Table of contents

Recent results obtained during the first part of the JET D-T phase are presented. Emphasis is placed on conditions carefully set to be similar to those required in ITER. Wall loading changeover from 100% deuterium to 90% tritium was achieved in about twenty discharges with 100% tritium injection. H mode threshold power was found to have inverse isotopic mass dependence whereas there was little mass dependence of global energy confinement time in the ELM free or ELMy H mode discharges. Preliminary results are consistent with the gyro-Bohm physics form of global energy confinement time. This form has a weak negative mass dependence but a stronger density dependence than the ITERH93-P form. The second harmonic ICRH ITER reference scenario (2ω_CT with and without the addition of a small amount of ³He) was assessed in JET D-T plasmas. High performance combined heating (neutral beam + ICRH) hot ion and optimized shear regime experiments were carried out with only half the installed neutral beam power. Nevertheless, internal confinement barriers in the optimized shear regime were also demonstrated in D-T plasmas.

The paper summarizes issues and presents results of the ITER physics work in the area of energetic particles such as fusion alpha particles, energetic ions produced by plasma auxiliary heating, and runaway electrons which can be generated during plasma disruption.

Confined trapped alpha energy spectra and differential radial density profiles in TFTR D-T plasmas were obtained with the pellet charge exchange (PCX) diagnostic, which measures high energy (E_α = 0.5-3.5 MeV) trapped alphas (v_||/v = -0.048) at a single time slice (Δt ≈ 1 ms) with a spatial resolution of Δr ≈ 5 cm. Tritons produced in D-D plasmas and RF driven ion tails (H, ³He or T) were also observed and energetic tritium ion tail measurements are discussed. PCX alpha and triton energy spectra extending up to their birth energies were measured in the core of MHD quiescent discharges where the expected classical slowing down and pitch angle scattering effects are not complicated by stochastic ripple diffusion and sawtooth activity. Both the shape of the measured alpha and triton energy distributions and their density ratios are in good agreement with TRANSP predictions, indicating that the PCX measurements are consistent with classical thermalization of the fusion generated alphas and tritons. From calculations, these results set an upper limit on possible anomalous radial diffusion for trapped alphas of D_α ⩽ 0.01 m²·s^-1. Outside the core, where the trapped alphas are influenced by stochastic ripple diffusion effects, the PCX measurements are consistent with the functional dependence of the Goldston-White-Boozer stochastic ripple threshold on the alpha energy and the q profile. In the presence of strong sawtooth activity, the PCX diagnostic observes significant redistribution of the alpha signal radial profile wherein alphas are depleted in the core and redistributed to well outside the q = 1 radius, but apparently not beyond the energy dependent stochastic ripple loss boundary. The helical electric field produced during the sawtooth crash plays an essential role in modelling the sawtooth redistribution data. In sawtooth free discharge scenarios with reversed shear operation, the PCX diagnostic also observes radial profiles of the alpha signal that are significantly broader than those for supershots. ORBIT modelling of reversed shear and monotonic shear discharges is in agreement with the q dependent alpha profiles observed. Redistribution of trapped alpha particles in the presence of core localized toroidal Alfvén eigenmode (TAE) activity was observed and modelling of the PCX measurements based on a synergism involving the α-TAE resonance and the effect of stochastic ripple diffusion is in progress.

Recent results on investigations of Alfvén eigenmodes, fast ion confinement and fast ion diagnostics in JT-60U are presented. It was found that toroidicity induced Alfvén eigenmodes (TAEs) were stable in negative shear discharges with a large density gradient at the internal transport barrier (ITB). If the density gradient was small at the ITB, multiple TAEs appeared around the q = 2 surface (pitch minimum) and showed a large frequency chirping (Δf ≈ 80 kHz). In low-q positive shear discharges, the location of the TAEs changed from outside to inside the q = 1 surface, owing to a temporal change of the q profile. A significant depression of the megaelectronvolt ion population was observed only with high-n (n up to 14) multiple TAEs inside the q = 1 surface. Non-circular triangularity induced Alfvén eigenmodes were observed for the first time. Considerable depression of the triton burnup was observed in negative shear discharges. Orbit following Monte Carlo simulations indicated that ripple loss was responsible for the enhanced triton losses. The fast ion stored energies in ICRF heated negative shear discharges were comparable to those of positive shear plasmas. Tail ion temperatures in high (second to fourth) harmonic ICRF heating experiments were first analysed with an MeV neutral particle analyser. The behaviour of MeV ions produced by ICRF heating was studied with gamma ray diagnostics. A scintillating fibre detector system for detecting the 14MeV neutron emission was developed for the triton burnup studies. Ion cyclotron emission measurements discriminating between parallel and perpendicular components of the electric field were carried out for the first time.

Linear and non-linear models for anomalous alpha particle transport due to collective instabilities are reviewed. The linear stability analysis is applied to the comparison of measured and computed Alfvén eigenmodes in tokamaks (`spectroscopy'). Scenarios for non-linear wave evolution and fast ion redistribution are discussed.

The paper summarizes those aspects of the 5th IAEA Technical Committee Meeting on Alpha Particles in Fusion Research which relate to experimental measurements of fast ion confinement and heating, the measurement and interpretation of fast particle distribution functions, thermal particle transport and the measurement and interpretation of ion cyclotron emission.

