Table of contents

Steady-state plasma flow in the scrape-off layer of a toroidal limiter is discussed. The force balance along the torus minor radius is taken into account, from which follows that the plasma pressure gradient is balanced by the ponderomotive force (l/c) x which arises in the presence of a current density component perpendicular to the magnetic field. The limiter has an important effect on the electric current flow in the scrape-off layer. It is shown that the electric potential and plasma density values differ from one side of the limiter to the other; this leads to plasma drift along the minor radius. The characteristic length of change in the plasma density is found to be of the order of the ion cyclotron radius calculated for a poloidal magnetic field.

Spheromaks with lifetimes of 1 ms are produced in the CTX experiment. This paper describes the diagnostics and measurements on plasmas which, for CTX-produced plasmas, are the hottest and longest-lived discharges using a solid copper flux conserver. These spheromaks are formed using a static hydrogen background gas filling the entire vacuum system before the discharge. The density rapidly decays in 150–300 μs from an initial value of (1–3) × 10¹⁴ cm⁻³ to a steady-state plateau with a value of (1–4) × 10¹³cm⁻³,determine d by the pressure of the gas fill. A multi-point Thomson scattering system measures the radial profiles of electron temperature and density. Peak temperatures of over 40 eV are observed, and the average temperature increases in time by Ohmic heating from 15 eV to over 30 eV. Equilibrium models for the magnetic field structure are used to calculate values of peak local beta (8–13%), volume-averaged beta (3–8%), and engineering beta (10–25%). The operation with a filling gas results in a reduction of the impurity radiation power as measured by spectroscopy. Improved vacuum practices, discharge cleaning and the use of the static gas fill have resulted in discharges in which the radiation power loss is not dominating the energy balance late in time. Particle loss and the associated ionization and heating of the neutral particles required to maintain the density plateau appear to be the major energy loss processes in the spheromak.

ICRF heating experiments were carried out in the JFT-2 tokamak with pure high-field-side excitation. Almost pure electron and ion heating regimes were made possible in the experiment so that the metal impurities were reduced. Power balance of the plasma heated by ICRF is revealed by a transport analysis of the experimental results. RF power deposition profiles of each heating regime are obtained. The results of this analysis may serve as an experimental basis for refining the present theoretical treatment of ICRF heating.

Results of electron cyclotron emission (ECE) measurements during low-density non-thermal ASDEX tokamak operation are presented. Particular attention is given to the transition between the thermal plasma state, where the electron velocity distribution is Maxwellian, and the non-thermal state, where the distribution function may be treated as a superposition of low- and high-energy components. The study is aided by the simultaneous use of two complementary ECE diagnostics, a rapid-scan Michelson interferometer for measurement of the entire ECE spectrum and a four-channel polychromator for registration of time-resolved intensities at different wavelengths. Numerical simulations confirm the two-component description of the non-thermal distribution and provide estimates of the population and energy of the high-energy part. By comparing X-mode and O-mode radiation, additional information regarding the optical depth of the low-energy component is obtained.

The transport of a currentless ECRH plasma in Heliotron E is studied numerically, using a 1-D transport code with an assumed RF power deposition profile. The investigated parameter range is as follows: 300 ⪅ T_e ⪅ 1000 eV, T_i ⪅ 120 eV, 2 < _e< 10 X 10¹² cm⁻³ and P^RF ≈ 90 kW at B = I T. Four typical models for the transport coefficients are used in the numerical calculations. It is found that a neoclassical model results in a remarkable discrepancy of the time evolution of the electron temperature T_e(r) and density n_e(r) profiles with regard to the measured values. The neoclassical model which includes the effects of the helical field ripple seems to simulate the saturation of T_e(0), in agreement with the experimental results. This saturation is due to the dependence of . However, a consistent description of T_e(r) and n_e(r) cannot be given by this model. In order to reproduce the measured profiles, it is necessary to use radially increasing transport coefficients. Adding Alcator-like anomalous transport terms (independent of T_e) to the neoclassical transport model, it is found that _e and P_RF dependences of T_e(0) can be adequately explained. The fourth model investigated has a temperature-dependent . For ions the _e dependence of an anomaly factor, ,is also examined.

The problem of ripple transport in a tokamak operating at low collisionality is re-examined. Such a calculation is highly sensitive to the particular condition imposed to determine the perturbed distribution function. The condition employed here is derived in a fashion fully analogous to that previously employed in simpler transport problems. In contrast, previous work on ripple transport has invoked more ad-hoc conditions based on heuristic considerations, which often turn out to be incorrect. The difference between imposing the present and previous conditions (which are themselves not in agreement) can completely change the resultant appropriate expression for the transport coefficients.

The influx of heavy impurities in the Alcator C tokamak is determined as a function of plasma parameters from observations of intrinsic impurities, in conjunction with an empirically derived anomalous impurity diffusion model. The influx of molybdenum as a function of electron density is found to decrease dramatically as the electron density is raised above 1 × 10¹⁴ cm⁻³. Sputtering (by neutrals, ions and impurities) is probably the dominant molybdenum release mechanism in Ohmically heated discharges.

A set of equations averaged over a well-defined cylindrical plasma layer is derived. A new snowplow equation, which is based on energy conservation, is derived from the set and is examined. This equation gives descriptions for the later phase of a pinch and differs from the 'snowplow' equation which is based on momentum conservation. It is shown that the plasma collapses to a smaller radius in a Z-pinch when the initial imploding velocity increases. A Z-pinch can be enhanced by superposing a pre-current, which gives similar initial conditions.

The formation of an FRC (Field-Reversed Configuration) is considered by using a hybrid simulation model that treats the electrons as a zero-inertia thermal fluid and the ions either kinetically or as a simple fluid. The unexpected spontaneous generation of a substantial toroidal magnetic field B_θ near magnetic reconnection points requires explanation. Although ion kinetic effects are responsible for some of the internal reconnection, simulations with a fluid ion representation also show similar self-generation of the toroidal field. Further investigation reveals that the origin of the toroidal field can be found in some of the terms that arise from retention of the Hall term – an automatic feature of the two-component model. Conceptually, the phenomena can be understood by noting that, if the magnetic field is 'frozen into the electron fluid', a sheared toroidal electron flow will stretch the poloidal field to make the toroidal field.

A variational calculation of the trapping rate and trapped-ion density in thermal barriers is presented. The effects of diffusion in energy as well as pitch-angle scattering are retained. The variational formulation uses the actual trapped/passing boundary in velocity space. The boundary condition is that the trapped-ion distribution function match the passing-ion distribution function, which is taken to be a Maxwellian, on the boundary. The results compare well with the two-dimensional Fokker-Planck code calculations by Futch and LoDestro. The CPU time for a variational calculation is less than 0.1 s using the CRAY-I computer, while a typical Fokker-Planck code calculation takes 10–20 min.

The observation of mode conversion during ICRF heating in a deuterium-hydrogen plasma is described. The mode-converted ion Bernstein wave is detected near the ion-ion hybrid resonance layer by 2-mm microwave scattering. In addition, a fast magnetosonic wave is also observed by magnetic probes inserted into the plasma. The plasma dispersion of both waves as determined experimentally agrees well with the results of calculations for a hot plasma.

By phasing the injection of frozen pellets into a tokamak plasma, it is possible to generate current. The current occurs when the electron flux to individual members of an array of pellets is asymmetric with respect to the magnetic field. The utility of this method for tokamak reactors, however, is unclear; the current, even though free in a pellet-fuelled reactor, may not be large enough to be worth the trouble. Uncertainty as to the utility of this method is, in part, due to uncertainty as to proper modelling of the one-pellet problem.