Table of contents

The results of a study of the statistical space/time properties of the turbulence observed in the edge region of the Pretext tokamak are reported. A local estimate of the wavenumber frequency spectra S(κ,ω) for poloidal (k_y)an d toroidal (k_z) wavenumbers is determined for the range 0.5 cm⁻¹ < k < 7.5 cm⁻¹, for both potential and density fluctuations obtained from spatially fixed Langmuir probe pairs. The S(k_y, ω) spectra aredominated by low frequencies ( < 200 kHz) and small wavenumbers (< 3 cm⁻¹) and appear broadened about an approximately linear statistical dispersion relation, (ω). The broadening is characterized by a spectral width σ_k (ω) (root-mean-square deviation about (ω)) which is of the order of (ω). The turbulence appears to propagate poloidally with an apparent mean phase velocity of (1–2) × 10⁵ cm·s⁻¹ in the ion diamagnetic drift direction. Measurements of the fluctuation-induced particle transport reveal the particle flux to be outward and resulting primarily from the low-frequency, long-wavelength components of the turbulence. A particle diffusion coefficient D_x, estimated from the measured particle flux and density profiles, is of the order of the coefficient estimated for Bohm diffusion, but appears to increase with density.

A general variational principle for an ICRF antenna radiating into a slab plasma is presented. The model is three-dimensional and includes the effect of connections to a transmission line. It also assumes an extent of absorption in the plasma that is sufficient to suppress eigenmodes. The variational principle gives the self-consistent currents flowing in the antenna, the fields excited inside the plasma and the antenna impedance at the generator terminals. Numerical computations are made for a TFR and a JET antenna. A study of power coupling shows that the optimum operating frequency lies near the plasma-modified resonance. The fields excited inside the plasma are found to disperse far more in the toroidal than in the poloidal direction. Also, at frequencies far from resonance, the large currents in the connections excite fields of considerably larger spatial extent than at resonance.

The longitudinal confinement of a plasma in a pulsed multi-mirror fusion reactor is investigated. It is shown that the initial plasma energy W_p per unit of reactor cross-section and the initial plasma temperature T₀ serve as similarity parameters. One-dimensional numerical calculations of plasma flow along the reactor axis were performed for various W_p and T₀ and were used to determine the plasma gain function Q_E(W_P, T₀). The region of maximum values of Q_E lies near T₀ = 5 keV for all W_p. Plasm a heating by α-particles becomes substantial for values of Q_E exceeding unity. At W_p = 20 MJ·cm⁻², Q_E calculated with plasma heating by α-particles exceeds Q_E as calculated without including such heating by a factor of five. If the mirror ratio is increased by enlarging the cross-section towards the reactor ends, Q is further increased by a factor of two. A value of Q_E = 1 is achieved at an initial plasma energy of W_p ≈ 5 MJ·cm⁻², and Q_E = 10 is achieved for W_p W_p≈12 MJ·cm⁻².

The suppression behaviour of magnetic island growth using electron cyclotron heating and lower hybrid current drive is analysed. A quasi-linear model is developed for island suppression, including the local peaking effect of the resistivity and the RF-driven current within the island. Pulsed launching of lower hybrid waves synchronized with the plasma diamagnetic rotation frequency can strongly suppress the magnetic island.

Microballoons filled with an equimolar deuterium-tritium mixture and coated with a plastic ablator of variable thickness are imploded by the eight-beam Octal laser (λ = 1.06 μm; 0.6 TW). An X-ray diagnostic with space-time resolution is used to analyse the implosion of the targets designed to the highest ∫ ρdr. The experimental results are compared with numerical simulations performed by a one-dimensional Lagrangian code. A DT density of about 2 g·cm⁻³ is obtained; the transition from an exploding-pusher regime to a more ablative one is analysed on the basis of the evolution of the preheat, the hydrodynamic efficiency and the density and temperature performance.

An extensive numerical study has been carried out for internal modes with toroidal mode number unity. These are internal kink modes, when the q = 1 surface falls within the plasma, and have a ballooning characteristic when q_axis > 1 Both modes show a dependence on the pressure and have a second region of stability at high β. A parameter survey has been conducted, varying the geometry, i.e. aspect ratio, ellipticity, triangularity, etc. and the current profiles, through the pressure and safety factor. The principal results show that the modes are dependent on the geometry and are strongly stabilized by high-order, noncircular effects. Broader pressure profiles and reduced shear are favourable for limiting the instability.

