Table of contents

The paper present an analysis of the loss cone boundary in phase space for toroidal helical devices. The effects of the radial electric field and the modulation of the helical field ripple are taken into account. The minimum energy of particles entering the loss cone is calculated. Modulation of the helical ripple is not always effective in reducing the loss when a radial electric field is present. The particle loss due to the loss cone is estimated in the collisionless limit. The impact of the radial electric field on the loss cone is discussed. It is more difficult to find a solution in the case of a positive radial electric field, which is required for the high ion temperature mode. This difficulty is great for systems with a small helical field ripple.

The change in the fusion power of a tokamak reactor due to distortions in the reaction component distribution functions has been studied. A numerical solution of the two-dimensional kinetic equation is used to determine the distribution of deuterium, tritium and alpha particles. Neoclassical ripple-plateau radial diffusion of deuterium and tritium components and stochastic transport of alpha particles as well as Coulomb collisions between these components and electrons are taken into account. It is shown that neoclassical transport of fast ions has a weak effect on the power generation. The most important process is ripple diffusion. For a reasonable ripple value, diffusion can reduce the energy input of the D-T reaction to the plasma by 15-30%.

In the JIPP T-IIU tokamak, a high power ICRF heating experiment has been conducted, up to an extremely high power density (∼2 MW·m⁻³), with a total RF power of P_RF = 2 MW. Great attention has initially been paid to the problem of impurities, and it has been found that (a) the adoption of low Z materials for the limiter, (b) in situ carbon coating (i.e. carbonization) and (c) adequate gas puffing synchronized to the RF pulse are very effective in suppressing radiation loss. With these methods, a remarkable reduction in metal impurities (especially in iron impurity) has been achieved; the total radiation loss has been reduced to less than 30-40% of the input power. In these reduced radiation loss plasmas, the characteristics of ICRF heated plasmas have been studied intensively. With an increase in the ICRF heating power, a deterioration of the energy confinement time has been observed, indicating quantitative agreement with the Kaye-Goldston L-mode scaling. It is shown that the so-called profile consistency, which is the leading feature in neutral beam heated plasmas, also holds in ICRF heated plasma. It has been observed that the electron temperature profile only responds to the safety factor q(a) and does not change when the deposition profile is controlled by tailoring the k₁ spectrum.

The low beta stability of a toroidal circular cross-section tokamak is re-analysed. Although the structure of the unstable modes can be considerably modified by toroidal coupling, the overall stability picture is well represented by the 'straight tokamak model', with the major exception that the internal kink, although stabilized by toroidal effects, still has a free-boundary counterpart – the toroidal kink mode. The toroidal kink is unstable for q₀ < q_c ≤ 1 and is destabilized by toroidal coupling, having a growth rate γ that scales with inverse aspect ratio a/R as γ – (a/R)^2[q_s], where [q_s] is the integer part of the edge safety factor q_s.

A new method of obtaining a dispersion equation for ballooning flute modes in a tokamak is proposed. The author derives a generalized dispersion equation for these modes in a tokamak with circular magnetic surfaces and an arbitrary pressure profile. It is shown that this equation behaves differently in the s > 1/2 region and in the region with s < 1/2, where s = (1/4 + U_o)^1/2 − 1/2, and U_o is the Mercier coefficient. It is established that the stability limit for ballooning flute modes strongly depends on the plasma pressure profile. Finally, the author demonstrates that the plasma self-stabilizing effect can substantially improve the stability of the plasma against these modes.

The neoclassical fluxes induced by a radiofrequency field are calculated using the gyrokinetic equations. In general, the fluxes consist of a convective part and a diffusive part. The convective part is non-vanishing when the heating is asymmetric with respect to the magnetic field. The adjoint technique developed by Antonsen and Yoshioka is used to calculate the fluxes in the collisionless regime. Explicit expressions for convective fluxes are obtained for fast waves and ion Bernstein waves in the ion cyclotron range of frequencies. It is found that the convective flux due to ion Bernstein waves can be quite significant. A possible scheme for controlling impurity influx is through direct asymmetric heating by ion Bernstein waves.

