Search

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.

The tunnel probe is a new kind of Langmuir probe for use in the tokamak scrape-off layer. Two-dimensional kinetic analysis of the ion current distribution on the concave conductors is used to calibrate the probe for making fast dc measurements of ion flux and electron temperature. Qualitative agreement with classical Langmuir-probe measurements is found, but the electron temperature given by the tunnel probe is several times lower. This discrepancy is partly caused by secondary electron emission, and can easily be corrected. Strong eduction of ion-flux and electron-temperature fluctuations is observed during electrode biasing.

A simple fluid model is used to interpret two edge flow phenomena that are observed in the scrape-off layer of the Tore Supra tokamak. (1) The Mach number of the parallel flow midway between the two sides of the toroidal limiter is always a significant fraction of the ion sound speed, typicallyM♭=−0.3÷−0.6. directed towards the high field side. In Tore Supra theE×B drift is relatively unimportant, so the esixtence of alarge parallel flow implies a significant poloidal aysmmetry of the SOL source. (2) Massive injection of neutral gas leads to a recycling source that is poloidally localized near the nozzle. The parallel Mach number changes during injection. The sign of the change is positive for injection on the high field side, and negative for injection on the low field side. The ionization source that is needed to produce the change is about 5÷10% of the total amount of injected gas, consistent with atomic physics calculations.

An essential ingredient of particle-in-cell (PIC) codes is a numerically accurate and stable integration scheme for the particle equations of motion. Such a scheme is the well known time-centered leapfrog (LF) method [1] accurate to 2nd order with respect to the timestep Δt. However, this scheme can only be used forces independent of velocity unless a simple enough implicit implementation is possible. The LF scheme is therefore inapplicable in Monte-Carlo treatments of particle collisions [2] and/or interactions with radio-frequency fields [3]. We examine here the suitability of the 2nd order Runge-Kutta (RK) method. We find that the basic RK scheme is nummerically unstable, but that conditional stability can be attained by an implementation which preserves phase space area. Examples are presented to illustrate the performance of the RK schemes. We compare analytic and computed electron orbits in a traveling nonlinear wave and also show self-consistent PIC simulations describing plasma flow in the vicinity of a lower hybrid antenna.

The tunnel probe is a new kind of Langmuir probe for use in the tokamak scrape-off layer. It provides simultaneous measurements of electron temperature and parallel ion current density with high frequency at the same point in space. We describe ongoing work to characterize the ion flows within the probe, and to calibrate the diagnostics using 2D kinetic simulations.

New experimental evidence of the correlation between edge sheared E×B flow and reduction of turbulence has been measured in the CASTOR tokamak (R = 0.4 m, a = 0.085 m, B T = 1 T). A biasing electrode is placed at the separatrix in a configuration which has demonstrated strongly sheared electric fields and consequent improvement of the global particle confinement. A set of movable electrostatic probes (rake, Langmuir, Gundestrup, and rotating Mach) provide redundant, simultaneous measurements of poloidal flow, toroidal flow, electron temperature, density, and radial electric field with high temporal resolution and at the same poloidal location. Particular effort has been made in the optimization of the Gundestrup probe collector geometry in order to reduce the relative uncertainty of Mach number measurements in plasmas with weak flow (M ∥, M ⊥ < 0.1). The measurements from the rake, Gundestrup, and rotating Mach probes give three independent radial profiles of E×B shear in ohmic and biased modes. Good agreement is obtained both for the profile shape and its absolute magnitude. The plasma flows, especially the poloidal E×B drift velocity, are strongly modified in the sheared region, reaching Mach numbers as high as half the sound speed. The corresponding shear rates (≈5×106 s-1) derived from both the flow and electric field profiles are in excellent agreement and are at least an order of magnitude higher than the growth rate of unstable turbulent modes as estimated from fluctuation measurements. In addition, we compare the measured E×B ion mass flow with the phase velocity of fluctuations moving poloidally across the Gundestrup collectors. Given the poloidal separation of the collecting plates and the sampling frequency (5 MHz), the maximum detectable phase velocity turns out to be rather modest compared to the measured bulk poloidal flow speed. Therefore the two quantities are only compared when the poloidal speed is low; in that case, they show similar behaviour in response to the applied bias.

Particle collection by the ALT-II belt limiter in the TEXTOR-94 tokamak is exclusively limited to the parallel outflux because the scoop walls that are oriented parallel to the field lines obstruct the poloidal E×B mass flow. With normal B tor direction (E×B towards the scoops), a threefold decrease of plenum pressure is measured during negative biasing, while with reversed B tor (E×B away from the scoops), a 60% pressure increase is observed. This behaviour is exactly opposite to that observed in X-point divertors. A simple fluid model explains this apparent contradiction, and gives good quantitative agreement with measurements of the parallel Mach number in the SOL. The essential physics is governed by the Bohm-Chodura criterion.

The interaction of high power lower hybrid waves with tokamak plasma results in a range of favourable but also unfavourable effects. This article deals with the effects observed by Langmuir probes on the small tokamak CASTOR. The most pronounced effects found are positive biasing of the whole plasma periphery and the formation of a negative "well" of floating potential in front of the launcher.

The 2D multifluid code TECXY has been used to model the experimental biasing of the toroidal belt limiter ALT-II on the tokamak TEXTOR-94 with respect to the first wall. It is well known that the edge flow pattern can be influenced by the poloidal electric drifts from imposing radial electric fields. The modelling with TECXY introduces imprinted bias currents in the scrape-off layer for the case of negative (limiter) biasing, and imprinted bias potentials for the case of positive biasing. This allows to simulate sufficiently well the experimental current-voltage characteristics for either biasing and also reproduces the essential features and trends of the observed plasma profiles in the SOL. A moderate negative biasing is found to be optimal for the exhaust efficiency of the pump limiter.

The superconducting tokamak Tore Supra will be equipped with an actively cooled toroidal pump limiter (TPL), in the framework of the CIEL (Composants Internes Et Limiteurs) project, dedicated to plasma facing component design for steady state operation. The TPL is equipped with throats, located only on the high field side, for particle collection allowing the control of plasma density which is essential for long plasma discharges. The present design work of the CIEL includes a biasing system in order to enhance the particle pumping. A fluid model, based on the classical fluid equation, is used to estimate the effects of the electric field on the particle flows in the Scrape-Off Layer (SOL). The modifications of the density, the particle flow (toroidal and poloidal) and the position of the stagnation point are discussed as a function of the bias voltage. The model clearly illustrates the different resulting effects on particle pumping for a divertor and a limiter configuration which are designed respectively for poloidal or parallel particle collection.