Table of contents

Analysis of experiments with electron cyclotron resonance heating (ECRH) requires a good knowledge of the ECRH power profile. This profile is reconstructed by analysis of the transient process after on-axis ECRH switching on in special experiments with suppressed sawtooth oscillations in the T-10 tokamak. The calculations show that the absorbed ECRH power, , determined by the change in time derivative of the electron temperature at the region of ECRH power input, and the absorbed ECRH power, , determined by the magnetic measurements, are several times different. Depending on the plasma density and plasma current, their relation, , changes from 0.2 to 0.4. Analysis of different explanations for this effect shows that adequate description of the transient process demands introduction of a ballistic jump in the total heat flux just after on-axis ECRH switching on. The effective heat diffusivity increases up to values of 10–15 m² s⁻¹ in the first 100–200 µs and decreases down to values of 1.5–2.0 m² s⁻¹ during the following 1–2 ms. Note that such a non-monotone dependence of the effective heat diffusivity cannot be described by the modern critical gradient models. It seems that plasma reacts directly to the deposited power but not to the corresponding consequences (the increase in temperature or gradients). Different physical mechanisms could be proposed for this process (partial destruction of magnetic surfaces, fast transition of information through the turbulent cell connections), but each of them needs further confirmation.

L–H transition experiments have recently been performed on JET to characterize the local edge ion parameters. Evidence of a threshold in the edge ion temperature to access the H-mode is presented and comparisons are made with edge electron temperature. These results provide the first scaling of pedestal ion temperature on JET. The threshold edge ion temperature scales approximately linearly with toroidal magnetic field and shows no dependence on electron density in the range explored. Reference shots have been repeated with the ion ∇ B drift away from the X-point and counter neutral beam injection. The L–H transition is also considered in terms of dimensionless parameter space. The effective edge ion collisionality does not appear to play a significant role in the L–H transition, providing no clear distinction between the L- and H-modes. In contrast, the normalized poloidal ion gyroradius provides a sharp H-mode boundary at any given field and current, consistent with a threshold ion temperature for H-mode access being dependent on toroidal field but not on density. However, comparison of ρ^* and with forward and reversed fields across a range of fields and currents demonstrates that these dimensionless parameters simply describe an operation trajectory for the JET L–H transition.

Active current drive with lower hybrid (LH) waves in the advanced scenario plasmas at JET-EFDA was successful after a systematic study of the coupling problems that derive from the H-mode features of the edge plasma, namely very low density and ELM activity. The LH coupling has been improved compared to the past, by making the edge plasma in front of the LH launcher denser and more uniform. Injecting deuterated methane (CD₄) from a nearby gas pipe increases the density in front of the LH launcher at least by a factor of 1.5, above the cut-off value for the LH frequency. A better matching of the plasma shape to that of the LH antenna makes the plasma ahead of the LH launcher more regular along the poloidal angle. These two techniques together have permitted a balanced supply of the three LH grills, with an average reflection below 4%, as in the previous L-mode operation. CD₄ does not affect the performances nor does it contaminate the main plasma up to the maximum flow rate so far used, and now it is routinely applied in JET (up to 4 MW have been injected for longer than 8 s) with very encouraging results for LHCD. Even though CD₄ is not suitable for ITER for tritium retention, the possibility of controlling locally and safely the scrape-off plasma density has been demonstrated.

The distribution function of ions heated in the ion cyclotron range of frequencies is obtained by solving the Fokker–Planck equation. For non-minority heating scenarios, the full non-linear Coulomb collision operator must be used in this equation. A method of resolution accounting for the complexity of this operator is presented. We consider a plasma immersed in a homogeneous magnetic field. The adopted method of resolution is based on an expansion of the distribution function in a series of Legendre polynomials. Results of the corresponding code non-linear Fokker–Planck in two-dimensional velocity space (NLFP-2D) are discussed for experimentally relevant JET-like parameters. The convergence of the Legendre expansion has been tested and the importance of the non-linear effects has been shown. Three different regimes, characterized by different modes of indirect ion heating and depending on the absorbed RF power density as well as the minority concentration, were identified. The NLFP-2D code has been used to study the validity of the Maxwellian approximation of the self-collision operator. One finds that this model is only correct in a very limited range of parameters, and leads mostly to an overestimation of the indirect ion heating.

Impurity transport during FTU electron internal transport barrier (ITB) discharges has been studied by means of a one-dimensional time dependent transport model to reproduce plasma line and continuum emissions. The impurity behaviour has been explored in two different experimental frames in which the formation and maintenance of an ITB is obtained in FTU. In the first scenario the lower hybrid and electron cyclotron resonance heating waves are launched during the current (I_p) flat-top phase, while in the second scenario the RF power is injected early during the I_p ramp-up phase.

A diffusion coefficient enhanced inside the barrier, by a factor of 10, and an inward pinch velocity linearly increasing with the radius up to 3.5 m s⁻¹, as found in the Ohmic case, are necessary to reproduce FTU plasma emission in the first scenario. A diffusion coefficient lowered inside the barrier (by a factor of 2) and about the same inward pinch velocity (v(a) = 5 m s⁻¹ in this case) have to be assumed to interpret the impurity behaviour if the RF power is injected during the I_p ramp-up phase. These diffusion coefficient profiles are similar to the ion thermal diffusivity profiles predicted by the JETTO code: an improved electron thermal conductivity at the centre but degraded ion thermal conductivity is predicted for the flat-top ITB case, while improved electron thermal conductivity in the centre with a slightly improved ion diffusion inside the barrier results in the ramp-up ITB.

In the H-mode (high-confinement mode) pedestal region of a static or a toroidally rotating D-shaped tokamak plasma, the separatrix or the X-point affects the high-n ballooning stability (n: toroidal mode number) only in a thin layer in the pedestal region. Therefore, D-shaping effectively stabilizes the ballooning modes even for a toroidally rotating plasma with a separatrix.

It is shown that in magnetohydrodynamics, the shapes of the magnetic energy and magnetic helicity spectra can influence whether the magnetic helicity resistive dissipation timescale is longer, equal to, or shorter than that of magnetic energy. The calculations highlight that magnetic helicity need not always dissipate significantly more slowly than magnetic energy. While this may be implicitly understood in detailed studies of magnetic helicity in plasma devices, it is not sufficiently emphasized in elementary discussions comparing magnetic helicity vs magnetic energy evolution. Measuring magnetic energy and helicity decay times may provide a posteriori insight into the initial spectra.