A. Kendl

Direct measurement of zonal flows and geodesic acoustic mode oscillations in ASDEX Upgrade using Doppler reflectometry

Zonal flows (ZFs) and associated geodesic oscillations are turbulence-generated time-varying Er × BT rigid poloidal plasma flows with finite radial extent. They are of major interest for tokamak confinement since they are thought to moderate drift-wave turbulence and hence edge transport. However, detection of ZFs (believed to be driven by Reynolds stress) and Geodesic acoustic modes (GAMs) (linked with poloidal pressure asymmetries) is challenging since they appear predominantly as low frequency (few kilohertz) potential or radial electric field Er fluctuations. Presented here are measurements of GAM/ZF properties in ohmic, L-mode and H-mode ASDEX Upgrade tokamak discharges using a new Doppler reflectometry technique to measure Er fluctuations directly.

Shear flow generation and energetics in electromagnetic turbulence

Zonal flows are recognized to play a crucial role for magnetized plasma confinement. The genesis of these flows out of turbulent fluctuations is therefore of significant interest. Here the relative importance of zonal flow generation mechanisms via the Reynolds stress, Maxwell stress, and geodesic acoustic mode (GAM) transfer in drift-Alfven turbulence is investigated. By means of numerical computations the energy transfer into zonal flows owing to each of these effects is quantified. The importance of the three driving ingredients in electrostatic and electromagnetic turbulence for conditions relevant to the edge of fusion devices is revealed for a broad range of parameters. The Reynolds stress is found to provide a flow drive, while the electromagnetic Maxwell stress is in the cases considered a sink for the flow energy. In the limit of high plasma β, where electromagnetic effects and Alfven dynamics are important, the Maxwell stress is found to cancel the Reynolds stress to a high degree. The geodesic ...

The Helias reactor HSR4/18

The Helias reactor is an upgraded version of the Wendelstein 7-X experiment. A straightforward extrapolation of Wendelstein 7-X leads to HSR5/22, which has 5 field periods and a major radius of 22 m. HSR4/18 is a more compact Helias reactor with 4 field periods and an 18 m major radius. Stability limit and energy confinement times are nearly the same as in HSR5/22, thus the same fusion power (3000 MW) is expected in both configurations. Neoclassical transport in HSR4/18 is very low, and the effective helical ripple is below 1%. The article describes the power balance of the Helias reactor, and the blanket and maintenance concepts. The coil system of HSR4/18 comprises 40 modular coils with NbTi superconducting cables. The reduction from 5 to 4 field periods and the concomitant reduction in size will also reduce the cost of the Helias reactor.

The European turbulence code benchmarking effort: turbulence driven by thermal gradients in magnetically confined plasmas

A cross-comparison and verification of state-of-the-art European codes describing gradient-driven plasma turbulence in the core and edge regions of tokamaks, carried out within the EFDA Task Force on Integrated Tokamak Modelling, is presented. In the case of core ion temperature gradient (ITG) driven turbulence with adiabatic electrons (neglecting trapped particles), good/reasonable agreement is found between various gyrokinetic/gyrofluid codes. The main physical reasons for some deviations observed in nonlocal simulations are discussed. The edge simulations agree very well on collisionality scaling and acceptably well on beta scaling (below the MHD boundary) for cold-ion cases, also in terms of the non-linear mode structure.

Radial convection of finite ion temperature, high amplitude plasma blobs

We present results from simulations of seeded blob convection in the scrape-off-layer of magnetically confined fusion plasmas. We consistently incorporate high fluctuation amplitude levels and finite Larmor radius (FLR) effects using a fully nonlinear global gyrofluid model. This is in line with conditions found in tokamak scrape-off-layers (SOL) regions. Varying the ion temperature, the initial blob width, and the initial amplitude, we found an FLR dominated regime where the blob behavior is significantly different from what is predicted by cold-ion models. The transition to this regime is very well described by the ratio of the ion gyroradius to the characteristic gradient scale length of the blob. We compare the global gyrofluid model with a partly linearized local model. For low ion temperatures, we find that simulations of the global model show more coherent blobs with an increased cross-field transport compared to blobs simulated with the local model. The maximal blob amplitude is significantly higher in the global simulations than in the local ones. When the ion temperature is comparable to the electron temperature, global blob simulations show a reduced blob coherence and a decreased cross-field transport in comparison with local blob simulations.

