Физика плазмы

Abstract

The T-15MD tokamak is equipped with a gyrotron setup, which currently includes one gyrotron with an operating outlet frequency of 82.6 GHz and a power of 1 MW. The length of the waveguide path from the gyrotron to the tokamak is 37 m. A significant result obtained earlier has been the measurement of HF‑radiation power using a small calorimetry load (0.95 MW at a pulse duration of 125 ms). The results of the first joint tests of a gyrotron and a waveguide path for a dummy load in a long pulse operation from a “V-iktoriya” high-voltage power supply are presented. The pulse duration of 9.4 s is achieved. The estimated microwave radiation power is 0.85 MW.

Abstract

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. Increasing the number of coils \(n\) of the toroidal field reduces the effect of ripple as \( {\sim} {\text{1/}}{{n}^{2}}\) . The applicability of the standard theory to finding the frequency and spatial structure of GAM in large tokamaks is shown.

Abstract

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.

Abstract

We investigated the correlation between density perturbations (bursts), having stochastic or intermittent character and magnetic fluctuations detected by Mirnov coils. To achieve this, we designed and constructed a novel Mixed-Probe system in the IR-T1 tokamak. Our experimental approach involved two steps: first with Resonant Helical magnetic Fields (RHF) and then without RHF. The conditional averaged analysis of the probe ion saturation current ( \({{\tilde {I}}_{{{\text{sat}}}}}\) ) revealed that stochastic density bursts or intermittencies were more likely to exist during the time interval between t = 10 and 12 ms. Before applying RHF, the intermittencies exhibited a strong cross-correlation with magnetic fluctuations. The predominant frequencies observed are below 7 kHz. Interestingly, after applying RHF, the peaks in the frequency of the \({{\tilde {I}}_{{{\text{sat}}}}}\) signal disappeared, and there was no predominant peak in the FFT of \({{\tilde {I}}_{{{\text{sat}}}}}\) . We further analyzed the radial and poloidal velocities of bursts and magnetic fluctuations using diagnostic data. The results suggested that magnetic fluctuations induced oscillations in the background plasma, affecting the radial motion of the density perturbations. Notably, the poloidal velocity of the magnetic field perturbations exceeded that of the bursts, allowing it to shear the poloidal plasma flux and generate the density bursts. The differing velocities confirmed their coupling for a specific time, followed by decoupling. Overall, in the IR-T1 tokamak, applying RHF reduced the cross-correlation between magnetic fluctuations and stochastic density bursts.

Abstract

The analysis of possible divertor working regimes and edge plasma parameters for TRT tokamak project is performed basing on modeling. It is shown that for the separatrix power of 18 MW corresponding to approximately twice higher full input power the low divertor integral heat flux 5 MW/m2 can be provided for the separatrix plasma density lower than 7 × 1019 m–3 and the effective charge Zeff lower than 2. These parameters are realistic for this device. In case of bigger separatrix power the working regime is possible with higher divertor heat load still within the technological limits of the machine. Modeling also shows positive effect of the increase of the distance between the separatrix and the vacuum vessel structures and better performance of the corner divertor configuration comparing to the “ITER-like” one.

Abstract

It is demonstrated experimentally that the thermophoretic force acting upon microparticles in the thermal field in complex plasma can be used for effective control of a cloud of charged microparticles formed in an electrostatic trap in the positive-column stratum of a glow discharge. Variation in the thermal-field temperature gradient is found to cause changes in the cloud location in plasma volume, its shape and size, along with suppression of oscillations of microparticles in the directions transverse with respect to this gradient. Microparticles of larger size experience stronger thermal action, and damping of microparticle oscillations occurs in conjunction with changes in the cloud spatial position. Obtained experimental results are consistent with theoretical concepts of the phenomena under consideration.

Abstract

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 jet-screen 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.

Abstract

A phenomenological gasdynamic model of the compression of the gas Z-pinch neck through whose ends the plasma flows out at a high velocity was considered. Calculations showed that in this process, conditions are created under which the relaxation of the ion plasma component is delayed compared to the macroscopic compression dynamics. Therefore, the description of the Z-pinches at their maximum compression stage has to account for the ion kinetics. This approach can explain the mechanism of the ion acceleration to high energies as well as the high intensity of the neutron radiation at the final stage of the neck compression.

Abstract

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.

Abstract

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 in 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 one to consider 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 the extraordinary mode at the second harmonic are considered. For each case, the ranges of the setup parameters are determined in which a considered heating scheme can be effective; the optimal focusing of the quasi-optical microwave beam for heating efficiency is analyzed.

Abstract

The derivation of the complete differential expansion of the integral of elastic collisions of electrons with heavy neutral particles is performed for the case when the electron distribution function is not symmetric with respect to some direction. The derivation is made under the assumption that the kinetic energy of electrons greatly exceeds the energy of thermal motion of atoms and molecules. It is shown how the resulting expansion can be used to derive equations for the moments of the electron distribution function.

Abstract

The results of a study of plasma pyrolysis of methane using a DC plasma torch with hollow electrodes are presented. Data on the effect of the ratio of methane consumption in the plasma torch and the reactor connected to it on the composition of pyrolysis products have been obtained. The arc power of the plasma torch is 40–50 kW, the methane consumption is ~0.7–1.6 g/s. The ratio of methane flows supplied to the reactor and the plasma torch varied within the range of 0–1.63. Results demonstrate that increasing this flow ratio decreases the hydrogen concentration while increasing the proportion of unconverted methane. The acetylene yield exhibits a maximum within the ratio range of 0.6–1.3, reaching a volumetric concentration of 10.52%. The methane conversion rate in the plasma torch reaches 98–99%, and the volumetric hydrogen concentration ranges from 92 to 97%.

Abstract

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.

Abstract

The results of heat and particle transport simulations for ohmic plasma in the T-10 tokamak with a circular limiter and for D-shaped plasma in the COMPASS tokamak with a divertor are presented. In addition, the H-mode with ohmic heating and with additional heating by the neutral beam injection (NBI) in the COMPASS was simulated. The simulations were carried out with the Canonical profile transport model (CPTM) using the ASTRA code. The obtained electron temperature and density profiles agree with the measured ones with standard deviations within the experimental accuracy of 10–15%. The calculations demonstrated very similar density profiles in the H-mode both with the ohmic and with additional NBI heating. The electron temperature profiles in the H-mode with additional heating have higher pedestals than in the ohmic H-mode, that agree with the measurements. The comparison showed that the ohmic regimes in COMPASS and T-10 can be described by the same stiffness coefficients in the heat and particle transport equations.

Abstract

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 O \(_{2}^{ + }\) , N \(_{2}^{ + }\) , O \(_{2}^{ - }\) , and 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 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.

Abstract

The characteristics of water spray have been measured in the presence of dielectric barrier discharge in the region where the film of liquid flowing out of the nozzle hole disintegrates into droplets. The discharge has been initiated in the region between the water film and the high-voltage electrode surrounded by a dielectric material. The measurements were performed using the direct shadow method based on obtaining multiple shadow instantaneous microphotographs of droplets. Current and voltage oscillograms in the barrier discharge initiation circuit have been recorded. The average parameters, such as the average diameter and Sauter mean diameter, of droplets have been compared in two cases: without discharge initiation and when it has been created in the region where the spray is formed, at a voltage frequency of 5 kHz and its amplitude of 10 kV. It is shown that in the case of discharge initiation, the average parameters of droplets significantly decrease and their number increases.

Abstract

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.