Detailed studies of sawtooth activity in Ohmic plasmas in JET have revealed significant discrepancies both with previously reported observations of the phenomenon and with conventional models of the internal disruption. ‘Compound’ sawteeth, which display an intermediate collapse during the ramp phase, are observed in the majority of discharges. These usually exhibit no precursor activity, in contrast to observations in smaller tokamaks, but are often accompanied by successor oscillations. Furthermore, the collapse time of such sawteeth is much shorter than expected. These results suggest that the conventional model of the sawtooth is inadequate to explain sawtooth activity in large tokamaks.

Measurements of second-harmonic electron cyclotron emission (ECE) were performed on T-10 with a new six-channel grating polychromator, based on the conical diffraction principle. From these measurements, highly time-resolved (25–200 μs) and space-resolved (Δx = 4 cm) direct information on the relative electron temperature was obtained. The temperature scale was calibrated by Thomson scattering and soft-X-ray measurements. Third-harmonic measurements confirmed the second-harmonic Te-profiles and from these measurements an effective wall reflection coefficient r = 0.85 for ECE radiation was deduced. Sawtooth activity gave direct information on the inversion radius, which is related to the q = 1 surface. The location of the q = 1 surface was also independently calculated from Te-profiles, using Spitzer's resistivity. The agreement between the two approaches is good, if one assumes proximity of the inversion layer and the q = 1 surface. The change of the temperature profile observed during the fast relaxation of a sawtooth excludes the evolution to a flat profile in T-10 as predicted by Kadomtsev's full reconnection model. For correlation between the central electron density ne(0), the plasma current I, and the sawtooth period ∇τsaw, the experimental scaling law ∇τsaw = (0.8 ± 0.1) × 10−36 /I was found for plasmas with Te(0) ≈ 1.2 keV. An analysis of the propagation of the heat pulse generated by internal disruptions gave an electron heat conductivity profile i n agreement with profiles deduced from power balance calculations. The absolute magnitude of the electron heat conductivity corresponds to 0.6 times the Alcator-INTOR value. The evolution of the electron temperature profile during first-harmonic ordinary-mode electron cyclotron resonance heating (ECRH) is also given. It was observed for the first time that the sawteeth are stabilized during heating outside the q = 1 radius. Furthermore, it is concluded that not only does high-intensity ECRH heat the bulk of the resonant electrons, but additionally a small (⪅0.35%) population of suprathermal trapped electrons (Tst > 6 keV) is created. The effect of pellet injection on the temperature profile is also shown.

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Two antennae have been installed in JET and operated to the maximum design capability of the generators. 4.5 MW, 10 MJ have been coupled to the plasma which heated up to a maximum stored energy of 3 NJ with central temperatures of TeO = 5 keV and Ti0 = 4 keY without increase of the relative impurity concentration. Degradation of energy confinement is observed according to an L mode scaling. Hydrogen and Helium 3 minority heating regimes give similar results.The effect of k/ shaping is discussed using a quadrupole antenna.