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Radio frequency (RF) plasmas are commonly used for surface treatments and plasma heating processes. Controlling the heat flux from the plasma to the RF electrode is a crucial issue for optimizing these processes and is, therefore, the subject of considerable research in the low- and high-temperature plasma physics communities. In an asymmetric capacitively coupled plasma discharge, the ions accelerated by the direct current (DC) self-bias are the prime factor of the wall heating process. In this work, investigations have been performed with the aim to act on the DC self-bias in a linear magnetized RF environment. The lateral side and one face of the electrode have been covered by ceramic in order to limit the electron flux toward these surfaces. The variations in DC self-bias voltage as a function of the gas pressure, coupled RF power, and tilt angle between the RF electrode and the axial magnetic field have been studied. A new regime was discovered at low pressures, higher magnetic fields, and grazing angles for which the self-bias is positive. An analytical model was developed, which is in agreement with the experimental results.

DC currents flowing along magnetic field lines connected to powered ICRF antennas have been observed experimentally. Negative current is collected on the powered ICRF antenna structure, while positive current is collected by magnetically connected Langmuir probes. Models based upon a double probe configuration are tested with the 2D electrostatic PIC code OOPIC and the 2D fluid code SEM. These models can be excited in a symmetrical (two electrode in antiphase) or an asymmetrical way (one electrode). Transverse RF currents can be exchanged with surrounding flux tubes. Simulations show that only “asymmetric” drive can draw net DC currents. As in experiments, they are negative on the “driven” electrode and positive on the grounded one. An electrostatic and low frequency analytical model explains how DC current is produced. The perpendicular width of current structures L⊥ depends on the parallel length L//.

Using numerical simulation, a first estimate is made of power fluxes caused by radio-frequency sheaths on plasma facing components surrounding the ITER Ion Cyclotron Range of Frequencies wave launcher. Three toroidal strap phasings were considered, and four plasma scenarios covering a broad range of Scrape-Off Layer densities. Parallel heat fluxes up to~16 MW/m 2 are obtained, localized in poloidal peaks whose radial extension (initially 4-35 mm) increases when nearby flux tubes are coupled by exchanging RF currents. Quantitative results strongly depend on the local density, both via the unperturbed input profiles and via subsequent RF-induced modifications by E x B 0 convection. Sources of uncertainty and aspects requiring further validation are identified.

Unexpected hot spots were observed around the edges of vertical ports on Tore-Supra, caused by fast electrons, accelerated by the lower hybrid waves used to drive the current, and trapped in the local ripple wells. Trajectory calculations, with the magnetic fields alone, show that no electrons should reach such locations. However, electrostatic potentials, at a kilovolt level, can induce a toroidal pinch of the trajectories, allowing particle deposition in these normally shadowed areas. Their origin is attributed first to a strong increase of the sheath potential at the port entrance, due to the fast electron flux itself, and second to its reduction within the port, when the sheath width becomes larger than the port width, due to the too low value of the local density. An increase of the capability of the cooling panels within the port, to cope with these additional fluxes, has been implemented.

Hot-spot formation on the corners of the ICRH antenna can be explained by high DC potential structures, which accelerate ion fluxes and generate strong convective fluxes to the antenna surface. This comes from RF sheaths at the end of open magnetic lines, which rectify RF potential resulting from parallel electric fields. As these electric fields are not homogeneous in front of the antenna, transverse potential gradients generate transverse polarization currents which modify the potential structure. These potentials are studied with a simple flux-tube model and then a 2D-fluid model was elaborated to obtain analytical expressions for rectified potential with respect to these transverse currents. We compare them to numerical results coming from a 2D-fluid code executed in a poloidal plane in front of the antenna. Then we build a potential peak criterion to determine the peaking of DC-potential structures for typical parameters in Tore Supra. Finally, current interaction between different magnetic line lengths is approached.

20th International Conference on Plasma-Surface Interactions in Controlled Fusion Devices (PSI), Forschungszentrum Julich, Aachen, GERMANY, MAY 21-25, 2012; International audience; The influence of RF electric fields on retarding field analyzer (RFA) measurements of sheath potential, V-sh is investigated. One-dimensional particle-in-cell simulations show that the RFA is able to measure reliably the rectified dc sheath potential only for ion plasma frequencies omega(pi) similar to the rf wave frequency omega(rf), while for real SOL conditions (omega(pi) > omega(rf)), when the RFA is magnetically connected to an RF antenna, it is strongly underestimated. An alternative method to investigate RF sheaths effects is proposed that uses broadening of the ion distribution function as evidence of the rf electric fields in the sheath. RFA measurements in Tore Supra indicate that the average effects of rf potentials do indeed propagate from the antenna 12 m along magnetic field lines.

This paper treats self‐consistently the interplay between slow magnetosonic wave penetration and edge plasma DC biasing using a two‐field fluid approach. The RF and DC parts are coupled by non‐linear RF and DC sheath boundary conditions at lateral boundaries of the simulation domain. Within this approach two mechanisms of RF‐induced DC current flows are outlined, depending on the toroidal symmetry of the RF drive. The numerical implementation of the model using COMSOL, its validation against explicit simplified solutions and its planned improvements are discussed.

