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Owing to its detailed diagnostics, feasibility of parameter scans, and relatively simple configuration, the basic plasma physics experiment TORPEX is an ideal testbed in which to perform experiment/simulation comparisons and to contribute to the establishment of the validation methodology. Focusing on observables related to Langmuir probe measurements, we consider a number of physical quantities that can be used as observables for experiment/simulation comparison. We classify the observables according to a hierarchy that sums the number of model assumptions and measurement combinations used to obtain an observable from experimental measurements and simulation results. The more assumptions needed, the less stringent the comparison with respect to this observable and, thus, its weight in the evaluation of the agreement between experiment and simulation should be decreased. We then propose a metric to quantify the agreement between experiment and simulation, based on three logical steps. First, relative to each observable chosen to perform the comparison, we quantify the agreement between experiment and simulation. Second, the levels of agreement relative to each observable are cast into a single real number, referred to as the composite metric, with the goal of quantifying the global agreement between experiment and simulation. This process requires that each observable is weighted correctly. Third, we introduce an index that quantifies the quality of the comparison between experiment and simulation. This methodology has been used to validate two models that have been developed to simulate TORPEX turbulence: a three-dimensional two-fluid model, able to describe the global evolution of TORPEX plasma, and a reduced two-dimensional two-fluid model, able to describe only the evolution of flute modes. We show that the validation metric reveals the unsatisfactory agreement of the two-dimensional model and the experiment when non-flute modes are present in the experiment.

The TORoidal Plasma EXperiment (TORPEX) is a basic plasma physics device where a Simple Magnetized Torus configuration (SMT) can be obtained superimposing a small vertical magnetic field component on the main toroidal field, resulting in open magnetic helical field lines. This configuration reproduces some of the main features of a tokamak plasma edge, including density and magnetic field gradient and curvature. In the last years, a complete study of electrostatic fluctuations and turbulence has been completed on TORPEX, taking advantage of its high degree of plasma reproducibility and of its extended diagnostic set. To produce magnetic field configurations closer to those accessible with tokamaks, a new system has recently been implemented. This consists in a toroidal copper wire suspended inside the vacuum vessel by four vertical supports and four horizontal supports to keep fix its position. A dedicated external power supply provides a current up to 1kA that flows in the wire, generating a poloidal magnetic field. This opens the possibility of investigating plasma fluctuations in the presence of a rotational transform in either confined regions with closed magnetic flux-surfaces, or Scrape-Off Layer (SOL) regions with open field lines. A set of poloidal coils for the vertical magnetic field component permits several magnetic field configurations, from plasma limited by the vessel itself (on the high- or low-field side), to more complicated solutions of Single or Double-Null X-points, or even advanced divertor concepts, such as magnetic Snowflakes. The toroidal in-vessel copper wire system will be presented. The characterization of the main background plasma parameters in some of the new magnetic field configurations will be shown, as well as the spectral and statistical analysis of the fluctuations.

The TORoidal Plasma EXperiment (TORPEX) is a Simple Magnetized Torus (SMT) where open helical magnetic field lines are obtained from the superposition of a small vertical component on the main toroidal field. The SMT configuration features the main driving mechanisms of plasma instabilities characterising the Scrape-Off Layer (SOL) of larger tokamak devices, making TORPEX a very attractive machine to perform fusion oriented studies of plasma turbulence. A new experimental set-up has been recently developed to generate a poloidal magnetic field. It consists in a toroidal copper wire suspended in the middle of the vacuum vessel by seven 1mm stainless steel wires, three vertical and four lateral, plus the vertical electrical coaxial feed-through. A current up to 1kA can be driven in the system, giving us the possibility to include a rotational transform and to deal with a SOL much closer to the one of a fusion device. Moreover plasma core and closed-to-open field line region studies could be performed. The technical details of the set-up will be given together with the experimental results obtained during the first campaigns with Hydrogen plasmas. A magnetic configuration with almost closed flux surfaces has been chosen, resulting from the poloidal magnetic field together with a small vertical component. The radial profiles of the main plasma parameters will be presented, such as plasma density n_e, plasma temperature T_e and plasma potential V_pl. The spectral properties of plasma dominant coherent modes at the position of maxima fluctuations will be provided too, including power spectral density estimates and measurements of vertical and toroidal wave-numbers obtained for different values of poloidal magnetic field. This allows us to calculate the poloidal and toroidal mode-numbers which can be used together with fluid simulations to asses the kind of instability we are dealing with. An overview of the new accessible magnetic field configurations will be showed as well, ranging from Single Null X-points to magnetic Snowflakes.

TORPEX is a basic plasma physics device where a simple magnetized torus (SMT) is produced by adding a small vertical magnetic field to a main toroidal component, generating helical field lines. The SMT configuration features the main ingredients of a tokamak Scrape-Off Layer (SOL), namely density gradients in the presence of magnetic field curvature and gradient. Recently, a toroidal in-vessel copper conductor has been installed in the TORPEX device. A current up to 1kA can be driven in the toroidal conductor. This produces a poloidal magnetic field, which closes the field lines together with the toroidal field, resulting in a monotonic safety factor profile from 1 to 12 along the radial direction with an almost constant magnetic shear of 2. Several magnetic field geometries are accessible using the vertical field coils, from wall-limited plasmas with a SOL on the high-field side or low-field side, to single or double-null X-points as well as magnetic snowflakes. This way, more fusion relevant configurations of increasing complexity are achieved, while maintaining plasma parameters in ranges that allow a complete spatio-temporal diagnostic coverage across the plasma cross-section. Magnetic configurations with almost circular and concentric flux surfaces are considered for the analysis of plasma coherent structures. Quasi-coherent modes with a strong poloidal asymmetry are measured. A dominant localization on the bad-curvature region of the plasma volume (low field side) suggests a ballooning nature for these instabilities. A spectral characterization at the position of maximum fluctuation amplitudes is performed, including measurements of the power spectral density, indicating mode frequencies in the range 15-30 kHz. Measurements of vertical and toroidal correlations for several values of the poloidal magnetic field have been performed, allowing us to calculate the poloidal and toroidal mode-numbers. Field aligned modes with a toroidal mode number n=1 are identified. This is being compared with the numerical results obtained with the linear version of the Global Braginskii Solver (GBS) to assess the nature of the observed dominant instabilities. Initial measurements of plasma turbulence in TORPEX plasmas with a single null Xpoint will also be discussed, including analysis of the blob dynamics in this configuration performed using imaging techniques and conditional average sampling.

