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In this paper we implement the stretched-coordinate Perfectly Matched Layer (PML) technique in [28] to emulate full power absorption outside the simulation domain for time-harmonic electromagnetic wave propagation in presence of gyrotropic dielectric tensor and curved geometry relevant for magnetized plasma devices. We recall the PML formulation as an artificial inhomogeneous lossy medium, following the stretching into the complex plane of a general system of three orthogonal curvilinear coordinates. We apply the general method in cylindrical and toroidal geometries. We then assess this technique in a simple case combining gyrotropy and coordinate curvature. Our test problem analytically quantifies the reflection of Transverse Electric (TE) cylindrical eigenmodes in a gyrotropic medium by a radial PML in cylindrical geometry. The obtained reflection coefficient involves wave, PML and geometric parameters at the PML location. The new coefficient generalizes the one obtained earlier with Cartesian coordinates, and becomes equivalent when the effects of the local cylindrical curvature at the PML (stretched) location can be neglected. These curvature effects are outlined and the limitations they impose on the properties of the PML are quantified as a function of the relevant parameters. Peculiarities related to the gyrotropy are also highlighted. Finite element calculations of the test problem in two-dimensional cylindrical geometry are exploited to verify these properties numerically. Indications are finally given on how to choose the PML parameters in order to obtain a minimal wave reflection at given numerical cost, taking into account errors associated with the numerical scheme.

International audience; Airborne characterization of ice crystals has important applications. The extreme difficulty of realizing in situ tests requires the development of a complete instrumentation in the laboratory. Such an installation should enable design, development, test, and calibration of instruments in conditions as close as possible to real ones. We present a set of numerical and experimental tools that have been developed to realize ice crystal sensors based on interferometric particle imaging. The set of tools covers the development of complementary simulators for crystal growth and interferometric particle imaging predictions, experimental generation of “programmable” ice crystals, and instrumentation of a freezing column where different techniques as in-focus imaging, out-of-focus imaging, and digital in-line holography can be combined simultaneously for test and calibration.

Using high-resolution diagnostics, this paper investigates experimentally two ways of influencing the radiofrequency (RF) sheath patterns (magnitude and spatial distribution) developing around active Tore Supra ion cyclotron antennae. Firstly two types of Faraday screen electrical designs were compared. The new design aimed at reducing the parallel RF electric field integrated along “long field lines” passing in front of the antenna, by interrupting all parallel RF current paths on its front face. This proved inefficient for attenuating the RF-sheaths on the screen itself, and only weakly modified their distribution. Secondly the ratio of strap RF voltage amplitudes was varied, either during spontaneous bifurcations of the antenna electrical operational point or through the power balance between RF generators. This affected the left–right symmetry of screen heat loads: the local fluxes tended to evolve as the RF voltages on the nearest strap. Both experiments question the method used to optimize the new screen.

Plasma heating with waves in the ion cyclotron range of frequency (ICRF) affects the edge plasma and the edge plasma affects the ICRF heating. In simulations, these nonlinear ICRF?edge plasma interactions have been self-consistently simulated by running the EMC3-EIRENE, RAPLICASOL and SSWICH codes in an iterative way on ASDEX Upgrade for the first time. In experiments, the edge plasma convection induced by powered 3-strap antennas is measured with the antenna embedded reflectometers for the first time. Both the simulation and experimental results indicate that the ICRF induced convective cells are most significant on the top and bottom of the antennas; the edge plasma convection induced by 3-strap antennas in optimized antenna feeding configuration (dipole phasing, power ratio between the center and outer straps ~1.5) is smallest among the studied cases. The simulation results also suggest that compared to the 2-strap antennas, the 3-strap antennas can significantly reduce the plasma convection associated with the radio-frequency sheaths, even with unfavorable power balance between the straps in dipole phasing.

