J.P. Christiansen

Plasma confinement in JET H?mode plasmas with H, D, DT and T isotopes

The scaling of the energy confinement in H mode plasmas with different hydrogenic isotopes (hydrogen, deuterium, DT and tritium) is investigated in JET. For ELM-free H modes the thermal energy confinement time τth is found to decrease weakly with the isotope mass (τth ~M-0.25±0.22), whilst in ELMy H modes the energy confinement time shows practically no mass dependence (τth ~M0.03±0.1). Detailed local transport analysis of the ELMy H mode plasmas reveals that the confinement in the edge region increases strongly with the isotope mass, whereas the confinement in the core region decreases with mass (τthcore ∝ M-0.16), in approximate agreement with theoretical models of the gyro-Bohm type (τgB ~M-0.2).

The scaling of transport with normalized Larmor radius in JET

From the first principles of plasma physics, the local thermal diffusivity can be expressed in terms of the Bohm diffusivity χB and a function, F, of dimensionless parameters. Tests of transport models, theoretical, empirical or heuristic, expressed in terms of such parameters often produce ambiguous answers, which arise from collinearities in the experimental data. In experiments on dimensionally similar discharges, the scaling of confinement with one single dimensionless parameter can be examined in a more unambiguous fashion. Experiments on JET have been carried out in which only the normalized Larmor radius rho is varied. This is achieved by varying the plasma current, the toroidal field, the density, the ion cyclotron frequency and the power level in a predetermined pattern, such that other dimensionless parameters, for example the normalized collisionality ν*, β and the safety factor qψ, are kept constant. The experiments on JET demonstrate that L mode confinement, both globally and locally, scales with ρ* according to the long wavelength or Bohm scaling. The implications for theory and future experiments are outlined

Local Transport in Joint European Tokamak Edge-Localized, High-Confinement Mode Plasmas with H, D, DT, and T Isotopes

The edge-localized, high-confinement mode regime is of interest for future Tokamak reactors since high performance has been sustained for long durations. Experiments in the Joint European Tokamak [M. Keilhacker et al., Nuclear Fusion 39, 209 (1999)] have studied this regime using scans with the toroidal field and plasma current varied together in H, D, DT, and T isotopes. The local energy transport in more than fifty of these plasmas is analyzed, and empirical scaling relations are derived for energy transport coefficients during quasi-steady state conditions using dimensionless parameters. Neither the Bohm nor gyro-Bohm expressions give the shapes of the profiles. The scalings with β and ν* are in qualitative agreement with Ion Temperature Gradient theory.

High fusion power steady state operation in JET DT plasmas

Because of its large size, single null divertor and flexible magnetic geometry, JET is capable of producing the most reactor relevant plasmas of any present generation tokamak. In recent DT experiments, the fusion performance of these plasmas was tested for the first time. Over 4 MW of fusion power was produced in a high power, steady state pulse of 5 s, limited by the duration of the heating power. The fusion QE, defined simply as the fusion energy produced divided by the input energy over this 5 s interval, was 0.18. These DT ELMy H mode discharges performed up to expectations based on DD preparation pulses and thus establish a firm basis for extrapolating to a next step machine. Operation at low q95 is possible in JET with no degradation in the confinement enhancement factor and provides an improved margin to ignition when extrapolated to ITER. Considerable uncertainties remain, nonetheless. In particular, access to high density, relative to the Greenwald limit, and operation in close proximity to the H mode threshold may both result in a degradation of the confinement in the next step machine.

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.