The problem of RF field excitation and absorption in an inhomogeneous plasma cylinder with axial uniform direct current is investigated. The RF power absorption is connected with the conversion of the incident wave generated by a helical antenna in the slow Alfvén (kinetic) mode in the vicinity of the point of 'local Alfvén resonance' r = r_A. Effects of the plasma gyrotropic properties are included. Analytical expressions for RF field amplitudes and absorbed RF power, W, are obtained in terms of series expansions. If the axial current is large enough, sharp resonance curves, W(r_A), are possible, with the maxima near the plasma centre. These maxima virtually coincide with the kink mode frequencies of the plasma column.

The ion cyclotron harmonic damping effects on slow and fast waves in the lower hybrid frequency range for tokamak reactor parameters are studied. Inclusion of the higher-order terms in the hot-plasma dielectric tensor introduces ion cyclotron harmonic damping; these terms also contribute to the real part of the dispersion relation and affect the wave trajectories. However, wave absorption by 15 keV deuterium and tritium ions can be avoided by choosing the slow-wave frequency above the lower-hybrid frequency and the fast-wave frequency below the lower hybrid frequency. But preliminary estimates show that energetic alpha particles tend to absorb both the slow and the fast waves. This absorption may become a serious obstacle to fusion-reactor current drive in the lower hybrid frequency range.

The mechanism of collisionless transformation of the orbits of trapped and localized particles as a result of toroidal drift is studied. The probabilities of such a transformation are calculated. The plasma regions in which this mechanism leads to the formation of qualitatively different orbits are determined. A method for a kinetic description of the effects associated with the transformation is proposed. Localized particle transport in situations where the collision frequency is low is discussed. Expressions are obtained for fluxes of localized particles in the internal region of the plasma in which closed orbits are formed.

After more than five years of operation of the T-10 tokamak a visual inspection of the internal liner surface was made and many arc tracks were found and photographed. Both the distribution and the direction of the tracks suggest the existence of an m = 1, n = 1 plasma mode lasting for at least 1 ms. A simple model is proposed to match the observed structures and to yield information about the plasma column.

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. Early measurements of the spectrum of 15 MeV protons produced by reactions between energetic ³He ions and relatively cold deuterons during fast-wave minority heating in the PLT tokamak found an asymmetric, downward-shifted fusion spectrum. New spectral measurements with a thicker detector that collects the full proton energy indicate that the velocity distribution of fast ³He ions is peaked perpendicular to the tokamak magnetic field.

Beryllium rail limiters were inserted into the plasma of the tokamak Unitor to study the behaviour of this material. The power load onto the limiter surface was about 2 k^W·cm⁻². The occurrence of heavy (Cr) and light (O) impurities was monitored spectroscopically. It was found that the contamination by heavy impurities stemming from the first wall (Cr) was reduced by a factor of ten in the course of a few subsequent shots. In comparison with other limiter materials (graphite, TiC, spinel, Ni, Al₂O₃) the number of disruptive discharges was considerably reduced. After breaking the vacuum and opening the vessel, the beryllium deposition on the wall and the various components (pumps, mass spectrometer, etc.) was studied. No volatile beryllium dust could be detected by samplers. The released beryllium was found to be quantitatively condensed on the torus wall with a thickness of ten monolayers after 1000 tokamak shots.

The confinement time for an Ohmically heated, low-β tokamak is well described by the scaling law τ ∝ n. This implies that β should increase linearly with additional auxiliary heating power. In fact, the increase is much slower, and confinement deteriorates with increasing auxiliary heating. The possibility that this deterioration is due to the localized onset of MHD ballooning instabilities, with confinement elsewhere continuing to follow the law established for Ohmic discharges, is examined. It is found that the overall effect of the ballooning instability is indeed gradual and the resultant confinement time corresponds to a scaling law in good agreement with that reported for auxiliary-heated tokamaks.

This tenth Symposium in a biennial series which concentrates on engineering aspects of fusion research was attended by 693 participants, demonstrating the continuing emphasis in the USA on the technological development of present-day physics experiments and future reactor systems. Organized by Professor R.G. Mills and his colleagues from the Princeton Plasma Physics Laboratory, the meeting lasted three and a half days. Following the opening plenary session, there were five parallel sessions each morning and afternoon, consisting of four oral sessions and a poster session with up to 80 presentations; in all, 472 contributions were presented.

The Fifth IAEA Technical Committee Meeting on Large Tokamak Experiments was held at the Princeton Plasma Physics Laboratory on 16–20 January 1984. Representatives from all of the large tokamak devices world-wide participated in a broad-ranging discussion of the status of each major project and the plans for the future.