The fast ␣-particle kinetic effects in fusion plasmas of deuterium and tritium are studied in the perspective that they can give rise to minority populations of fast fuel ions. The resulting modification of the neutron emission spectrum is computed for a plasma in the state of steady thermonuclear burn of conditions similar to those envisaged for the planned ITER tokamak. The nuclear interaction in these scattering ␣-particle knock-on processes is taken into account explicitly and the dependence on plasma parameters is investigated. The findings provide evidence that neutron spectrometry is a potential diagnostic of the fast ␣-particle population in burning fusion plasmas. ͓S1063-651X͑97͒06602-6͔

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Suprathermal fuel ions from alpha-particle knock-on collisions in fusion DT plasmas are predicted to cause a weak feature in the neutron spectrum of d+t-->alpha+n. The knock-on feature has been searched for in the neutron emission of high ( >1 MW) fusion-power plasmas produced at JET and was found using a magnetic proton recoil type neutron spectrometer of high performance. Measurement and predictions agree both in absolute amplitude and in plasma-parameter dependence, supporting the interpretation and model. Moreover, the results provide input to projecting alpha-particle diagnostics for future self-heated fusion plasmas.

The first three moments of the energy distributions of products from fusion reactions in thermonuclear plasmas with Maxwellian ion velocity distributions are determined analytically. Relativistic kinematics is used allowing the desired accuracy to be reached, which is 2 to 3 orders of magnitude better than previous analytical results. In particular, neutron spectra of the reactions D(d,n)3He and D(t,n)α for plasma ion temperatures 0 < Ti < 100 keV are studied for which the results are also given in tabulated form with interpolation formulas for the purpose of practical use. The neutron energy distributions are also calculated with numerical methods, in order to assess the analytical results. High accuracy calculations are motivated by the crucial role that neutron measurements are envisaged to play in the next step fusion experiments on burning plasmas, which are also discussed.

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

Fusion product measurements planned for ITER are reviewed from the viewpoint of alpha particle-related physics studies. Recent advances in fusion plasma physics have extended the desirable measurement requirements to the megahertz region for neutron emission rate, better resolution of neutron profiles for the study of internal transport barriers (ITBs), etc. Employing threshold counters and/or scintillation detectors confers megahertz capability on neutron emission rate measurement. The changes in the neutron/alpha particle birth profile due to the formation of ITB and its deviation from uniformity on the magnetic flux surface can be measured by addition of eight viewing chords in an equatorial port plug and seven viewing chords from the divertor to the original radial neutron camera. On the other hand, it is still difficult to measure the distributions of confined and escaping alpha particles. Several proposals to resolve these difficulties are currently under investigation.

Recent JET experiments have been devoted to the study of (3 He)-D plasmas involving radio frequency (RF) heating. The present paper starts by discussing the RF heating efficiency theoretically expected in such plasmas, covering both relevant aspects of wave and of particle dynamics. Then it gives a concise summary of the main conclusions drawn from recent experiments that were either focusing on studying RF heating physics aspects or that were adopting RF heating as a tool to study plasma behaviour. Depending on the minority concentration chosen, different physical phenomena are observed. At very low concentration (X[ 3 He] < 1%), energetic tails are formed which trigger MHD activity and result in loss of fast particles. Alfvén cascades were observed and gamma ray tomography indirectly shows the impact of sawtooth crashes on the fast particle orbits. Low concentration (X[ 3 He] < 10%) favors minority heating while for X[ 3 He] >> 10% electron mode conversion damping becomes dominant. Evidence for the Fuchs et al. beating effect [Fuchs et al., Phys. Plasmas 2 (1995) 1637-1647] on the absorption is presented. RF induced deuterium tails were observed in mode conversion experiments with large X[ 3 He] (18%). As tentative modeling shows, the formation of these tails can be explained as a consequence of wave power absorption by neutral beam particles that efficiently interact with the waves well away from the cold D cyclotron resonance position as a result of their substantial Doppler shift. As both ion and electron RF power deposition profiles in (3 He)-D plasmas are fairly narrow-giving rise to localized heat sourcesthe RF heating method is an ideal tool for performing transport studies. Various of the experiments discussed here were done in plasmas with internal transport barriers (ITBs). ITBs are identified as regions with locally reduced diffusivity, where poloidal spinning up of the plasma is observed. The present know-how on the role of RF heating for impurity transport is also briefly summarized. IntroductIon: Why doIng 3 he experIments? For experiments with ion cyclotron resonance heating (ICRH), using 3 He as a minority gas has a number of advantages. Because of its relatively high mass, it is more difficult to heat 3 He than the most commonly used minority gasH to high energies. Hence RF heated tails are less likely to form for a given amount of coupled RF power. Because the 3 He population's critical velocity is high, the power collisionally transferred from the heated minority to the bulk mainly ends up in the ion channel. Increasing the minority concentration results in fading out the minority heating and fadingin mode conversion heating, which allows creating a highly localized electron heat source through the very efficient electron absorption that the essentially electrostatic ion Bernstein wave undergoesas soon as it is excited near the ion-ion hybrid resonance layer. Both in the minority and in the mode conversion heating schemes relatively narrow power deposition profiles are obtained: 3 He ions stick

The neutron emission from (3He)D plasmas with RF heating is calculated using a model that includes supra-thermal (knock-on) components of the deuteron population. The RF generation of fast 3He ions is described and the knock-on components were determined with the help of newly derived 3He + d scattering cross sections. Results are presented on the neutron emission spectrum and its contributions from different deuteron velocity components. It is shown that knock-on leaves an observable feature in the spectrum with a clear dependence on absorbed RF power. The importance of the nuclear interaction in the elastic cross section is demonstrated. The results represent a step forward in the use of neutron emission spectroscopy to diagnose fusion plasmas with minority supra-thermal components in their fuel ion composition.