Through a series of controlled experiments on the DITE tokamak the main impurity production mechanisms at graphite limiters are investigated. Three principal processes can release carbon impurities: deuterium chemical sputtering, physical sputtering and oxygen chemical erosion. By varying, in turn, the limiter temperature, the bombarding particle energies and the local oxygen flux the rate of each erosion process is changed independently. From these experiments it is found that oxygen erosion dominates. Hence, carbon impurities leave the graphite surface primarily in the form of CO gas. The observed radial transport of intrinsic carbon and oxygen originating from the limiter is confirmed to be similar to that resulting from the localized puffing of CO. Metal impurities produced at the graphite limiter are physically sputtered, predominantly by carbon and oxygen impurity ions. The observed transport of chromium from the limiter has been modelled with a physical sputtering transport code.

A simple transport model is developed to study the dynamical evolution of an auxiliary heated tokamak plasma during the transition to a high beta, ballooning mode second-stable equilibrium. The effect of the ballooning mode stability on the transport is incorporated by prescribing an enhanced thermal diffusion in the unstable region. The resultant highly non-linear transport equation is solved numerically as an initial value problem, and also analytically by means of boundary layer theory. In particular, the auxiliary heating power P required for global transition of a flux-conserving tokamak plasma to the second stability regime is found to scale as where χ_max is the instability induced thermal conductivity enhancement factor and τ_E is the confinement time in the ballooning stable regime.

H-mode discharges with ion cyclotron resonance heating (ICRH) alone have been achieved in JET, up to a power level of 12 MW. These H-modes, which have reached durations of up to 1.5 s, have all the characteristics of H-modes with neutral beam injection. In particular, the energy confinement is similar to that of neutral beam cases and typically corresponds to two times Goldston L-mode scaling. The ICR heated H-modes are characterized by long sawtooth free periods. The use of beryllium gettering is an important factor in these experiments.

The paper is a review of investigations of the H-mode on ASDEX performed since its discovery in 1982. The topics discussed are: (1) the development of the plasma profiles, with steep gradients in the edge region and flat profiles in the bulk plasma, (2) the MHD properties resulting from the profile changes, including an extensive stability analysis, (3) the impurity development, with special emphasis on the MHD aspects and on neoclassical impurity transport effects in quiescent H-phases, and (4) the properties of the edge plasma, including the evidence of threedimensional distortions at the edge. The part on confinement includes scaling studies and the results of transport analysis. The power threshold of the H-mode is found to depend weakly on the density, but there is probably no dependence on the toroidal field or the current. For the operational range of the H-mode, new results for the limiter H-mode on ASDEX and the development of the H-mode under beam current drive conditions are included. A number of experiments are described which demonstrate the crucial role of the edge electron temperature in the L-H transition. New results of magnetic and density fluctuation studies at the plasma edge within the edge transport barrier are presented. Finally, the findings on ASDEX are compared with results obtained on other machines and are used to test various H-mode theories.

The Fifth International Conference on Megagauss Magnetic Field Generation and Related Topics was hosted by the Lavrentyev Institute of Hydrodynamics, Siberian Division of the USSR Academy of Sciences, which has performed this service for the world scientific community for the second time; Megagauss III was held in Novosibirsk in 1983. The International Advisory Committee of Megagauss V was chaired by Academician E.P. Velikhov, with Professor V.M. Titov, Director of the Lavrentyev Institute of Hydrodynamics, serving as Vice-Chairman and Dr. G.A. Shvetsov as Scientific Secretary. The conference again provided a forum for discussion of the physics, technology and the application of high magnetic fields and of related pulsed electrical power systems involving multimegaampere currents. The conference was preceded by the International Workshop on Opening Switch Physics and Techniques, chaired by Academician G.A. Mesyats. The participants visited several institutes in Leningrad, Moscow, Tomsk and Novosibirsk before and after the workshop and the conference.