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Optimum confinement is realized in Wendelstein 7-AS by wall conditioning and by properly adjusting the parameters determining the magnetic field configuration. The effective heating of net current free plasmas by ECRF and neutral beam injection (NBI) involves different plasma parameters and transport regimes. Stationary plasmas are generally produced by ECRF, whereas density and impurity control is a severe problem during NBI. This has initiated different kinds of impurity and particle control scenarios. An extended parameter range with electron temperatures of 200 eV

Wendelstein 7-AS was the first modular stellarator device to test some basic elements of stellarator optimization: a reduced Shafranov shift and improved stability properties resulted in β-values up to 3.4% (at 0.9 T). This operational limit was determined by power balance and impurity radiation without noticeable degradation of stability or a violent collapse. The partial reduction of neoclassical transport could be verified in agreement with calculations indicating the feasibility of the concept of drift optimization. A full neoclassical optimization, in particular a minimization of the bootstrap current was beyond the scope of this project. A variety of non-ohmic heating and current drive scenarios by ICRH, NBI and in particular, ECRH were tested and compared successfully with their theoretical predictions. Besides, new heating schemes of overdense plasmas were developed such as RF mode conversion heating—Ordinary mode, Extraordinary mode, Bernstein-wave (OXB) heating—or 2nd harmonic O-mode (O2) heating. The energy confinement was about a factor of 2 above ISS95 without degradation near operational boundaries. A number of improved confinement regimes such as core electron-root confinement with central Te ≤ 7 keV and regimes with strongly sheared radial electric field at the plasma edge resulting in Ti ≤ 1.7 keV were obtained. As the first non-tokamak device, W7-AS achieved the H-mode and moreover developed a high density H-mode regime (HDH) with strongly reduced impurity confinement that allowed quasi-steady-state operation (τ ≈ 65 · τE) at densities (at 2.5 T). The first island divertor was tested successfully and operated with stable partial detachment in agreement with numerical simulations. With these results W7-AS laid the physics background for operation of an optimized low-shear steady-state stellarator.

The plasma edge region of the W 7-AS stellarator has a complex structure due to “natural” islands. Guiding centre orbit calculations by a Monte Carlo code indicate that there are locally fixed patterns of enhanced particle flow to the wall. Power loss simulations by the DEGAS code satisfactorily reproduce measured power depositions on the main limiters, but in the total power balance including radiation a significant deficiency is found. The role of limiters with respect to impurity fluxes is discussed on the basis of spectroscopic data. A long-term collector probe analysis shows inhomogeneous poloidal and toroidal distributions of deposited materials which are influenced by plasma shots as well as conditioning discharges.

This paper is a summary of some of the major results from the Wendelstein 7-AS stellarator (W7-AS). W7-AS [G. Grieger et al., Phys. Fluids B 4, 2081 (1992)] has demonstrated the feasibility of modular coils and has pioneered the island divertor and the modeling of its three-dimensional characteristics with the EMC3/EIRENE code [Y. Feng, F. Sardei et al., Plasma Phys. Controlled Fusion 44, 611 (2002)]. It has extended the operational range to high density (4×1020m−3 at 2.5T) and high ⟨β⟩ (3.4% at 0.9T); it has demonstrated successfully the application of electron cyclotron resonance heating (ECRH) beyond cutoff via electron Bernstein wave heating, and it has utilized the toroidal variation of the magnetic field strength for ion cyclotron resonance frequency beach-wave heating. In preparation of W7-X [J. Nuhrenberg et al., Trans. Fusion Technol. 27, 71 (1995)], aspects of the optimization concept of the magnetic design have been successfully tested. W7-AS has accessed the H-mode, the first time in a “non-to...

Plasma build-up and heating of net-current-free plasmas in W VII-A was investigated by ECRH. Experiments were performed at two ECR-frequencies (28 and 70 GHz) and different heating scenarios were investigated such as first harmonic ordinary mode heating and second harmonic extraordinary mode heating. The basic effects predicted by theory, i.e. localized wave absorption and optical thickness of the plasma were verified. The electron heat conduction was found to be governed by neoclassical losses in the plasma core for high enough temperatures, whereas enhanced losses have to be assumed in the outer plasma regions. Generation of a target plasma with sufficient parameters to allow further heating by NBI was successfully demonstrated. Configuration studies showed a beneficial influence of small shear on the confinement, where internal currents have to be taken into account.

