A. I. Smirnov

Weakly ionized plasmas in aerospace applications

This paper is an overview of the activity and state-of-the-art in the field of plasma aerospace applications. Both experimental results and theoretical ideas are analysed. Principal attention is focused on understanding the physical mechanisms of the plasma effect on hypersonic aerodynamics. In particular, it is shown that drag reduction can be achieved using a proper distribution of heat sources around a flying body. Estimates of the energetic efficiency of the thermal mechanism of aerodynamic drag reduction are presented. The non-thermal effect caused by the interaction of a plasma flow with a magnetic field is also analysed. Specifically, it is shown that appropriate spatial distribution of volumetric forces around a hypersonic body allows for complete elimination of shock wave generation. It should be noted that in an ideal case, shock waves could be eliminated without energy consumption.

Scalar equation for wave beams in a magnetized plasma

A heuristic procedure is proposed for deriving quasi-optical equations for wave beams in anisotropic gyrotropic media with allowance for aberrations, spatial dispersion, and absorption. To solve such equations numerically, a method is developed that generalizes the operator exponent method. The applicability limits of the aberration-free approximation for simulating the propagation of beams in absorbing media are determined. Numerical examples of the propagation of beams in the vicinity of the electron cyclotron resonance in plasmas in actual devices are presented.

Effect of dissipation on the propagation of wave beams in inhomogeneous anisotropic and gyrotropic media

For wave beams propagating in inhomogeneous anisotropic absorbing media with spatial dispersion, a quasi-optical approximation is developed that makes it possible to account for the combined influence of the refraction, diffraction, and dissipation effects. It is shown that, in the aberration-free approximation, the problem of calculating the beam structure is reduced to that of solving a set of ordinary differential equations for the parameters of the kernel of an integral transformation and calculating the convolution with the spatial Fourier spectrum of the initial field distribution. In particular, the case of a Gaussian beam is analyzed. The applicability limits of the aberration-free solution, which are especially relevant to the ECR absorption regime, are discussed. The effect of aberrations associated with the Hermitian and anti-Hermitian parts of the dielectric tensor of the medium is considered. It is found that the beam deviates toward the region of weaker absorption and that, during the deviation, the beam may become wider or narrower, depending on the type of the inhomogeneity. It is demonstrated that, when absorption is taken into account correctly, the width of the power deposition region during plasma heating in controlled fusion devices can turn out to be substantially larger than that given by the existing estimates.

Two-Wire Microwave Resonator Probe

Results are presented from theoretical and experimental studies of the influence of ponderomotive effects on the operation of a two-wire plasma microwave resonator probe. It is shown that the nonlinear regime of probe operation can be used to measure not only the plasma density, but also the plasma temperature.

H-mode studies on TUMAN-3 and TUMAN-3M

The focus of the TUMAN-3 and TUMAN-3M tokamaks programme is on issues of improved confinement. The transition from an ordinary ohmic regime into improved confinement mode has been found in circular limiter configuration in a vessel with all-metallic walls and limiters. The signatures of the H-mode in auxiliary heated tokamaks have been observed in this regime. The crucial role of the radial electric field was found in experiments with internal probe biasing. Other techniques were demonstrated to trigger H-mode: short increase of the working gas puffing rate, minor radius magnetic compression and pellet injection. The scaling of the energy confinement time in ohmic H-mode was obtained, which differs dramatically from the scaling for the ordinary ohmic regime. A strong dependence of on plasma current was found. The scaling for the ohmic H-mode is consistent with the scaling proposed for devices with powerful auxiliary heating (JET/DIII-D H-mode scaling). The result shows that H-mode physics is universal in tokamaks with different geometries and heating methods. In 1994 a new vacuum vessel was installed in the TUMAN-3 tokamak. The modified device, TUMAN-3M, is able to produce higher and , up to 2 T and 0.2 MA, respectively. During the first operational period a plasma current of 0.15 MA was achieved at T, which corresponded to . The impact of the quality of wall coating on confinement was asserted. The longest energy confinement time (30 ms) was observed under the conditions of best boronization.

Quasi-optical diffraction grill for excitation of lower-hybrid waves in tokamaks

A theoretical investigation of infinitely periodic quasi-optical grill structures for excitation of lower-hybrid waves in toroidal plasmas has been undertaken for cylinder grill elements with an arbitrary cross section. The basis for investigation is the expansion of the scattered RF field into the infinite series of spatial harmonics which makes it possible to obtain the integral equation for the distribution of magnetic current over the surface of grill elements and to calculate the efficiency of LH wave excitation depending upon the LH wave slowing-down and parameters of the grill structure and of the plasma layer. The numerical simulation performed for the single-layer grill, with elements of elliptical cross section irradiated from vacuum by infinite plane waves, demonstrates the possibility of achieving a high efficiency of up to 25 - 30% of the unidirectional LH wave excitation with the slowing-down , which is relevant to large fusion installations. The calculations have also been performed for the grill structure irradiated by quasi-optical RF beams with finite cross section; it is shown that the coupling efficiency degrades insignificantly if the RF beam aperture (which may be of the order of the size of the RF port in the direction of the toroidal magnetic field) essentially exceeds the vacuum wavelength of radiation. The results obtained seem to be encouraging for construction of a double-layer grill with high efficiency close to unity and low Q-value of the quasi-optical cavity, resulting in a comparatively low value of RF field intensity near the grill elements and in a weak dependence of grill efficiency upon plasma parameters.

One-dimensional model of a helicon discharge

A simplified model describing the steady state of a helicon discharge in a low-pressure plasma is considered. The electron Langmuir frequency of the plasma produced by the discharge is shown to be much higher than the electron gyrofrequency. It is found that the gas medium is ionized and the electrons are heated primarily by the extraordinary mode. The calculated electron density depends nonmonotonically on the magnetic field, in agreement with the results of numerous experiments.

Formation of an internal transport barrier in the ohmic H-mode in the TUMAN-3M tokamak

In experiments on studying the ohmic H-mode in the TUMAN-3M tokamak, it is found that, in high-current (Ip∼120–170 kA) discharges, a region with high electron-temperature and density gradients is formed in the plasma core. In this case, the energy confinement time τ E attains 9–18 ms, which is nearly twice as large as that predicted by the ELM-free ITER-93H scaling. This is evidence that the internal transport barrier in a plasma can exist without auxiliary heating. Calculations of the effective thermal diffusivity by the ASTRA transport code demonstrate a strong suppression of heat transport in the region where the temperature and density gradients are high.