]  Joseph A. Johnson,et al.  Energy confinement and magnetic field generation in the SSPX spheromaka) , 2008 .

[62]  K. Gibson,et al.  Ion energy measurements in the SPHEX spheromak , 1995 .

[63]  J. B. Taylor,et al.  Relaxation of toroidal plasma and generation of reverse magnetic fields , 1974 .

[64]  N. Hicks,et al.  Recent results from the HIT-SI experiment , 2011 .

[65]  C. Sovinec,et al.  The Effect of Artificial Diffusivity on the Flute Instability , 2005 .

[66]  S. Jardin.  Ideal magnetohydrodynamic stability of the spheromak configuration , 1982 .

[67]  Finn,et al.  Chaotic scattering and self-organization in spheromak sustainment , 2000, Physical review letters.

[68]  D. N. Hill,et al.  Simulation of Spheromak Evolution and Energy Confinement , 2005 .

[69]  J. Moller,et al.  Neutral particle analyzer measurements on the SSPX spheromak. , 2007, The Review of scientific instruments.