The deuterium-tritium (D-T) fusion reaction will be the energy source of a thermonuclear reactor: 2D + 3T -> 4He + 1n with 17.6 MeV energy gain released in the form of kinetic energy of the two products. The power for a self-sustained burning plasma will be provided by the slowing down of 4He ions (called alpha particles or alphas) that are produced with an energy of 3.5 MeV. Investigations into the behaviour of these fusion alphas are therefore of crucial importance for burning plasma physics. While the measurement of the 14.1 MeV D-T fusion produced neutrons 1n has been routinely carried out for several years with various neutron detectors, detecting alpha particles has remained a major challenge. The worldwide unique capabilities of JET (confinement of 3.5 MeV alphas and the use of Beryllium first wall) have recently provided a ‘natural’ solution to this challenge, as explained below. The new technique can be used not only in D-T plasmas, but also in experiments where 110 keV 4He nuclei (injected with helium doped neutral beam injectors in JET) are accelerated to energies of several MeV using Ion Cyclotron Resonance Heating, thus simulating the behaviour of fusion alphas without actually using the D-T reaction. The technique used also enables discrimination of fast alphas from fast deuterium ions, as will be needed for ITER. A simultaneous observation of both fast particles is thus possible.

The JET neutron and gamma detection system (Fig.1) consists of two fanshaped collimator arrays (cameras), one horizontal (with 10 detector channels) and one vertical (with nine detector channels). Each channel is equipped with three detectors:

• a NE213 * liquid scintillation detector, for detection of 2.5MeV D-D fusion neutrons
• a plastic scintillation detector for detection of 14MeV D-T fusion neutrons
• a special CsI(Tl) detector for detection of gamma rays

Observation of fusion alphas has become possible by detecting the gamma rays emitted by an excited 12C nucleus, resulting from a nuclear reaction between beryllium and fast alpha particles – the threshold energy for the reaction is 1.7 MeV. The presence of beryllium in JET thus allows to trace fast alphas, such as the 3.5 MeV D-T fusion alphas. In addition a two-dimensional reconstruction of both gamma-ray and neutron emissivity is possible on JET, allowing detailed spatial information on their behaviour. Fast deuterium ions are detected using another nuclear reaction between a 12C nucleus and fast deuterium ions (energy>0.8 MeV), which emits gamma rays with a different energy.

This new diagnostic method demonstrated its full potential in fusion alpha simulation experiments earlier this year. For the first time ever on a tokamak detailed information was obtained on the trajectories of fast alpha particles, as shown in Fig.1. The figure shows in addition that the alpha particles are well confined in the centre of the JET machine, and is in agreement with simulations. A reconstruction of the location of the fast deuterium ions, measured simultaneously, is shown in Fig. 2.

The density evolution of fast particles as a function of time can also be studied by observing the time evolution of the gamma-ray intensity. This offers the possibility of measuring the confinement of e.g. fast alpha particles in various plasma regimes. A classical slowing down of alpha particles in discharges at high plasma current and monotonic q-profile is observed, confirming predictions. In addition, degraded confinement of the alpha particles is observed in low plasma current discharges and plasmas with a so-called ‘Current Hole’, an effect for specific JET discharges.

* NE213 liquid scintillation detector: Neutron spectrometry is a tool for obtaining important information on the fuel ion composition, velocity distribution and temperature of fusion plasmas. A compact NE213 liquid scintillator was installed and operated at JET during two experimental campaigns (C8-2002 and Trace Tritium Experiment-TTE 2003).