The interaction between a powered ICRH antenna and plasma scrape‐off layer is investigated by Langmuir probes. When a reciprocating Langmuir probe is magnetically connected to a powered ICRH antenna, large RF‐induced potentials are observed along the entire leading edge of the antennaSs poloidal protection limiters. Recent experiments show that a key parameter affecting the amplitude of these potentials is plasma density. The detailed 2D map of the potentials induced in front of the ICRH antenna is presented as well as the influence of the density on these potentials and parallel flow.

We investigate possible mechanisms for radial DC and RF current flow by running a set of simulations based on a fluid modeling with flute hypothesis. They clearly demonstrate the impact of DC transverse currents in perpendicular potential gradient regions. We propose a modeling coupling parallel currents driven by RF sheaths, both RF and DC transverse currents, and plasma density in biased flux tubes to reconstruct IV probe characteristics when they are connected to an antenna.

Experimental measurements have previously demonstrated the existence of DC electron currents collected on powered ICRF antenna structures, while at the same time ion current is measured by Langmuir probes at the other end of open flux tubes magnetically connected to the antenna. Such a configuration is tested numerically in this work by using the 2D electrostatic PIC code OOPIC. In simulations, the ICRF antenna is replaced by an RF electrode at one extremity of an open flux tube. At the other extremity of the open flux tube, the probe is modeled by a collecting electrode maintained at ground potential. RF and DC currents are computed on each boundary of the simulated box containing a magnetized plasma typical of tokamak SOL plasma parameters. Scans of RF voltage and frequency give clues about the nature of these currents.

Measurements show that the vicinity of powered Ion Cyclotron Range of Frequency (ICRF) antennae is biased positively with respect to its environment. This is attributed to RF-sheaths. The radial penetration of DC potentials into Tokamak SOL determines the power deposition on the walls and especially on the antenna structure, which is a key point for long time clean discharges. Within independent flux tube models of RF-sheath rectification the radial penetration of DC potentials is determined by the skin depth x{sub 0} = c/{omega}{sub pe} for the slow wave. When self-consistent exchanges of transverse RF currents are allowed between neighboring flux tubes, such a structure can be broadened radially up to a characteristic transverse length L. Broadening arises as soon as L>r{sub 0}. A linear modeling gives a first evaluation of the theoretical length L{approx_equal}(L{sub parallel} {rho}{sub ci}/){sup 1/2}. Within the 'flute assumption' it scales with the length L{sub parallel} of open flux tubes and the ion Larmor radius {rho}{sub ci}. This has been confirmed by the SEM code which takes into account non-linear rectifications. Applying our model to several potential maps generated by an ITER antenna, it comes out that L ranges between 1 and 10 cm depending on localmore » L{sub parallel} and on typical ITER plasma parameters. Langmuir probe measurements on Tore Supra suggest that the broadening is lower than predicted by the code, which supposes that currents do not occur all over the parallel magnetic lines but on a fraction of it.« less

International audience; This paper summarizes recent experimental characterization of radio frequency (RF)-induced scrape-off layer (SOL) modifications in ASDEX-Upgrade (AUG), JET and Tore Supra (TS). Geometrical aspects are emphasized: complex SOL patterns are observed by several indicators visualized in one or two dimensions transverse to the magnetic field lines. Results are ascribed to inhomogeneous RF-induced SOL biasing around powered ion cyclotron range of frequencies antennas and associated E × B 0 density convection (D'Ippolito et al 1993 Phys. Fluids B 5 3603). Within a simple RF sheath model (Perkins 1989 Nucl. Fusion 29 583), the shape of convective cells on TS can be interpreted in terms of RF-sheath generation by parallel RF currents. Some lessons are drawn for future machines.

Long pulse operation on the Tore Supra tokamak has entered a new phase, characterized by the use of heating power level in excess of 10?MW, during pulses lasting several tens of resistive times. This has been made possible by the use of ion cyclotron range of frequency (ICRF) heating (9?MW coupled to the plasma at 57?MHz), combined with lower hybrid current drive (LHCD: 3?MW at 3.7?GHz) and efficient fuelling techniques (supersonic gas injection, pellets). This paper addresses key technological, operational and physics issues related to the long pulse operation of the Tore Supra ICRF system and required for a reactor: R&D on the ICRF plant, real-time control and safety procedures, integration with other tokamak subsystems, experimental investigation and theoretical modelling of the edge ICRF physics (wave coupling, heat loads on antenna front faces). As far as possible lessons are drawn from the experience gained on Tore Supra for the design and operation of a next-step device.

The propagation of light in a dielectric moving at relativistic velocities is investigated. The analysis shows that, in that case, the medium may support two original propagative modes, analogously to birefringence in crystal optics. The conditions at which the kinematic birefringence occurs and its main features are exhibited and compared with the usual birefringence. In particular, it is shown that this peculiar effect is of second order in relative velocity β=v/c, and the kinematic ordinary refractive index exhibits angular dependence contrary to what happens for the usual ordinary index. Moreover, the existence of negative values for the ordinary and extraordinary indices is manifested in the ultrarelativistic regime.