Table of contents

The author obtains analytical expressions for the collisional losses of electrons and their energy from an adiabatic trap when the plasma has a high positive potential. The expressions are derived on the basis of an approximate solution of the Fokker-Planck equation, which yields more reliable results than those obtained with the estimative formulas used in a number of other works. In addition, an expression is obtained for the mean energy of the electrons escaping from the trap.

Recent plasma confinement experiments in Heliotron-D are described in connection with neoclassical diffusion theory. Weakly and relatively strongly ionized plasmas are produced by coaxial-gun and Joule heating, respectively. In both cases, the observed diffusion coefficients are found to be in good agreement with the theory, under the conditions of weak magnetic field. Diffusion processes in two different magnetic configurations, ε_t ∼ ε_h and ε_t ≪ ε_h, are also compared experimentally, where ε_t is the ratio r/R of the distance from the magnetic axis to the major radius, and ε_h is the modulation depth due to the helical field.

The effect of collisions on the onset of the current-driven ion-wave instability is experimentally investigated in a diffusion plasma. The critical wavenumber of the instability decreases when the collisional parameter ω_piτ_i increases from 20 to 120, in good agreement with theory.

An investigation is made of the heating of plasmas by an oscillating electric field close to the lower hybrid frequency. In the presence of low-frequency short-wavelength background fluctuations, the electric field can become anomalously absorbed because of resonant transfer of energy to damped lower hybrid modes. This results in an efficient heating of the ions in the plasma.

The stability problem of a linear diffuse pinch is studied by a normal-mode analysis using guiding-centre theory. The general normal-mode equation capable of determining the growth rate is obtained. The guiding-centre Suydam instability and the associated eigenvalue (growth rate) are examined.

The nuclear performance of vanadium as a structural material in a deuterium-tritium fusion reactor is discussed. For comparison, calculations are also presented for niobium. The areas considered include: (1) tritium breeding; (2) nuclear heating; (3) radiation effects relating to material damage; and (4) neutron-induced activity. It is concluded that: (1) the tritium breeding characteristics of vanadium are somewhat superior to those of niobium; (2) nuclear heating in the first wall of the blanket will be substantially lower with vanadium than with niobium; (3) helium and hydrogen production in the first wall will be significantly greater with vanadium than with niobium; and (4) vanadium will exhibit an afterheat and biological hazard several orders of magnitude lower than that of niobium for times-after-shutdown of ∼ 100 days and greater.

The excitation of absolute instabilities in three-wave parametric couplin g processes in inhomogeneous plasmas has been studied analytically . A detailed examination of the full fourth-order set of differential equations describing such processes in infinite media confirms the WKB-criterion that absolute instabilities arise only if . A model calculation using the WKB-approximation has also been carried out to study the effects of finite plasma extent and pump depletion; conditions for the excitation of absolute instabilities in this case have also been obtained.

The hydrodynamic theory and properties of spherical homogeneous isentropic compression are discussed. Computer results are described showing that a close approximation to compressions of this type may be accomplished by light-induced pellet ablation if the light absorbed by the pellet is properly programmed with time, and is spatially uniform over the pellet surface. Pellet compressions in excess of ten thousand-fold are computed.

Five microwave interferometers are used to measure radial and longitudinal electron density profiles versus time in the 2XII minimum-B mirror containment experiment. The mean plasma radius determined from a radially offset interferometer agrees with electrostatic probe and laser measurements. The longitudinal profile is consistent with a collisional ion angular distribution with a density at the mirrors of approximately 5% of the central density. After compression the plasma shape remains approximately constant. Plasma densities beyond the mirrors were less than 5% of the central density.

Electrons are injected into an afterglow plasma confined in an ℓ = 3 stellarator in order to raise the velocity of poloidal rotation. A steady shock-like structure characterized by a steep density gradient is observed near the critical rotational speed. Strong enhancement of the diffusion rate occurs at higher radial electric fields, accompanied by large-amplitude density fluctuations. Vortices are generated at the three corners of the triangular magnetic surfaces when the radial electric field is reversed inside the plasma.

Observations of theta-pinch plasmas were followed up by calculating the energy losses due to resonance line radiation from oxygen impurities. A plasma, homogeneous in density and temperature, was assumed to be heated instantly from a low initial temperature to a certain high temperature. The oxygen atoms are thereby ionized over different ionization stages until equilibrium conditions are reached. On the assumption of prevailing binary collisions a normalized density-independent time and a normalized density-independent emission coefficient can be introduced. Three-body collisions and stepwise excitation in lithium-like ions limit this scaling to electron densities below 10¹⁶ cm⁻³. With the help of these results an estimate of the energy losses by line radiation due to oxygen impurities in non-stationary plasmas like pinches, tokamaks and stellarators is possible provided the impurity concentration and the electron density and temperature are known from measurements.

