2D DC potential structures induced by RF sheaths coupled with transverse currents in front of ICRF antennas

12th International Congress on Plasma Physics, 25-29 October 2004, Nice (France)Sheaths are space charge regions at the plasma-wall. They are induced by the differential inertia between ions and electrons, and without external perturbation, they create a floating potential between the neutral plasma and the walls. In Tokamaks, these sheaths are locally enhanced by the RF (radiofrequency) electric field generated by the ICRF (ion cyclotron resonance frequency) antennas used to heat magnetic fusion plasmas at very high temperature. RF sheaths are located at the connection points of magnetic field lines to the wall, or to the bumpers which protect the antenna or to any part of the antenna structure. The asymmetric behaviour of these oscillating sheaths rectifies RF potentials in the plasma in front of antenna, to finally create nonlinearly a DC potential which can be much higher than the floating potential. We study specifically how the space-time distribution of these RF and DC rectified potentials is modified when nearby flux tubes are allowed to exchange perpendicular polarization current. To simulate that, a 2D fluid code has been implemented to compute the 2D RF potential map in a plane perpendicular to magnetic lines, and within the flute approximation the whole 3D potential map is deduced. In simulation, we consider a homogeneous transverse conductivity and use a “test” potential map having, in absence of transverse currents, a Gaussian shape characterized by its width r0 and its amplitude f0. As a function of these 2 parameters (normalized respectively to a characteristic length for transverse transport and to the local temperature), we can estimate the peaking and the smoothing of the potential structure in the presence of polarization current. So, we are able to determine, for typical plasmas, the amplitude of DC potential peaks , particularly on antenna's corners , where hot spots appear during a shot. In typical Tore Supra conditions near antenna corners potential structures less than centimetric are involved in the 2D effects. The next step will consist in studying space transition between several areas characterized by different perpendicular conductivities, which can be modelled via effective connection lengths in our 2D fluid code. This more precise approach will be useful to obtain the potential structures in front of each part of the complex antenna's geometry and to minimize potential peaks generating many spurious perturbations in the plasma edge for long duration discharge as in ITER reactor

Plasma sheath properties in a magnetic field parallel to the wall

International audienceParticle in cell simulations were carried out with a plasma bounded by two absorbing walls and a magnetic field applied parallel to them. Both the sheath extent and the potential drop in it were derived from simulations for different plasma parameters such as the electron and ion temperature T i , particle density and ion mass. Both of them exhibit a power law dependent on the Larmor to plasma ion pulsation ratio Ω i. For increasing values of the magnetic field, the potential drop within the sheath decreases from a few T i /e down to zero, where e stands for the electron charge. The space charge extent increases with Ω i and saturates to 2.15 ion Larmor radius. A simple model of sheath formation in such a magnetic field configuration is presented. Assuming strongly magnetized electrons, and neglecting collisions and ionizations, a new typical length is evidenced, which depends on the ratio Ω i. The charge separation sheath width is theoretically found to increase from a combination of the electron gyroradius and the ion Debye length for low Ω i ratios up to several ion gyroradii for strongly magnetized ions. Both the calculated sheath extent and plasma potential show a fair agreement with the numerical simulations

The plasma-wall transition layers in the presence of collisions with a magnetic field parallel to the wall

International audienceThe plasma-wall transition is studied by mean of a particle-in-cell (PIC) simulations in the configuration of a parallel to the wall magnetic field (B), with collisions between charged particles vs. neutral atoms taken into account. The investigated system consists in a plasma bounded by two absorbing walls separated by 200 electron Debye lengths (λ d). The strength of the magnetic field is chosen such as the ratio λ d /r l , with r l the electron Larmor radius, is smaller or larger than the unity. Collisions are modelled with a simple operator that reorients randomly ion or electron velocity, keeping constant the total kinetic energy of both the neutral atom (target) and the incident charged particle. The PIC simulations show that the plasma-wall transition consists in a quasi-neutral region (pre-sheath), from the center of the plasma towards the walls, where the electric potential or electric field profiles are well described by an ambipolar diffusion model, and in a second region at the vicinity of the walls, called the sheath, where the quasi-neutrality breaks down. In this peculiar geometry of B and for a certain range of the mean-free-path, the sheath is found to be composed by two charged layers, a first, positive, close to the walls, and a second one, negative, towards the plasma and before the neutral pre-sheath. Depending on the amplitude of B, the spatial variation of the electric potential can be non-monotonic and presents a maximum within the sheath region. More generally, the sheath extent as well as the potential drop within the sheath and the pre-sheath are studied with respect to B, the mean-free-path and the ion and electron temperature

Plasma turbulence measured by fast sweep reflectometry on TORE SUPRA

Traditionally devoted to electron density profile measurement we show that fast frequency sweeping reflectometry technique can bring valuable and innovative measurements onto plasma turbulence. While fast frequency sweeping technique is traditionally devoted to electron density radial profile measurements we show in this paper how we can handle the fluctuations of the reflected signal to recover plasma density fluctuation measurements with a high spatial and temporal resolution. Large size turbulence related to magneto-hydrodynamic (MHD) activity and the associated magnetic islands can be detected. The radial profile of the micro-turbulence, which is responsible for plasma anomalous transport processes, is experimentally determined through the fluctuation of the reflected phase signal.Comment: 12th International Congress on Plasma Physics, 25-29 October 2004, Nice (France

Experimental measurements of the RF sheath thickness with a cylindrical Langmuir probe

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