Confinement in ASDEX with neutral beam and RF heating

Abstract

Xeutral beam (NI), ion cyclotron resonance (ICRH) and lower hybrid resonance (LIIKV) heating on ASDEX are discussed with regard to their effect on plasma confinement. Comparison of SI and ICRH shows that the L and H-regimes are universal confinenent nodes of auxiliary-heated tokamak plasmas (i.e. independent of the heating nethod), and that the edge electron temperature (or a related parameter) dictates which mode prevails. In this connection it is noted that carbonization of the vessel walls impedes transition to the H-mode in the case of SI heating. Studies of energy confinement in the intermediate regime from ohmic to SI heating reveal a gradual transition from ohnic (-se) to neutral injection L-mode (-I) scaling. At the same tine a renarkable invariance of electron tenperature profile shape with increasing heating power is observed. Changing the XI power deposition profiles fron central to off-axis leaves gross energy confinement tines unchanged while central confinement is substantially inproved. Detailed investigations of the O-linit confirn the relation &$ [ %] = 2.8 I/(a B) [HA*n-l*T-l]. They show that the &limit is independent of heating method (NI, ICRH) and confinenent regine (L or H), but strongly dependent on the current density profile. indirect evidence suggests ballooning (and possibly surface kink) nodes to be responsible for the limitation in plasma pressure. LH heating and current drive show improved energy confinement (conpared to OH) at low densities (necl -2 x 1013 ~n -~, suprathermal electrons), but a degradation in confinement at higher densities.

Possible applications of a resonant helical magnetic field in the tokamak boundary

“…When |b r |/B t ~ 1 X 10~4, the effective thermal conductivity, Xeff((\b T \/B) 2 ^Ri^he) t^], due to externally imposed ergodic magnetic structure, is ~1.0 m 2 • s" 1 for T e = 200 eV, which is an acceptable level. (Note that the observed value in the high power L-mode discharge is greater than 10 m 2 -s" 1 [34]. )…”

“…In the ASDEX H-mode discharges [ 14,34], the energy confinement during the quiescent phase (no ELM) is extremely high, and rapid heat flow takes place only during the ELM phase with a short period (several milliseconds). When the ELM is eliminated, the transport energy confinement increases significantly.…”

“…Good edge energy confinement results in steep VT and, hence, steep VP or Vj at the edge. It is conjectured [34] that the MHD instability causes the ELM because VP or Vj at the edge become too high. With enhanced central radiative power due to impurities, a smaller fraction of the input power flows through the edge region, and the VP or Vj at the edge can be maintained within the MHD stability limit.…”

Analytical modelling of impurity transport in toroidal devices

“…Experimental evidence of such a convective term, which leads to impurity accumulation in the plasma centre in the case of inward flow (v ± < 0), has been found in a number of investigations [4][5][6][7]. Impurity accumulation is found to be enhanced for deuterium background plasmas [5] and in the case of counter injection heating [6].…”

“…Impurity accumulation is found to be enhanced for deuterium background plasmas [5] and in the case of counter injection heating [6]. Severe accumulation problems which lead to a radiation collapse of the discharge also occurred during the excellent confinement phase (quiescent H-phase) of Nl-heated ASDEX plasmas [7].…”