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The paper describes a theoretical and experimental study of the structure of local Alfvén resonance (AR) and absorption of Alfvén waves (AWs) in an inhomogeneous toroidal plasma. An analysis is made of expressions for the RF field distribution in the AR region and for the power absorbed by the plasma, taking into account finite ion Larmor radius, inertia of electrons, the gyrotropy effect and Cherenkov absorption of waves by electrons. The dispersion characteristics of the AWs excited have been studied in the currentless regime on the OMEGA toroidal device. Amplitude and phase space-time measurements of the Ẽ and B̃ components of the RF field in the AR region for travelling and standing AWs have been made. AW absorption in the AR region and the dynamics of plasma production using AWs have been studied on the OMEGA device and on the URAGAN-3 torsatron. With an input of about 500 kW of RF power in the frequency region ω < ωci. in the U-3 torsatron, a dense hydrogen plasma which had a clearly defined boundary and divertor layers was produced with the following parameters: n̄e = 1.1 × 1013, T̄e ≈ T̄i ≈100 eV. In these experiments, which were performed with weak magnetic fields (Bo ≤ 4.2 kG), the beta achieved (≈0.5%) is comparable with the theoretical maximum equilibrium value for the device.

A three-dimensional equilibrium code (NEAR) is used to study plasma equilibrium in torsatron configurations having various winding laws. The harmonic spectra describing the variation of |B⃗| along magnetic field lines are calculated at finite plasma pressure and then used to calculate the geometric factors that determine the magnitudes of the neoclassical transport coefficients in the helical ripple trapping regime, where the particle and heat fluxes are inversely proportional to the collision frequency. The results show that an appropriate choice of winding law can reduce helical ripple transport by a factor of about 2 in the vacuum field and at high plasma pressure.

Neoclassical transport in realistic torsatron magnetic fields of reactor size is estimated by use of the Monte Carlo simulation technique. The configurations examined differ in the winding law which describes their helical conductors — a difference which has been predicted analytically to yield differing transport levels. This effect is confirmed not only in the 1/ν regime, for which the analytic calculations were originally done, but also at much lower collision frequencies. For the torsatrons examined, the overall transport rates differ by a factor of 1.5-3. It is noted that the configuration with the more favourable transport characteristics bears some similarity to an advanced stellarator.

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