Overview of physics studies on ASDEX Upgrade

The ASDEX Upgrade (AUG) programme, jointly run with the EUROfusion MST1 task force, continues to significantly enhance the physics base of ITER and DEMO. Here, the full tungsten wall is a key asset for extrapolating to future devices. The high overall heating power, flexible heating mix and comprehensive diagnostic set allows studies ranging from mimicking the scrape-off-layer and divertor conditions of ITER and DEMO at high density to fully non-inductive operation (q(95) = 5.5, beta(N)

Measurement of edge electrostatic turbulence in the TCV tokamak plasma boundary

Jan Horacek

Almost since the first density profile measurements were made in the scrape-off layer (SOL) of the early tokamaks, it has been recognized that the rate of particle transport perpendicular to magnetic surfaces exceeds that expected on the basis of classical collisional diffusion by as much as three orders of magnitude. Plasma turbulence has rightfully been claimed as the origin of such large discrepancies, much as it has for enhanced (over classical or neoclassical) transport rates observed in the confined plasma. But in the SOL, the "bursty" or "blobby" nature of the measured density fluctuations is of a much higher amplitude than that found in the core, making large-scale, convective fluid turbulence a strong candidate mechanism. This thesis will demonstrate quantitatively and unequivocally, for the first time, that such interchange motions are indeed the driver for the edge density and particle flux fluctuations observed on the Tokamak a Configuration Variable (TCV). Since the principle driver of this turbulence is a curved magnetic field, with gradient direction matching that of the local edge plasma pressure profile, together with a region of open magnetic field lines, the interchange mechanism identified here is very likely to be the very same process at the root of transport in all tokamak SOLs. In showing that the measured turbulence driven cross-field particle flux in TCV is quantitatively consistent with interchange physics, a path is opened by which the anomalous transport rates might be estimated in a predictive way for larger tokamaks, like the ITER device, which are yet to be built but for which concerns are now being raised that such transport might lead to excessive plasma-wall interactions. Using a fast reciprocating Langmuir probe, fluctuation measurements have been made in the TCV low-field-side SOL across a wide range of ohmic discharges comprising variations in plasma shape and configuration (limiter and divertor), plasma current, confinement mode (L and H), plasma density, toroidal magnetic field direction and plasma fuel species (deuterium and helium). Statistical analysis of the time series is used to demonstrate a remarkable degree of similarity across the database and to show that the radial dependence of the probability distribution functions (PDFs) of flux and density fluctuations can be well approximated by the known Gamma and Lognormal analytic PDFs, characterized in terms only of the relative fluctuation levels. In the vicinity of the SOL-main chamber interface, where particles interact with the walls, the density fluctuations exhibit clear evidence of self-similarity over two orders of magnitude in frequency and a PDF which is universal in shape. The observed constancy of the correlation between density and poloidal field fluctuations in turn implies a universal PDF for the radial particle flux which moreover is found to scale almost linearly with the local mean density. Careful comparison of one particular case inside the experimental database with the results of 2D fluid turbulence simulations of the TCV SOL using the ESEL code developed at the Risø National Laboratory, Denmark has shown a remarkable level of agreement between theory and experiment when the simulation output time series is analyzed in exactly the same way as that applied to the tokamak data. Quantitative agreement between model and experiment has been found for radial profiles of mean values, fluctuation levels, PDF shapes, timescales and power spectra of both density and turbulent driven flux throughout the main SOL and even partially inside the separatrix. Automatically, this level of agreement also implies that the code output conforms quite closely to the Gamma and Lognormal distributions. Parallel SOL flow data have also been gathered simultaneously with the turbulence measurements. An extensive database of radial Mach flow profiles has been assembled, most notably including a direct comparison of the density dependence of the flow dynamics in carefully matched discharges with forward and reversed toroidal field. These constitute the first measurements of their kind in TCV and reveal the presence of very strong flows, up to Mach numbers of ∼ 0.6. The magnitude and direction of the measured flows is found to be surprisingly well described by neoclassical return parallel flows (Pfirsch-Schlüter) compensating the poloidal ExB and diamagnetic drifts. Combining the forward and reversed field data uncovers a slight, field independent offset thought to originate from the excess transport, driven by the interchange motions, in the outboard midplane vicinity. The flow and fluctuation data have been combined to test the possible link between flow generation and turbulence first demonstrated from similar data on JET. No such correlations have been found on TCV throughout most of the SOL, supporting the finding that the neoclassical component can account for the majority of the measured parallel flow.

EPFL2006

Sources and detectors of fast ions for basic devices

Ambrogio Fasoli, Ivo Furno, Gennady Plyushchev

The physics of supra thermal test ions in turbulent plasmas can be conveniently studied in basic plasma physics devices, which allow high-resolution measurements of plasma and fast ion parameters and wave fields throughout the whole plasma cross-section. We describe recent advances in the development of an experimental setup consisting of a non-perturbative Li 6+ miniaturized ion source and a detector for the investigation of the interaction between supra thermal ions and drift/interchange–driven turbulence on the TORPEX device. We present first measurements of the spatial and energy space distribution of the fast ion beam in different plasma scenarios, in which the plasma turbulence is fully characterized. We also discuss the possibility of using the fast ion source in basic plasma devices for fusion. This work is partly funded by the Fonds National Suisse de la Recherche Scientifique.

2009

Plasma turbulence in the tokamak scrape-off layer

Federico David Halpern, Sébastien Jolliet, Joaquim Loizu Cisquella, Annamaria Mosetto, Mario Podesta, Paolo Ricci, Fabio Riva, Christoph Wersal

In the tokamak scrape-off layer (SOL) the magnetic field lines are open, channeling particles and heat onto plasma facing components, and constraining their lifetime. This is a critical issue for ITER and future devices -- safe operation of fusion devices will require a better understanding of the SOL plasma dynamics \cite{Loarte2007}. The steady-state heat load onto the tokamak plasma facing components depends on the SOL width, which results from a balance between plasma injection from the core region, turbulent transport, and losses to the divertor or limiter. Recently, we have gained deep insights into the tokamak SOL dynamics in the inner-wall limited configuration (like the ITER start-up plasmas), by means of massively-parallel fluid electromagnetic turbulence simulations. SOL turbulence, in fact, is characterized by the presence of large amplitude meso-scale turbulence, which requires the use of a global, flux-driven approach. In order to address this turbulent system, we have developed GBS, a flux-driven global turbulence code implementing the drift-reduced Braginskii equations. Our investigations have pointed out, among the others, the mechanisms regulating the turbulence level and therefore the SOL width, the turbulent regimes, and the mechanisms driving the rotation in this region. GBS simulations at realistic size and plasma parameters display features normally observed in the SOL of limited discharges, for instance, low frequency drift-wave or ballooning turbulence, fluctuations with an amplitude of the order of 30 percent and a poloidal width of about 10 ion sound larmor radii. Moreover, we have recovered strongly skewed fluctuation PDFs revealing the presence of intermittent transport events. In the present talk, we address, in particular, (a) the mechanisms establishing the SOL width, which regulates the steady-state heat load and (b) the physics of coherent filamentary plasma structures (blobs) that can travel across the SOL carrying heat and particles. The simulated non-linear dynamics have been compared with analytical estimates, that have highlighted the key physics mechanisms at play in the SOL, and with experimental measurements taken in a number of tokamak worldwide, showing good agreement.