Table of contents

The theory of homogeneous isentropic compression is extended to the case of hollow shells, and exact results for the growth of Rayleigh-Taylor instability during such compressions are obtained. Earlier results on the optical power P_L required to achieve a given measure of inertial confinement ρR of a laser-driven isentropically-compressed DT pellet are extended to include the case of hollow shells. – It is found that a five-fold reduction in peak power should be attainable by imploding a hollow shell having an unablated-wall thickness equal to 10% of its radius, instead of a solid pellet. Further reductions in wall-thickness and peak optical power appear to be strongly limited by Rayleigh-Taylor instability.

Toroidal DT plasmas in which the D and T ions make up two distinct, quasi-thermal velocity distributions, oppositely displaced in velocity along the magnetic axis, can be set up by introducing all ions by tangential injection of neutral beams, and by removing ions from the plasma shortly after they have decelerated to an energy <̃2T_e by Coulomb drag on the plasma electrons. A simple physical model for this counterstreaming-ion operation allows one to deduce the ion velocity distributions and required energy and particle confinement times that are in good agreement with the results of previous Fokker-Planck calculations. These distributions are used to calculate fusion reactivities, from which the variation of fusion power gain and power density with injection energy and T_e are determined. The practical problems of implementing counterstreaming operation in a tokamak include charge-exchange losses, impurities, and the need to minimize plasma re-cycling.

Several properties of the counterstreaming system that make it advantageous for near-term reactor applications are (1) optimal performance at relatively low injection energy (40–60 keV), because of head-on nuclear collisions, (2) small electron energy confinement time for Q ∼ 1 (e. g. n_eτ_E ≈ 2 × 10¹² cm⁻³·s at T_e ≈ 3.5 keV), (3) fuelling solely by the injected beams, and (4) possible maintenance of the plasma current by the beams. For realistic tokamak parameters, neutron wall loading is limited to 0.5 MW/m² or less. Plasma parameters appropriate to radiation test reactors and fusion-fission hybrid reactors are discussed.

We determine the parameters of a tokamak reactor plasma that attains ignition in the same regime of collisionality, ν_eiA/ω_be, as present-day Ohmic-heated tokamak plasmas. The 'collisional' regime is attractive because the presently observed confinement scaling, nτ ∝ n², can be employed, and fusion power densities can be very large (∼10 W/cm³). The use of Nb₃Sn toroidal field coils and a plasma elongation ⪆ 1. 5 are necessary to attain the high plasma density (n ≈ 5 × 10¹⁴ to 10¹⁵ cm⁻³) required for ignition in this low-temperature collisional regime.

An analysis is made of the breakdown and the ionization of the filling gas in toroidal devices. A model is set up to try and reproduce the main features of experimental results, which include upper- and lower-pressure and electric-field thresholds, discharge formation time, particle loss and injection, and initial rates of increase of current and temperature. The model is compared with experiments on devices built at Fontenay-aux-Roses (TA2000 and TFR). Some of the discrepancies between model and experiment may be due to the existence of early run-aways. The absence of an efficient, uniform preionization on TFR could be responsible for the appearance of these run-aways. Finally, it is concluded that impurities from the walls and limiter flow into the TFR plasma right from the start; the rate of influx is estimated for the first few milliseconds. This influx itself is considered indicative of early run-aways.

The coupling efficiency of a phased multi-waveguide structure (the "Grill") designed to launch HF-waves at the lower hybrid resonance to heat large toroidal plasmas, while satisfying the accessibility condition, is studied. To find the reflection and transmission coefficients, as well as the k_||-spectrum of the excited field, the waveguide field, represented as a superposition of eigenmodes, is matched to the field in the plasma, which is evaluated on the assumption of a linear density profile near the plasma edge. It is found that the reflection coefficient can be made acceptably low and is not sensitively dependent on the plasma parameters. It is concluded that it is possible to design a Grill capable to launch lower hybrid waves at the power level required for the ignition of a reactor plasma.

During the slowing-down process, fast ions injected into a tokamak plasma may pitch-angle-scatter into orbits whose deviations from flux surfaces are of order a, the limiter radius. Ions on such orbits may hit the wall or limiter or be charge-exchanged out of the system and are thus in the "loss region". The boundary of this region is calculated and its effect on plasma heating by neutral-beam injection is evaluated by using the Fokker-Planck equation. In present injection experiments, the loss region may have a significant effect if the impurity level of the plasma is high. Counter-injected ions are more strongly affected by the loss region than are co-injected ions since they have to scatter through a smaller pitch angle to reach it.

Self-modulation and filamentation of an electromagnetic wave is considered as a problem of the nonlinear interaction between electromagnetic and ion waves. A new electro-magnetic modulational instability is obtained, whose threshold is the same as that of the oscillating two-stream instability. A simple geometrical model is given of filamentation when the non-linearity is due to the ponderomotive force. The relationship between the filamentation and electromagnetic modulational instabilities and other parametric instabilities is considered. In particular, it is shown that both electromagnetic modulational and filamentation instabilities can occur at the critical density where they have the same threshold as the modulational instability of a Langmuir wave. Finally, a conservation relation (a generalization of the Manley-Rowe relation) for the wave action density is obtained for the filamentation instability. This shows clearly that this instability results from a four wave interaction.

