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Electron cyclotron resonance heating (ECRH) is the main heating system for W7-X. A 10-MW ECRH plant with continuous wave (cw) capability is under construction to support the W7-X operation, which aims at demonstrating the steady-state capability of stellarators at reactor-relevant plasma parameters. The ECRH system consists often radio-frequency (rf) modules with 1 MW power each at 140 GHz. The rf beams of the individual gyrotrons are transmitted in common to the W7-X torus via open multibeam mirror lines. The losses of individual components of the transmission system were measured with both low- and high-power methods. Integrated full-power, cw measurements of the long-distance transmission losses are reported and compared to theoretical design estimates.

Electron Cyclotron Resonance Heating (ECRH) is the main heating method for the Wendelstein 7-X stellarator (W7-X) which is presently under construction at IPP Greifswald. The mission of W7-X is to demonstrate the inherent steady state capability of stellarators at reactor relevant plasma parameters. A modular 10 MW ECRH-plant at 140 GHz with 1 MW CW-capability power for each module is also under construction to support the scientific objectives. The commissioning of the ECRH-plant is well under way; three gyrotrons are operational. The strict modular design allows to operate each gyrotron separately and independent from all others. The ECRH-plant consists of many devices such as gyrotrons and high voltage power supplies, superconductive magnets, collector sweep coils, gyrotron cooling systems with many water circuits and last but not least the quasi-optical transmission line for microwaves with remote controlled mirrors and further water cooled circuits. All these devices are essential for a CW operation. A steady state ECRH has specific requirements on the stellarator machine itself, on the microwave sources, transmission elements and in particular on the central control system. The quasi steady state operation (up to 30 min) asks for real time microwave power adjustment during the different segments of one stellarator discharge. Therefore, the ECRH-plant must operate with a maximum reliability and availability. A capable central control system is an important condition to achieve this goal. The central control system for the 10 MW ECRH-plant at W7-X comprises three main parts. In detail these are the voltage and current regulation of each gyrotron, the interlock system to prevent the gyrotrons from damages and the remote control system based on a hierarchy set of PLCs and computers. The architecture of this central control system is presented.

Electron Cyclotron Resonance Heating (ECRH) is the main heating method for the Wendelstein 7-X Stellarator (W7-X), which is under construction at IPP-Greifswald. A 10 MW ECRH plant with CW-capability at 140 GHz is under construction to meet the scientific objectives. The microwave power is generated by 10 gyrotrons with 1 MW each two gyrotrons are operational at IPP in Greifswald. The tubes are equipped with a single-stage depressed collector for energy recovery and operate with an output power modulation between 0.3 and 1 MW with a sinusoidal frequency of up to 10 kHz which is achieved by modulating the depression voltage and is an interesting feature for NTM control at ITER. The general features of the ECRH-plant such as frequency power, cw-capability, flexibility and the experimental experience are of high relevance for the ITER system. Each gyrotron is fed by two high-voltage sources. A high-power supply for driving the electron beam and a precision low-power supply for beam acceleration. The high-power facility consists of modular solid state HV-supplies (−65 kV 50/100 A) providing fast power control and high flexibility. The low-power high-voltage source for beam acceleration is realized by a feed back controlled high-voltage servo-amplifier driving the depression voltage. A protection system with a thyratron crowbar for fast power removal in case of gyrotron failure by arcing is installed. Both the high power and low-power high-voltage sources have the capability to supply a 2 MW ITER gyrotron without any modification. Analogue electronic devices control the fast functions of the high-voltage system for each gyrotron and a hierarchy of industrial standard PLCs and computers supervise the whole ECRH-plant.

Since the end of 2005 a liquid lithium limiter has been installed on FTU. The liquid lithium is confined by capillarity in a mat of stainless steel or other refractory material and the capillary force counteracts the electromagnetic force. In this paper we review some of the most important results obtained in the experimental campaigns led on FTU. Peaked electron density profiles are spontaneously formed when the density exceeds 1.0 x 10(20) m(-3). Despite to the fact that FTU is a fully metallic machine with a TZM toroidal limiter, the only impurity that is present in the plasma is lithium so that very clean plasma are obtained and the beneficial effects are discussed. Heat loads in excess of 5 MW/m(2) are withstood by the limiter without any damage also because the radiative losses from the evaporated lithium are able to dissipate most of the incoming heat flux. (C) 2010 Elsevier B.V. All rights reserved.