The basic plasma physics device TORPEX features some of the main ingredients of the Scrape-Off Layer (SOL) of fusion devices, including gradients of density and magnetic field, as well as magnetic field curvature. A simple magnetized torus configuration (SMT) is produced using a small vertical magnetic field component superimposed to the main toroidal field, resulting in helical open magnetic field lines. A comprehensive study of electrostatic instabilities and turbulence in TORPEX plasmas has been performed in the last years. A new experimental set-up has been recently implemented on the TORPEX device to produce a poloidal magnetic field and a rotational transform. This set-up is based on an in-vessel toroidal copper wire of 1 cm of radius, suspended in the middle of the vacuum vessel of TORPEX by several horizontal and vertical supports. These supports can also be used to position the current carrying wire at different vertical positions up to the top of the chamber to recover the original SMT configuration. The wire is powered by an external power supply providing a maximum current of 1 kA during a flat-top of about 0.5s. A 0.2s dynamics for the ramp-up and the ramp-down phases is given by the power supply temporal response. The current flowing in the toroidal wire generates a poloidal magnetic field that leads to a rotational transform. In the resulting magnetic field lines configuration the characterization of the plasma turbulence and instabilities can be performed on both the core region of closed flux-surfaces and the SOL region of open field lines. A proper value of the vertical magnetic field is used to precisely define the geometry of closed/open field lines, with the vessel walls acting as a limiter. An overview of the new experimental set-up will be presented, together with the background plasma parameters and profile. We will discuss the first measurements of plasma fluctuations in the SOL and their spatio-temporal evolution. The more advanced magnetic configurations that are accessible with this new experimental set-up will also be presented.

TORPEX is a toroidal device located at the CRPP-EPFL in Lausanne. In TORPEX, a vertical magnetic field superposed on a toroidal field creates helicoidal field lines with both ends terminating on the torus vessel. The turbulence driven by magnetic curvature and plasma gradients causes plasma transport in the radial direction while at the same time plasma is progressively lost along the field lines. The relatively simple magnetic geometry and diagnostic access of the TORPEX configuration facilitate the experimental study of low frequency instabilities and related turbulent transport, and make an accurate comparison between simulations and experiments possible. We first present a detailed investigation of electrostatic interchange turbulence, associated structures and their effect on plasma using high-resolution diagnostics of plasma parameters and wave fields throughout the whole device cross-section, fluid models and numerical simulations. Interchange modes nonlinearly develop blobs, radially propagating filaments of enhanced plasma pressure. Blob velocities and sizes are obtained from probe measurements using pattern recognition and are described by an analytical expression that includes ion polarization currents, parallel sheath currents and ion-neutral collisions. Then, we describe recent advances of a non-perturbative Li 6+ miniaturized ion source and a detector for the investigation of the interaction between supra thermal ions and interchange–driven turbulence. 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. The experiments are interpreted using two-dimensional fluid simulations describing the low-frequency interchange turbulence, taking into account the plasma source and plasma losses at the torus vessel. By treating fast ions as test particles, we integrate their equations of motion in the simulated electromagnetic fields, and we compare their time-averaged and statistical properties with experimental data. Finally, we discuss future developments including the possibility of closing the magnetic field lines and of performing magnetic reconnection experiments.

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.

We present unprecedented space and time-resolved measurements of the transport of a suprathermal ion beam injected in the toroidal device TORPEX. Experiments are performed in turbulence dominated by an ideal-interchange mode using a Li6+ ion source and a set of energy analyzers. Depending on the suprathermal ion energy, the transport exhibits subdiffusive or superdiffusive behaviors. The fast ion current fluctuations are quasi-Gaussian in the former regime and strongly intermittent in the latter. In the superdiffusive case, using conditional sampling, we show that the transport is associated with intermittent field-aligned blobs. In the subdiffusive case, supra-thermal ions average the turbulent structures during their gyro-motion and their vertical drift. These results complement our investigations of the supra-thermal ion transport using 3D time-averaged measurements, which are in agreement with numerical simulations. Numerical modeling is performed by computing the trajectories of tracers in a turbulent electrostatic field generated by a 2D global fluid model. Gyro- and drift-averaging reduce the transport. The evolution of the radial distribution of fast ions can be modeled by a fractional diffusion equation describing fractional Lévy motion.

TORPEX is a basic plasma physics toroidal device located at the CRPP-EPFL in Lausanne. In TORPEX, a vertical magnetic field superposed on a toroidal field creates helicoidal field lines with both ends terminating on the torus vessel. We review recent advances in the understanding and control of electrostatic interchange turbulence, associated structures and their effect on suprathermal ions. These advances are obtained using high-resolution diagnostics of plasma parameters and wave fields throughout the whole device cross-section, fluid models and numerical simulations. Furthermore, we discuss future developments including the possibility of generating closed field line configurations with rotational transform using an internal toroidal wire carrying a current. This system will also allow the study of innovative fusion-relevant configurations, such as the snowflake divertor.