The presentations at the 5th IAEA Technical Committee Meeting on alpha driven instabilities are summarized. Particular emphasis is given to the comparison between experimental results and theoretical predictions. Progress in linear and non-linear theories of alpha driven instabilities is also briefly described.

This paper summarizes the contents of the 5th IAEA Technical Committee Meeting on Alpha Particles in Fusion Research regarding alpha particle transport, and the confinement of alpha particles.

The paper summarizes the implications for ITER of the results of the 5th IAEA Technical Co-operation Meeting on Alpha Particles in Fusion Research.

Following impurity injection by laser ablation (LA) in JET plasmas, the electron temperature, T_e, is observed to drop at a rate that cannot be accounted for by changes in radiated power, ΔP_rad. T_e starts to drop promptly over a large fraction of the plasma volume, which can be explained by a non-local change in electron heat diffusivity. The change in diffusivity, Δχ_e, is generally short lived, lasting a few milliseconds, even if ΔP_rad can persist for longer times. No clear relation between ΔP_rad and the strength of the prompt plasma response to LA can be observed, but only those events with ΔP_rad/P_rad > 0.5 give rise to detectable T_e perturbations. In hot ion H mode plasmas, Δχ_e is found to be spatially non-uniform and increasing exponentially with the time delay of the LA relative to the onset of the H mode. Examples of non-monotonic Δχ_e profiles are found in plasmas of lower heating power. Energy transport models combining local and non-local (or strongly non-linear) features are possible candidates for an explanation of these observations.

There are various conditions in JET under which the energy confinement of a plasma is reduced from H mode towards L mode levels. Examples are reported here for which the loss of confinement is associated with a reduction of the electron pressure pedestal. This loss is brought about by strong gas puffing that increases the frequency of edge localized modes (ELMs). The effect of ELMs on confinement is modelled, and used to explain the observed degradation of confinement. The model is generalized for application to other machines.

The current penetration in the startup phase of the SINP tokamak discharge is investigated by using soft X ray diagnostics. It is found that the current penetration during the current rise phase is anomalous, and the results show that current driven ion acoustic instability must be the most prominent candidate for rapid current penetration.

Simplified methods of the magnetic diagnostics of tokamak equilibria are revisited. These methods allow interpretation of external magnetic measurements in the form of simple analytical formulas for physical parameters, as linear functions of measurements, similar to the ones used for high aspect ratio circular plasmas. At present, these methods can be considered as simplified versions of general ones, such as toroidal harmonics, local field expansion and function parametrization methods. A simplified function parametrization method is developed for simple non-circular plasma shapes. An error analysis of the various methods is performed by numerical simulations for the TCA/BR tokamak with moderate aspect ratio and plasma elongation up to 1.4.

In lower hybrid current drive (LHCD) experiments in JET, up to 7.3 MW of lower hybrid (LH) power has been coupled to X point plasmas, resulting in sawtooth suppression and full current drive up to 3 MA. The current drive efficiency reached η_CD = 0.26 × 10²⁰ m^-2·A/W in these experiments. The LH power deposition and driven current profiles can be quite well reproduced with the ray tracing and Fokker-Planck code in JET, even when multipass absorption of the LH wave is dominant. Profile control with off-axis LHCD has been used for increasing q above 1, thereby suppressing sawteeth before the edge localized mode (ELM)-free hot ion H mode. In optimized shear plasmas, a broad q profile with central negative shear was formed with moderate LH power (∼2 MW), applied in the low density phase during the current rampup. An internal transport barrier with improved electron confinement was produced, which resulted in a peaking of the electron temperature profile and T_e0 up to 10 keV at n_e0 ⩽ 1.5 × 10¹⁹ m^-3.

Quasi-axially symmetric tokamaks (QA tokamaks) are an extension of the conventional tokamak concept. In these devices the magnetic field strength is independent of the generalized toroidal magnetic co-ordinate even though the cross-sectional shape changes. An optimized plasma equilibrium belonging to the class of QA tokamaks has been proposed by Nührenberg. It features the small aspect ratio of a tokamak while allowing part of the rotational transform to be generated by the external field. In this article, two particular aspects of the viability of QA tokamaks are explored, namely the feasibility of modular coils and the possibility of maintaining quasi-axial symmetry in the free-boundary equilibria obtained with the coils found. A set of easily feasible modular coils for the configuration is presented. It was designed using the extended version of the NESCOIL code (Merkel, P., Nucl. Fusion 27 (1987) 867). Using this coil system, free-boundary calculations of the plasma equilibrium were carried out using the NEMEC code (Hirshman, S.P., Van Rij, W.I., Merkel, P., Comput. Phys. Commun. 43 (1986) 143). It is observed that the effects of finite β and net toroidal plasma current can be compensated for with good precision by applying a vertical magnetic field and by separately adjusting the currents of the modular coils. A set of fully three dimensional (3-D) auxiliary coils is proposed to exert control on the rotational transform in the plasma. Deterioration of the quasi-axial symmetry induced by the auxiliary coils can be avoided by adequate adjustment of the currents in the primary coils. Finally, the neoclassical transport properties of the configuration are examined. It is observed that optimization with respect to confinement of the alpha particles can be maintained at operation with finite toroidal current if the aforementioned corrective measures are used. In this case, the neoclassical behaviour is shown to be very similar to that of a conventional tokamak.