The first plasma experiments on the W7AS advanced stellarator were conducted in October 1988, after magnetic surface mapping. The characteristics of the device are described. During the first phase of operation, 70 GHz ECF was used to generate and heat a 'currentless' plasma which was maintained in quasi-steady state for typically 0.5 s. Effects of the magnetic configuration on the confinement and measures to deal with the observed plasma current (bootstrap current and ECF-driven current) were investigated. Preliminary results of transport analysis are presented and compared with predictions of transport models.

Steady-state electrotransport measurements were performed for H and D interstitials in V, Nb, and Ta. The measurements were made in the temperature range between 49\ifmmode^\circ\else\textdegree\fi{}C and 245\ifmmode^\circ\else\textdegree\fi{}C and with hydrogen concentrations up to an atom ratio of 0.054. The results yield strong experimental evidence for the existence of a direct field force.

Plasma generation and heating of OH-current free plasmas by strong ECR-wave irradiation were studied on the Garching W VII-A stellarator for three kinds of wave launching: direct irradiation of the gyrotron modes (mainly TE02 mode corresponding to a 50% O-mode and 50% X-mode mixture) from the low field side, and advanced wave launching (linearly polarized TE11 and HE11 mode) in O-mode orientation (E//Bo, Bo, k perpendicular to Bo). The nonabsorbed fraction was reflected into the plasma in the X-mode (E perpendicular to Bo) by a focusing polarization twist reflector mounted to the inner torus wall. The experiments were performed at 28 GHz (1 Tesla resonance B-field) with 200 kW RF power and a pulse duration of

This work reports calculations and measurements on mode converters for the transformation of TEOn gyrotron mode mixtures (primarily TEO2) into the linearly polarized HE11 hybrid mode at 70 GHz. This mode is ideal for quasi-optical launching systems for ECRH of plasmas. Mode transducers with axisymmetric radius perturbations convert the gyrotron TEOn mode composition into the TEO1 mode. Proper matching of the phase differences between the various modes and of the perturbation amplitudes of the several converter sections is required. The TEO1 wave is used for long-distance transmission through smooth-walled overmoded waveguides. A mode converter with constant diameter and periodically perturbed curvature transfers the unpolarized TEO1 mode into the polarized TE11 wave. The experimentally determined ΣTEOn-to-TEO1 conversion efficiency is (98.5±1)% (99% predicted) while the TEO1-to-TE11 transformer has a (94±2)% efficiency (93% theoretically); ohmic losses are included. The “Gaussian-like” HE11 mode with axisymmetric power distribution and almost no cross polarization is produced in a circumferentially corrugated TE11-to-HE11 mode transducer with a measured conversion efficiency of (98.3±1.5)% (98.5% predicted). The overall efficiency of the complete mode converter system in the desired mode was determined to be (91±2.5)%. High-power operation (200kW, 100ms) has been successfully demonstrated.

Both in ohmically and beam-heated L-mode discharges of ASDEX, the electron-temperature (T/sub e/) profile shape can be varied over a wide range by the choice of the safety factor q/sub a/. The power-deposition profile, on the contrary, has no effect on the T/sub e/-profile shape. In current-free WVII-A stellarator plasmas, no such invariance property is found. An independent constraint seems to fix the current distribution j(r) of the tokamak, which defines the conditions of electron heat transport.

WENDELSTEIN VII-A has been operated for ten years. It is a low-shear, high-aspect-ratio device. The confinement properties have been thoroughly studied for both ohmically heated and net-current free plasmas. For the latter case, NBI- and ECF-maintained plasmas were of particular importance. It was found that under optimized conditions the core of high-pressure, net-current free plasmas is mainly governed by collisional effects.

For electron cyclotron resonance heating of plasmas in the stellarator W VII-AS, high-power microwave sources with high mode purity are needed. Therefore, the mode compositions of four commercial 200 kW/70 GHz gyrotrons, specified for this project, were analysed with a wave number spectrometer. The measurements confirm the specified spurious mode content of less than 10%.

