Search

Temporal characteristics of density bursts in the scrape-off layer of the CASTOR tokamak were investigated. Intermittent positive bursts of density are observed by means of a radial array of Langmuir probes. Globally (comparing the closest and furthermost radial positions with respect to the center of the discharge chamber) we observe a radial decrease of average burst rate together with a radial increase of the average burst duration. Recently, we observed a monotonic radial decrease of average burst rate together with an increase of the average burst duration in the Tore Supra tokamak [1, 2], but the decay length is significantly shorter than in CASTOR. This is most probably due to radially elongated turbulent events (streamers), which govern the radial transport at the edge of the CASTOR tokamak [3] and are responsible for the appearance of density bursts.

The 2D boundary layer code TECXY has been used to simulate edge plasma parameters in the Tore Supra tokamak. Experimental situations have been analysed when the plasma contact point is shifted to different poloidal positions (limiter at bottom, top, outboard and inboard midplane) as well as the edge plasma conditions during lower hybrid RF power scan experiments. With the results of the calculations it was possible to reproduce experimentally observed changes to the flow pattern in the scrape-off-layer (SOL) for different plasma positions. These changes can be associated with the dependence of the radial electric field and the corresponding E × B drift velocity on the limiter position as well as to the poloidal asymmetries of the input particle flux. However, the code results with 'standard radial transport' did not show the large changes to the SOL widths as observed in the experiment. In order to reproduce in the calculations the observed differences in the SOL width, it was necessary to assume strong convective (or diffusive) radial plasma flow localized at the outboard midplane.

The measurements of ion saturation current fluctuations in the scrape-off layer of the Tore Supra tokamak are presented. As the radial distance from last closed flux surface increases more and more rare, but larger positive bursts become dominant in the signal and its probability density function becomes strongly asymmetric with pronounced positive tail. At the same time burst duration and inter-burst time increase dramatically. We explain this phenomenon by radial propagation and dynamics of the ensemble of different size large scale coherent structures.

Near-sonic parallel flows are systematically observed in the far scrape-off layer (SOL) of the limiter tokamak Tore Supra, as in many L-mode X-point divertor tokamak plasmas. The poloidal variation of the parallel flow has been measured by moving the contact point of a small circular plasma onto limiters at different poloidal angles. The resulting variations of flow are consistent with the existence of a poloidally localized enhancement of radial transport concentrated in a 30° sector near the outboard midplane. If the plasma contact point is placed on the inboard limiters, then the SOL expands to fill all the space between the plasma and the outboard limiters, with density decay lengths between 10 and 20 cm. On the other hand, if the contact point lies on the outboard limiters, the localized plasma outflux is scraped off and the SOL is very thin with decay lengths around 2–3 cm. The outboard radial transport would have to be about two orders of magnitude stronger than inboard to explain these results.

ICRF-induced SOL modifications are mapped for the first time in 2-D around Tore Supra ICRF antennas using reciprocating Langmuir probes. When probe heads are magnetically connected to powered antennas, radical modifications of floating potentials Vfloat, effective temperatures Teff and ion saturation currents are observed. Vfloat perturbations are located radially near antenna limiters, with a typical extension 2 cm. Poloidally they are locally minimal near the equatorial plane, and maximal near antenna box corners. Two possible interpretations for increased Teff are proposed: localised electron heating and RF loop voltage induced along probe circuit. Both interpretations rely on the generation of parallel RF fields by parallel RF currents on the antenna structure. The topology of such currents could explain the 2-D structure of Teff maps. Both interpretations also imply a positive DC biasing of the antenna environment. Differential biasing of nearby flux tubes drives DC E × B0 convection that could explain 2-D density patterns.

We present the measurements of plasma characteristics in the scrape-off layer (SOL) of the Tore Supra tokamak performed by means of reciprocating Langmuir probe. The probe is inserted into the machine from top. As the radial distance from last closed flux surface (LCFS) increases, ion saturation current exhibits stronger bursty character and its probability density function becomes increasingly skewed towards positive values. At the same time, burst duration and inter-burst time increase dramatically. We explain this phenomenon by radial propagation and dynamics of the ensemble of coherent turbulent structures of different size. The results of two-dimensional fluid modelling based of flux-driven interchange instability mechanism are in excellent agreement with experimental results. We obtained clear experimental evidence that most of the coherent structures are formed in poloidally localized region of the SOL around the outboard midplane. If the probe is magnetically connected to this region, the SOL is very wide and we detect bursty behaviour in the far SOL. On the other hand, if the probe is not magnetically connected to the outboard midplane region (magnetic field lines are intercepted by the outboard limiter), then the SOL is very thin and bursty behaviour is much less prominent. Detection of bursty behaviour in the far SOL is correlated with existence of wide SOL pointing on important role of bursty transport by means of coherent turbulent structures in establishing the width of the SOL in tokamaks. The measurements of parallel flow in the SOL shows that plasma particle radial flux coming from confinement region to the SOL is mostly poloidally localized around the outboard midplane. Our estimations show that more than 80% of plasma particle radial flux is coming from confinement region to the SOL in poloidally localized region — approximately ±15° — around the outboard midplane.