Flux-surface shaping effects on tokamak edge turbulence and flows

Shaping of magnetic flux surfaces is found to have a strong impact on turbulence and transport in tokamak edge plasmas. A series of axisymmetric equilibria, with varying elongation and triangularity, and a divertor configuration are implemented into a computational gyrofluid turbulence model. The mechanisms of shaping effects on turbulence and flows are identified. Transport is mainly reduced by local magnetic shearing and an enhancement of zonal shear flows induced by elongation and X-point shaping.

Radial and zonal modes in hyperfine-scale stellarator turbulence

Electromagnetic plasma turbulence at hyperfine (i.e., electron gyroradius) scales is studied in the geometry of an advanced stellarator fusion experiment, Wendelstein 7-AS [H. Renner, Plasma Phys. Controlled Fusion 31, 1579 (1989)], by means of nonlinear gyrokinetic simulations. It is demonstrated that high-amplitude radial streamers may also exist in non-tokamak devices, raising the electron heat flux to experimentally relevant values. Moreover, some statistical characteristics of the fully developed turbulence are computed, highlighting the (co-)existence, nature, and role of self-generated zonal flows and fields.

Investigation of the parallel dynamics of drift-wave turbulence in toroidal plasmas

The three-dimensional structure of drift-wave turbulence is studied in the core of the toroidal low-temperature plasma in the torsatron TJ-K. The results are compared with simulations from the GEM3 turbulence code. In experiment and simulation, the dimensionless parameters are similar to those of fusion edge plasmas. Arrays with 64 probes are used to measure parallel wavenumbers, propagation velocities and the tilt of the turbulent structures with respect to the field line. A parallel wavelength of 15 m and a parallel velocity in between the ion-sound and the Alfven velocity confirm the three-dimensional nature of drift-wave turbulence. Quantitative agreement between experiment and simulation is found. The comparison with the drift-wave dispersion relation gives evidence for the coupling of the density perturbation to the shear-Alfven wave.

Measurement of Poloidal Flow, Radial Electric Field and E × B Shearing Rates at ASDEX Upgrade

The theoretical model of transport reduction by shear decorrelation is tested exper- imentally for ASDEX Upgrade discharges. The radial force balance is used to determine the radial electric eld from charge exchange recombination spectroscopy measurements. As the eective rate coecient for photon emission of the charge exchange process depends on the collision energy, the alignment of the lines of sight with respect to the neutral beam gives rise to apparent velocities and temperatures. In addition, the gyro-motion of the observed species along with the nite lifetime of the observed excited state leads to lineshifts in spectra measured in the poloidal direction. Both eects require corrections, which will be discussed. The corrections are tested using the measurements of a discharge with a locked mode. From the proles of an H mode discharge with improved connement and a discharge with an internal transport barrier (ITB) the ion heat transport coecients, shearing rates and the maximum linear growth rates of the instabilities are calculated. Comparison of these results supports the assumption that turbulent transport due to the ion temperature gradient instability is suppressed inside the transport barrier in the ITB discharge. However, due to the dom- inant influence of the toroidal rotation velocity on the central shear, they do not prove the shear decorrelation model, because Er naturally rises during the phases with improved connement if unbalanced neutral beam heating is applied.

Analysis of the temperature influence on Langmuir probe measurements on the basis of gyrofluid simulations

The influence of the temperature and its fluctuations on the ion saturation current and the floating potential, which are typical quantities measured by Langmuir probes in the turbulent edge region of fusion plasmas, is analysed by global nonlinear gyrofluid simulations for two exemplary parameter regimes. The numerical simulation facilitates a direct access to densities, temperatures and the plasma potential at different radial positions around the separatrix. This allows a comparison between raw data and the calculated ion saturation current and floating potential within the simulation. Calculations of the fluctuation-induced radial particle flux and its statistical properties reveal significant differences to the actual values at all radial positions of the simulation domain, if the floating potential and the temperature-averaged density inferred from the ion saturation current is used.