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

The scaling of the energy confinement in H-mode plasmas with different hydrogenic isotopes (H, D, D-T and T) is investigated in JET. For ELM-free H-modes the thermal energy confinement time τ th is found to decrease weakly with the isotope mass (τ th ~ M-0.25 ± 0.22) whilst in ELMy H-modes the energy confinement time shows practically no mass dependence (τ th ~ M 0.03 ± 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 (τ thcore ∝ M-0.16) in approximate agreement with theoretical models of the gyro-Bohm type (τ gB ~ M-0.2).

An experiment has been performed at the Joint European Torus (JET) which has demonstratedclear self-heating of a deuterium-tritium plasma by alpha particles produced in fusion reactions. Since the alpha power was approximately 10% of the total power absorbed by the plasma, the heating was distinguished from other changes, due to isotopic effects, by scanning the plasma and neutral beam mixtures together from pure D to nearly pure T in a hot ion H-mode with 10.5MW neutral beam power. At an optimum mixture of 60±20% T, the fusion gain (=P fusion / P absorbed ) was 0.65 and the alpha heating showed clearly as a maximum in electron temperature.

The combination of two regimes of enhanced performance, the H-mode and the pellet enhanced performance (PEP) mode, has been achieved in JET. The strong enhancement of the central plasma parameters, obtained with pellet injection and subsequent auxiliary heating, is found to persist well into the H-mode phase. A characteristic of the PEP regime is that an improvement of the fusion reactivity over non-pellet discharges is obtained under the condition of nearly equal electron and ion temperatures. A maximum neutron production rate of 0.95 × 10l6 s−1 was obtained in a double-null X-point discharge with 2.5 MW of neutral beam heating and 9 MW of ion cyclotron resonance heating, with central ion and electron temperatures of about 10 keV and a central deuterium density of 8.0 × 1019 m−3. The corresponding fusion product nD(0)τETi(0) is between 7.0 and 8.6 × 1020 m−3·s·keV. The enhanced neutron production is predominantly of thermonuclear (Maxwellian) origin. The compatibility of these regimes is an important issue in the context of tokamak ignition strategies. Several technical developments on JET have played a role in the achievement of this result: (1) the use of low voltage plasma breakdown (0.15 V/m) to permit pellet injection in an X-point configuration before the formation of a q = 1 surface; (2) the elimination of ICRH specific impurities with antenna Faraday screens made of solid beryllium; (3) the use of a novel system of plasma radial position control that stabilizes the coupling resistance of the ion cyclotron heating system.

Because of its large size, single null divertor, and exible magnetic geometry, JET is capable of producing the most reactor-relevant plasmas of any present generation tokamak. In the recent deuterium-tritium experiments the fusion performance of these plasmas was tested for the rst 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 Q E , de ned simply as the fusion energy produced divided by the input energy over this 5 s interval, was 0.18. The performance of our DT ELMy Hmode discharges extrapolates to ignition in ITER and thus provides increased con dence in its current design. Operation at low q95 is possible in JET with no degradation in con nement and provides an improved margin to ignition when extrapolated to ITER.

A new type of MHD mode , provisionally termed the Wash Board (WB) mode, has been observed during H-mode plasmas in JET. It occurs in all types of H-mode discharges, but is not seen during L-mode even at high values of β. The WB mode appears to be linked with saturation in the plasma confinement and central plasma temperatures. These modes have high m and n numbers and are localised in the outer part of the plasma, typically from the q=2 surface to the plasma edge. They rotate with the electron diamagnetic frequency and have a strong ballooning character. There is a good correlation between increasing plasma pressure and the growth of both the spectral extent and amplitude of the WB modes. Changes in the electron temperature profile also correlate well with changes in the amplitude of these modes. They are therefore regarded as a possible candidate to explain the power degradation of the empirically established H-mode scaling laws.

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 β and collisionality as the ignited ITER. The key issue of the scaling of the transport with the main dimensionless parameter ρ * is discussed in detail. Finally, possible shortcomings of the technique are examined.

Pellet enhanced performance (PEP) has been observed in a number of JET discharges at various plasma conditions, in both L and H modes, with the H multiplier (the confinement enhancement factor over the Goldston confinement time) covering the range from 1 to 4, and with plasma currents from 1 MA to 4.1 MA. Most of the PEP plasmas have been created by refuelling with pellets of 4 mm diameter injected at 1.2 km/s. PEPs show an improved central confinement with an effective heat conductivity reduced by factors of approximately 2-5 relative to otherwise comparable discharges. This is possibly related to the inverted shear in the plasma core due to the large local bootstrap current density. The limitations in the PEP performance seem to be set by at least two mechanisms: impurity behaviour, MHD activity or a combination of both. In certain discharges, MHD modes seem to be able to check the often observed impurity accumulation. Too much MHD mode activity, however, easily destroys the enhanced confinement of the PEP discharge. The stability of the ballooning modes has been studied and the PEP plasma core is found to be in the second stability region against ballooning modes or close to marginal stability. In a number of discharges complex high (m,n) modes have been observed with the soft X-ray cameras. The behaviour of the low (m,n) MHD modes can only be understood by considering the detailed evolution of the inverted q profile, which exists in a given discharge