A study is made of the magnetic field of a solenoid formed by current-carrying loops, the plane of which is perpendicular to a helical line. Analytical expressions are derived for the scalar potential and the components of the magnetic field strength inside a straight solenoid with a dense winding. The toroidal case was investigated using a BESM-6 computer. The results are presented of an experimental study of the magnetic field of a rectilinear system with a helical magnetic axis. The paper discusses the possible design of a toroidal trap with the magnetic configuration in question.

An investigation is made of a magnetic configuration formed by a combination of fields from a solenoid with a helical magnetic axis and a plane-geometry axis and stellarator helical conductors. The paper contains analytical expressions for the determination of the characteristics of the magnetic configuration, examples of magnetic surfaces of the system in a rectilinear approximation and the results of a numerical calculation of a toroidal variant of the configuration.

Previously, it has been shown that in a cold, inhomogeneous, magnetized plasma half-space the lower-hybrid resonance is accessible to the transverse-magnetic (TM) plane waves incident on the vacuum-plasma interface at an approximately grazing incidence, provided that at the hybrid layer ω_pe/ω_ce ≲ 0.4. In this paper, these results are extended to the slow-wave case when n_z, the refractive index in the static magnetic field direction, exceeds unity. It is found that the plasma is indeed accessible to the slow waves if Golant's accessibility criterion n_z > 1 + (ω_pe/ω_ce)² is satisfied. The following recommendations can be made for coupling r.f. energy to the lower-hybrid resonance: (i) if ω_pe/ω_ce ≲ 0.4, efficient coupling is possible by launching TEM-like waves on the plasma column, (ii) if ω_pe/ω_ce ≳ 0.4 and if the transverse machine dimensions exceed the r.f. vacuum wavelength, it is possible to couple TM waves using passive slow-wave structures inside the machine walls, (iii) if ω_pe/ω_ce ≳ 0.4, but for smaller machine dimensions, recourse must be taken to transverse-electric slow-wave coupling with current-carrying coils of appropriate periodicity . If, as was pointed out by Glagolev, propagation from the plasma edge to the hybrid layer is not materially affected by the inclusion of finite-temperature effects, by far the most elegant solution (with potential application to thermonuclear plasmas) for coupling r.f. energy from the second to the twentieth ion-cyclotron harmonic waves is by launching TEM-like waves in the coaxial waveguide formed by the plasma columnand the containing walls.

Two decades ago, Dr. von Engel organized the International Conference on Phenomena in Ionized Gases in Oxford for the first time. It was proposed there that the conference should be held every two years. In the meantime, research in plasma and gas-discharge physics has undergone an intense development, the variety of phenomena with which it is concerned increasing rapidly. The realization of magnetohydrodynamic conversion of thermal energy into electric power and the solution of the problem of controlled nuclear fusion became extremely important scientific and technological tasks. Quite naturally, specialized meetings became necessary, the conference on controlled fusion and plasma physics now being held regularly every year. Therefore, the organizing committee of the eleventh conference did not accept papers directly related to MHD- generators and thermonuclear fusion. The restricted number of topics resulted in a reasonable number of participants (about 500) from a large number of countries. In the two previous conferences held at Oxford (1971) and Bucharest (1969), some 750 participants took part. On the other hand, the number of contributed papers (about 500) has remained almost unchanged.

The International Fusion Research Council (IFRC) was founded in 1971 within the framework of the International Atomic Energy Agency. The main function of the Council is the promotion of international co-operation in controlled nuclear fusion research and its application. Among the eleven members of the Council there are representatives of the following countries: Australia (M. H. Brennan), France (M. Trocheris), FRG (von Gierke), Italy (B. Brunelli), Japan (K.Husimi), Netherlands (C.M. Braams), Sweden (B. Lehnert), UK (R.S. Pease), USA (R.L. Hirsch) and USSR (E. Velikhov). There is also a member of IFRC from EURATOM (D. Palumbo). The Chairman of the Council is C.M. Braams (Netherlands). The Council meets once a year. The third IFRC meeting was convened during the Sixth European Conference on Controlled Fusion and Plasma Physics, Moscow, 30 July – 3 August 1973. Previous meetings were held in Madison, USA (1971) and Grenoble, France (1972).

A translation of the abstracts of all the articles for this issue into French, Russian and Spanish.