Type-I edge-localized modes (ELMs) have been mitigated at the JET tokamak using a static external n=1 perturbation field generated by four error field correction coils located far from the plasma. During the application of the n=1 field the ELM frequency increased by a factor of 4 and the amplitude of the D(alpha) signal decreased. The energy loss per ELM normalized to the total stored energy, DeltaW/W, dropped to values below 2%. Transport analyses shows no or only a moderate (up to 20%) degradation of energy confinement time during the ELM mitigation phase.

No abstract

High fusion power experiments using DT mixtures in ELM-free H mode and optimized shear regimes in JET are reported. A fusion power of 16.1 MW has been produced in an ELM-free H mode at 4.2 MA/3.6 T. The transient value of the fusion amplification factor was 0.95±0.17, consistent with the high value of nDT(0)τEdiaTi(0) = 8.7 × 1020±20% m-3 s keV, and was maintained for about half an energy confinement time until excessive edge pressure gradients resulted in discharge termination by MHD instabilities. The ratio of DD to DT fusion powers (from separate but otherwise similar discharges) showed the expected factor of 210, validating DD projections of DT performance for similar pressure profiles and good plasma mixture control, which was achieved by loading the vessel walls with the appropriate DT mix. Magnetic fluctuation spectra showed no evidence of Alfvénic instabilities driven by alpha particles, in agreement with theoretical model calculations. Alpha particle heating has been unambiguously observed, its effect being separated successfully from possible isotope effects on energy confinement by varying the tritium concentration in otherwise similar discharges. The scan showed that there was no, or at most a very weak, isotope effect on the energy confinement time. The highest electron temperature was clearly correlated with the maximum alpha particle heating power and the optimum DT mixture; the maximum increase was 1.3±0.23 keV with 1.3 MW of alpha particle heating power, consistent with classical expectations for alpha particle confinement and heating. In the optimized shear regime, clear internal transport barriers were established for the first time in DT, with a power similar to that required in DD. The ion thermal conductivity in the plasma core approached neoclassical levels. Real time power control maintained the plasma core close to limits set by pressure gradient driven MHD instabilities, allowing 8.2 MW of DT fusion power with nDT(0)τEdiaTi(0) ≈ 1021 m-3 s keV, even though full optimization was not possible within the imposed neutron budget. In addition, quasi-steady-state discharges with simultaneous internal and edge transport barriers have been produced with high confinement and a fusion power of up to 7 MW; these double barrier discharges show a great potential for steady state operation. © 1999, Euratom

Edge Localised Modes (ELMs) are universally recognised as one of the greatest threats to the viability of ITER and future fusion power plants based on the tokamak concept. They are plasma relaxations driven by MHD modes and are thought to originate in the steep pressure gradient region of the edge transport barrier characteristic of H-mode plasmas. In ITER, extrapolations from JET predict that Type I ELMs in the Q DT = 10 baseline scenario will expel between 3-8% of the 350 MJ plasma stored energy, depositing energy fluxes of 0.6 -3.4 MJm -2 on the divertor targets [1]. Only at the lowest values of this energy range would the subsequent target erosion be tolerable. Of late, concerns are being raised not just for the divertor targets, where most of the ELM energy is intercepted, but also for the main chamber walls to where ELM power fluxes are now known to extend.The mechanisms governing the ELM origin location and non-linear evolution within the H-mode pedestal and the subsequent cross-field propagation within the scrape-off layer (SOL) remain the subjects of keen debate. Once in the SOL, however, the thermal energy within the filament is removed predominantly by parallel losses to divertor targets, a process which is better understood but which is nevertheless complex, comprising both kinetic and fluid effects. This contribution aims to demonstrate how experiments and modelling at JET are significantly advancing our understanding of the ELM SOL parallel transport, providing many of the key elements required for an integrated, quantitative treatment of the ELM energy fluxes and their subsequent consequences for plasma-wall interaction.Infra-red thermography is extensively employed for divertor target measurements at JET and has recently been complemented by a unique new wide angle view of the main chamber wall surface. Such measurements are technically challenging due to the fast transient nature of the ELM and the presence of thin surface layers, which can differ radically from surface to surface. Taking proper account of this reveals that ELMs deposit energy preferentially (~ factor 2) in the outer target at low pedestal collisionality (ν*) but that this ratio inverts in favour of inner target energy deposition (up to ~ factor 3) with rising ν*.These target plate measurements of the ELM heat flux transient, in combination with fast triple Langmuir probe data, have provided the first known experimental evidence for

Combined analysis of divertor thermocouple and IR camera measurements during JET disruptions can provide valuable information on the distribution of the energy loads, even if the stored energy of the JET plasmas is small compared to that foreseen for the next-generation tokamaks. Typically the energy collected at the divertor represents a small fraction of the pre-disruption plasma energy; this is consistent with the high level of radiation observed and with part of the magnetic energy being transferred to the plasma-coupled conductors.The data for this paper are taken from the whole set of disruptive plasmas of JET operation in the years 2000 and 2001. In most of the MkIIGB disruptions, the plasma displaces upwards (away from the divertor); therefore, only a small number of downward events are available for analysis. However, divertor heat loads seem to be more strongly correlated to the delay of the loss of the X-point with respect to the thermal quench than the direction of the plasma displacement.When the plasma thermal energy is lost with the plasma still in X-point configuration, the septum and the tiles wetted by the strike-points, often more than one tile per strike-point, experience a sharp increase in temperature, equivalent to up to 1 MJ m −2 . When the thermal quench occurs at the same time as, or after, the loss of plasma vertical control, no significant divertor tile temperature increase can be observed for both upwards and downwards events. Most of the disruptions purposely made to produce runaway electrons went towards the divertor and, although not systematically, lead to local (mostly at the septum) temperature increase equivalent to a load up to 2 MJ m −2 , often toroidally asymmetric.