Toroidal Magnetic Devices Research Articles

Fine-structure of electromagnetic radiation reflected from the O–X mode conversion zone in magnetized plasma

The linear coupling between the ordinary and extraordinary waves in the vicinity of the plasma cutoff is used for the heating of overdense plasma in toroidal magnetic devices. If the mode coupling is not perfect, there is reflected ordinary wave radiation. In this paper, we analyze the structure of this radiation in a realistic case of an incident beam with a finite aperture. It is shown that reflection from the evanescent layer in inhomogeneous gyrotropic plasma results in wave distributions with a non-zero-angular momentum. A helicity of the reflected beam results in unusual effects, at least in the plasma electrodynamics, which are identified and discussed.

Effect of suprathermal electrons flux on the ambipolar electric field in stellarators

Low collisionality plasmas in non-axisymmetric toroidal magnetic devices have significant radial particle fluxes due to trapped particles in the magnetic ripples. The associated ambipolar radial electric field cannot be quantitatively reproduced by standard neoclassical models since the measured values are systematically larger. Here the contribution of suprathermal electrons is included in the radial electric field. The radial flux of suprathermal particles is calculated following a kinetic analysis of the particle transport in which a non-Maxwellian velocity distribution is assumed for the high velocity electrons in the low collisionality regime. The electric field modification produced by the suprathermal electrons is then obtained and an analytical expression for the radial electric field is found which is compared with the one obtained from the thermal particles. An estimate of the needed contribution of the suprathermal electrons is given. Experimental data of the TJ-II Heliac-stellarator are taken for the computations since good Heavy Ion Beam Probe measurements are available.

On the possibility of reducing the instability threshold of a parametric decay of an extraordinary wave into two upper hybrid waves in an inhomogeneous plasma

A parametric decay instability (PDI) of an extraordinary wave leading to excitation of two upper hybrid (UH) plasmons at frequencies close to half the pump wave frequency is analyzed. It is shown that the two-plasmon PDI power threshold can be significantly reduced under conditions of electron cyclotron resonance heating (ECRH) experiments in toroidal magnetic devices, where the plasma density profile is often nonmonotonic, which leads to the localization of UH waves.

Analysis of ICRE (ω ⩽ ωci) plasma production in large scale tokamaks

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

Kinetic theory of runaway production in toroidal magnetic devices

A kinetic equation is derived for fast electrons in toroidal, tokamak- or stellarator-type magnetic devices. It is of a different type for different groups of electrons, i.e. passing or trapped electrons, in a magnetic field. The solution of the kinetic equation is found, and analytic expressions for the electron distribution function are obtained for the case of an ideal (non-rippled) torus and an electric field weaker than the critical Dreicer field. The runaway flux of electrons is found. The case of a rippled torus is investigated by taking account of the vertical drift resulting in electrons blocked in the ripples being lost to the chamber walls. Considerable plasma loss is shown to be possible only for electrons of an energy exceeding some quantity ℰd, which depends on the magnetic field, the depth of the ripple, the plasma density, and the parameters of the device.

Toroidal magnetic devices for plasma confinement

Vacuum magnetic fields appear suitable for the confinement of highly energetic plasmas for times that may become adequate to make controlled thermonuclear reactors scientifically feasible. To achieve satisfactory confinement, these magnetic fields must possess certain characteristics. The properties that toroidal magnetic fields must have are reviewed and examples presented of apparatus in use in the laboratory to provide these magnetic fields.