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On the W7-AS stellarator, the subdivertor neutral pressure in an up-down divertor pair as well as at two points in the vicinity of a lower divertor module in the main chamber are measured. Results are presented for + a = 5/9 island divertor discharges under conditions of normal confinement (NC) and the HDH-mode for: n e ∼ 0.1-4 x 10 20 m -3 , P ecrh = 0.5-1.5MW, P nbi = 2MW, and H + and D + plasmas, with both normal- and reversed-B t for H + . Subdivertor pressures are in the range 1-2 x 10 -3 mbar for HDH conditions. For plasma detachment at the target plates a strong up-down pressure asymmetry arises, with p up /p down ≤ 5. The asymmetry reverses with reversed B t . Main vessel pressures are a factor of 5-10 lower than the average subdivertor pressure for H + , with D + plasmas exhibiting still lower values.

Electron Cyclotron Resonance Heating (ECRH) systems for next step large fusion-devices operate at a Continuous Wave (CW) power well beyond 10 MW generated by a large number of gyrotrons with typically 1 MW power per unit. The combination of the power of two (or more) gyrotrons and switching of the power between different launchers for different physics applications is an attractive feature for such systems. The combination of beams from different gyrotrons would reduce the number of transmission lines and the requirements on port space. Fast switching between two antennas synchronously with the Magneto-Hydro Dynamic (MHD) modes frequency would increase the efficiency of mode stabilization. Both combination and switching as well as power sharing between different ports can be performed with high-power four-port diplexers using small frequency differences or small frequency-shift keying of the gyrotrons, respectively. Fast directional switches (FADIS) and beam combiners (BC) can be designed on the basis of different physical mechanisms: some selected design variants were investigated and the results are presented. Considerations on the integration of FADIS/BC's into large ECRH systems and their use in test arrangements are presented.

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The addition of a divertor into the W7-AS stellarator has allowed access to a high density regime where the radiation profiles reach a steady state. In earlier limiter discharges, the plasma suffered a radiative collapse at high densities. In contrast to limiter experiments, where the impurity confinement time measured by Al laser blow-off increased with increasing line integrated density, in divertor discharges, above a density threshold, the impurity confinement time decreased with increasing line integrated density. The observation that the divertor plasma radiates mainly at the plasma edge rather than the plasma centre is a further indication that changes to the impurity transport coefficients at these high densities are the basis for the achievement of steady state discharges in the divertor configuration of W7-AS. The maximum line integrated density reached with a divertor is compared to that reached with a limiter. The previously derived scaling law for the density limit with a limiter shows that the achieved densities do not exceed those predicted when the higher deposited power is taken into account. In a divertor the radiated power is located at the plasma edge and increasing the density, cooling the plasma edge and radiating sufficient power to cause plasma detachment determines the density limit.

An experimental analysis is discussed of different phases of ECRH H?mode discharges on COMPASS-D, from L-H transition triggering to steady state. Comprehensive high resolution measurements in the transport barrier region have enabled significant progress to be made in assessing H?mode transition models, together with additional experiments aimed at resolving controlling trigger mechanisms. The application of four possible transition models to the data indicates that all demonstrate limiting or critical parameter values at the transition. One of them, based on Alfv?n drift wave turbulence suppression, exhibits precursor behaviour in the stability regime diagram and predicts the density and magnetic field dependences of the H?mode power threshold observed on COMPASS-D. Analysis of the evolution of the local radial electric field and its shear indicates that significant shear only develops after the L-H transition. Stationary ELMy H?modes are achieved at high ?* with good confinement, and dimensionless scaling over a range of ?* has been carried out, providing valuable confinement data in a regime where heat is deposited primarily to the electrons.