A new array of B-dot probes was installed on ASDEX Upgrade. The purpose of the new diagnostic is to study Ion Cyclotron Range-off Frequencies (ICRF) wave field distributions in the evanescent scrape-off layer (SOL) plasma region on the low field side of ASDEX Upgrade. The vacuum measurements (no gas, B-T = 0 T) reveal ICRF wave field measurements consistent with the profiles expected from the newly installed 3-strap ICRF antennas outside the antenna box: the shape of the toroidal distribution of both the amplitude and the phase is the same for the case of only the central straps being active, as for the case of only the side straps being active. These profiles become strongly modified during plasma operations. The modifications can be separated into two types: " Inter-edge localized mode (ELM)" and " During-ELM" periods. The phase distribution of the ICRF wave fields remains well-defined during the Inter-ELM period; however, it becomes more spread out over the entire 360. range during ELMs. The observed modulations cannot be explained by the observed changes in the ICRF power, as monitored in the transmission line. However, they are consistent with ICRF coupling changes introduced by plasma filaments: the plasma density perturbations due to the filaments are high enough to change the nature of the fast ICRF wave field from evanescent to propagating. The coverage of the present diagnostic is being expanded to include both the low field side and the high field side probes. Additionally, a manipulator probe head is being developed to measure ICRF wave field radial profiles across the SOL region. Published by AIP Publishing.

Équipe 107 : Physique des plasmas chauds; International audience; Firstly, a brief overview will be given on different models that are able to describe the behaviour of wave propagation as a function of specific frequency ranges. Each range corresponds to different heating systems, namely, 20-100 MHz for the ion cyclotron resonant heating, 2-20 GHz for lower-hybrid heating or current drive, and 100-250 GHz for electron cyclotron resonant heating or current drive systems. The specification of every system will be explained in detail, including the typical set of equations and the assumptions needed to describe the properties of these heating or current drive systems, as well as their specific domains of validity. In these descriptions, special attention will be paid to the boundary conditions. A review of specific physical problems associated with the wave heating systems will also be provided. The review will detail the role of simulation in answering questions that arise from experiments on magnetized plasma devices devoted to fusion. A few examples that will be covered are the impact of edge turbulence on wave propagation and its consequences on heating system performance, the effects of fast particles and ponderomotive effects, among others. A study that is more focused on radio-frequency sheath effects will also be discussed. It shows that such simulations require very sophisticated tools to gain a partial understanding of the observations undertaken in dedicated experiments. To conclude this review, an overview will be given about the requirements and progress necessary to obtain relevant predictive simulation tools able to describe the wave heating systems used in fusion devices.

IShTAR (Ion cyclotron Sheath Test ARrangement) is a linear magnetised plasma test facility for RF sheaths studies at the Max-Planck-Institut fur Plasmaphysik in Garching. In contrast to a tokamak, a test stand provides more liberty to impose the parameters and gives better access for the instrumentation and antennas. The project will support the development of diagnostic methods for characterising RF sheaths and validate and improve theoretical predictions. The cylindrical vacuum vessel has a diameter of 1 m and is 1.1 m long. The plasma is created by an external cylindrical plasma source equipped with a helical antenna that has been designed to excite the m=1 helicon mode. In inductive mode, plasma densities and electron temperatures have been characterised with a planar Langmuir probe as a function of gas pressure and input RF power. A 2D array of RF compensated Langmuir probes and a spectrometer are planned. A single strap RF antenna has been designed; the plasma-facing surface is aligned to the cylindrical plasma to ease the modelling. The probes will allow direct measurements of plasma density profiles in front of the RF antenna, and thus a detailed study of the density modifications induced by RF sheaths, which influences the coupling. The RF antenna frequency has been chosen to study different plasma wave interactions: the accessible plasma density range includes an evanescent and propagative behaviour of slow or fast waves, and allows the study of the effect of the lower hybrid resonance layer.

IShTAR (Ion cyclotron Sheath Test ARrangement) is a linear magnetized plasma test facility dedicated to the investigation of RF wave/plasma interaction [1] in the Ion Cyclotron Range of Frequencies (ICRF). It provides a better accessibility for the instrumentation than tokamaks while being representative of the neighboring region of the wave emitter. It is equipped with a magnetized plasma source (1 m long, 0.4 m diameter) powered by a helical antenna up to 3 kW at 11 MHz. We present the results of the first analysis of the plasma characteristics (plasma density, electron temperature) in function of the operating parameters (injected power, neutral pressure and magnetic field) as measured with fixed and movable Langmuir probes, spectrometer and cameras. The plasma is presently produced only by the helical antenna (no ICRF). We show that the plasma exists in three regime depending on the power level: the first two ones are stable and separated by a jump in density; a first spatial profile of the plasma density has been established for these modes; The third mode is unstable, characterized by strong oscillations of the plasma tube position.

