Table of contents

The current-driven (CD) drift instability of a collisionless plasma with both electron and ion temperature inhomogeneities in a sheared magnetic field is investigated. It is found that the CD mode can become unstable with the universal mode is stable, and that the positive ion temperature gradient affects the stability by suppression (or reduction) of convective damping through magnetic shear. The resulting particle and heat fluxes due to the CD mode are also obtained.

A magnetic confinement system in the form of a stuffed toroidal line-cusp (Tormac) is proposed as the basis for a thermonuclear reactor. The interior (stuffing) region is assumed to consist of closed of closed magnetic flux surfaces and a Maxwellian DT plasma at high levels of beta. The particles in the surrounding sheath, on the other hand, are mirror-confined on open field lines and thus are lost into a divertor region at the ion-ion collision rate. The confinement based on this assumption is given by nτ ∝ r_p BT_i, where r_p denotes the plasma minor radius. In this paper, power gain and net output of Tormac reactors for a wide range of parameters are calculated. The net power gain Q_E, the gross electric output power/circulating electric power, is found to have values between 2 and 10 for reasonable reactor dimensions and very modest magnetic field strengths. For example, a plant yielding 500 MW net electrical power using ions at 40 keV thermal energy and a magnetic field of 40 kG would require a plasma of minor radius r_p = 1 m and a major radius of 5 m. Q enhancement by direct recovery of particle energy has not yet been included.

The properties of the toroidal plasma produced by the stabilized double-theta pinch are investigated experimentally and theoretically. The toroidal plasma produced has a strongly elongated crosssection with the half-axis ratio b/a ∼ 14 and an aspect ratio R₀/a ∼ 8 at t = 3 μs, when the values of β and β_p are approximately 0.23 and 0.30, respectively. It is shown that in the vicinity of the median plane of the plasma the Suydam criterion S ≥ 0 for the high m-mode is satisfied throughout the cross-section of the plasma produced. With the one-dimensional approximation, an analytic solution of MHD equilibrium is presented which always satisfies the Suydam criterion.

The physics of a wall-confined, fully ionized plasma is studied in the case where β ≫ 1. When electron thermal diffusion is magnetically reduced, but still dominates over ion diffusion (1 ≲ ω_eτ_e ≲ (m_i/m_e)^¼), the Nernst effect is shown to increase the diffusion losses substantially. Simple criteria are derived to estimate the validity of the 'equipressure' condition. The results are checked by a onedimensional hydro-code including the complete magnetic-field evolution. These results depend strongly on the wall boundary condition for the magnetic field.

A quasi-linear numerical model for RF heating is developed in a form so as to be applicable both to tokamak and to magnetic mirror plasma confinement devices. It is then combined with a two-dimensional multi-species Fokker-Planck code to yield detailed information on the distortion of the ion distribution function due to an imposed level of RF. For a two-component tokamak, it is found that while "fast-wave" ion heating will not increase the fusion output, as much as 40% of the beam power may be replaced by RF in a strongly driven two-component tokamak without significantly degrading the fusion output. For magnetic mirror devices, results indicate that RF in combination with relatively low-energy beams may be expected to heat the ions to well above the beam injection energy as well as to increase confinement.

The characteristics of TFR plasmas have been investigated for limiter radii from a = 12.5 to a = 19 cm and for two limiter materials, molybdenum and graphite. The central electron temperature is roughly proportional to a. The energy confinement time scales approximately as a². It is not clear whether this is true geometrical scaling or the consequence of a particular behaviour of the radiation losses. The concentration of light impurities decreases with increasing plasma-to-wall distance. The same is true for heavy elements (molybdenum) layered on the wall when a light element (graphite) is used as the limiter material.

Magnetic field lines near a given closed field line which is taken as a magnetic axis are calculated analytically. Existence properties are obtained for equilibria whose rotational transform on a magnetic axis is an integer multiple of ½. The results are applied to a tokamak with a perturbation of the main field, the ℓ = 2 stellarator, the stability properties of equilibria with vanishing rotational transform, and the scaling of the equilibrium β-value in configurations without net longitudinal current and with small rotational transform.

Three regimes in the formation of a simple pinch are delineated. For a rapidly rising current, the formation of the pinch by a strong shock is first considered. A slug model is developed which, unlike the snowplough model, describes the radial structure between piston and shock. The final equilibrium pinch is described and radial profiles of density and temperature are obtained. Under the conditions of a strong shock, a weakly compressed pinch is formed in which the pinch radius is related to the wall radius and is independent of the current applied. By programming the current in time, regimes of singular compression and adiabatic compression can be obtained. A much more highly compressed plasma is produced. Such a current profile can readily be produced by a line storage energy source.