The three-dimensional magnetic configuration of a stellarator offers two specific mechanisms for a transition to improved particle and energy confinement. One route goes through the so-called electron-root confinement regime, which leads to a reduction of neoclassical transport via strong radial electric fields. In this Letter evidence for a second route is presented. It opens due to the layer of a strongly varying radial electric field which is present in the transitional region from neoclassical electron to ion-root confinement. This type of improvement acts on turbulent transport.

Neoclassical tearing modes (NTMs) are studied in the ASDEX Upgrade tokamak [O. Gruber, H.-S. Bosch, S. Gunter et al., Nucl. Fusion 39, 1321 (1999)]. The onset βN for NTMs scales with the normalized ion gyroradius ρ*, independent of the normalized collisionality ν* for a wide range of ν*. This scaling is in accordance with both polarization current and χ⊥/χ∥ model, if for the latter, the heat flux limit on parallel heat conductivity is taken into account. Analysis of NTM stabilization experiments indicates that the typical seed island size is 1–1.5 cm, again in agreement with both models. Mode coupling to the q=1 surface is a crucial element in determining the impact of the NTM on the discharge performance. Complete stabilization using electron cyclotron current drive (ECCD) has been demonstrated at βN=2.5 and with about 10% of the total heating power. The results are in good qualitative agreement with modeling using the Rutherford equation and in quantitative agreement with a two-dimensional nonlinear cyl...

Quasi-stationary, high β tokamak plasmas (βNdia ≈ 2) have been developed in the COMPASS-D tokamak using ECRH. These regimes are sustained for 20 energy confinement times and are limited by the duration of the heating pulse. For the first time, fast particle instabilities are observed with strong electron heating only. The plasmas are sawtooth free and the most dangerous (m = 2, n = 1) neoclassical tearing modes are avoided. The scenario is a result of the optimization of the timing of the heating pulse and application of LHCD.

Density-limit discharges in the W7-AS stellarator, with constant line-integrated density and a duration of up to 2 s, were studied at three values of the toroidal magnetic field (B = 0.8, 1.25 and 2.5 T). The central factor governing the physics of the density limit in stellarators was demonstrated to be the decreasing net power to the plasma when the centrally peaked radiated power density profile exceeds that of the deposited power density. The process was further accelerated by the peaking of electron density under these conditions. In discharges with B = 2.5 T, simulations of the centrally peaked radiation power density profiles could be shown to be due to peaked impurity density profiles. Laser blow off measurements clearly inferred an inward pinch of the injected aluminium. These discharges had the electron density profile form found in the improved confinement H-NBI mode on W7-AS. The aim of producing steady-state discharges at the highest possible density in stellarators is naturally of special interest for reactor operation. Such a scenario has been best achieved in H-mode discharges, in which ELMs restricted the impurity influx to the plasma and an equilibrium in the plasma parameters with suitably low radiation power levels was possible. A density scan in ECRH discharges highlights the need to control impurity sources and choose electron densities well below the density limit in order that steady-state operation can be attempted in discharges without ELMs. A simple model of bulk radiation predicted that the limiting density should depend on the square root of heating power and this was experimentally confirmed. The magnetic field scaling of the limiting density found experimentally in this simple model will partly depend on the term concerning the radial profile of the impurity density, which in turn is a function of the diffusion coefficient and inward pinch of the impurity ions. Theoretical studies have shown that an assumption about the B dependence of the thermal conductivity leads to density limit scaling laws with an explicit B dependence.

The 10 MW, 140 GHz, CW ECRH‐plant for W7‐X is in an advanced state of commissioning and the installation was used to investigate advanced applications for extended heating‐ and current drive scenarios. The operation of the TED gyrotrons was recently extended to a 2nd frequency of 103.6 GHz at reduced output power and first results are presented. An improved collector sweep system for the W7‐X gyrotrons with enhanced power capability and smooth power distribution was developed, results are reported.