Most present Ion Cyclotron Resonant Frequency (ICRF) heating codes and antenna codes assume the antenna sitting in a vacuum region and consider the fast wave only, which implicitly performs an abrupt density transition from vacuum to above lower hybrid (LH) resonance. We studied the impact of densities that decay continuously inside the antenna box on near field patterns and power coupling. A new full wave code based on the COMSOL Finite Element Solver has been developed to investigate this topic. It is shown that: up to the memory limits of the adopted workstation, the local RF field pattern in low-density regions below the LH resonance changes with the grid size. Interestingly and importantly, however, the total coupled spectrum is independent to the mesh size and is weakly affected by the presence of the density profile inside the antenna box in dipole phasing. Thus one can drop out this density for coupling studies. Simulation also shows that varying the density gradient in the fast wave evanescence region has no significant effect on wave coupling.

Tore Supra is dedicated to long pulse operation at high power, with a record in injected energy of 1 GJ (2.8 MW × 380 s) and an achieved capability of 12 MW injected power delivered by 3 RF systems: Lower Hybrid Current Drive (LHCD), Ion Cyclotron Resonance Heating (ICRH) and Electron Cyclotron Resonance Heating (ECRH). The new WEST project (W [tungsten] Environment in Steady-state Tokamak) aims at fitting Tore Supra with an actively cooled tungsten coated wall and a bulk tungsten divertor. This new device will offer to ITER a test bed for validating the relevant technologies for actively cooled metallic components, with D-shaped H-mode plasmas. For WEST operation, different scenarii able to reproduce ITER relevant conditions in terms of steady state heat loads have been identified, ranging from a high RF power scenario (15 MW, 30 s) to a high fluence scenario (10 MW, 1000 s). This paper will focus on the evolution of the RF systems required for WEST. For the ICRH system, the main issues are its ELM resilience and its CW compatibility, three new actively cooled antennas are being designed, with the aim of reducing their sensitivity to the load variations induced by ELMs. The LH system has been recently upgraded with new klystrons and the PAM antenna, the possible reshaping of the antenna mouths is presently studied for matching with the magnetic field line in the WEST configuration. For the ECRH system, the device for the poloidal movement of the mirrors of the antenna is being changed for higher accuracy and speed.Tore Supra is dedicated to long pulse operation at high power, with a record in injected energy of 1 GJ (2.8 MW × 380 s) and an achieved capability of 12 MW injected power delivered by 3 RF systems: Lower Hybrid Current Drive (LHCD), Ion Cyclotron Resonance Heating (ICRH) and Electron Cyclotron Resonance Heating (ECRH). The new WEST project (W [tungsten] Environment in Steady-state Tokamak) aims at fitting Tore Supra with an actively cooled tungsten coated wall and a bulk tungsten divertor. This new device will offer to ITER a test bed for validating the relevant technologies for actively cooled metallic components, with D-shaped H-mode plasmas. For WEST operation, different scenarii able to reproduce ITER relevant conditions in terms of steady state heat loads have been identified, ranging from a high RF power scenario (15 MW, 30 s) to a high fluence scenario (10 MW, 1000 s). This paper will focus on the evolution of the RF systems required for WEST. For the ICRH system, the main issues are its ELM resil...