The possibility and convenience of identifying the mode composition at the output of high power gyrotrons and analyzing the mode conversion properties of overmoded waveguide components (tapers, bends, mode converters) by measuring the radiation pattern from an open-ended circular waveguide in the far field region are studied. The method has been simplified remarkably by using spatial correlations of the field amplitudes at selected angles.

The 1 MW/70 GHz ECRH-System for the modular stellarator W VII-AS is described. Measurements on the gyrotrons and the transmission lines show the high-power long-pulse performance of the system. A description of the launching antenna is given and possible heating scenarios are discussed.

Experiments are done in the Wendelstein VII‐A Stellarator to get technical experience in handling ECRH as well as in getting data on both wave absorption phenomena and plasma behavior in a shearless configuration.(AIP)

The design of the Advanced Stellarator Wendelstein W VII‐AS is based on optimization of the vacuum magnetic field configuration. Essential features of the underlying concept are the reduction of the Pfirsch‐Schluter currents and of the neoclassical heat transport losses. This optimization in general leads to a non‐axisymmetric complex magnetic field configuration, which was realized by a set of modul twisted coils. Plasma operation started in October 1988. In the first experimental campaign the experiments concentrated on ECRH alone, combination with NBI is foreseen as the next step.

For plasma start-up and heating in the new Garching W VII-AS stellarator an ECRH microwave system is under construction which comprises five subsystems each designed to generate, transmit and radiate into the plasma 200 kW of millimeter-wave power with pulse length up to 3 s at a frequency of 70 GHz. Each subsystem consists of a microwave power source module, a transmission line and a quasi-optical wave launching antenna for plasma irradiation in the almost perfectly linearly polarized “Gaussian-like” HE11. mode. The overall efficiency in the desired mode of a complete transmission line is expected to be approximately 87%. The microwave power and the mode composition at the gyrotron output and in the transmission line are measured by special calorimeters and wavenumber spectrometers, respectively. Design and realization of the system are described together with extensive low-power and first high-power measurements.

This work reports measurements on mode converters to transform circular electric gyrotron TEg on -mode compositions (mainly TE 02 ) into the linearly polarized hybrid HE 11 wave which is in many respects ideal for use in quasioptical launching systems for electron cyclotron resonance plasma heating by means of high power gyrotrons. Periodically modulated wall mode converter sections (sinusoidal m=0 radius modulation) convert TE on mode mixtures into a nearly pure TE 01 mode with a measured efficiency of about 98 % at 28 GHz. The TE 01 wave is used for long distance transmission through smooth overmoded waveguideswith optimized tapers and corrugated bends. In a second converter with constant diameter and periodically modulated curvature in one plane (m=1 distortion) the circular symmetric TE 01 mode is transferred into the nearly linearly polarized TE 11 mode with an efficiency of about 97 %. The Gaussian HE 11 mode with almost no cross-polarization is produced in a circumferentially corrugated TE 11 to HE 11 mode converter. The overall efficiency of the complete microwave system is about 90 %.

Calibration of a W-band 8-channel heterodyne receiver for the measurement of the electron cyclotron emission (ECE) from W VII-A stellarator may be realized by adding a further channel with SIS mixer of high sensitivity as a reference in which ECE, attenuated to about 1000 K, is compared with the emission of well known blackbody sources at 77 K and 290 K.

A design is presented of a 140 GHz 2nd harmonic electron cyclotron resonance heating (ECRH) system to be used on the advanced stellarator W VII‐AS at IPP Garching. The primary objectives of these ECRH experiments will be (1) to extend the density range for ECRH and for combined heating (NBI and/or ICRH), (2) to investigate the local electron heat transport in high density plasmas by the heat‐wave technique, (3) to develop new millimeter‐wave transmission line concepts and antenna systems for reactor‐compatible multi‐megawatt heating of future large‐size stellarators. The planned 140 GHz ECRH facility will use a 200 kW/100 to 200 ms TEO3‐mode gyrotron from KfK Karlsruhe combined with a quasi‐optical fundamental Gaussian‐mode transmission system employing focusing metallic mirrors as phase correcting elements. The unpolarized gyrotron output mode will be converted directly into the linearly polarized Gaussian free‐space beam with the help of a quasi‐optical coupler. A universal quasi‐optical polarizer will ...

