Table of contents

Flaking of co-deposited layers on the inner limiter tiles was recently observed in TFTR. This phenomenon was unexpected and has occurred since the termination of plasma operations on 4 April 1997. Flaking affects approximately 15% of the observable tiles and appears on isotropic graphite but not on carbon fibre composite tiles. Photographic images of the flakes and precise measurements of the limiter geometry are reported. The mobilizability of tritium retained in co-deposited layers is an important factor in safety analyses of future DT reactors. A programme to analyse the flakes and tiles is underway.

Plasma was successfully built up by neutral beams alone in LHD. After injecting 1.6 MW neutral beams through a low density neutral gas target of 10^-3Pa for about 0.3 s, fully ionized plasma of 10¹⁸ m^-3 was generated. Keeping the beams on, additional gas puff could increase the plasma density sufficiently for absorption of the beam, and a dense plasma of 5 × 10¹⁹ m^-3 was sustained afterwards. This technique is specific to a helical system that has a vacuum confining magnetic field structure, and no other assisting heating tools are required such as ECH, which has been used for producing target plasma with NBI.

Tokamak energy confinement scaling in TFTR L mode and supershot regimes is discussed. The main result is that TFTR L mode plasmas fit the supershot scaling law for energy confinement. In both regimes, plasma transport coefficients increased with increased edge deuterium influx. The common L mode confinement scaling law on TFTR is also inversely proportional to the volume of wall material that is heated to a high temperature, possibly the temperature at which the deuterium sorbed in the material becomes detrapped and highly mobile. The deuterium influx is increased by: (a) increased beam power due to a deeper heated depth in the edge components and (b) decreased plasma current due to an increased wetted area as governed by the empirically observed dependence of the SOL width upon plasma current.

Increases in alpha particle loss due to MHD activity in TFTR were observed for both coherent modes and transient reconnection events using an array of scintillator detectors near the wall. The magnitude of the coherent MHD induced alpha loss was normally comparable to the MHD quiescent first orbit or TF ripple loss, but the magnitude of the alpha loss during reconnection events was up to 1000 times higher than this for a short time. Modelling suggests that the coherent MHD loss mechanism will be less significant for future reactor scale DT tokamaks due to the smaller ratio of the alpha gyroradius to the minor radius.

A natural diamond detector (NDD), developed in the Russian Federation, has been used for the first time for fast charge exchange H⁰ atom spectrometry and flux evolution measurements during H⁺ minority ICRH experiments in deuterium plasmas in TFTR. The behaviour of deeply trapped RF driven H⁺ ions was studied in quiescent plasmas and under the influence of sawtooth oscillations during central and off-axis ICRH. H⁺ ion energy spectra measured by the NDD yielded data on the efficiency of H⁺ minority ion acceleration by ICRF waves and the H⁺ ion confinement at different ICRH powers and resonance layer positions. The evolution of the hydrogen charge exchange atom flux measured by the NDD was compared with fast proton losses measured by escaping charged particle detectors to demonstrate that fast H⁺ ions are redistributed from the plasma core to well beyond the q = 1 surface. The redistribution is sufficiently far from the plasma centre for fast ions to leave the plasma due to stochastic ripple diffusion. In contrast with non-deeply-trapped fast ions, Alfvén eigenmode (AE) activity does not significantly affect the spatial and energy distributions of deeply trapped fast ions.

Simulations have demonstrated that magnetic islands having the widths expected in major disruptions cause collisionless loss of relativistic electrons and that the resultant loss rate is high enough to avoid or to suppress runaway generation. This is because, for the magnetic fluctuations in the disruptions, the loss of electron confinement due to the breakdown of toroidal momentum conservation overwhelms the runaway electron confinement due to the phase averaging effect of relativistic electrons. The simulation results provide strong support for the JT-60U experiments on fast plasma shutdown avoiding runaway generation.

Results are presented of experiments carried out in JET with the Mark II divertor to study ELMy H modes at high density. In these experiments the effective ion mass of the plasma (pure hydrogen, deuterium and tritium), input power, plasma current and edge triangularity were varied. The variation of density was achieved by gas fuelling. The focus is on two main issues: the variation of global energy confinement with density and the scaling of the H mode pedestal. It is found that the density dependence in the ITER97-P(y) scaling is not confirmed by experiment when the density limit is approached. For the high density regime in ELMy H modes a density independent scaling is proposed that includes the plasma triangularity in the regression variables. The core energy content of the plasma is correlated to the edge pedestal parameters, consistently with a mixed Bohm-gyroBohm transport model for H mode plasmas. It is found that with type I ELMs, the edge pedestal pressure increases with the ion mass of the plasma, the input power and the edge magnetic shear. A scaling is derived for the pressure at the top of the edge pedestal which is consistent with a model where the pedestal width is assumed to be proportional to the ion gyro-radius and the pedestal gradient is limited by ideal MHD.

