C. Theiler

Simulation of plasma turbulence in scrape-off layer conditions: the GBS code, simulation results and code validation

Based on the drift-reduced Braginskii equations, the Global Braginskii Solver, GBS, is able to model the scrape-off layer (SOL) plasma turbulence in terms of the interplay between the plasma outflow from the tokamak core, the turbulent transport, and the losses at the vessel. Model equations, the GBS numerical algorithm, and GBS simulation results are described. GBS has been first developed to model turbulence in basic plasma physics devices, such as linear and simple magnetized toroidal devices, which contain some of the main elements of SOL turbulence in a simplified setting. In this paper we summarize the findings obtained from the simulation carried out in these configurations and we report the first simulations of SOL turbulence. We also discuss the validation project that has been carried out together with the GBS development.

Electrostatic instabilities, turbulence and fast ion interactions in the TORPEX device

Electrostatic turbulence, related structures and their effect on particle, heat and toroidal momentum transport are investigated in TORPEX simple magnetized plasmas using high-resolution diagnostics, control parameters, linear fluid models and nonlinear numerical simulations. The nature of the dominant instabilities is controlled by the value of the vertical magnetic field, Bv, relative to that of the toroidal field, BT. For Bv/BT > 3%, only ideal interchange instabilities are observed. A critical pressure gradient to drive the interchange instability is experimentally identified. Interchange modes give rise to blobs, radially propagating filaments of enhanced plasma pressure. Blob velocities and sizes are obtained from electrostatic probe measurements using pattern recognition methods. The observed values span a wide range and are described by a single analytical expression, from the small blob size regime in which the blob velocity is limited by cross-field ion polarization currents, to the large blob size regime in which the limitation to the blob velocity comes from parallel currents to the sheath. As a first attempt at controlling the blob dynamical properties, limiter configurations with varying angles between field lines and the conducting surface of the limiter are explored. Mach probe measurements clearly demonstrate a link between toroidal flows and blobs. To complement probe data, a fast framing camera and a movable gas puffing system are installed. Density and light fluctuations show similar signatures of interchange activity. Further developments of optical diagnostics, including an image intensifier and laser-induced fluorescence, are under way. The effect of interchange turbulence on fast ion phase space dynamics is studied using movable fast ion source and detector in scenarios for which the development from linear waves into blobs is fully characterized. A theory validation project is conducted in parallel with TORPEX experiments, based on quantitative comparisons of observables that are defined in the same way in the data and in the output of numerical codes, including 2D and 3D local and global simulations.

Langmuir probe-based observables for plasma-turbulence code validation and application to the TORPEX basic plasma physics experiment

The methodology for plasma-turbulence code validation is discussed, with focus on the quantities to use for the simulation-experiment comparison, i.e., the validation observables, and application to the TORPEX basic plasma physics experiment [A. Fasoli , Phys. Plasmas 13, 055902 (2006)]. The considered validation observables are deduced from Langmuir probe measurements and are ordered into a primacy hierarchy, according to the number of model assumptions and to the combinations of measurements needed to form each of them. The lowest levels of the primacy hierarchy correspond to observables that require the lowest number of model assumptions and measurement combinations, such as the statistical and spectral properties of the ion saturation current time trace, while at the highest levels, quantities such as particle transport are considered. The comparison of the observables at the lowest levels in the hierarchy is more stringent than at the highest levels. Examples of the use of the proposed observables are applied to a specific TORPEX plasma configuration characterized by interchange-driven turbulence.

Practical solutions for reliable triple probe measurements in magnetized plasmas.

The triple probe method to obtain local, time-resolved measurements of density, electron temperature and plasma potential is investigated in detail. The difficulties in obtaining reliable measurements with this technique are discussed and overcome. These include phase delay errors, ion sheath expansion and limited bandwidth due to stray capacitance to ground. In particular, a relatively simple electronic circuit is described to strongly reduce stray capacitance. Measurements with the triple probe are presented in a plasma characterized by interchange-driven turbulence in the TORPEX device. The measured time-averaged and time-dependent, conditionally averaged parameters are cross-checked with other Langmuir probe based techniques, and show good agreement. Triple probe measurements show that electron temperature fluctuations are sufficiently large, such that the identification of plasma potential fluctuations with fluctuations of the floating potential is not a good approximation. Over a large radial region, the time-averaged fluctuation-induced particle flux can, however, be deduced from floating potential only. This is because the phase shift between density and electron temperature is close to zero there and temperature fluctuations do not give rise to a net radial particle transport.

Studies of blob formation, propagation and transport mechanisms in basic experimental plasmas (TORPEX and CSDX)

The findings of previous blob studies in the interchange-dominated regime of TORPEX helium (Muller et al 2007 Phys. Plasmas 14 110704) and hydrogen plasmas (Furno et al 2008 Phys. Rev. Lett. 100 055004) are summarized and compared. The onset of blobs is studied as a function of the vertical magnetic field Bz, proving the existence of blobs also in the drift-interchange-dominated regime characterized by Bz < 1 mT. These blobs, despite being inherently three-dimensional and sheath-disconnected, exhibit statistical properties similar to the blobs in the interchange regime. Using conditionally averaged density and potential measurements, the entire time line of an interchange instability leading to the formation of wave-like structure patterns and blobs could be experimentally observed. These results show that a background E × B shear flow is not essential for the generation of blobs and that the phase shift between density and potential during the generation of blobs is π/2 in all studied cases, demonstrating the interchange nature of blobs in TORPEX. Fast-camera and Langmuir probe measurements of blobs in the linear device CSDX (Tynan et al 2004 Phys. Plasmas 11 5195) support the hypothesis that an interchange instability is also responsible for the generation of blobs in the linear geometry, where the necessary effective gravity is provided by centrifugal forces.

