Fabio Avino

Basic investigations of electrostatic turbulence and its interaction with plasma and suprathermal ions in a simple magnetized toroidal plasma

Progress in basic understanding of turbulence and its influence on the transport both of the plasma bulk and of suprathermal components is achieved in the TORPEX simple magnetized torus. This configuration combines a microwave plasma production scheme with a quasi-equilibrium generated by a toroidal magnetic field, onto which a small vertical component is superimposed, simulating a simplified form of tokamak scrape-off layers. After having clarified the formation of blobs in ideal interchange turbulence, TORPEX experiments elucidated the mechanisms behind the blob motion, with a general scaling law relating their size and speed. The parallel currents associated with the blobs, responsible for the damping of the charge separation that develops inside them, hence determining their cross-field velocity, have been measured. The blob dynamics is influenced by creating convective cells with biased electrodes, arranged in an array on a metal limiter. Depending on the biasing scheme, radial and vertical blob velocities can be varied. Suprathermal ion transport in small-scale turbulence is also investigated on TORPEX. Suprathermal ions are generated by a miniaturized lithium source, and are detected using a movable double-gridded energy analyser. We characterize vertical and radial spreading of the ion beam, associated with the ideal interchange-dominated plasma turbulence, as a function of the suprathermal ion energy and the plasma temperature. Experimental results are in good agreement with global fluid simulations, including in cases of non-diffusive behaviour. To investigate the interaction of plasma and suprathermal particles with instabilities and turbulence in magnetic configurations of increasing complexity, a closed field line configuration has recently been implemented on TORPEX, based on a current-carrying wire suspended in the vacuum chamber. First measurements indicate the creation of circular symmetric profiles centred on the magnetic axis, and instabilities driven in the region of strong gradients, with a strong ballooning character.

Electromagnetic turbulent structures: A ubiquitous feature of the edge region of toroidal plasma configurations

Electromagnetic features of turbulent filaments, emerging from a turbulent plasma background, have been studied in four different magnetic configurations: the stellarator TJ-II, the Reversed Field Pinch RFX-mod, a device that can be operated also as a ohmic tokamak, and the Simple Magnetized Torus, TORPEX. By applying an analogous diagnostic concept in all cases, direct measurements of both field-aligned current density and vorticity were performed inside the filament. The inter-machine comparison reveals a clear dependence of the filament vorticity upon the local time-averaged E × B flow shear. Furthermore, a wide range of local beta was explored allowing concluding that this parameter plays a fundamental role in the appearance of filament electromagnetic features.

Plasma turbulence, suprathermal ion dynamics and code validation on the basic plasma physics device TORPEX

The TORPEX basic plasma physics device at the Center for Plasma Physics Research (CRPP) in Lausanne, Switzerland is described. In TORPEX, simple magnetized toroidal configurations, a paradigm for the tokamak scrape-off layer (SOL), as well as more complex magnetic geometries of direct relevance for fusion are produced. Plasmas of different gases are created and sustained by microwaves in the electron-cyclotron (EC) frequency range. Full diagnostic access allows for a complete characterization of plasma fluctuations and wave fields throughout the entire plasma volume, opening new avenues to validate numerical codes. We detail recent advances in the understanding of basic aspects of plasma turbulence, including its development from linearly unstable electrostatic modes, the formation of filamentary structures, or blobs, and its influence on the transport of energy, plasma bulk and suprathermal ions. We present a methodology for the validation of plasma turbulence codes, which focuses on quantitative assessment of the agreement between numerical simulations and TORPEX experimental data.

The new TORPEX in-vessel toroidal conductor for the generation of a poloidal magnetic field

TORoidal Plasma EXperiment (TORPEX) is a Simple Magnetized Torus featuring open helical magnetic field lines obtained from the superposition of a small vertical component on the main toroidal field. This work introduces the experimental setup developed to include a poloidal magnetic field. The toroidal and poloidal fields generate a rotational transform, making the magnetic geometry of TORPEX closer to that of a tokamak. This upgrade opens the possibility to deal with closed and open flux surfaces, as well as with the transition region across the last closed flux surface. The main technical solutions are discussed together with the physical considerations at the basis of the system design. Selected examples of the magnetic configurations accessible with the set of magnetic field coils available on TORPEX are discussed, ranging from single-null X-points to magnetic snowflakes. The simplest magnetic configuration of quasi-circular concentric flux surfaces is tested experimentally. Measurements of the two-dimensional electron plasma density profiles and the particle confinement time are presented, together with the first steps towards the understanding of plasma production mechanisms.

Dual Langmuir-probe array for 3D plasma studies in TORPEX

We have designed and installed a new Langmuir-probe (LP) array diagnostic to determine basic three-dimensional (3D) features of plasmas in TORPEX. The diagnostic consists of two identical LP arrays, placed on opposite sides of the apparatus, which provide comprehensive coverage of the poloidal cross section at the two different toroidal locations. Cross correlation studies of signals from the arrays provide a basic way to extract 3D information from the plasmas, as experiments show. Moreover, the remarkable signal-to-noise performance of the front-end electronics allows us to follow a different approach in which we combine information from all probes in both arrays to reconstruct elementary 3D plasma structures at each acquisition time step. Then, through data analysis, we track the structures as they evolve in time. The LP arrays include a linear-motion mechanism that can displace radially the probes located on the low field side for experiments that require fine-tuning of the probe locations, and for operational compatibility with the recently installed in-vessel toroidal conductor.

X-Point Effect on Plasma Blob Dynamics

Plasma blob dynamics on the high-field side in the proximity of a magnetic field null (X point) is investigated in TORPEX. A significant acceleration of the blobs towards the X point is observed. Close to the X point the blobs break apart. The E×B drifts associated with the blobs are measured, isolating the background drift component from the fluctuating contribution of the blob internal potential dipole. The time evolution of the latter is consistent with the fast blob dynamics. An analytical model based on charge conservation is derived for the potential dipole, including ion polarization, diamagnetic, and parallel currents. In the vicinity of the X point, a crucial role in determining the blob motion is played by the decrease of the poloidal magnetic field intensity. This variation increases the connection length that short circuits the potential dipole of the blob. Good quantitative agreement is found between the model and the experimental data in the initial accelerating phase of the blob dynamics.

Basic characterization of TORPEX electrostatic modes in closed field line configurations

Electrostatic coherent modes are studied in the TORPEX device [Fasoli et al., Plasma Phys. Controlled Fusion 52, 124020 (2010)], in closed flux surfaces. The accessibility to this magnetic geometry is provided by a current-carrying in-vessel toroidal conductor developed to generate a poloidal magnetic field [Avino et al., Rev. Sci. Instrum. 85, 033506 (2014)]. The background plasma parameters are measured, and the ion saturation current fluctuations are characterized in terms of power spectral density to identify the dominant coherent modes and their spatial localization. A statistical approach is implemented to determine the mode spectral properties by computing the statistical dispersion relation. The poloidal wave number kθ and the toroidal wave number kϕ are obtained, as well as the corresponding mode numbers. A three-dimensional linear code based on the drift-reduced Braginskii equations is used to investigate the nature of the instabilities. The linear analysis suggests a dominant ballooning charact...