Volume 50, Nº 12 (2024)

The influence of the toroidal magnetic field ripples on the spatial structure and frequency of the geodesic acoustic mode (GAM) in tokamak plasma has been investigated. It is shown that the toroidal asymmetry of the magnetic configuration leads to coupling of oscillations of the GAM electric potential with toroidal and poloidal inhomogeneous perturbations of the plasma pressure. For tokamaks with a large aspect ratio, the GAM dispersion law is derived, taking the non-uniformity of the ripple in the tokamak cross-section into account. As the number of the toroidal field coils n increases, the effect of ripples ceases to depend on n.The applicability of the standard theory to finding the frequency and spatial structure of GAM in large tokamaks is shown.

The question of the existence of internal transport barriers (ITBs) around low-rational number values of the stability factor q = 1, 2, … in the usual L-mode of tokamaks with central additional heating and positive magnetic shear remains open. To clarify the existence of such ITBs, experiments were carried out with a programmed linear time shift of the plasma column, in which the positions of the electron temperature measurements Te of the emission at the second electron cyclotron harmonic are shifted relative to the plasma column, which allows one to study the Te profile details. A series of experiments with perpendicular input of EC radiation with a power of 0.4 and 0.85 MW at a central ECRH and a fast (60 ms) shift of the column by 0.13a (minor plasma radius a) were carried out on the T-10 tokamak in plasma with a carbon limiter. In both cases, outside the q = 1 surface narrow (about 0.03a wide) and weak (a decrease in the coefficient of electron thermal conductivity χe by approximately two times) ITBs detected, which disappeared when the discharge parameters changed. These ITBs are 2–3 times narrower and an order of magnitude weaker than the ITBs near the surface q = 1, which was previously proposed to explain the RTP tokamak results. ITBs were not detected in the L-mode in experiments with simultaneous generation of a co-current and counter-current in the plasma center by two gyrotrons with a total power of 1.5 MW in a plasma with a tungsten limiter (column shift by 0.1a in 30 ms). This article appears to be the first journal publication on the study of Te profile details with rapid movement of the plasma column.

The possibility of using the method of dual-wavelength digital holographic interferometry to assess the wear of protective elements of the Globus-M2 spherical tokamak after working plasma discharges is demonstrated. At this stage of the work, the protective elements were removed from the tokamak discharge chamber and used as samples in the holographic setup. A diagram of a holographic interferometer for recording primary holographic images is presented, in which control of the radiation wavelength recording and monitoring systems is carried out through a hardware and software complex in real time. The results of measurements of the shape of tokamak elements are presented. It is shown that when the difference in wavelengths changes, the sensitivity of the measurement method changes, and in the proposed configuration of the optical scheme it is possible to determine the minimum value of the shape change at a level of 10–30 μm. At the same time, the error in determining the phase difference, by which the surface profile is assessed, in the digital method can reach about 2π/40.

Possible scenarios of auxiliary electron cyclotron plasma heating in various configurations of the next-generation open magnetic trap GDMT (Gas-Dynamic Multiple-mirror Trap) designed at the Budker Institute of Nuclear Physics are considered. For this purpose, a full-wave impedance approach is used to model the interaction of electromagnetic waves with hot plasma, which allows taking into account the interaction of electromagnetic and quasi-electrostatic modes in the vicinity of the electron cyclotron resonance in an axisymmetric magnetic configuration. Heating scenarios using the ordinary electromagnetic mode at the fundamental harmonic and extraordinary mode at the second harmonic are considered. For each case, the ranges of the setup parameters in which such a heating scheme can be effective are determined; optimal for the heating efficiency focusing parameters of the quasi-optical microwave beam are analyzed.

In a number of experiments, the surfaces of condensed matter, for example, the electrodes of pulsed power facilities, are exposed to powerful pulsed X-ray radiation with an energy flux density of ~1 TW/cm². The source of this radiation can be, for example, Z-pinches formed by current compression of multi-wire liners. Under the effect of this radiation, evaporation and plasma formation processes can occur on the surface of the electrodes. This paper provides a theoretical examination of these processes. In the case where the plasma layer thickness is small compared to the characteristic dimensions of the electrodes, plasma formation can be described by one-dimensional equations of magnetohydrodynamics taking radiation transfer into account. One-dimensional calculations performed for the experimental conditions at the Angara-5-1 facility (energy flux density coming from the pinch, ~0.2 TW/cm², radiation exposure time ~15 ns, electrode material Fe), have shown that the characteristic plasma temperature in this case is ~40 eV, density ~3 mg/cm³, and its expansion speed is ~60 km/s. It is interesting that the magnetic fields in these experiments, which are relatively small (~0.8 MG) and are incapable to lead to plasma formation, restrain the expansion of the plasma with their pressure and affect its characteristic values and expansion speed. The speed obtained in the calculation is somewhat less than that measured experimentally using an X-ray electron- optical converter (~90 km/s), that may be due to not one dimensional turbulent plasma expansion or due to experimental errors.

The interaction of a circularly polarized electromagnetic wave with a layer of photoionized inert gas plasma in the magnetic field has been studied. A detailed analysis of the reflection and transmission coefficients of the wave under conditions where the wave frequency is the same as the photoelectron cyclotron frequency is given. The possibility of a strong increase in the reflection and transmission coefficients, when negative small absolute values of the imaginary part of the photoionized plasma dielectric permittivity are released, has been revealed. It is shown that in the photoionized plasma layer obtained in the process of fast multiphoton ionization of xenon atoms at atmospheric pressure, there is a possibility of increasing the field strength of terahertz radiation by more than two orders of magnitude.

The results of a study of the interaction of a powerful flow of hydrogen plasma with a supersonic gas jet in front of a tungsten target are presented. Nitrogen or neon injected in front of the target surface provides a reliable method of shielding tungsten from direct exposure to hydrogen plasma. It has been experimentally shown that the resulting plasma of the gas jet is a powerful source of short-wave line radiation. Energy density absorbed by a tungsten target ≈25 J/cm² is half the energy absorbed by tungsten during pulsed action of a hydrogen plasma flow without a gas jet ≈50 J/cm². The maximum temperature achieved by the tungsten surface is ≈3700 K with the use of a gas jet and ≈5800 K without a gas jet. The presence of a gas jetscreen in front of the tungsten leads to the localization of evaporated tungsten near the target at distances of up to 1 cm from the surface.

Using an approximate approach that is valid for a large ratio of the ion mass to the mass of the neutral particle, the mobility of ${\mathrm{O}}_2^{+}$, ${\mathrm{N}}_2^{+}$, ${\mathrm{O}}_2^{-}$, and ${\mathrm{O}}_4^{-}$ ions was calculated in helium and good agreement was obtained with the available results of Monte Carlo calculations and experimental data at high values of the reduced electric field. This simplified approach was used to determine the average energy of ${\mathrm{O}}_4^{-}$ ions and the rate constants of dissociation of these ions in helium. The obtained ion characteristics were compared with the results of more accurate calculations using the Monte Carlo method. Good agreement has been obtained between these two approaches for the average ion energy, but for the dissociation rate constant the difference is quite significant, reaching an order of magnitude or more. It was shown that this difference is associated with the peculiarities of the energy distribution of ions for a large ratio of the ion mass to the mass of the neutral particle.