The role of the density gradient on intermittent cross-field transport events in a simple magnetized toroidal plasma

Intermittent cross-field particle transport events (ITEs) are studied in the basic toroidal device TORPEX [TORoidal Plasma EXperiment, A. Fasoli et al., Phys. Plasmas 13, 055902 (2006)], with focus on the role of the density gradient. ITEs are due to the intermittent radial elongation of an interchange mode. The elongating positive wave crests can break apart and form blobs. This is not necessary, however, for plasma particles to be convected a considerable distance across the magnetic field lines. Conditionally sampled data reveal two different scenarios leading to ITEs. In the first case, the interchange mode grows radially from a slab-like density profile and leads to the ITE. A novel analysis technique reveals a monotonic dependence between the vertically averaged inverse radial density scale length and the probability for a subsequent ITE. In the second case, the mode is already observed before the start of the ITE. It does not elongate radially in a first stage, but at a later time. It is shown that this elongation is preceded by a steepening of the density profile as well. ©2008 American Institute of Physics

Blob current structures in TORPEX plasmas: experimental measurements and numerical simulations

In the simple magnetized torus TORPEX, field-aligned blobs originate from ideal interchange waves and propagate radially outward due to ∇B and curvature induced drifts. Time-resolved two-dimensional measurements of the field-aligned current density J∥ associated with blobs are obtained from conditionally sampled data from a single-sided Langmuir probe and a specially designed current probe. The profile of J∥ exhibits an asymmetric dipolar structure, which originates from the polarization of the blob and is consistent with sheath boundary conditions. The asymmetry results from the non-linear dependence of J∥ at the sheath edge upon the floating potential. Using internal measurements, we directly confirm the existence of two regimes, in which parallel currents to the sheath do or do not significantly damp charge separation and thus blob radial velocity. To investigate the effect of the observed asymmetry of J∥ on the blob motion, we carried out numerical simulations of seeded blobs, using a two-field fluid model, which evolves electron density and vorticity. Simulations are performed spanning a wide range of blob sizes covering both regimes. We use either the complete or a linearized form for the sheath dissipation term in the vorticity equation. The structure of the parallel current density and plasma potential is found to be different in the two cases. Asymmetric profiles are observed in simulations with the complete form, while symmetric profiles are obtained when a linearized form is used. Negligible effects are, however, observed in terms of blob radial velocity. The relevance of the present results for fusion devices is also discussed.

Statistical properties of electrostatic turbulence in toroidal magnetized plasmas

Note: invited paper Reference CRPP-CONF-2007-059 URL: http://www.eps2007.ifpilm.waw.pl/ Record created on 2008-05-13, modified on 2017-05-12

Blob motion and control in simple magnetized plasmas

The radial propagation of plasma blobs and possibilities of influencing it are investigated in the TORPEX toroidal experiment [Fasoli et al., Phys. Plasmas 13, 055902 (2006)]. The effect of changing the connection length and the neutral background pressure on blob velocity is measured and trends are found to agree with predictions from a previous study [Theiler et al., Phys. Rev. Lett. 103, 065001, (2009)]. Effects on blob motion due to a change in limiter material and geometry are also discussed.

Imaging of turbulent structures and tomographic reconstruction of TORPEX plasma emissivity

In the TORPEX [ A. Fasoli et al., Phys. Plasmas 13, 055902 (2006) ], a simple magnetized plasma device, low frequency electrostatic fluctuations associated with interchange waves, are routinely measured by means of extensive sets of Langmuir probes. To complement the electrostatic probe measurements of plasma turbulence and study of plasma structures smaller than the spatial resolution of probes array, a nonperturbative direct imaging system has been developed on TORPEX, including a fast framing Photron-APX-RS camera and an image intensifier unit. From the line-integrated camera images, we compute the poloidal emissivity profile of the plasma by applying a tomographic reconstruction technique using a pixel method and solving an overdetermined set of equations by singular value decomposition. This allows comparing statistical, spectral, and spatial properties of visible light radiation with electrostatic fluctuations. The shape and position of the time-averaged reconstructed plasma emissivity are observed to be similar to those of the ion saturation current profile. In the core plasma, excluding the electron cyclotron and upper hybrid resonant layers, the mean value of the plasma emissivity is observed to vary with (Te)^α(ne)^β, in which α = 0.25–0.7 and β = 0.8–1.4, in agreement with collisional radiative model. The tomographic reconstruction is applied to the fast camera movie acquired with 50 kframes/s rate and 2 μs of exposure time to obtain the temporal evolutions of the emissivity fluctuations. Conditional average sampling is also applied to visualize and measure sizes of structures associated with the interchange mode. The ω-time and the two-dimensional k-space Fourier analysis of the reconstructed emissivity fluctuations show the same interchange mode that is detected in the ω and k spectra of the ion saturation current fluctuations measured by probes. Small scale turbulent plasma structures can be detected and tracked in the reconstructed emissivity movies with the spatial resolution down to 2 cm, well beyond the spatial resolution of the probe array.