Équipe 107 : Physique des plasmas chauds; International audience; A novel method to simulate ion cyclotron wave coupling in the edge of a tokamak plasma with the finite element technique is presented. It is applied in the commercial software COMSOL Multiphysics. Its main features include the perfectly matched layer (PML) technique to emulate radiating boundary conditions beyond a critical cutoff layer for the fast wave (FW), full-wave propagation across the inhomogeneous cold peripheral plasma and a detailed description of the wave launcher geometry. The PML technique, while widely used in numerical simulations of wave propagation, has scarcely been used for magnetized plasmas, due to specificities of this gyrotropic material. A versatile PML formulation, valid for full dielectric tensors, is summarized and interpreted as wave propagation in an artificial medium. The behavior of this technique has been checked for plane waves on homogeneous plasmas. Wave reflection has been quantified and compared to analytical predictions. An incompatibility issue for adapting the PML for forward (FW) and backward (slow wave (SW)) propagating waves simultaneously has been evidenced. In a tokamak plasma, this critical issue is overcome by taking advantage of the inhomogeneous density profile to reflect the SW before it reaches the PML. The simulated coupling properties of a Tore Supra ion cyclotron resonance heating (ICRH) antenna have been compared to experimental values in a situation of good single-pass absorption. The necessary antenna elements to include in the geometry to recover the coupling properties obtained experimentally are also discussed.

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 year TORE-SUPRA celebrated its 25 years of operation. During this long time a number of technologies have been developed [1]. First of all it was mandatory to develop reliable superconducting magnets at ∼ - 4 K, with superfluid helium as efficient coolant. For the production of steady state discharge, 3 types of Radio Frequency (RF) additional heating systems have been developed: Lower Hybrid Current Drive (LHCD), Ion Cyclotron Resonance Heating (ICRH) and Electron Cyclotron Resonance Heating (ECRH) [2]. To cope with long lasting discharges (up to 380 s × 2.8 MW) and large RF additional heating power (12.3 MW × 3 s), Actively Cooled (AC) Plasma Facing Components (PFC) were deployed in TORE-SUPRA for the first time in a Tokamak environment. TORE-SUPRA is now being modified into an axisymmetric tokamak with actively cooled tungsten main chamber walls and a divertor, the WEST project (W - for tungsten - Environment in Steady-state Tokamak) [3]. This new facility has the objective to offer ITER a test bed for validating the relevant actively cooled metallic technologies in D-shape H-mode plasmas. In contrast to other metallic devices such as JET and ASDEX, WEST will rely only on RF additional power systems. A set of plasma scenarios have been identified, ranging from a high total RF power scenario up to 15 MW during 30 seconds, to a high fluence scenario of 1000 seconds with up to 10 MW of injected RF power. These scenarios are able to reproduce ITER relevant conditions of steady state heat loads of 10 to 20 MW/m, to test tungsten actively cooled divertor technologies with relevant power heat fluxes and particle fluence. The paper presents the main issues regarding WEST project and especially the additional RF power injection systems (2 LHCD antennas, 3 + 4 = 7 MW continuous wave and 3 ICRH antennas, 3 × 3 = 9 MW-30 s or 3 MW-1000 s) for WEST. The front face of the LHCD antennas will be modified to account for the different plasma position and smaller toroidal field ripple, due to the more inward antenna position in the vessel. No other modifications are needed on the Passive-Active Multijunction (PAM) or the Fully-Active Multijunction (FAM) LHCD antennas, or the associated generator (2 × 8 klystrons, 600 kW each CW). Concerning the ICRH system, the main challenges are its ELM-resilience, its compatibility with continuous operation, and the interaction of the RF near fields with neighbouring plasma facing components. 3 new actively cooled antennas are being designed to be matched with an ELMs resilient electric circuit. The proposed solution is based on the JET-EP antenna and CEA prototype tested in 2007, both having identical internal conjugate-T electrical layout and a demonstrated load resilience capacity to plasma edge transients during ELMs. © 2013 IEEE.

The ongoing design of the ITER Ion Cyclotron Heating and Current Drive system (20 MW, 40-55 MHz) is rendered challenging by the wide spectrum of requirements and interface constraints to which it is subject, several of which are conflicting and/or still in a high state of flux. These requirements include operation over a broad range of plasma scenarios and magnetic fields (which prompts usage of wide-band phased antenna arrays), high radio-frequency (RF) power density at the first wall (and associated operation close to voltage and current limits), resilience to ELM-induced load variations, intense thermal and mechanical loads, long pulse operation, high system availability, efficient nuclear shielding, high density of antenna services, remote-handling ability, tight installation tolerances, and nuclear safety function as tritium confinement barrier. R&D activities are ongoing or in preparation to validate critical antenna components (plasma-facing Faraday screen, RF sliding contacts, RF vacuum windows), as well as to qualify the RF power sources and the transmission and matching components. Intensive numerical modeling and experimental studies on antenna mock-ups have been conducted to validate and optimize the RF design. The paper highlights progress and outstanding issues for the various system components. (C) 2013 ITER Organization. Published by Elsevier B.V. All rights reserved.

