D.J. Campbell

Plasma detachment in JET Mark I divertor experiments

The experimental characteristics of divertor detachment in the JET tokamak with the Mark?I pumped divertor are presented for ohmic, L?mode and ELMy H?mode experiments with the main emphasis on discharges with deuterium fuelling only. The range over which divertor detachment is observed for the various regimes, as well as the influence of divertor configuration, direction of the toroidal field, divertor target material and active pumping on detachment, will be described. The observed detachment characteristics, such as the existence of a considerable electron pressure drop along the field lines in the scrape-off layer (SOL), and the compatibility of the decrease in plasma flux to the divertor plate with the observed increase of neutral pressure and D? emission from the divertor region, will be examined in the light of existing results from analytical and numerical models for plasma detachment. Finally, a method to evaluate the degree of detachment and the window of detachment is proposed, and all the observations of the JET Mark?I divertor experiments are summarized in the light of this new quantitative definition of divertor detachment.

Improved performance of ELMy H-modes at high density by plasma shaping in JET

We present the results of experiments in JET to study the effect of plasma shape on high density ELMy H-modes, with geometry of the magnetic boundary similar to that envisaged for the standard Q = 10 operation in ITER. The experiments described are single lower null plasmas, with standard q profile, neutral beam heating and gas fuelling, with average plasma triangularity ? calculated at the separatrix ~0.45-0.5 and elongation ?~1.75. In agreement with the previous results obtained in JET and other divertor Tokamaks, the thermal energy confinement time and the maximum density achievable in steady state for a given confinement enhancement factor increase with ?. The new experiments have confirmed and extended the earlier results, achieving a maximum line average density ne~1.1nGR for H98~0.96. In this plasma configuration, at 2.5?MA/2.7?T (q95~2.8), a line average density ~95%?nGR with H98 = 1 and ?N~2 are obtained, with plasma thermal stored energy content Wth being approximately constant with increasing density, as long as the discharge maintains Type I ELMs, up to nped~nGR (and ne~1.1nGR). A change in the Type I ELMs behaviour is observed for pedestal densities nped70%?nGR, with their frequency decreasing with density (at constant Psep), enhanced divertor D? emission and increased inter-ELM losses. We show that this change in the ELM character at high pedestal density is due to a change in transport and/or stability in the pedestal region, with the ELMs changing from Type I to mixed Type I and Type II. The similarity of these observations with those in the Type II ELM regime in ASDEX?Upgrade and with other small ELM regimes in DIII-D, JT-60U and Alcator C-MOD is discussed. Finally, we present the first results of experiments by studying in more detail the effects of the plasma boundary geometry, in particular by investigating separately the effect of the upper and lower triangularity, at high average ?. We show that the changes to the lower ? (or of the radial position of the x-point) affect the pedestal parameters, the size of ELM energy losses as well as the global energy confinement of the plasma.

Plasma stored energy and momentum losses during large MHD activity in JET

Substantial losses of plasma stored energy and toroidal ion momentum are observed in JET during large amplitude oscillating or quasi-stationary MHD activity when mode coupling effects become important. The degradation in the diamagnetic stored energy due to low m,n MHD modes increases with amplitude, reaching ΔW/W > 30% at a mode amplitude of r/Bθ > 0.4%. Favourable comparisons are made with the degradation in the incremental energy confinement time during such MHD activity as predicted by Chang and Callen. The reduction in the plasma ion toroidal momentum, from charge exchange measurements on C 6+ ions, depends on the extent of mode coupling within the plasma and on the oscillation frequency of the n = 1 mode. When r/Bθ > 0.1% for more than about 300 ms, toroidal coupling between low m,n modes together with coupling of the plasma ions to the modes by a force equilibrates the toroidal ion rotation frequency with the MHD oscillation frequency over substantial regions of the plasma, depending on the radius of the rational q surface of the coupled MHD mode. This ion mode coupling force becomes particularly apparent when the mode frequency drops to nearly zero and the ion toroidal rotation frequency also drops to zero within 100–300 ms, despite continued neutral beam injection. In such cases, the toroidal ion momentum appears to be lost electromagnetically via the MHD modes to the external structure or to fixed stray fields of the tokamak, while the plasma stored energy losses must be accounted for by other processes.

