B. Labit

Electrostatic turbulence and transport in a simple magnetized plasma

Gradient driven electrostatic instabilities are investigated in TORPEX [A. Fasoli, B. Labit, M. McGrath, S. H. Muller, M. Podesta, and F. M. Poli, Bull. Am. Phys. Soc. 48, 119 (2003)], a toroidal device (R=1m, a=0.2m) in which plasmas are produced by microwaves (P⩽20kW) with frf=2.45GHz, in the electron cyclotron frequency range. Typical density and temperature are ne⩽1017m−3 and Te≃5eV, respectively. The magnetic field is mainly toroidal (⩽0.1T), with a small vertical component (⩽4mT). Instabilities that can be generally identified as drift-interchange waves are observed and characterized for different levels of collisionality with neutrals. The frequency spectrum and the spatial profile of the fluctuation-induced flux are measured. An 86-tip probe is used to reconstruct the spatio-temporal evolution of density structures across the plasma cross section. The measured structures are characterized statistically, and related quantitative observables are constructed.

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.

Basic turbulence studies on TORPEX and challenges in the theory-experiment comparison

TORPEX [A. Fasoli, B. Labit, M. McGrath, S. H. Muller, M. Podesta, and F. M. Poli, Bull. Am. Phys. Soc. 48, 119 (2003)] is dedicated to the study of electrostatic instabilities, turbulence, and transport. Plasmas are produced by waves in the electron cyclotron frequency range and are confined by a toroidal magnetic field of about 0.1T to which a small vertical component Bz is added. The crucial role of Bz for the basic confinement scheme through the generation of parallel flows has been studied previously. This paper focuses on the effects of Bz on turbulence. The observed strong dependence indicates an intrinsic coupling between average profiles, confinement, and turbulence regulated by the action of Bz. Two approaches to characterize turbulence are adopted, via time series statistics and via the direct measurement of spatiotemporal structures, made possible by the novel hexagonal turbulence imaging probe diagnostic, which is described in detail. Analysis methods to condense the large amount of data of s...

Experimental characterization of drift-interchange instabilities in a simple toroidal plasma

Low frequency electrostatic instabilities are investigated on TORPEX [Fasoli, Labit, McGrath, Muller, Podesta, and Poli, Bull. Am. Phys. Soc. 48, 119 (2003)], a toroidal device for basic plasma physics experiments with a toroidal magnetic field 100mT and a small vertical magnetic field (⩽4mT). A two-dimensional (2D) profile of the frequency and amplitude of density and potential fluctuations is reconstructed using electrostatic probes with high space and time resolution. The measured phase velocity, corrected for the Doppler shift induced by the E×B drift, is consistent with the electron diamagnetic drift velocity. The local dispersion relation, measured along and across the magnetic field, is in agreement with the predictions of a linear kinetic slab model for drift waves. Unstable modes are generated in regions of unfavorable curvature, where the pressure gradient is colinear with the magnetic field gradient. It is demonstrated that the curvature of the magnetic field lines is essential for driving the ...

Plasma production by low-field side injection of electron cyclotron waves in a simple magnetized torus

The mechanisms leading to ionization of neutral gas by means of microwaves in the electron cyclotron range of frequencies in a simple toroidal plasma configuration are investigated in the TORPEX device (Fasoli A et al 2003 Bull. Am. Phys. Soc. 48 119). The role of the upper hybrid resonance, where most of the ionization events occur once the plasma is formed, is clearly demonstrated. The fast phenomena related to ionization are discriminated from the slower mechanisms leading to the stationary background profiles by measuring the density perturbation caused by a modulation of the injected microwave power. The same method permits reconstruction of the spatial profile of the particle source. The ionization processes are then related to experimental control parameters such as the injected microwave power.

Theory-based scaling of the SOL width in circular limited tokamak plasmas

A theory-based scaling for the characteristic length of a circular, limited tokamak scrape-off layer (SOL) is obtained by considering the balance between parallel losses and non-linearly saturated resistive ballooning mode turbulence driving anomalous perpendicular transport. The SOL size increases with plasma size, resistivity, and safety factor q. The scaling is verified against flux-driven non-linear turbulence simulations, which reveal good agreement within a wide range of dimensionless parameters, including parameters closely matching the TCV tokamak. An initial comparison of the theory against experimental data from several tokamaks also yields good agreement.

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.

Plasma blobs in a basic toroidal experiment: Origin, dynamics, and induced transport

Detaching plasma blobs with very similar properties to tokamaks are observed in the basic toroidal plasma experiment TORPEX [A. Fasoli et al., Phys. Plasmas 13, 055902 (2006)]. The blobs originate from the breaking of wave crests of a drift-interchange wave, which span over regions characterized by strongly inhomogeneous background parameters. Once decoupled from the wave, the blobs follow a predominantly radial trajectory pattern. The blob-induced cross-field transport can instantaneously exceed the steady-state parallel fluxes by one order of magnitude, while accounting for only 10% of the time-average device losses. If the particles were confined in the parallel direction, as is the case in tokamaks, blobs would constitute the dominant loss mechanism in TORPEX. The presented results show that the presence of gradB is sufficient and neither a magnetic-topology change nor the presence of limiters, both absent in TORPEX, are necessary for the generation of blobs.

Global numerical study of electron temperature gradient-driven turbulence and transport scaling

A three-dimensional, electromagnetic, fluid code with flux boundaries conditions is used to study the scaling of electron thermal transport caused by electron temperature gradient-driven (ETG) turbulence. It is found that close to the ETG threshold, the thermal transport depends weakly on β, which differs from the heuristic formula χe∝1/β. It is shown also that electron thermal losses scale like gyro-Bohm. The observed electron temperature profiles appear more resilient than the ion temperature profiles obtained from previous ITG simulations in similar conditions.