In the edge plasma of the ASDEX tokamak, electrostatic fluctuations were observed with Langmuir probes and in Halpha light with high poloidal and temporal resolution. These fluctuations contribute a significant fraction to the 'anomalous' radial particle transport in the scrape-off layer (SOL). The basic properties and the dependence of the fluctuation parameters on the discharge conditions are documented. A model for an instability mechanism specific to the SOL is introduced and the experimentally observed fluctuation parameters are compared with the predictions of the linearized version of this model. For plasma temperatures above approximately 10 eV in the SOL the observed parameter dependences of the fluctuations are well reproduced by the model. By mixing length arguments the radial transport and the resulting density and pressure gradients in the SOL are estimated from the model. Their dependence on plasma temperature and density qualitatively agrees with the behaviour observed in ohmic discharges on ASDEX

Doppler reflectometry is characterized by a finite tilt angle of the probing microwave beam with respect to the normal onto the cutoff surface. According to the Bragg condition the diagnostic selects density perturbations with wave number K ⊥ in the reflecting layer. From the Doppler shift of the returning microwave the propagation velocity of these perturbations v ⊥ can be obtained directly. The signal intensity contains information about the perturbation amplitude. The diagnostic potential of Doppler reflectometry is demonstrated both numerically by the use of two-dimensional full-wave codes and experimentally by an antenna system with variable tilt angle installed at the W7-AS stellarator. During stationary plasma conditions the measured profile of the propagation velocity v ⊥ (r) is dominated by the E × B velocity of the plasma, which is obtained from passive spectroscopy. Transient states of the plasma can be followed with a temporal resolution of less than 50 µs. Thus, Doppler reflectometry allows us to investigate the interdependence of sheared flow and turbulence on that timescale.

High spatial resolution radial profiles of the perpendicular plasma rotation velocity u ⊥ using a dual channel 50-75 GHz Doppler reflectometer system on the ASDEX Upgrade tokamak are presented for a variety of discharge scenarios, including Ohmic, L-mode, H-mode, etc with forward and reversed magnetic field and co-and counter neutral beam injection. The reflectometers have steppable launch frequencies f o = c/λ o , with selectable O-or X-mode polarization, giving tokamak edge to mid-radius coverage. Low-field-side antennae (hog-horn antenna pairs) with deliberate tilting (primarily poloidally) produce a Doppler shifted spectrum directly proportional to the perpendicular velocity f D = u ⊥ k ⊥ /2π = u ⊥ 2 sin θ t /λ o . The incident angle θ t between the beam and cut-off layer normal varies with plasma shape, cut-off layer position and refraction. However, typical angles range from 5˚to 27˚giving a probed turbulence wavenumber, k ⊥ , range of 1.8-14.3 cm −1 , with resulting Doppler shifts f D of up to 5 MHz. The measured perpendicular velocity is u ⊥ = v E×B + v phase , which for a typical H-mode is slightly positive in the tokamak scrape-off-layer with a deep negative well across the H-mode steep pressure gradient pedestal region and then following the perpendicularly projected toroidal fluid velocity in the core, should be dominated by the E × B velocity, as the intrinsic phase velocity is predicted to be small, which may allow u ⊥ to be interpreted directly as the radial electric field E r profile.

Abstract.The technique of correlation Doppler reflectometry for providing radial correlation length L r measurements is explored in this paper. Experimental L r measurements are obtained using the recently installed dual channel Doppler reflectometer system on ASDEX Upgrade. The experimental measurements agree well with theory and with L r measured on other fusion devices using different diagnostic techniques. A strong link between L r and plasma confinement could be observed. During an L-H transition, an increase in the absolute value of E r shear was detected at the same plasma edge region where a decrease in L r was measured. This observation is in agreement with theoretical models which predict that an increase in the absolute shear suppresses turbulent fluctuations in the plasma, leading to a reduction in L r . Furthermore, L r decreases from plasma core to edge due to the more highly confined plasma pedestal region. Measurements of L r versus plasma triangularity δ were also obtained, showing a decrease of L r with increasing δ. This indicates that plasma confinement is improving with triangularity. In connection with the experimental results, an investigation of the correlation Doppler reflectometer response function using a 2-dimensional finite difference time domain (FDTD) code was performed. The simulation results confirm that Doppler reflectometry provides robust radial correlation lengths of the turbulence with high resolution and suggests that L r is independent of the turbulence wavenumber k ⊥ and its fluctuation level.

A finite-difference time-domain code is used to obtain the full-wave solution of the O-X mode conversion process for typical parameters of the TJ-II stellarator in a cylindrical geometry. This reduction of the complicated stellarator geometry to a cylindrical geometry is chosen since the conversion process occurs only over a limited radial plasma volume. In the calculations, Gaussian antenna beams are studied with the option of different beam waists in the poloidal and toroidal direction. Optimum conversion efficiency is found if the wavefront of the incident antenna beam is matched to the local curvature of the O-X conversion layer. Finally, the code is used to calculate the complete O-X-B conversion process into a Bernstein wave.

The next step in the Wendelstein stellarator line is the large superconducting device Wendelstein 7-X, currently under construction in Greifswald, Germany. Steady-state operation is an intrinsic feature of stellarators, and one key element of the Wendelstein 7-X mission is to demonstrate steady-state operation under plasma conditions relevant for a fusion power plant. Steady-state operation of a fusion device, on the one hand, requires the implementation of special technologies, giving rise to technical challenges during the design, fabrication and assembly of such a device. On the other hand, also the physics development of steady-state operation at high plasma performance poses a challenge and careful preparation. The electron cyclotron resonance heating system, diagnostics, experiment control and data acquisition are prepared for plasma operation lasting 30 min. This requires many new technological approaches for plasma heating and diagnostics as well as new concepts for experiment control and data acquisition.

Doppler reflectometry selects electron density perturbations with a finite wave vector K ⊥ by a line of sight which is non-perpendicular with respect to the reflecting layer. This provides, simultaneously, a local measurement of their propagation velocity v ⊥ (K ⊥ ) and of changes in their fluctuation amplitude ñ(K ⊥ ). In the multichannel Doppler reflectometer at the Stellarator W7-AS the antenna system and signal detection are optimized for maximum temporal resolution in order to study transport bifurcations such as the transitions between L-and H-mode. For the conditions in W7-AS the quantities v ⊥ and ñ can be measured with a temporal resolution of less than 10 µs as shown for the example of an H to L back-transition. The dual antenna system installed allows us to supplement Doppler reflectometry with a simultaneous time-offlight measurement, which is the complementary method to determine v ⊥ .

Runaway of the reflectometer output phase can be produced by transversely propagating density fluctuations if the antenna system is operated in the tilt-mode, i.e. away from perpendicular incidence. The observed characteristics of the phase runaway are explained with an analytical model. The two-dimensional (2D) effects resulting from poloidally propagating density fluctuations are calculated using a time-independent full-wave 2D code. Numerical simulations for both monochromatic and turbulent density fluctuations are presented. Good agreement with the characteristics of phase runaway measured with reflectometry in the W7-AS stellarator is obtained. The potential of a reflectometer in the tilt-mode to measure the radially resolved poloidal propagation velocity of density fluctuations is analysed.