The xenon spectrum, excited in ECR- and NBI-heated plasmas with central electron densities of around 1020 m-3, and central electron temperatures from 0.7 to 2.5 keV, has been studied photoelectrically with a multi-channel grazing-incidence spectrometer. Besides numerous well-known lines of Zn- and Cu-like Xenon, more than 50 additional lines which have not yet been published in the literature have been found and partly identified,.

The electron cyclotron current drive (ECCD) with extraordinary mode at the second cyclotron harmonic launched from the high-field-side (HFS) is investigated in W7-AS stellarator. Trapped particle effects and quasi-linear effects are taken into account in linear ray-tracing and quasi-linear bounce-averaged Fokker–Planck computations of the driven current. The results for both low-field-side (LFS) and HFS injection in W7-AS are compared with the experimental results. A slightly lower current drive efficiency is obtained, both theoretically and experimentally, in the case of HFS injection, confirming the LFS launch as the preferred mode of operation for ECCD.

Europe is devoting significant joint efforts to develop and to manufacture MW-level gyrotrons for electron cyclotron heating and current drive of future plasma experiments. The two most important ones are the stellarator Wendelstein W7-X at Greifswald and the Tokamak ITER at Cadarache. While the series production of the 140 GHz, 1 MW, CW gyrotrons for the 10-MW electron cyclotron resonance heating system of stellarator W7-X is proceeding, the European GYrotron Consortium is presently developing the EU-1 MW, 170 GHz, CW gyrotron for ITER. The initial design had already been initiated in 2007, as a risk mitigation measure during the development of the advanced ITER EU-2-MW coaxial-cavity gyrotron. The target of the ITER EU-1-MW conventional-cavity design is to benefit as much as possible from the experiences made during the development and series production of the W7-X gyrotron and of the experiences gained from the earlier EU-2-MW coaxial-cavity gyrotron design. Hence, the similarity of the construction will be made visible in this paper. During 2012, the scientific design of the ITER EU-1-MW gyrotron components has been finalized. In collaboration with the industrial partner Thales electron devices, Velizy, France, the industrial design of the technological parts of the gyrotron is being completed. A short-pulse prototype is under development to support the design of the CW prototype tube. The technological path toward the EU ITER-1 MW gyrotron and the final design will be presented.

The first series tube of the gyrotrons for the 10-MW electron cyclotron resonance heating system of the stellarator W7-X was tested at Forschungszentrum Karlsruhe (FZK) and yielded a total output power of 0.98 MW, with an efficiency of 31% (without a single-stage depressed collector) in short-pulse operation and of 0.92 MW in pulses of 180 s (efficiency of almost 45% at a depression voltage of 29 kV). The Gaussian mode output power was 0.91 MW. The pulselength at full power (1 MW) is limited at FZK by the available power supply. At a reduced electron beam current, it is possible to operate at longer pulselengths. At an output power of 0.57 MW (electron beam current of 29 A), the pulselength was increased to 1893 s. There was no physical reason for a limitation of this pulse: The pressure increase during the pulse was less than a factor of two and ended up at a very low value in the 10-9 mbar range. The tube was delivered to Max-Planck-Institut fuumlr Plasmaphysik Greifswald for tests at full power and up to 30-min pulselength. The Gaussian mode RF output power, measured in a calorimetric load after a 25-m-long quasi-optical transmission line (seven mirrors), was 0.87 MW at a total output power of 0.92 MW in 30-min pulses. Again, no indications for a limitation in pulselength were found. The second series tube was tested in short-pulse operation and showed a strange behavior concerning a mode hopping which has not yet been understood. The third series gyrotron delivers up to now 0.65 MW at a pulse duration of 180 s. Preliminary operation of the prototype tube as a two-frequency gyrotron delivered 0.41 MW in 10-s pulses at 103.8 GHz (TE21,6 mode)