In the course of the Preliminary Tritium Experiment in JET, where combined deuterium and tritium neutral beam injection generated a DT fusion power of 1.7 MW, ion cyclotron emission (ICE) was measured in the frequency range Y I 180 MHz. The ICE spectra contain superthermal, narrow, equally spaced emission lines, which correspond to successive cyclotron harmonics of deuterons or alpha particles at the outer midplane, close to the last closed flux surface at major radius R -4.0 m. Above about 100 MHz the lines merge into a relatively intense continuum. The ICE signal fluctuates rapidly in time, and is extinguished whenever a large amplitude edge localized mode (ELM) occurs.In pure deuterium and mixed DT discharges ICE spectra are similar in form, but on changing from pure D to mixed D + T neutral beam injection at constant power, the intensity of the ICE rises in proportion to the increased neutron flux: this indicates that fusion alpha particlesand not beam ionsprovide the free energy to generate ICE. The JET ICE database, which now extends over a range of six decades in signal intensity, shows that the time averaged ICE power increases almost linearly with total neutron flux. The rise and fall of the neutron flux during a single discharge is closely followed by that of the ICE signal, which is delayed by a time of the order of the fusion product slowing down time. This feature is well modelled by a TRANSP code simulation of the density of deeply trapped fusion products reaching the plasma edge. Calculations reveal a class of fusion products, born in the core, which make orbital excursions of sufficient size to reach the outer midplane edge. There, the velocity distribution has a ring structure, which is found to be linearly unstable to relaxation to obliquely propagating waves on the fast Alfv6n-ion Bernstein branch at all ion cyclotron harmonics. The paper shows how ICE provides a unique diagnostic for fusion alpha particles.

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.

In tokamaks and stellarators, measurements of electromagnetic fluctuations in the presence of resonant particle drive, including fusion-produced α's, reveal the excitation of Alfvén eigenmodes (AE), related under certain conditions to a degradation in the fast-particle confinement. The balance between the drive and the background damping is investigated using active diagnostic systems to excite and measure the AE spectrum in terms of frequencies and damping rates. At JET, saddle-coil antennae drive low toroidal mode number (n < 4) AE in the range 30-500 kHz, including toroidal AE, kinetic AE, elliptical AE and global AE. Conditions for weak damping (γ /ω damp < 1%) are identified. Low-n AE appear to be strongly damped (γ /ω damp > 1%) during the creation of the magnetic X-point. In the presence of resonant fast particles, information on the instability drive is obtained: low-n modes are found to be stable in the presence of NBI with v /v A < 1. Fast ions generated by ICRH are observed to produce a drive for P ICRH > P thresh , with 2.5 MW < P thresh < 5 MW; under these conditions, intrinsically driven TAE and EAE are clearly observed in the magnetic fluctuation spectra, with no measurable effect on the plasma performance.

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.

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).