Edge localized modes and edge pedestal in NBI and ICRF heated H, D and T plasmas in JET

On the basis of experiments carried out in JET in D:T mixtures varying from 100:0 to 5:95 and those carried out in hydrogen plasmas, the isotopic mass dependence of ELM parameters and the edge pedestal pressure in NBI and ICRF heated H mode plasmas is presented. The ELM frequency is found to decrease with the atomic mass number in both ICRH and NBI discharges. However, the frequency in the case of ICRH is about 8-10 times higher than that in the case of NBI. Assuming that ELMs occur at a critical edge pressure gradient, limited by the ballooning instability, the scaling of the maximum edge pressure is most consistent with the assumption that the width of the transport barrier scales as the ion poloidal Larmor radius governed by the average energy of fast ions at the edge. The critical edge pressure in NBI heated discharges increases with the isotopic mass, which is consistent with the higher deduced width of the edge transport barrier in tritium than those in deuterium and hydrogen. The critical edge pressure in ICRH discharges is smaller, presumably, due to the smaller fast ion contribution to the edge region. As a consequence of the edge pressure scaling with isotopic mass, the edge operational space in an ne-Te diagram increases with operation in tritium. If the evidence that the edge pedestal width is governed by the average energy of fast ions in the edge prevails, the pedestal in ITER would be controlled by the slowing down energy spectrum of alpha particles in the edge.

Performance near operational boundaries

The performance of ELMy H-mode operation in ASDEX Upgrade and JET is compared. Special attention is paid to variations (usually reductions) in this performance near the operational limits which will need to be approached in a next-step device. In JET it is found that input powers substantially above the H-mode threshold power are required to obtain discharges with energy confinement enhancement factors at or above the usual ELMy H-mode scalings. Such a margin (as much as a factor of two in JET) is not observed in ASDEX Upgrade. It is proposed that this difference may be due to the higher edge collisionality in ASDEX and the results are compared to a recent theory based on interchange instabilities and magnetic flutter. In ASDEX Upgrade, the confinement in type I ELMy discharges degrades as the density is raised due to a stiffness of the temperature profiles which leads to a degradation of the core confinement. This type of stiffness is observed in JET only at relatively high edge densities. In JET, the edge confinement degrades as the density is increased by external gas fuelling, consistent with a constant edge pressure gradient and an edge barrier width which reduces in proportion to the edge ion poloidal Larmor radius. In both machines, H-mode performance is limited at high density by a transition first to the type III ELM regime and then to the L-mode. The confinement penalty, relative to good type I ELM discharges, of operating with type III ELMs is about 25-30%. The maximum densities for operation with type I or type III ELMs can be substantially increased by increasing the plasma triangularity in both machines.

Studies of D-D fusion reactivity in high temperature jet plasmas

The D-D fusion reactivity from JET plasmas has been optimized by using combined neutral beam injection and radiofrequency heating. Reaction rates of up to 2.9 x 10(16) reactions per second (equivalent to neutron emission rates of 1.45 x 10(16) neutrons per second) have been achieved. The best results were obtained in the double-null magnetic configuration or with plasmas limited by the inner wall. By far the largest fraction of the neutron production originates from beam induced reactions, although, in some cases, about 25% of the production can be attributed to the acceleration of deuterium ions by the radiofrequency. The instantaneous neutron yield was limited by a strong influx of carbon impurity ions due to excessive, but local, wall heating.

ITER simulation experiments on JET of the H-mode power threshold, confinement scaling and beta saturation

The results of a series of ITER simulation experiments on JET are described. A series of H-mode threshold experiments are shown to reproduce one of the standard power threshold scaling expressions that is being used to predict the power threshold in ITER. Then, from a series of experiments in which the Larmor radius scaling of ELMy H-modes is examined, it is concluded that the scaling of the confinement is gyro-Bohm-like provided the power levels are well above the threshold. Finally, we show that at high there is a dramatic reduction in the confinement at .

The testing of transport models against data

Many different models for energy transport in a tokamak have been tested against subsets of the wealth of data accumulated from tokamak experiments and claimed to describe adequately the data used in the tests. Yet no single transport model has emerged as an obvious candidate for accurate extrapolations from todays large tokamak to a reactor. The principles involved in testing a model against data are examined. It is shown how: (i) collinearities in the data, (ii) confinement degradation with power and (iii) measurement errors in the data, can make tests insensitive to particular details of transport models. This lack of sensitivity is illustrated via tests of models against global confinement data from several tokamaks (ITER L mode database) and via tests of different models against local confinement data from one single tokamak (JET database). The latter tests demonstrate that the testing of transport models must include data from two or more tokamaks of different size