The ITER ion-cyclotron range of frequency (ICRH) heating system is required to couple 20 MW of power in the frequency range 40-55 MHz for a large range of scenarios with Edge Localized Modes. To mitigate the associated risks, it is foreseen to design and install on ITER two port-plug antennas for a total of 20 MW coupled power on long pulse operation [1]. The CEA activity to this antenna design within the CYCLE consortium was focused on the Faraday screen design (Fig. 1) and associated radio frequency (RF) sheath modelling, the reflectometers design for edge density measurement in the antenna vicinity and a contribution to the remote handling/tooling of the antenna. This paper is an overview of each item and proposes some R&D activities on key components. © 2013 Elsevier B.V. All rights reserved.

A comparison of the ASDEX Upgrade 3-strap ICRF antenna data with the linear electro-magnetic TOPICA calculations is presented. The comparison substantiates a reduction of the local electric field at the radially protruding plasma-facing elements of the antenna as a relevant approach for minimizing tungsten (W) sputtering in conditions when the slow wave is strongly evanescent. The measured reaction of the time-averaged RF current at the antenna limiters to the antenna feeding variations is less sensitive than predicted by the calculations. This is likely to have been caused by temporal and spatial fluctuations in the 3D plasma density distribution affected by local non-linear interactions. The 3-strap antenna with the W-coated limiters produces drastically less W sputtering compared to the W-coated 2-strap antennas. This is consistent with the non-linear asymptotic SSWICH-SW calculations for RF sheaths. © 2017 IOP Publishing Ltd.

International audience; Most present Ion Cyclotron Resonant Frequency (ICRF) heating codes and antenna codes assume the antenna sitting in a vacuum region and consider the fast wave only, which implicitly performs an abrupt density transition from vacuum to above lower hybrid (LH) resonance. The impact of the appearance of the LH resonance is entirely overlooked in their simulations. We studied the impact of densities that decay continuously inside the antenna box on near field patterns and power coupling. A new full wave code based on the COMSOL Finite Element Solver has been developed to investigate this topic. It is shown that: up to the memory limits of the adopted workstation, the local RF field pattern in low-density regions below the LH resonance changes with the grid size. Interestingly and importantly, however, the total coupled toroidal spectrum is almost independent on the mesh size and is weakly affected by the presence of the density profile inside the antenna box in dipole toroidal strap phasing. This suggests one can drop out this density for coupling studies to speed up the computation. Simulation also shows that varying the density gradient in the fast wave evanescence region has no significant effect on wave coupling.

An exploratory study was carried out in the long-pulse tokamak Tore Supra, to determine if electric fields in the plasma around high-power, RF wave launchers could be measured with non-intrusive, passive, optical emission spectroscopy. The focus was in particular on the use of the external electric field Stark effect. The feasibility was found to be strongly dependent on the spatial extent of the electric fields and overlap between regions of strong (>∼1 kV/cm) electric fields and regions of plasma particle recycling and plasma-induced, spectral line emission. Most amenable to the measurement was the RF electric field in edge plasma, in front of a lower hybrid heating and current drive launcher. Electric field strengths and direction, derived from fitting the acquired spectra to a model including time-dependent Stark effect and the tokamak-range magnetic field Zeeman-effect, were found to be in good agreement with full-wave modeling of the observed launcher.

A minimal two-field fluid approach is followed to describe the radio-frequency (RF) wave propagation in the bounded scrape-off layer plasma of magnetic fusion devices self-consistently with direct current (DC) biasing of this plasma. The RF and DC parts are coupled by non-linear RF and DC sheath boundary conditions at both ends of open magnetic field lines. The physical model is studied within a simplified framework featuring slow wave (SW) only and lateral walls normal to the straight confinement magnetic field. The possibility is however kept to excite the system by any realistic 2D RF field map imposed at the outer boundary of the simulation domain. The self-consistent RF + DC system is solved explicitly in the asymptotic limit when the width of the sheaths gets very large, for several configurations of the RF excitation and of the target plasma. In the case of 3D parallelepipedic geometry, semi-analytical results are proposed in terms of asymptotic waveguide eigenmodes that can easily be implemented n...