Radially peaked density profiles were observed in the stellarator W7-AS in discharges with small central particle sources. Particle balance analyses and density modulation experiments revealed the existence of a convective inward particle flux. The peaked density profiles appear only in discharges with flat central electron temperature profiles. In discharges with peaked temperature profiles the convective inward flux is canceled by outward directed thermodiffusion.

Electron Cyclotron Resonance Heating (ECRH) is the main heating system for the W7‐X stellarator and the only one for CW‐operation in the first stage. The mission of W7‐X, which is presently under construction at IPP‐Greifswald, is to demonstrate the inherent steady state capability of stellarators at reactor relevant plasma parameters. W7‐X is therefore equipped with a superconducting coil system operating at 3 T and a divertor for 10 MW steady state heat removal. A 10 MW ECRH plant with CW‐capability at 140 GHz is under construction to meet the scientific objectives. The commissioning of the ECRH plant is well under way, the status of the project and first full power, cw test results are reported.

Experimental and theoretical investigations of lower hybrid (LH) turbulence in the W7-AS stellarator are presented. The turbulence is excited by an ion beam, which is generated by a weak neutral hydrogen beam injected transversely to the confining magnetic field. The instability is detected by collective Thomson scattering of powerful gyrotron radiation. From the measured density dependence of the frequency it was identified as an LH type of instability. The spectrum is characterized by a narrow bandwidth in spite of the inherently poor radial resolution of the backscattering geometry. The theoretical model of an LH instability driven by a transverse fast ion component under the double resonance condition (coincidence of the LH frequency with a high cyclotron harmonic of the fast ions) is developed. The instability growth rate is derived. The stabilizing effect of high bulk ion temperatures was observed experimentally, in accordance with theoretical modelling. An instability saturation mechanism similar to the well known stochastic ion heating is proposed.

Collective Thomson scattering (CTS) of electromagnetic radiation from thermal plasma fluctuations in principle allows the velocity distribution of plasma ions and its composition in the plasma to be measured. The use of powerful microwave radiation from gyrotrons opens new perspectives for the application of CTS, which is considered to be a promising candidate for alpha-particle diagnostics in reactor-size tokamaks with D/T operation. We have performed the first experiments at W7-AS with different scattering geometries to prove the applicability of gyrotrons for CTS. The experiments were performed with a 140 GHz gyrotron which is routinely used for ECRH, delivering a power of 0.45 MW. The receiver antenna and detection system for the registration of CTS spectra were especially designed for the scattering experiment. In backscattering experiments, which have inherently no spatial resolution, we have measured a transversely propagating, non-thermal lower-hybrid turbulence, which is driven by perpendicularly injected fast particles from a diagnostic neutral beam. The instability is excited by the beam ions under double-resonance conditions, where the LH frequency coincides with some harmonic of the beam ion gyrofrequency. For scattering geometries with the scattering wavevector not perpendicular to the magnetic field, thermal density fluctuations in the plasma were experimentally detected. The ion temperatures derived from these thermal spectra agree well with other diagnostics. A modified scattering geometry ( scattering) allows local measurements of the ion temperature and is considered a prototype for the design of a routine diagnostic for ion-temperature measurements.

An experimental overview on pumping and particle exhaust studies for deuterium as well as for helium and neon in the ASDEX Upgrade divertor tokamak is presented. Strong turbomolecular pumps connected to the divertor region allow effective pumping of all these gases. Deuterium, however, is pumped effectively by the carbon walls in ASDEX Upgrade which dominate over the external pumping. Noble gases are hardly pumped by the walls, and their pumping efficiency and exhaust rate depends strongly on their respective density in the divertor chamber, i.e. on the effectivity of scrape-off layer transport and divertor retention. Compression of helium and neon in the divertor chamber increases with the divertor neutral gas density, and in high-power, high-density discharges in H- or CDH-mode pumping is very efficient. Helium exhaust in these scenarios is sufficiently fast to fulfill the requirements for ITER or a fusion reactor.