For parameters appropriate to high-density pinch experiments (n_e ⪆ 10¹⁴ cm⁻³) it is shown that both classical transport processes and anomalous transport processes can play an important role during the implosion. The fluid-numerical model of Liewer and Krall has been modified and extended to include (in addition to turbulent transport) the effects of classical transport (e.g. frictional force, thermal force, heat conduction, heat convection, resistive heating and thermalization), as well as the effects of ionization, neutral-particle diffusion and charge exchange. Numerical studies of the early and intermediate stages of implosion are presented for parameters appropriate to Scylla 1-B and Scyllac. It is found that the relative importance of the various transport processes depends in detail on radial location, initial ionization level, bias field, etc. As a general remark, however, anomalous transport processes tend to dominate in the sheath region and behind the piston (where the current and temperature are relatively high), whereas classical transport processes are important in front of the piston (where the density is high and the temperature is relatively low). Moreover, for sufficiently large initial neutral density, the effects of charge exchange, neutral-particle diffusion, and ionization, are found to be significant during the implosion.

Measurements on the beam-plasma interaction occurring when an energetic ion beam is injected into a plasma are presented. Excitation of ion cyclotron harmonic waves propagating azimuthally across the applied magnetic field is observed. The excitation is attributed to the small velocity component of the ion beam directed perpendicular to the magnetic field, independent of the parallel velocity component. The experimental results are compared with the linear theory of electrostatic instabilities in a Maxwellian plasma traversed by a monoenergetic beam with d. c. energy components parallel and perpendicular to the magnetic field.

The paper considers the possibility of generating energy in a hybrid fission and fusion reactor in which deuterium is used as the thermonuclear fuel. It studies the energy balance of a plasma for the case of injection and non-injection of a beam of high-energy deuterons (hundreds of keV). The authors determine the conditions under which the requirements in respect of the plasma parameters are least rigorous. They show that the D-D nuclear fusion reaction can be used in a hybrid reactor with a deeply subcritical blanket.

A method has been developed for investigating the stability of a steady-state thermonuclear reaction in an inhomogeneous plasma. It has been shown that "thermonuclear burning" is unstable with respect to the excitation of axisymmetric temperature fluctuations (m = 0) homogeneous along the magnetic field (k_z = 0) if the temperature T_m at the axis of the plasma column is lower than some value T_*. This value is determined by the condition a (T_m = T_*) = a_min, where a_min is the smallest plasma column radius at which the energy losses of the plasma can be compensated by the energy released as a result of thermonuclear reactions. The stability of the plasma energy balance in relation to temperature fluctuations with m≠0 and k_z≠0 is discussed.

It has been assumed that the temperature at the plasma surface is lower than the ignition temperature of the thermonuclear reaction. It has also been assumed that the plasma energy confinement time is short in comparison with the particle confinement time.

The author has obtained the dispersion equation for an inhomogeneous turbulent magnetoactive plasma. The frequencies and wave vectors of oscillations are assumed to be arbitrary. The effect of ion-cyclotron oscillations on flute oscillations is considered. It is shown that when the amplitude is sufficiently large, cyclotron oscillations determine the flute oscillation frequency, which is equal in order of magnitude to the ratio between the electric drift velocity and the characteristic length of the inhomogeneity of the plasma. The flute oscillations become stable (unstable) in the field of cyclotron Oscil lations of a wavelength greater (shorter) than the ion Larmor radius.

The m = 2, n = 1 kink mode is shown to be capable of destabilizing the m = 1, n = 1 internal kink. A nonlinear Lagrangian theory is developed for the coupling of modes of different pitch and is applied to the interaction of these modes. The coupling to the m = 2 mode provides sufficient additional destabilization to the internal mode to permit it to account even quantitatively (where it had failed when considered by itself) for many of the features of the disruptive instability.

At this Conference organized by the Ecole Polytechnique Fédérale de Lausanne and the European Physical Society, and attended by more than 400 participants, approximately 200 talks were presented, including 21 invited papers. These papers were approximately equally divided into experimental and theoretical contributions although the invited papers mostly concerned experimental work. As was to be expected from the present trends in funding and interest, the Conference proceedings were dominated by reports on tokamak research, a topic which concerned more than half of the invited talks and approximately 30% of the contributed papers. In contrast, the representation of other, normally strong areas of research, as laser fusion, high-β plasmas, and stellarators was significantly reduced owing to topical conferences in these fields which were held at about the same time, and because some major machines are still in the construction stage. The Conference included new results on TFR which tend to explain the confinement-time limitations observed in that machine, extensive new work on internal and macroscopic disruptions in tokamaks and an order-of-magnitude advance in plasma confinement time observed in the 2XIIB mirror machine at Livermore.

The high level of interest in the prediction and evaluation of radiation effects in fusion reactors was underlined by the success of the Gatlinburg Conference. Of the more than 300 total conference attendants, 150 to 200 attended the Radiation Effects sessions. There was a total of 56 papers on radiation effects, contributed from the United States of America (41), the United Kingdom (5), the Federal Republic of Germany (5), Japan (3), Canada (1) and South Africa (1). In classifying the papers by subject, the greatest emphasis was on the examination of the microstructures of irradiated samples, with 18 papers, closely followed by papers that could be grouped as theory, modelling or analytical treatments of radiation effects, with 15 papers. Experimental techniques for the evaluation of radiation effects were highlighted in 7 papers, and 7 papers dealt with radiation-produced changes in physical properties. The fraction of effort devoted to the effect of irradiation on mechanical properties was disappointingly small, with only 5 papers on this important topic. The final subject division of design analysis included 4 papers that considered the effects of different reactor design options on the radiation response of materials.