Hot ion H mode discharges at JET are analysed using interpretative and predictive transport simulation techniques. The simulations are based on a multifluid model for ion temperature gradient driven modes, collisionless trapped electron modes and impurity modes, including effects of electromagnetic perturbations and fast particle dilution in a simple model equilibrium. It is shown that the stabilizing effects of finite-β electromagnetic perturbations are important for the simulations and allow for a unified description of bulk transport in L mode, H mode and hot ion H mode discharges at JET. In particular, it is shown that the ion heat diffusivity is substantially reduced for large values of β away from the ideal threshold, which enables the simulations to reproduce the transient ELM-free phase of the pulses with ion temperatures reaching 15-25 keV.

The ICRF heating efficiency and the global energy confinement time during ICRF heating are investigated, including the effect of energetic ion loss in heliotrons. The approximate formula of ICRF heating efficiency is derived using results based on Monte Carlo simulations (Murakami, S., et al., Fusion Eng. Des. 26 (1995) 209). The global energy confinement time including the energetic ion effect can be expressed in heliotrons in terms of ICRF heating power, plasma density and magnetic field strength. Results in plasmas at CHS show a systematic decrease of the global energy confinement time due to energetic ion loss from the assumed energy confinement scaling law, which is consistent with the experimental observations. The model is also applied to ICRF minority heating in LHD plasmas in two cases of typical magnetic configurations. A clear increase of the global energy confinement time due to the stored energy of energetic tail ions is obtained in the `orbit improved' configuration, while a decrease is observed in the `standard' configuration.

Central impurity toroidal rotation has been observed in Alcator C-Mod ICRF heated plasmas, from the Doppler shifts of argon X ray lines. Rotation velocities of up to 1.3 × 10⁵ m/s in the co-current direction have been observed in H mode discharges with no direct momentum input. There is a strong correlation between the increase in the central impurity rotation velocity and the increase in the plasma stored energy, induced by ICRF heating, although other factors may be involved. This implies a close association between energy and momentum confinement. Co-current rotation is also observed during purely ohmic H modes. In otherwise similar discharges with the same stored energy increase, plasmas with lower current rotate faster. For hydrogen minority (D(H)) heating, plasmas with the highest rotation have an H/D ratio between 5 and 10% and have the resonance location in the inner half of the plasma, i.e. in the same conditions that are conducive to the best ICRF absorption and heating. Comparisons with neoclassical theory indicate that the ion pressure gradient is an unimportant contributor to the central impurity rotation and the presence of a substantial core radial electric field is inferred during the ICRF pulse. An inward shift of ions induced by ICRF waves could give rise to a non-ambipolar electric field in the plasma core.

A first-flight neutral transport model to describe the dependence of pump plenum neutral pressure on plasma parameters and magnetic geometry is presented. It is shown that the model is in excellent agreement with neutral pressure data from a low recycling DIII-D tokamak discharge. It is also shown that the main contribution to plenum pressure arises from the part of the ion particle flux profile which is closest to the plenum entrance. This work illustrates the sufficiency of a simple model in divertor plenum hardware design studies to maximize particle exhaust for density control.

Non-Maxwellian structures superposed on Maxwellian Thomson spectra are observed with various sizes in wavelength space and in real space. These features are always seen but in some cases they are more pronounced. A new robust fitting method is presented. With this fitting procedure the non-Maxwellian structures can be investigated better than before. In the fitting method presented a transformed relative error is used for calculation of the weight given to the measured data points, instead of the normally used absolute error. The non-Maxwellian structures are investigated in relation to ablating pellets, the temperature profile and the sawtooth cycle. The detected features seem to be related to transport barriers, the qprofile, temperature profile and sawtooth cycle. Although a two temperature distribution is a rough approximation it describes some of the characteristics of the observed coherent distortions. A Thomson spectrum for a two temperature Maxwellian velocity distribution is simulated and can partly parameterize the observed features.