V.P. Budaev

High energy jets in the Earth’s magnetosheath: Implications for plasma dynamics and anomalous transport

High energy density jets in the magnetosheath near the Earth magnetopause were observed by Interball-1 [1]. In this paper, we continue the investigation of this important physical phenomenon. New data provided by Cluster show that the magnetosheath kinetic energy density during more than one hour exhibits an average level and a series of peaks far exceeding the kinetic energy density in the undisturbed solar wind. This is a surprising finding because the kinetic energy of the upstream solar wind in equilibrium should be significantly diminished downstream in the magnetosheath due to plasma braking and thermalization at the bow shock. We suggest resolving the energy conservation problem by the fact that the nonequilibrium jets appear to be locally superimposed on the background equilibrium magnetosheath, and, thus, the energy balance should be settled globally on the spatial scales of the entire dayside magnetosheath. We show that both the Cluster and Interball jets are accompanied by plasma superdiffusion and suggest that they are important for the energy dissipation and plasma transport. The character of the jet-related turbulence strongly differs from that of known standard cascade models. We infer that these jets may represent the phenomenon of the general physical occurrence observed in other natural systems, such as heliosphere, astrophysical, and fusion plasmas [2–10].

Intermittent transport in the plasma periphery of the T-10 tokamak

Plasma periphery investigation performed in the T-10 tokamak has shown an essential increase of the perpendicular anomalous particle flux in the scrape-off layer (SOL) with an average plasma density rise. The strengthening of the radial transport is found to occur at an average electron density above a threshold level, which depends on a plasma current Ip. The value of the threshold level is about 0.3 times the Greenwald density. Langmuir probe measurements of SOL plasma parameters indicate that intermittent events can play a significant role in the cross-field transport. Intermittent behaviour of the plasma parameters is associated with formation and propagation of the plasma regions (or structures) with high density. The structures move in radial and poloidal directions. Radial movement is predominantly directed to the vacuum vessel wall in the SOL. The radial velocity of the high density plasma structures reduces from 1000 m s−1 near the last closed flux surface to 200 m s−1 at the wall of the vacuum chamber. The radial size of the structures also decreases with minor radius from 3 to 0.5 cm. The poloidal velocity is equal to 1000–1300 m s−1 and is directed towards an ion diamagnetic velocity; the poloidal size of the plasma structures is 2–3 cm. The observed plasma structures can be responsible for more than 50% of the total radial turbulent particle flux. T-10 results support the hypothesis that intermittent convection rather than diffusion can define the cross-field transport.

Effect of rotating helical magnetic field on the turbulence fractal structure and transport in the tokamak edge plasma

Fractal structure of the edge turbulence and enhanced turbulent transport have been studied in the tokamak HYBTOK-II with a variation of the rotating helical magnetic field (RHF) frequency in the range 5–30 kHz. Edge fluctuations have non-Gaussian statistics caused by intermittent bursts with a time scale of 40–100 µs. The variation in the RHF frequency has a selective effect on the fractal structure of edge turbulence and the turbulent flux, demonstrating a selective control of the transport process. A delayed synchronization control of resonant drift wave modes by the RHF is considered as a candidate mechanism to explain the dependence of the effect on the RHF frequency.

Intermittency and extended self-similarity in space and fusion plasma: boundary effects

A comparative study of fluctuation features in the edge plasma of fusion devices and in turbulent boundary layers (TBLs) of the Earths magnetosphere has demonstrated similar statistical characteristics including scalings of structure functions and multifractal spectra. The detected intermittency and anomalous transport of mass and momentum is carried by sporadic plasma flux bursts with nonGaussian probability of flux magnitude. The turbulence exhibits a generalized (extended) self-similarity in an extended scale range. The experimental scalings of the structure functions are rather well fitted by the log-Poisson model considering quasi-1D singular dissipative structures. It appears that the turbulence in the edge plasma of fusion devices and in the TBL of the Earths magnetosphere is governed by cross-field motions similar to hydrodynamic turbulence. Here the experimental scalings from the plasma are available for a comparison with experimental results from neutral fluids. The plasma scalings display universal properties of intermittent turbulence. A statistical approach permits us to evaluate turbulent transport scalings. The time dependence of an average squared particle displacements δx2 ∝ τα infers superdiffusion with α ≈ 1.4–1.87 > 1.

