E. Holzhauer

Doppler Reflectometry for the Investigation of Propagating Density Perturbations

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.

Plasma rotation profile measurements using Doppler reflectometry

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 fo = c/λo, 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 fD = u⊥k⊥/2π = u⊥ 2sinθt/λo. The incident angle θt 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 fD of up to 5 MHz. The measured perpendicular velocity is u⊥ = vE × B + vphase, 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 Er profile.

Theoretical and experimental investigation of the phase-runaway in microwave reflectometry

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.

Radial correlation length measurements on ASDEX Upgrade using correlation Doppler reflectometry

The technique of correlation Doppler reflectometry for providing radial correlation length Lr measurements is explored in this paper. Experimental Lr measurements are obtained using the recently installed dual channel Doppler reflectometer system on ASDEX Upgrade. The experimental measurements agree well with theory and with Lr measured on other fusion devices using different diagnostic techniques. A strong link between Lr and plasma confinement could be observed. From the L- to the H-mode, an increase in the absolute value of Er shear was detected at the same plasma edge region where a decrease in Lr 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 Lr. Furthermore, Lr decreases from the plasma core to the edge and decreases with increasing plasma triangularity δ. The experimental results have been extensively modelled using a 2-dimensional finite difference time domain code. The simulations confirm that Doppler reflectometry provides robust radial correlation lengths of the turbulence with high resolution and suggests that Lr is independent of the turbulence wavenumber k⊥ and its fluctuation level.

Operational conditions and characteristics of ELM-events during H-mode plasmas in the stellarator W7-AS

H-mode operation in the low-shear stellarator W7-AS is achieved for specific plasma edge topologies characterized by three `operational windows of the edge rotational transform. An explanation for this strong influence of the magnetic configuration could be the increase of viscous damping if rational surfaces and thus island structures occur within the relevant plasma edge layer, thereby impeding the development of an edge transport barrier. Prior to the final transition to a quiescent state, the plasma edge passes a rich phenomenology of dynamic behaviour such as dithering and ELMs. Plasma edge parameters indicate that a quiescent H-mode occurs if a certain edge pressure is achieved.

Confinement regime transitions in ASDEX

The authors give an overview of the different confinement regimes observed on ASDEX and compare the changes during the transition phases with qualitative tendencies suggested by theoretical models. The transitions discussed are those between purely Ohmic heating and additional heating in the L-regime between the L- and the H-regime and between discharges with flat and peaked electron density profiles.

A new quasi-stationary, very high density plasma regime on the W7-AS stellarator

Stellarators have the intrinsic property of steady state operation. However, on present-day stellarators the pulse length is usually not only limited due to technical reasons, but also by physical problems. Lack of density control and a subsequent radiation collapse terminate the discharges quite often at high densities. To improve the control of the plasma-wall interaction, the island divertor concept was developed for optimized stellarators. To test this divertor concept on W7-AS, all limiters were removed and replaced by ten divertor modules. In subsequent divertor experiments a promising new plasma operational regime has been discovered which is termed high density H-mode (HDH-mode). During the transition into that regime a clear reduction of ELM-like events and turbulent fluctuations is observed. The HDH-mode combines good energy confinement with very low impurity confinement resulting in low core radiation, but high edge-localized radiation. Consequently, stationary discharges at densities of typically 2 x 10 20 m -3 can be performed within the accessible pulse length of about 1 s. At densities above 3 x 10 20 m -3 a controlled transition from attached to partially detached plasmas is observed. The still edge-localized radiation reaches 90% of the heating power so that the power load onto the divertor target plates is further reduced. At a lower toroidal field of 0.9 T average β-values could be raised from earlier 2% to more than 3% in magnetic field configurations with rather smooth flux surfaces at the plasma boundary. The recently obtained results render excellent prospects for W7-X, the larger superconducting successor experiment of W7-AS.

Coupling of turbulence and reflectometer simulation codes and comparison with experiment

First proof-of-principle results are presented from the coupling of a numerical (Landau-fluid) turbulence code and three two-dimensional reflectometer simulation codes (finite-difference time domain full-wave, time-invariant network full-wave, and physical optics). With a view to validating the numerical turbulence code, the results are favourably compared with real reflectometer signals from an L-mode edge discharge in the ASDEX Upgrade tokamak.

The H-mode in the ASDEX tokamak

The L-H transition in the ASDEX tokamak was investigated. The level of fluctuations both in the scrape-off layer and in a narrow region inside the separatrix drops on a 100 mu s timescale. Density turbulence coming from the edge and magnetic turbulence, as measured with magnetic probes, show a similar behavior. Before and during the transition no frequency shift in the fluctuation spectra could be detected which would indicate the rapid appearance of a radial E-field in the region of the H-mode barrier. Within milliseconds after the L-H transition new fluctuations propagating in the electron diamagnetic direction are destabilized which however exhibit no marked influence on transport across the barrier. In the bulk of the plasma fluctuation spectra show a frequency shift which is dominated by toroidal plasma rotation due to neutral beam injection. The initial transport barrier is deduced to lie inside the separatrix with a width of <or=3 cm.

Measurement and scaling of the radial correlation lengths of turbulence at the plasma edge of ASDEX Upgrade

With the single-frequency swept reflectometry diagnostic on ASDEX Upgrade radial correlation lengths of the plasma turbulence can be determined by a new quasi-simultaneous correlation technique. The scaling of the observed microturbulence is investigated in detail through the correlation lengths measured 2 cm inside the separatrix. During the low-temperature L-mode and near the L-H transition these scale according to the resistive ballooning turbulence model. For L- and H-mode plasmas with higher temperatures the correlation lengths scale with the effective Larmor radius as expected for dominantly i driven turbulence.