For "beam-target" fusion reactions, collimated measurements of the energy spectrum of one of the reaction products can provide information on the degree of anisotropy of the reacting beam ions. Analytical expressions relating the fusion spectrum to the velocity distribution of the beam ions are given. Application of the expressions to measurements of the spectrum of 15-MeV protons produced by reactions between energetic 3He ions and relatively cold deuterons during ICRF heating in the PLT tokamak indicate that the velocity distribution of fast 3He ions is peaked perpendicular to the tokamak magnetic field.

A summary of fast ion experiments in the DIII-D lokamak is given. Most of the experiments involve ∼80-keV deuterium beam ions. Deceleration of dilute fast-ion populations is accurately described by coulomb scattering theory. Fast waves with frequencies several times the deuterium cyclotron frequency interact with beam ions when the product of wave number and gyroradius k ⊥ρi is ≳1.4. Global confinement of fast ions is often excellent although sawteeth, tearing modes, and beam-driven instabilities can cause additional transport. Intense beamion populations often drive instabilities. Toroidicity-induced Alfvén eigenmodes (TAE) and somewhat lower frequency modes (originally called beta-induced Alfvén eigenmodes) are often observed in a wide variety of plasma conditions. Over 50% of the beam power is lost during strong activity. Damping mechanisms such as mode coupling or radiative damping are needed to explain the observed TAE stability threshold. The most unstable toroidal mode number agrees well with theoretical expectations, but the radial and poloidal structure of the mode and the observed beam-ion transport have not been adequately explained. The modes with frequencies below the TAE are probably two types of energetic particle modes: the resonant TAE and the resonant kinetic ballooning mode.

One of the neutral-particle spectrometers used on the Doublet III tokamak has been improved and modified for use as an active charge exchange diagnostic on the DIII-D tokamak. The spectrometer employs a He stripping cell, parallel E and B analyzer fields, and a microchannel-plate detector with 60 energy channels for each of two neutral-particle masses. Improvements to the diagnostic include better radio frequency and γ-ray shielding and complete instrumentation of all energy channels. Calibration results for H and D are similar to those reported previously. The instrument views across one of the DIII-D heating beams which can be modulated to provide local neutral doping. The analyzer is scanned shot to shot to measure neutrals with pitch angles between ∼5° and ∼110°.

During neutral-beam injection into the DIII-D tokamak, instabilities with frequencies that 'whistle' down a factor of two in a single 2 ms burst are observed between 50 and 200 kHz. The instabilities have toroidal mode numbers n=1-8 and cause the loss of beam ions from the plasma. In contrast to the usual Alfven modes, which are fluid modes of the background plasma, these instabilities seem to be beam modes that are nearly stationary in the plasma frame.

Much is known about the behavior of energetic ions in tokamak devices but much remains to be understood. Single-particle effects are well understood and provide a firm basis for extrapolation to a burning plasma. In contrast, collective effects involving fast ions are more poorly understood and extrapolations are unreliable. Collective modes of concern include toroidicity-induced and ellipticity-induced Alfvén eigenmodes, kinetic ballooning modes, and internal kink modes. In addition to these magnetohydrodynamic normal modes, there are also energetic particle modes characterized by strong dependence on the fast-ion distribution function. Although many issues are important areas of study in current experiments, five issues distinguish burning plasma experiments. First, the energetic alphas are not the dominant source of free energy for the instabilities unless the fusion power exceeds the heating power by a factor of 10. Second, the damping of the instabilities depends sensitively on mode coupling to other heavily-damped waves. The magnitude of this coupling is expected to depend on the normalized thermal gyroradius, which is much smaller in a reactor. Third, in a reactor, both the radial extent of the instabilities and the fast-ion orbit contract relative to current experiments, so the fast-ion transport will change. Fourth, when instability occurs, a larger number of modes are unstable, so the mechanism of nonlinear saturation could shift from fast-ion transport to mode coupling. Fifth, because of the extreme sensitivity of energetic particle modes to the distribution function, an isotropic alpha particle distribution function differs from anisotropic fast-ion populations. © 2002 American Institute of Physics.

The pitch-angle scattering rate of a dilute population of 75 keV deuterium ions is measured in a well-diagnosed, relatively quiet, magnetically-confined deuterium plasma. Neutral particle diagnostics detect the fast-ion density in velocity space following a short 10 ms pulse of injected beam ions. The data are compared to the classical theory of diffusion in velocity space caused by many, small-angle, Coulomb-scattering events. Within uncertainties of ≲15%, the data confirm the classical theory.

The spectra of 15 MeV protons and 14 MeV neutrons produced in the burnup of 0.8 MeV 3He ions and 1 MeV tritons through the d(3He, p)α and d(t, n)α fusion reactions contain information on the velocity distributions of the energetic 3He ions and tritons. © 1986.

The conceptual design of a neutral particle analyzer that is optimized for spatially resolved fluctuation measurements is described. Estimates indicate that a pair of these analyzers could detect fluctuations in beam-ion density associated with high β instabilities in the DIII-D tokamak.

Measurements of fluctuations in the neutron yield are made on the tokamak fusion test reactor (TFTR) with plastic scintillators. Light from the scintillators is coupled through acrylic rods or fiber-optic cables to photomultipliers operated in current mode. Discrimination against hard x rays is accomplished through comparison with the signal from a ZnS(6Li) scintillator. These measurements are useful in studies of deuterium pellet deposition, of the acceleration of beam ions during major radial compression, and of MHD instabilities. Techniques for measuring the neutral beam density profile and Qequivdt using neutron fluctuation measurements during pellet injection also have been proposed.