In the framework of the ion cyclotron resonance heating (ICRH) development led at CEA Cadarache, an actively cooled Faraday screen (FS) prototype with cantilevered horizontal bars and a slotted box has been designed to increase the heat exhaust capability (for high-power operation), reduce the parallel RF electric field along long field lines and qualify alternative mechanical solutions for ITER (bars are disconnected from the septum to reduce the stress level). The new FS has been installed on an existing ICRH antenna, and was tested during the 2011 Tore Supra experimental campaign. The antenna hosting the new screen exhibits high sensitivity to the edge plasma condition, some instabilities of electrical matching and improved heat exhaust capabilities in accordance with the thermo-mechanical design. RF-induced heat loads derived from IR thermography have been found to be about five times higher in the equatorial plane with the new design compared with the conventional design. The experimental results show that minimizing the parallel RF electric field along long field lines is not enough to reduce the wave–plasma interaction on the screen. This paper summarizes the experimental RF-induced heat load for several plasma scenarios and edge parameters (plasma current, density and heating power level) with emphasis on RF-sheath rectification and E × B convection generated in front of the antenna through the differential biasing of adjacent field lines.

New ICRF (Ion Cyclotron Range of Frequency) antennas, targeted to reduce impurity generation in the all-metal ASDEX Upgrade were designed and fabricated at different sites in an international cooperation between IPP, ASIPP (Hefei, China) and ENEA (Frascati, Italy). Installation and test were done by a multinational team at IPP. The antenna further integrates a powerful reflectometer system, designed and built by ENEA and IST (Portugal). The project, with the cooperation of four institutes in four countries on two continents was successfully completed within budget and schedule boundaries. Test results, gathered during recent experiment campaign, reveal that the new antennas have achieved their goal: A significant reduction of ICRF heating induced W impurity generation at the antennas.

Équipe 107 : Physique des plasmas chauds; International audience; In parallel to the direct contribution to the procurement phase of ITER and Broader Approach, CEA has initiated research & development programmes, accompanied by experiments together with a significant modelling effort, aimed at ensuring robust operation, plasma performance, as well as mitigating the risks of the procurement phase. This overview reports the latest progress in both fusion science and technology including many areas, namely the mitigation of superconducting magnet quenches, disruption-generated runaway electrons, edge-localized modes (ELMs), the development of imaging surveillance, and heating and current drive systems for steady-state operation. The WEST (W Environment for Steady-state Tokamaks) project, turning Tore Supra into an actively cooled W-divertor platform open to the ITER partners and industries, is presented.

The influence of outer top gas injection on the scrape-off layer (SOL) density and ion cyclotron range of frequency (ICRF) coupling has been studied in ASDEX Upgrade (AUG) L-mode plasmas for the first time. The three-dimensional (3D) edge plasma fluid and neutral transport code EMC3-EIRENE is used to simulate the SOL plasma density, and the 3D wave code RAPLICASOL is used to compute the ICRF coupling resistance with the calculated density. Improvements have been made in the EMC3-EIRENE simulations by fitting transport parameters separately for each gas puffing case. It is found that the calculated local density profiles and coupling resistances are in good agreement with the experimental ones. The results indicate that the SOL density increase depends sensitively on the spreading of the injected outer top gas. If more gas enters into the main chamber through the paths near the top of vessel, the SOL density increase will be more toroidally uniform; if more gas chooses the paths closer to the mid-plane, then the SOL density increase will be more local and more significant. Among the various local gas puffing methods, the mid-plane gas valve close to the antenna is still the best option in terms of improving ICRF coupling. Differences between the outer top gas puffing in AUG and the outer top gas puffing in ITER are briefly summarized. Instructive suggestions for ITER and future plans for ITER gas injection simulations are discussed. © 2017 EUROfusion Max Planck Institute for Plasma Physics.