In order to study impurity transport and radiation behavior nitrogen has been injected into the scrape-off plasma of limiter-dominated discharges by gas puffing or into natural magnetic islands at the plasma edge of separatrix-dominated discharges by a reciprocating erosion probe. Strong radiative plasma edge cooling with a reduction of the power flux to the limiter could be obtained. The accompanied degradation of the energy confinement and the observed decrease of the central electron temperature can partly be explained by the reduction of the effective heating power. At high plasma densities and sufficiently strong impurity injection phenomena well known from tokamaks, like plasma shrinking, plasma detachment from the limiters and MARFE's were transiently observed.

A new maximum entropy approach has been applied to analyse three types of transient transport experiments. For sawtooth propagation experiments in the ASDEX Upgrade and ECRH power modulation and power-switching experiments in the Wendelstein 7-AS Stellarator, either the time evolution of the temperature perturbation or the phase and amplitude of the modulated temperature perturbation are used as non-linear constraints to the profile to be fitted. Simultaneously, the constraints given by the equilibrium temperature profile for steady-state power balance are fitted. In the maximum entropy formulation, the flattest profile consistent with the constraints is found. It was found that determined from sawtooth propagation was greater than the power balance value by a factor of five in the ASDEX Upgrade. From power modulation experiments, employing the measurements of four modulation frequencies simultaneously, the power deposition profile as well as the profile could be determined. A comparison of the predictions of a time-independent model and a power-dependent model is made. The power-switching experiments show that the profile must change within a millisecond to a new value consistent with the power balance value at the new input power. Neither power deposition broadening due to suprathermal electrons nor temperature or temperature gradient dependences of can explain this observation.

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A diagnostic experiment of collective Thomson scattering (CTS) on thermal density fluctuations designed to provide spatially-resolved measurements of the ion temperature is since long being carried out in the high-field tokamak FTU using a high-power 140 GHz gyrotron as a source of probing radiation. A peculiar feature of CTS experiment on FTU is that the propagation at 140 GHz is below the fundamental electron cyclotron (EC) harmonic (195 GHz-220 GHz). With the only exception of the low-power experiment attempted in TFTR, this is the first proof-of-principle experiment of CTS below the EC resonance, hence with accessibility from the low-field side of both ordinary and extraordinary plasma modes. Thus, this is the only experiment capable for an exhaustive demonstration of CTS in the configuration relevant for alpha CTS in ITER, in which the extraordinary mode at 50-60 GHz propagating below the EC resonance is assumed as most liable candidate. In spite of long lasting efforts to detect CTS from thermal density fluctuations, strong anomalous spectra including spectral lines were systematically detected both in aligned- and in misaligned-antenna conditions at spectral power densities orders-of-magnitude higher than those predicted for the thermal CTS spectra. In the present communication, we report on the interpretation of the anomalous spectra observed at FTU following the results of two experimental campaigns of 2004 and 2005 expressly performed to investigate the anomalous spectra.

The Wendelstein 7X (W7-X) stellarator (R = 5.5 m, a = 0.55 m, B < 3.0 T), which at present is being built at Max-Planck-Institut fur Plasmaphysik, Greifswald, aims at demonstrating the inherent ste...

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A multi parameter statistical analysis of Lower Hybrid Current Drive (LHCD) efficiency has been carried out on all the data available for the Frascati Tokamak Upgrade (FTU). The volume of the parameters space scanned is quite large and of direct interest for ITER. The positive dependence of the CD efficiency with 〈Te〉 has been confirmed. Comparing the results with the expectations for the CD process, a reasonable agreement is found for the ion charge, Zeff, dependence (CD efficiency ∝1/(Zeff+5)), while the influence of other parameters (like qa, ne, BT, N∥) is discussed in detail. Clear evidence of the synergy between the lower hybrid and electron cyclotron (EC) waves is found, provided the EC power is directly absorbed on the fast e− tails. The effect of different N∥ (LH parallel refraction index) spectra is also investigated.

