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An important focus of the present research on thermonuclear fusion based on magnetic confinement is the control of rotating magnetic islands of low helical mode numbers m, n that are the nonlinear stage of resistive instabilities developing at magnetic surfaces where q = m/n. These perturbations are often driven by external resonant 'error fields' or mode coupling effects and maintained by local distortions of the current profile. The islands can be reduced in width or completely suppressed by a current driven by electron cyclotron waves accurately located within the island. A requisite for an effective control action is the ability to identify the relevant state variables (radial location, island amplitude, frequency and phase) in 'real time' and vary accordingly the control variables (wave beam power modulation and direction). Traditionally magnetohydrodynamic instabilities are monitored by external magnetic signals that however give no information about the radial location of the perturbation within the plasma. Here we describe the design of a real-time diagnostics/control device using an original algorithm for processing electron cyclotron emission signals to monitor temperature fluctuations and EC wave power deposition.

Significant progress has been made on ASDEX Upgrade during the last two years in the basic understanding of transport, in the extension of the improved H-mode in parameter space and towards an integrated operating scenario and in the development of control methods for major performance limiting instabilities. The important features were the understanding of particle transport and the control of impurity accumulation based on it, the satisfactory operation with predominantly tungsten-clad walls, the improved H-mode operation over density ranges and for temperature ratios covering (non-simultaneously) the ITER requirements on ν*, n/nGW and Te/Ti, the ELM frequency control by pellet injection and the optimization of NTM suppression by DC-ECCD through variation of the launching angle. From these experiments an integrated scenario has emerged which extrapolates to a 50% improvement in n T τ or a 30% reduction of the required current when compared with the ITER base-line assumptions, with moderately peaked electron and controllable high-Z density profiles.

Two experiments on DIII-D have been performed with the purpose of searching for evidence of a critical electron temperature gradient or gradient scale length. Both experiments employed off-axis electron cyclotron heating to vary the local value of inverse temperature scale length 1/L-Te, = -del T-e/T-e while holding the total heating power and thus edge temperatures constant. The incremental or heat pulse thermal diffusivity was monitored as a means of identifying the existence of a threshold or critical value of 1/L-Te, k(crit), where a large increase in heat pulse diffusivity may occur while going from below to above kcrit. No evidence of a critical value was found, but the existence of one cannot be ruled out by the experimental results. The results can be used to place an upper bound on k(crit). If k(crit) exists, the experimental results indicate kcrit < 3.8 m(-1) at rho = 0.45 and k(crit) < 2.5 m(-1) at rho = 0. 29 corresponding to temperature scale lengths relative to the plasma minor radius of LTe/a > 0.43 and 0.65, respectively. Over most of the plasma radius (rho >= 0.3) the temperature profile in the L-mode discharges studied exceeds these upper bounds on k(crit). A transport model based on a critical gradient is consistent with experimental observations but is not unique. Models other than one based on the existence of a critical gradient are also consistent with experimental observations.

Perturbative and steady-state heat transport of FTU tokamak in current ramp-up discharges are investigated by means of modulated electron cyclotron heating (MECH). Perturbative and steady-state transport experiments are coherent with an electron heat transport which switches from low to high values when electron temperature gradient length reaches a threshold value 1/L_Tc. The threshold value 1/L_Tc is shown to be proportional to the ratio s/q. The experimental findings are compared to predictions of an empirical model based on the assumption of a threshold gradient length, L_Tc (1/L_T = |grad Te/Te|), in the electron temperature Te below which electron thermal diffusivity, chi_e, switches from low to high values. Plasma responses to steady state and MECH are modelled assuming the electron diffusivity as chi_PB = chi_0 + alpha Te^(3/2) (1/L_T - 1/L_Tc)^(1/2); here Te^(3/2) reflects the gyro-Bohm assumption, chi_0 represents the heat transport for 1/L_T < 1/LT_c and the term (1/L_T - 1/L_Tc)^(1/2), which sets in for 1/L_T > 1/L_Tc , mimics an extra transport possibly due to electron temperature gradient (ETG) modes. In agreement with ETG threshold 1/L_Tc is shown to be correlated with the magnetic shear s.

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