The authors study the possibility of target plasma radiofrequency (RF) production in the ion cyclotron range of frequencies (ICRF) (ω ⩽ ωci) in large scale tokamaks before the startup of an Ohmic discharge. A number of experimental and theoretical studies on dense plasma production in the ICRF in toroidal magnetic devices are reviewed. The criteria for optimal development of the RF discharge stages are analysed, i.e., for RF breakdown of the neutral fill gas in the vicinity of the antenna (non-wave stage, ne << na, where ne and na are the densities of electrons and atoms), and the wave stages of initial ionization (ne < na) and neutral gas burnout (ne approximately na) in the whole volume of the plasma torus. A number of requirements for the design of the antenna system are formulated for all stages of dense plasma ICRF production. A scenario for plasma ICRF production with slot type antennas in the ITER tokamak is proposed. Numerical simulations with a 0-D transport code show that in ITER a target plasma with an electron density of approximately=3*1012 cm-3 can be produced by coupling 3-6 MW of RF power to the plasma at a frequency of approximately=3 MHz. Such a level of RF power is sufficient not only for full ionization of the neutral gas but also for heating the produced plasma to electron temperatures of approximately 80 eV

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.

ICRF power levels of up to 2.8 MW were achieved during the 1988 experimental run on TFTR. Metal impurity concentrations (Ti, Cr, Fe, Ni) and Z,, were monitored during ICRF heating by Xray pulse height analysis and UV spectroscopy. Antenna phasing was the key variable affecting ICRF performance. No increase in metallic impurities was observed for P,, 2 2.8 MW with the antenna straps phased 0-n. while a measurable increase in titanium (Faraday screen material) was observed for P, 9 1 .O MW with 0-0 phasing.

The use of radiofrequency (RF) fields in the megahertz range to produce the initial plasmas for nuclear fusion experiments can provide a broader operating regime for many stellarator devices and can result in a saving of volt-seconds in tokamaks. This process has been used for many years in various devices at the Institute of Physics and Technology at Khar'kov. The initial breakdown of neutral gas by non-resonant RF near-fields can be understood in terms of RF electric fields parallel to the static magnetic field and a ponderomotive trap with a long scale length parallel to the static magnetic field. If the parallel fields are not too strong for a given antenna geometry, the concept of a ponderomotive trap is valid and the trapped electrons can oscillate in the near-field region for many RF periods. Efficient breakdown can begin if the oscillation energy of these trapped electrons is sufficient to ionize the gas. Good agreement between the experimental observations and an analytic model for the early phase of the breakdown process is presented.