Results are presented from a series of dedicated experiments carried out on JET in tritium, DT, deuterium and hydrogen plasmas to determine the dependence of the H mode power threshold on the plasma isotopic mass. The Pthr ∝ Aeff-1 scaling is established over the whole isotopic range. This result makes it possible for a fusion reactor with a 50:50 DT mixture to access the H mode regime with about 20% less power than that needed in a DD mixture. Results on the first systematic measurements of the power necessary for the transition of the plasma to the type I ELM regime, which occurs after the transition to H mode, are also in agreement with the Aeff-1 scaling. For a subset of discharges, measurements of Te and Ti at the top of the profile pedestal have been obtained, indicating a weak influence of the isotopic mass on the critical edge temperature thought to be necessary for the H mode transition.

A number of experiments with heating of deuterium-tritium (D-T) plasmas using waves in the ion cyclotron range of frequencies (ICRF) have been carried out at the Joint European Torus (JET). The results of these experiments have been analysed by comparing experimentally measured quantities with results of numerical simulations. In particular, four scenarios have been examined: (1) heating of minority (~5−20%) deuterons at the fundamental ion cyclotron frequency, ω ω = cD ; (2) second harmonic heating of tritium, ω ω = 2 cT ; (3) fundamental minority heating of 3 He with a few percent of 3He, and (4) second harmonic heating of deuterium, ω ω = 2 cD . An important aim of the analysis is to assess if the present understanding of the ICRF physics is adequate for predicting the performance of ICRF in D-T plasmas. In general good agreement between experimental results and simulations is found which increases the confidence in predictions of the impact of ICRF heating in future reactors. However, when a relatively high deuterium concentration was used in the ω ω = cD scenario, discrepancies are observed. In order to increase confidence in the simulations, we have studied the sensitivity of the simulation results to various plasma parameters.

ABSTRACT.Reactor relevant ICRH scenarios have been assessed during D-T experiments on the JET tokamak using H-mode divertor discharges with ITER-like shapes and safety factors. Deuterium minority heating in tritium plasmas was demonstrated for the first time. For 9% deuterium, an ICRH power of 6 MW gave 1.66 MW of fusion power from reactions between suprathermal deuterons and thermal tritons. The Q-value of the steady state discharge reached 0.22 for the length of the RF flat top (2.7 s), corresponding to three plasma energy replacement times. The Doppler broadened neutron spectrum showed a deuteron energy of 125 keV which was optimum for fusion and close to the critical energy. Thus strong bulk ion heating was obtained at the same time as high fusion efficiency. Deuterium fractions around 20% produced the strongest ion heating together with a strong reduction of the suprathermal deuteron tail. The edge localised modes (ELMs) had low amplitude and high frequency and each ELM transported less plasma energy content

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).

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