Long-range correlations in the structure of fractal films

Hierarchically granulated films formed on a material subjected to the action of a high-temperature plasma have been microscopically studied. The statistical self-similarity of the structure of the surface from nanoscale to macroscale, as well as long-range correlations in the relief that are due to the statistically inhomogeneous structure of granularity, has been revealed.

The effect of the rotating helical fields on the plasma edge in the HYBTOK-II Tokamak

An experiment with rotating helical magnetic fields (RHMF) in the HYBTOK-II tokamak is reported. The effect of the RHMF rotation with a frequency of 5 kHz on the poloidal rotation of plasma density perturbation was studied by a movable multi-pin Langmuir probe. The wavelet technique has been used to study the poloidal rotation of the bursts. During the RHMF operation the poloidal direction of the plasma density perturbation rotation coincides with the direction of the RHMF rotation. The effect of the RHMF is found to be essential up to minor radius r = 6 cm.

Peculiarities of electrode biasing experiments on the T-10 tokamak in regimes with Ohmic and ECR heating

Two types of electrode biasing H-mode were observed on the T-10 tokamak in regimes with electron–cyclotron resonant heating (ECRH). These types differ mainly by the dynamics of the electron temperature. Both types are characterized by a strong (130 V cm−1) radial electric field formation, a decrease of Dα emission intensity, a rise of line-average plasma density and an increase in energy confinement time. However, distinguishing features of the type II H-mode are a time delay of 70–80 ms in the electron temperature growth, a slower increase of the plasma density, and a weak drop of Dα intensity. The differences between the two types of biased H-mode are likely to be connected with the conditioning of the vacuum chamber.

Control of rotating helical magnetic field penetration into tokamak plasmas using electrode biasing in HYBTOK-II

Detailed measurements of rotating helical field (RHF) have been carried out in a small tokamak device HYBTOK-II in order to know the interaction between the tokamak plasma and RHF. The magnetic pick-up coils were inserted deeply inside the plasma to detect the response profile of tokamak plasma to the RHF. It was found that a large relative velocity between the plasma and RHF poloidal rotation velocities gives rise to a strong attenuation of RHF in the plasma because the large screening current flows around the resonance surface against the externally applied RHF. The electrode biasing was applied so as to increase the relative velocity so that the RHF was strongly attenuated in comparison with unbiased case.

Intermittent structures in the high field side boundary of the HYBTOK-II tokamak

The spatial and temporal characteristics of the turbulence in the inboard and outboard scrape-off layer (SOL) of tokamak HYBTOK-II have been studied using poloidal probe array. Bursty behaviour with intermittent bursts was observed for both outboard and inboard SOL. In the inboard (high field side), high level of density fluctuations has been observed. The fluctuations in the high field side and low field side are identical in statistics that is non-Gaussian one.

PLASMA DEVICE AT NRU «MPEI» FOR TESTING OF REFRACTORY METALS AND CREATION OF HIGHLY POROUS MATERIALS OF NEW GENERATION

Плазменная установка НИУ «МЭИ» предназначена для испытаний тугоплавких металлов и создания высокопористых материалов нового поколения, в том числе для испытаний материалов в обеспечение отечественной программы создания термоядерного реактора (ТИН и ДЕМО) и международного термоядерного реактора ИТЭР. Установка представляет собой плазменную ловушку с линейной мультикасповой конфигурацией магнитного поля со стационарным плазменным разрядом с параметрами плазмы, обеспечивающими возможность мощной плазменно-тепловой нагрузки, ожидаемой на материалы в стационарных режимах термоядерного реактора. В экспериментах планируется разработать новую технологию создания высокоразвитой и высокопористой структуры поверхности тугоплавких металлов, в том числе так называемый вольфрамовый «пух» с размером пор и нановолокон ~50 нм, обладающих новыми физико-химическими свойствами, что представляет значительный интерес для ядерных, химических, энергетических, биомедицинских технологий.