JET takes off to new heights in fusion performance

JET (Joint European Torus) – flagship of the European Community Fusion Programme aimed at developing magnetic confinement fusion as a safe and environmentally-friendly energy source – is, of all existing fusion facilities, closest in size and working conditions to the International Thermonuclear Experimental Reactor (ITER). ITER is currently in the detailed engineering design phase, under the quadripartite ITER agreement between the European Union, Japan, Russia and the USA. JET’s results on the scaling of plasma behaviour, together with those from smaller tokamaks, have been crucial for predicting the size and heating requirements of ITER.

D–T EXPERIMENTS

A key aspect of fusion research – and one which JET alone can now study – is operation with the deuterium–tritium (D–T) fuel mixture of a future fusion power station. The autumn of 1997 saw a broad-based series of D–T experiments which allowed JET to address crucial issues of D–T technology and physics for ITER and to set new world records for fusion power, fusion energy and ratio of fusion power produced to input power (the “fusion amplification factor, Q”).

In November 1991, for the first time ever, controlled fusion power (peaking at 1.7MW and averaging 1MW over 2 seconds) was produced in a laboratory plasma at JET, using a dilute fuel mixture of just 10% tritium in deuterium. Since then, the US Tokamak Fusion Test Reactor (TFTR) has operated with D–T and, using the optimum D–T mixture (50% tritium in deuterium), has produced 10.7MW of fusion power and a fusion Q of 0.27. TFTR also showed that under certain conditions, the thermal insulation of D–T plasmas could be better than that of pure deuterium plasmas. TFTR has now shut-down, having reached the end of its useful lifetime.

During this same period, JET carried out several configuration changes to test the effect of divertor geometry for ITER and, in late September 1997, JET resumed D–T operation to address specific questions relating to ITER Technology, ITER Physics and Fusion Power Production.

DEMONSTRATING ITER TECHNOLOGIES

The technology mission of these D–T experiments was to demonstrate key ITER and reactor-relevant technologies of: tritium handling, processing and mixture control; remote handling and control; and heating systems suited to D–T operation.

With its Active Gas Handling System, JET has tested the first large scale plant for the supply and processing of tritium in a closed cycle which includes an operating tokamak. This plant has supplied and reprocessed around 100g of tritium, allowing the repeated use of the 20g of tritium on site for these experiments. It should be noted that the tritium actually consumed by the fusion reactions was very small, a total of about 1mg. Nevertheless, 675 MJ of fusion energy were produced.

An important aspect of D–T operation was the measurement and control of the D–T mixture in the plasma. This series of experiments demonstrated that the D–T mixture could be easily controlled, in line with the predictions of a model based on the JET D–T experiments of 1991. The model therefore provides a good basis for extrapolation to ITER. Furthermore, the two week period which followed the D–T experiments gave valuable information on the torus tritium inventory and the removal of tritium from plasma facing surfaces inside JET.

The injection of high energy neutral beams is currently the primary method of heating plasmas to the temperatures necessary for fusion, and has also proven itself in D–T plasmas. In addition, heating methods, based on various radio frequency schemes, have been used. In this series of D–T experiments, two ITER reference heating schemes at the ion cyclotron resonance frequency have now been successfully tested in D–T: heating at the second harmonic of tritium and at the fundamental resonance of a minority concentration of deuterium in a tritium plasma. In the first scheme, the addition of a small (10%) concentration of He3 has allowed high ion temperatures (150 million degrees) to be achieved and, in the second with 4.7MW of radio frequency heating, the fusion Q reached 0.25 and remained steady above 0.22 for 2s, terminating only when the heating power was switched off.

ITER PHYSICS

The physics mission of this series of D–T experiments had the crucial objective of studying the effect of the heavier D–T and pure tritium plasmas (the “isotopic effect”) on the heating power needed to access conditions with better thermal insulation (the high confinement “H-mode”) and on the dependence of the thermal insulation on plasma conditions. The experiments used the same plasma configuration as ITER (including a “divertor” which channels the plasma exhaust to a remote target and pump, see NEI, December 1993) and the standard mode of operation foreseen for ITER, the “ELMy H-mode”. The experiments were the first of their kind, and allowed more accurate assessments of the ignition margin and the heating requirements for ITER.

The most notable result was that, in comparison with previous experiments in pure deuterium, the heating power needed to access the H-mode is lower in D–T and pure tritium, roughly as the inverse of the atomic mass. This is a very significant result since it predicts a 50% reduction in the power needed to access conditions with high thermal insulation in a pure tritium plasma (for example, during the start-up of the discharge) and a 25% reduction in the power needed to maintain such conditions during high fusion power operation of ITER, thereby increasing operational flexibility.

On the other hand, these JET experiments with the standard ITER conditions did not show the better thermal insulation observed with D–T under some TFTR operating conditions. On the contrary, the JET data indicates that the favourable mass dependence, assumed in projections to ITER, should be removed – a result which is more in line with theory. Nevertheless, the prediction for the thermal insulation of ITER still stands, because the lack of a mass dependence is compensated by a more favourable plasma density dependence observed in these experiments.

FUSION POWER PRODUCTION

With high levels of heating power, the ITER standard ELMy H-mode has been maintained under steady-state conditions for the full duration (3 seconds) of the applied heating power. In doing so, a world record fusion energy (21.7MJ) was produced. It should be noted that JET is the only device which can study in D–T these conditions foreseen for ITER.

In the traditional mode of high performance operation, the “hot ion ELM-free H-mode”, world records of fusion power (peaking at 16.1MW and remaining above 10MW for 0.7s) and fusion Q (0.65 and 0.9, transiently) have been set in D–T (typically with 50% tritium). Under these conditions plasma self-heating by the a-particles produced in the D–T fusion reaction made a significant contribution to the plasma power balance. In fact, specially designed experiments showed that the plasma temperature and energy content were consistently higher with higher levels of a-particle heating. Thus, the process by which ignition and thermonuclear burn will occur in ITER has been experimentally confirmed in JET.

Finally, experiments in JET and other tokamaks with pure deuterium have shown the benefits of actively controlling the spatial distribution of plasma current and pressure. In particular, the thermal insulation of the core plasma can be improved in the “optimised shear mode” of operation which had not been, hitherto, demonstrated in D–T. A series of JET experiments in D–T showed that the active control scenario used with pure deuterium plasmas had to be modified for use with D–T. Full optimisation was not possible in the time available for these experiments, but improved thermal insulation of the core plasma was demonstrated for the first time in D–T and 8.2MW of fusion power was produced. The potential of this relatively new mode of operation has thus been demonstrated.

FUTURE JET PROGRAMME

This series of D–T experiments in JET has now been completed and the JET experimental programme has continued with a period of specific ITER physics studies in hydrogen (the “isotopic effect”) and deuterium. The third stage of the JET divertor programme for ITER began on 2 February 1998 with the fully remote handling installation of an ITER-specific divertor target assembly. This will take place (over 4 months) without man-access and will demonstrate, for the first time, one of the central technologies which is vital for both International Thermonuclear Experimental Reactor and a fusion power station.

The Joint European Torus experimental programme will then continue with ITER-specific divertor studies, general ITER physics studies and plasma optimisation in pure deuterium plasmas before a further period of D–T studies. The scope of this programme will be limited by the duration of the presently approved JET programme (that is, to the end of 1999) and the possible use of the JET facilities beyond 1999 as a part of the European Fusion Programme is therefore being discussed.