In tokamak plasmas subjected to substantial toroidal-field ripple, sufficiently energetic members of the bulk-ion population can diffuse rapidly away from the hot central region by the superbanana process. The ion velocity distribution is truncated at the velocity above which the escaping fast ions cannot be replaced sufficiently rapidly by Coulomb collisions. This tail-depletion effect causes a marked reduction in the fusion reactivity and can give rise to an anomalously low central ion temperature as deduced from the thermonuclear neutron flux. The application of this phenomenon to the Alcator and TFR plasmas is discussed.

Heating of a 26-keV trapped-ion beam by interaction with incident lower-hybrid RF power (as low as 5 kW) was observed in the ATC plasma. We suggest that ion-cyclotron damping of lower hybrid waves by beam ions can account for the increase in beam energy. This process can explain the main features of the experiment: (1) preferential absorption in the perpendicular direction, (2) lack of absorption by the background plasma ions, and (3) low power requirement for absorption. Theory requires k_⊥ρ_i≳(ω/ω_ci)^½. The relatively high perpendicular temperature of the beam ions (∼ 1 keV), combined with one of several possibilities for RF energy at large k_⊥, allows the condition on k_⊥ρ_i to be satisfied. Moreover, the large parallel energy of the beam ions plays a major role in broadening the harmonic resonances, thus making it possible for a large number of beam ions to resonate with the wave. Though the primary process is perpendicular absorption, there is also a net gain in parallel energy during a collision time due to pitch-angle scattering. The importance of this heating mechanism for large machines such as TFTR is discussed. For these machines, ions will be injected with large ρ_i, making ion heating possible even with moderate values of k_⊥.

Calculations were performed to investigate how the thermonuclear energy gain for inertially confined fusion varies as a function of the tritium inventory. Results showed that for large ρR pellets (25 g/cm²) the yield was relatively insensitive to the D: T ratio, so that a reduction by a factor of five in the amount of tritium produced only a 15% reduction in yield. An attempt was made to further reduce the sensitivity by replacing some of the deuterium by ⁶Li. The idea was to breed tritium by neutron capture within the pellet and then to burn it in situ. However, the breeding rate was found to be too slow for this process to make any significant contribution to the burn. Lithium was not able to improve upon deuterium as a fuel.

Particle and energy fluxes in the scrape-off layer produced by poloidal limiters are studied in the JFT-2 tokamak with mean electron densities of 1.3 × 10¹³ and 2.8 × 10¹³ cm⁻³. The measurements were made by a combined Langmuir/thermoprobe and an infrared camera. The electron density in the scrape-off layer decays exponentially with increasing minor radius, implying that the cross-field diffusion is of the order of the Bohm diffusion. It is shown that heat transmission to the limiter more than 1 cm behind the edge radius is determined by a sheath of Maxwellian particles. Near the edge radius, accelerated electrons play a major role, which is indicated by toroidal anisotropy of the heat flux in the lower-density case.

The containment properties of alpha particles during the slowing-down process in an axisymmetric tokamak reactor are studied by using Monte-Carlo calculations, in which the pitch-anglescattering effects are included. The loss of alpha particles during slowing-down is found to be safely negligible in a pure DT plasma, but increases proportionally to the effective charge number of the plasma and cannot be ignored in a highly contaminated plasma (Z_eff > 4), compared with the loss of 3.5-MeV particles.

The linear theory of drift waves in a collisionless plasma can give the radial particle and energy flux for a given level of fluctuations. In the ODE device, good agreement was found between direct transport measurements and linear-theory predictions. The experimental methods of diffusion measurement in the hydrogen plasma column are described.

Experimental studies on the origin of metal impurities in DIVA are described. Three processes of metal impurity release, i.e. ion sputtering, evaporation and arcing, have been identified. Among these processes, ion sputtering is the dominant process in the quiet phase of the discharge, which is characterized by the lack of spikes in the loop voltage as well as by the fact that no heat flux concentration towards a specific part of the first wall takes place. Moreover, it is demonstrated that a honeycomb structure can decrease the metal impurity release.

A new divertor concept is proposed that introduces minimal added complexity (and hence cost) to the design of a tokamak. This concept is based on the creation of a localized region of enhanced toroidal field ripple by increasing the bore size of one or more of the toroidal field coils. Plasma particles that are trapped in the enhanced-ripple region, which we call a blister, experience strong drift motions which provide a means for collecting these particles. The results of model calculations that demonstrate the feasibility of producing the required blister region and a preliminary evaluation of the divertor collection efficiency are presented.