The time behaviour of the thermal conductivity during L to H and H to L transitions in JET

The change in thermal transport across the L to H transition is studied in detail for those JET high performance H-modes which have a very fast transition. It is found that in these pulses the transport changes very rapidly (<4 msecs) over a very large radial region 0.5< rho <1, and a very large transport barrier is formed. The reasons for the formation of this barrier are discussed.

A review of the dimensionless parameter scaling studies

The theoretical basis of the dimensionless parameter scaling technique is derived and the limitations in its application are discussed. The use of the technique is illustrated by the production on JET of a steady-state ITER similarity pulse having the same and collisionality as the ignited ITER. The key issue of the scaling of the transport with the main dimensionless parameter is discussed in detail. Finally, possible shortcomings of the technique are examined.

Evolution of transport through the L-H transition in JET

The evolution of the energy, momentum and particle transport through the L-H transition are determined in JET NBI heated discharges. Both normal and periodic L-H transitions are studied. It is found that all of the transport coefficients drop at the transition over a wide radial region and not just in the edge region as was previously thought to be the case. Indeed it is shown by two different modelling techniques that the conventional model in which the transport changes in a narrow region at the edge cannot explain the time behaviour of the electron temperature. Measurements of the fluctuation level by reflectometry also show a very fast drop over a wide radial region

Low particle confinement H-mode observed during ICRF heating on JET

During H-mode experiments with ion cyclotron resonance frequency heating on JET it was observed that the influxes of deuterium and of impurities can substantially modify the quality of both energy and particle confinement. In particular, a low particle confinement H-mode (LPCH-mode) was triggered either by deuterium input into the X-point region or as a result of a moderate impurity influx into the plasma boundary. In the LPCH-mode the particle confinement is at least three times less than in a normal H-mode. The electron density, the deuterium density and the radiated power are also less than in a normal H-mode. However, the deuterium dilution reaches a steady state. The approximately 20% decrease in the plasma energy content is due to the drop in density. The H-mode duration is extended to 2.8 s by the LPCH-phase and is limited only by the length of the heating pulse. The LPCH-mode particle confinement is linked to L-mode-like particle diffusion within the radius 0.5 < ρ < 0.8 and to the H-mode-like barrier due to the large ratio of the inward convective velocity to diffusion coefficient in the plasma boundary. This mode has the potential for substantially extending the H-mode phase to make it more reactor relevant

Divertor performance on carbon and beryllium targets in JET

The dependence of impurity production and retention on the divertor density, on the power flow into this region as well as on the X-point to target distance are investigated. Model predictions suggest a good impurity retention above a certain divertor (scrape-off) density threshold, which is dependent on heating power. In our experiments pre-programmed midplane or X-point gas puffs were used to scan the density, as well as to avoid the depletion of particles from the divertor and the scrape-off during H-modes. The gas puffs reduce T e and increase N e in particular at the outer strike zone. In general the Be as well as the C influx increases with density, which is understood from the T e ( T i ) dependence of the sputtering yields. The impurity retention shows the expected improvement with increasing scrape-off (divertor) density as well as with increasing X-point to target distance (connection length).

High temperature L- and H-mode confinement in JET

The energy confinement properties of low density, high ion temperature L- and H-mode plasmas are investigated. For L-mode plasmas it is shown that, although the global confinement is independent of density, the energy confinement in the central region is significantly better at low densities than at higher densities. The improved confinement appears to be associated with the steepness of the density gradient. For the H-mode phase, although the confinement at the edge is dramatically improved, which is once again associated with the steep density gradient in the edge region, the central confinement properties are essentially the same as for the standard L-mode. The results are compared in a qualitative manner with the predictions of the ion temperature gradient instability theory and appear to be in disagreement with some aspects of this theory.

Comparison between experimental and theoretical conditions for the L-H transition in JET

The transition to H-mode is conditional to the achievement of certain critical conditions, generally dependent on both local quantities and global plasma parameters. A physical understanding of the L-H transition process cannot improve without the development of theoretical models and their testing against such experimental evidence. The aim of the present paper, therefore, is to compare some of the physics-based theoretical models with the experimental conditions for the L-H transition in JET plasmas. The results show that although the models considered can offer a partial understanding of the processes involved in the formation of the edge transport, none of them can uniquely explain all the results.