Plasma mechanism for the generation of toroidal magnetic field in the magnetosphere of Crab pulsar is presented. The mechanism is based on the development of parametric type instability in the relativistic electron-positron plasma of the pulsar magnetosphere. As a result of plasma corotation with pulsar and its magnetic field, the effect of plasma radial braking takes place and the time dependence of plasma particle radial velocity is harmonic. This triggers the development of parametric type instability in the relativistic plasma of the pulsar magnetosphere. The energy for this process is drawn from the slowing down of pulsar rotation.

Results of investigation of the shock wave dynamics under subatmospheric pressure in neutral gases and weakly ionized low temperature plasma are presented. The characteristics of spherical and plane configuration shock wave excitation and propagation in gases in the pressure region 1 Torr < p

A new plasma mechanism for the magnetic field generation in the Crab Nebula is put forward. The mechanism is based upon the development of a two-stream instability resulting from the supernova explosion which took place in 1054 and led to the formation of the Crab Nebula.

The properties of plasma injected into an open magnetic trap of uniform field from an independent UHF source have been investigated. Plasma is created in the UHF source at the frequency of 2400 MHz (power input 150 W) in the electron cyclotron resonance (ECR) regime at the pressure of neutral argon (10−5−10−2) torr. It is established that a rather quiescent target plasma with controlled density within the range of (2 × 108−2 × 1012) cm−3 and temperature 2–3eV is accumulated in the trap. It turned out that plasma lifetime in the trap is determined by a classical mechanism of particle escape at the expense of collisions, at fixed value of magnetic field in the trap it practically is not changed with the variation of neutral gas pressure and reaches the value ≈ 4×10−3 s at the magnetic field strength in the trap equal 1600 Oe.

In the present paper the results of the experiments on the shock wave dynamics under subatmospheric pressure in neutral gases are presented. The characteristics of spherical and plane configuration shock wave excitation and propagation are studied in the pressure region 1 torr

The equation of the motion of the relativistic plasma stream which corotates with the pulsar magnetosphere written in the noninertial frame of a rotating magnetic field line and the same equation written in the rest inertial frame are compared with each other. The possibility of the freezing-in condition use in the rest inertial frame is discussed.

A gedanken experiment of bead motion along a rotating pipe with relativistic velocities is discussed. It is shown that when v 0>c/√2,the centrifugal acceleration changes its sign and braking has begun. It is pointed out that in investigating pulsar magnetospheres, we must take into account the effect of rotating relativistic stream braking.

An investigation of the kinematics of a rotating relativistic plasma stream in the perpendicular rotator model of the pulsar magnetosphere is presented. It is assumed that the plasma (ejected from the pulsar) moves along the pulsar magnetic field lines and also corotates with them. The field lines are considered to be radial straight lines, located in the plane which is perpendicular to the pulsar rotation axis. The necessity of taking particle inertia into account is discussed. It is argued that the "massless" ("force-free") approximation cannot be used for the description of this problem. The frame selection is discussed and it is shown that it is convenient to discuss the problem in the noninertial frame of ZAMOs (Zero Angular Momentum Observers). The equation of motion and the exact set of equations describing the behaviour of a relativistic plasma stream in the pulsar magnetosphere is obtained. The possible relevance of this investigation for the understanding of the formation process of a pulsar magnetosphere is discussed., Plain LaTeX

The behaviour of a relativistic electron-positron plasma in the pulsar magnetosphere is investigated. The equation of the motion of the magnetospheric plasma is discussed, from which it follows that, if the plasma particle radial velocity ${V_r}>{c/{\sqrt 2}}$ (where c is the speed of light), the centrifugal acceleration changes its sign and the particle braking begins. The stability of the magnetospheric plasma with respect to the radially oriented potential perturbations is discussed and the possibility of the electrostatic field generation in this plasma along the pulsar magnetic field lines is shown., Plain LaTeX

Temporal characteristics of intermittent positive bursts of plasma density in the scrape-off layer of the CASTOR and Tore Supra tokamaks are investigated. A monotonic radial decrease of average burst rate together with an increase of average burst duration is observed in the Tore Supra tokamak [I. Nanobashvili et al., Czech. J. Phys. 56, 1339 (2006); I. Nanobashvili et al., J. Nucl. Mater. 363–365, 622 (2007)]. At the edge of the CASTOR tokamak the temporal characteristics stay nearly constant within a quite wide radial region. The reason is that radially elongated turbulent structures (streamers) govern the radial transport in CASTOR [P. Devynck et al., Phys. Plasmas 13, 102505 (2006)]. A comparative analysis allows to conclude that radial transport at the edge of the Tore Supra tokamak is governed by blob-like structures. Plasma biasing at the edge of CASTOR significantly modifies the burst characteristics, reduces plasma transport, and considerably improves the confinement.