Institute for Plasma Research, University of Stuttgart, D-70569 Stuttgart, Germany, e-mail: kasparek@ipf.uni-stuttgart.de Karlsruhe Research Center, Association EURATOM-FZK, Institute for Pulsed Power and Microwave Technology, D-76021 Karlsruhe, Germany Max-Planck-Institute for Plasma Physics, Ass. EURATOM-IPP, D-17491 Greifswald, Germany Max-Planck-Institute f. Plasma Physics, Ass. EURATOM-IPP, D-85748 Garching, Germany

Fast ion physics will play an important role on ITER where confined alpha particles will affect plasma dynamics and overall confinement. Fast ion collective Thomson scattering (CTS) using gyrotrons has the potential to meet the need for measuring the spatially localized velocity distributions of confined fast ions in ITER. Currently, CTS experiments are performed at TEXTOR using a 150 kW, 0.2 s, 110 GHz gyrotron and a receiver upgraded at the Ris phi National Laboratory. The gyrotron and receiver optics have also been upgraded for rapid scanning during a plasma shot. The receiver consists of a nine-mirror quasioptical transmission line including a universal polarizer and a 42-channel data acquisition system, which allows complete coverage of the double sideband scattered spectrum for localized (similar to 10 cm) time resolved (4 ms) measurements of the ion velocity distribution. At ASDEX Upgrade (AUG) a similar 50-channel CTS receiver has been installed. This CTS system will use the 105 GHz frequency of a dual frequency gyrotron. The gyrotron is presently being commissioned. CTS campaigns are scheduled for the summer of 2006 with a probe power of up to 1 MW for 10 s. This report presents the alignment of the quasioptical transmission line, calibration, and gyrotron tuning of the TEXTOR and AUG CTS systems. We will also review the progress on the design of the proposed fast ion CTS diagnostic for ITER. It is envisaged that scattered radiation from two 60 GHz probe beams launched from the low field side midplane port will be received by two arrays of receivers located on the low and high field sides of the plasma. This geometry will allow the ion velocity distribution near perpendicular and near parallel to the magnetic field to be measured in ten or more spatial locations covering the full plasma cross section. The temporal resolution can be significantly better than the required 100 ms. (c) 2006 American Institute of Physics.

The ECRH experiment (up to 2 MW for 0.5 s) planned on the FTU tokamak (major radius R=0.93 m; minor radius a=0.3 m; toroidal magnetic field Bt up to 8 T) has now reached its realization phase. A preliminary system with only a fraction of the full power (0.5 MW for 15 ms) has already been installed and tested. The simulations here presented refer to these data. Moreover predictive computations for the global experiment are discussed.

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A joint Euratom-CEA, Euratom-FOM electron cyclotron resonance heating (ECRH) programme was conducted on the TFR tokamak during 1985 and 1986, with 600 kW of ECR power at 60 GHz. The paper describes the experiments performed during this period and analyses the results. In spite of considerable electron heating, a degradation of the global energy confinement time has been observed in all cases, with incremental confinement times ranging from 1 to 2 ms, depending on the experimental conditions. The electron thermal conductivity has been determined in three different independent ways and the results are in reasonable agreement, but only 1/4-1/3 of the gyrotron power is found in the rate of change of the plasma energy. Other topics addressed are: impurity content and transport, sawtooth behaviour, MHD stabilization, suprathermal electron population and pellet ablation during ECRH.

This paper describes the construction of and the experience with a system comprising three 60 GHz, 200 kW, 100 ms gyrotrons (Varian VGE 8060). This system was built up on the tokamak in Fontenay-aux-Roses and has been working very reliable since February 1985.

Electron Cyclotron Resonance Heating (ECRH) is the main heating system for steady-state operation and was the only available heating system during the first operational phase (OP1.1) of the W7-X stellarator [1]. The ECRH was used for different tasks at W7-X such as wall conditioning, controlled plasma start-up from the neutral gas up to steady state plasma control [2] and performance optimization by temperature profile shaping. The ECRH plant was equipped with 7 gyrotrons and 6 of them were operational. The quasi optical transmission line was precisely adjusted in preparation of OP1.1. The operational experience of the ECRH plant will be discussed.