A. A. Skovoroda

The EPSILON experimental pseudo-symmetric trap

Within the framework of the Adaptive Plasma Experiment (APEX) conceptual project, a trap with closed magnetic field lines, the Experimental Pseudo-Symmetric Closed Trap (EPSILON), is examined. The APEX project is aimed at theoretical and experimental development of the physical foundations for a steady state thermonuclear reactor designed on the basis of an alternative magnetic trap with tokamak-like large β plasma confinement. A discussion is given of the fundamental principle of pseudo-symmetry, which a magnetic configuration with tokamak-like plasma confinement should satisfy. Examples are given of calculations in the paraxial approximation of pseudo-symmetric curvilinear elements with a poloidal modulus B isoline. The EPSILON trap, consisting of two direct axisymmetric mirrors linked by two curvilinear pseudo-symmetric elements, is considered. To increase the equilibrium β, the plasma currents are short-circuited within curvilinear equilibrium elements. An untraditional scheme of MHD stabilization for a trap with closed field lines by use of axisymmetric mirrors with a divertor is analysed. The experimental installation EPSILON-One Mirror Element (OME), which is under construction for experimental investigation of stabilization by divertor, is discussed. The opportunity for applying the ECR method of plasma production in EPSILON-OME in conditions of high density and low magnetic field is examined.

The next step in the development of a negative ion beam plasma neutralizer for ITER NBI

Deuterium atom beam injectors developed for ITER plasma heating and current drive are based on negative ion acceleration and further neutralization with a gas target. The maximal efficiency of the gas stripping process is 60%. The replacement of the gas neutralizer by a plasma one must increase the neutral yield to 80%. An overview of experimental studies of microwave discharges in a multicusp magnetic system chosen as the base device for plasma neutralizer (PN) realization and design development of PNs for ITER neutral beam injectors (NBIs) is presented. The experimental results achieved with the PN model PNX-U are discussed. Plasma confinement, gas flows and ionization degree were investigated. High density plasmas with ne ~1018 m-3 with low electron and ion temperatures ( ≈ 5-6 eV) and a high ionization degree (not less than 40%) at its centre have been generated in operation with argon.

On the resonance amplification of magnetic perturbations near the threshold of tearing instability in a tokamak

Using a cylindrical model, a relatively simple description is presented of how a magnetic field perturbation stimulated by a low external helical current or a small helical distortion of the boundary and generating magnetic islands penetrates into a plasma column with a magnetic surface q=m/n to which tearing instability is attached. Linear analysis of the classical instability with an aperiodic growth of the perturbation in time shows that the perturbation amplitude in plasma increases in a resonant manner as the discharge parameters approach the threshold of tearing instability. In a stationary case, under the assumption on the helical character of equilibrium, which can be found from the two-dimensional nonlinear equation for the helical flux, there is no requirement for the small size of the island. Examples of calculations in which magnetic islands are large near the threshold of tearing instability are presented. The bifurcation of equilibrium near the threshold of tearing instability in plasma with a cylindrical boundary, i.e., the existence of helical equilibrium (along with cylindrical equilibrium) with large islands, is described. Moreover, helical equilibrium can also exist in the absence of instability.

Hamiltonian formalism in the equilibrium problem for a plasma with islands

Hamiltonian formalism is applied to the equilibrium problem for a plasma with islands by using an analogy between the equilibrium problem for a plasma with one island and the nonlinear mechanics of a physical pendulum. A relationship is established between magnetic flux coordinates with straightened magnetic field lines and the action-angle variables. The flux and current representations of a magnetic field with islands are obtained, and the solution to the equilibrium problem for a narrow island is presented.

Local surface equilibrium equations for currentless magnetic configurations

Invariant local surface equilibrium equations are derived that interrelate the absolute value of the magnetic field B, the absolute value of the gradient of the magnetic flux |∇Φ|, the local shear s, and the plasma pressure on nested equilibrium magnetic surfaces in currentless configurations. Examples of applying these equations to analysis of symmetric and isodynamic equilibria are considered.

Laser spectroscopy for measuring the parameters of a plasma containing helium and argon

Laser spectroscopy diagnostics used in experiments on the PNX-U facility are described. The working gas was argon with an additive of helium. The 23P → 33D transition was excited by means of optical pumping, and helium fluorescence at wavelengths of 388 and 706.5 nm was observed. The Doppler temperature of helium atoms was determined by scanning the profile of the absorption line with the help of a tunable laser. The sum of the signals so obtained provides information on the local density of helium atoms. It was proposed to determine the local value of the electron density Ne(R) from the ratio between the fluorescence intensities at wavelengths of 388 and 706.5 nm. The ratio of these intensities as a function of Ne for He I was calculated in the collisional-radiative model, and relevant measurements of Ne in the PNX-U facility were performed. When diagnosing the argon component, the main attention was paid to measurements of the ion temperature Ti(R, t). In the course these measurements, anomalous heating of Ar II ions was revealed. The concentration of singly charged argon ions was estimated.

Stabilization of ballooning modes by nonparaxial cells

An analysis is made of the effect of high-curvature stabilizing nonparaxial elements (cells) on the MHD plasma stability in open confinement systems and in confinement systems with closed magnetic field lines. It is shown that the population of particles trapped in such cells has a stabilizing effect not only on convective (flute) modes but also on ballooning modes, which govern the maximum possible β value. In the kinetic approach, which distinguishes between the effects of trapped and passing particles, the maximum possible β values consistent with stability can be much higher than those predicted by the MHD model.

Toroidal mirror system

A study is made of a toroidally linked mirror system with a zero rotational transform and a three-dimensional magnetic field that ensures good confinement of charged particles. A toroidally linked magnetic mirror configuration at low plasma pressures is calculated by numerically solving the isometry equation for the magnetic field to second order in the small parameter of the paraxial approximation. The calculations carried out with the VMEC code for a particular linked magnetic mirror configuration demonstrate the possibility of achieving good confinement of drifting particles. The calculated results show that it is, in principle, possible to link mirror cells into a toroidal configuration capable of providing plasma confinement at a tokamak level.

Electron cyclotron resonance as a tool for plasma diagnostics in open traps

The authors consider resonance at the harmonics of the electron cyclotron frequency in a magnetic field whose value has an extremum. A calculation is given of the electromagnetic-wave absorption coefficients at electron cyclotron resonance with allowance for different broadening mechanisms for the second and higher harmonics. It is shown that the wave absorption at resonance near the magnetic field extrema is determined by the nature of the absorption line broadening, whereas the absorption coefficient sufficiently far from the extrema is independent of the broadening mechanism and can be defined by a single analytical expression. The possibility of using the results for making local measurements of the plasma density, the electron distribution function and the magnetic field strength in adiabatic traps is discussed.

Studies of hollow carbon nanospheres and grain boundary phase transitions in metals as examples of plasma material science

Results are presented from studies of the material of hollow carbon nanospheres (nanocapsules) that form in tokamaks as a result of plasma interaction with the chamber wall and are similar in structure to meteorite onions. The possibility is demonstrated of studying the material properties in the grain boundary phase transitions in solid materials (metals) by using the plasma-stimulated permeability for hydrogen. The term “plasma material science” is proposed for such material studies.