Exact solutions to the Grad–Shafranov equation for ideal magnetohydrodynamic (MHD) tokamak equilibria with dissimilar functional dependences of the pressure and poloidal current source profiles are presented. The current density profile has three free parameters, which is sufficiently flexible to describe equilibria consistent with external magnetic measurements. Experimental x-point and limiter plasma configurations can be represented by a superposition of solutions with the same eigenvalue. Both normal and reversed shear current profiles are allowed. An efficient algorithm for least squares fitting of numerically obtained experimental equilibria to the exact solution functions is described and applied to the ASDEX Upgrade (axially symmetric divertor experiment) tokamak [Plasma Physics and Controlled Nuclear Fusion Research 1992 (International Atomic Energy Agency, Vienna, 1993), Vol. I, p. 127].

Abstract.The flux surface topology of a toroidal plasma bounded by a magnetic separatrix allows edge moments of the toroidal current density profile to be identified in an MHD equilibrium reconstruction code using only external magnetic measurements. This is demonstrated analytically for simple plasma shapes and applied to experimental data on the ASDEX Upgrade tokamak where CLISTE reconstructions from magnetic data are shown to be consistent with those obtained from a more complete set of diagnostic data. An independent demonstration of edge current profile recoverability is obtained by analyzing the reconstruction errors for a database of Monte Carlogenerated equilibria.

Abstract. Recent studies at ASDEX Upgrade aim to further characterise and understand the physics of the improved H-mode scenario. The main focus is on the influence of the rampup phase of the plasma current and heating on energy confinement and MHD-activity during the subsequent flat-top phase. Depending on the ramp-up scenario two different stationary plasmas can be generated, which show different equilibrated current profiles, although external control parameters are the same in the flat-top phase. The difference of the current profiles in the flat-top phase seems to be due to different MHD modes. These MHD modes set in during relaxation of the current profile, which itself depends on the ramp-up scenario. Also the stored energy is different in the two cases as is the peaking of the temperature profiles.Three mechanisms seem to play a role in linking the observed changes in MHD-behaviour and current profile to the changes of the kinetic profiles: the increased transport due to the MHD modes themselves, the variation of the ratio of magnetic shear to safety factor , which modifies the critical temperature gradient-length for the onset of ITGs and effects of the H-mode pedestal pressure.

Internal transport barriers have been demonstrated to exist also under conditions with T(e) approximately T(i) approximately 10 keV and predominant electron heating of the tokamak core region. Central electron cyclotron heating was added to neutral beam injection-heated ASDEX Upgrade discharges with a preexisting internal transport barrier, established through programmed current ramping leading to shear reversal. Compared to a reference internal transport barrier discharge without electron cyclotron resonance heating, the electron heat conductivity in the barrier region was found not to increase, in spite of a fivefold increase in electron heat flux, and also angular momentum and ion energy transport did not deteriorate.

Advanced tokamak discharges on ASDEX Upgrade can generate an internal transport barrier (ITB) during the current ramp-up phase early in the discharge. Formation of the ITB has become more reliable with the discovery that a low density is necessary for it to form. These ITBs form in very low or negative central shear regions. There is no clear evidence of integer q magnetic surfaces triggering the ITB phase. The appearance of an integer q surface (usually q = 2) often leads to a second ITB (i.e. another steepening in the ion temperature gradient) inside the first ITB.Investigation of possible turbulence suppression mechanisms suggests that something is missing from the generally accepted model of sufficient E × B sheared flow suppressing the turbulence since the E × B shearing rate was not greater than, or comparable with, the maximum linear growth rate in about half of the ITB discharges analysed. Thus, it does not explain the reduced radial transport observed in these discharges.