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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) 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.

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.

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.

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.

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.

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...

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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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.