The neoclassical “Electron Root” feature in the Wendelstein-7-AS stellarator

The neoclassical prediction of the “electron root,” i.e., a strongly positive radial electric field, Er (being the solution of the ambipolarity condition of the particle fluxes), is analyzed for low-density discharges in Wendelstein-7-AS [G. Grieger, W. Lotz, P. Merkel, et al., Phys. Fluids B 4, 2081 (1992)]. In these electron cyclotron resonance heated (ECRH) discharges with highly localized central power deposition, peaked Te profiles [with Te(0) up to 6 keV and with Ti≪Te] and strongly positive Er in the central region are measured. It is shown that this “electron root” feature at W7-AS is driven by ripple-trapped suprathermal electrons generated by the ECRH. The fraction of ripple-trapped particles in the ECRH launching plane, which can be varied at W7-AS, is found to be the most important. After switching off the heating the “electron root” feature disappears nearly immediately, i.e., two different time scales for the electron temperature decay in the central region are observed. Monte Carlo simulati...

[1]  H. E. Mynick,et al.  Effect of the ambipolar potential on stellarator confinement , 1983 .

[2]  James F. Lyon,et al.  Monte Carlo studies of transport in stellarators , 1985 .

[3]  S. Kasilov,et al.  Solution of the drift kinetic equation in the regime of weak collisions by stochastic mapping techniques , 1997 .

[4]  R. Jaenicke,et al.  Confinement in W7-AS and the role of radial electric field and magnetic shear , 1997 .

[5]  J. Rax,et al.  Non-local current response in wave driven tokamaks , 1989 .

[6]  Allen H. Boozer,et al.  Monte Carlo evaluation of transport coefficients , 1981 .

[7]  R. Cano,et al.  Electron cyclotron emission and absorption in fusion plasmas , 1983 .

[8]  C. D. Beidler,et al.  Neoclassical Transport Scalings Determined from a General Solution of the Ripple-Averaged Kinetic Equation (GSRAKE) , 1995 .

[9]  E. C. Crume,et al.  Plasma transport coefficients for nonsymmetric toroidal confinement systems , 1986 .

[10]  Georg Kühner,et al.  Experimental and neoclassical electron heat transport in the LMFP regime for the stellarators W7‐A, L‐2, and W7‐AS , 1993 .

[11]  M. O'Brien,et al.  Fokker–Planck studies of high power electron cyclotron heating in tokamaks , 1986 .

[12]  H. Maassberg,et al.  Measurement and calculation of the radial electric field in the stellarator W7-AS , 1998 .

[13]  H. Sanuki,et al.  DYNAMIC BEHAVIOR OF POTENTIAL IN THE PLASMA CORE OF THE CHS HELIOTRON/TORSATRON , 1997 .

[14]  Monte Carlo simulation study of ICRF minority heating in the Large Helical Device , 1994 .

[15]  K. Itoh,et al.  REVIEW ARTICLE: The role of the electric field in confinement , 1996 .

[16]  S. Hirshman.  The ambipolarity paradox in toroidal diffusion, revisited , 1978 .

[17]  Daniel E. Hastings,et al.  A differential equation for the ambipolar electric field in a multiple‐helicity torsatron , 1985 .

[18]  V. Erckmann,et al.  Kinetic modelling of the ECRH power deposition in W7-AS , 1997 .

[19]  Charles F. F. Karney.  Fokker-Planck and Quasilinear Codes , 1986 .

[20]  H. Sanuki,et al.  Shock formation in a poloidally rotating tokamak plasma , 1991 .

[21]  Nathaniel J. Fisch,et al.  Theory of current-drive in plasmas , 1987 .

[22]  S. Hirshman,et al.  Variational bounds for transport coefficients in three-dimensional toroidal plasmas , 1989 .

[23]  L. Giannone,et al.  Physics optimization of stellarators , 1992 .

[24]  Ker-Chung Shaing,et al.  Stability of the radial electric field in a nonaxisymmetric torus , 1984 .

[25]  Paul Garabedian,et al.  Stellarators with the magnetic symmetry of a tokamak , 1996 .

[26]  Charles F. Kennel,et al.  Velocity Space Diffusion from Weak Plasma Turbulence in a Magnetic Field , 1966 .

[27]  Daniel E. Hastings,et al.  The ambipolar electric field in stellarators , 1985 .

[28]  W. Lotz,et al.  Monte Carlo computations of neoclassical transport , 1988 .