Sources and detectors of fast ions for basic devices

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

RF antennas as plasma monitors

Invited tal

Electromagnetic, complex image model of a large area RF resonant antenna as inductive plasma source

A large area antenna generates a plasma by both inductive and capacitive coupling; it is an electromagnetically coupled plasma source. In this work, experiments on a large area planar RF antenna source are interpreted in terms of a multi-conductor transmission line coupled to the plasma. This electromagnetic treatment includes mutual inductive coupling using the complex image method, and capacitive matrix coupling between all elements of the resonant network and the plasma. The model reproduces antenna input impedance measurements, with and without plasma, on a 1.2x1.2 m2 antenna used for large area plasma processing. Analytic expressions are given, and results are obtained by computation of the matrix solution. This method could be used to design planar inductive sources in general, by applying the termination impedances appropriate to each antenna type

Convective cells and blob control in a simple magnetized plasma

Blob control by creating convective cells using biased electrodes is demonstrated in simple magnetized toroidal plasmas. A two-dimensional array of electrodes is installed on a metal limiter to obtain different biasing schemes. Detailed two-dimensional measurements across the magnetic field reveal the formation of a convective cell, which shows a high degree of uniformity along the magnetic field. Depending on the biasing scheme, radial and vertical blob velocities can be varied significantly. A high level of cross-field currents limits the achievable potential variations to values well below the applied bias voltage. Furthermore, the strongest potential variations are not induced along the biased flux tube, but at a position shifted in the direction of plasma flows

Convective cells and blob control in a simple magnetized torus

In view of controlling wall and divertor heat loads in magnetic fusion devices, we investigate the possibility of creating convective cells by means of biased electrodes for turbulence and blob control in the simple magnetized toroidal plasmas of TORPEX. A two-dimensional array of 24 electrodes is installed on a metal limiter to test different biasing schemes. This allows influencing significantly the frequency of the dominant mode as well as radial and vertical velocities of blobs. Detailed measurements along and across the magnetic field provide a rather clear picture of the effect of the biasing. The biased electrodes produce perturbations of the plasma potential and density profiles that are fairly uniform along the magnetic field. Background flows influence the location where potential variations are induced. The magnitude of the achievable potential variations in the plasma is strongly limited by cross-field currents. A quantitative discussion on the origin of these currents is presented

Investigation of fast ion transport in TORPEX

Basic aspects of fast ion transport in ideal interchange-mode unstable plasmas are investigated in the simple toroidal plasma device TORPEX. Fast ions are generated by a miniaturized lithium 6+ ion source with energies up to 1 keV, and are detected using a double-gridded energy analyser mounted on a two-dimensional movable system in the poloidal cross-section. The signal-to-noise ratio is enhanced by applying a modulated biasing voltage to the fast ion source and using a synchronous detection scheme. An analogue lock-in amplifier has been developed, which allows removing the capacitive noise associated with the voltage modulation. We characterize vertical and radial transport of the fast ions, which is associated with the plasma turbulence. Initial experimental results show good agreement with numerical simulations of the fast ion transport in a global fluid simulation of the TORPEX plasma

Suprathermal ion transport theory and experiments in the SMT

Recent advances in the suprathermal ion diagnostic in the basic plasma experiment TORPEX have inspired our comprehensive theoretical study of suprathermal ion transport. TORPEX, an example of a simple magnetized toroidal plasma (SMT), is equipped with a flexible fast ion source and detector capable of exploring fast ion dynamics in a wide range of positions and energies. We simulate an ensemble of ion tracer trajectories as specified by ideal interchange-mode turbulence imported from a validated numerical simulation based on the drift-reduced Braginskii model. Using the variance of displacements, $\sigma^2(t) \sim t^{\gamma}$, we find that $\gamma$ depends strongly on suprathermal ion injection energy and the magnitude of turbulent fluctuations. When the beam interacts with the turbulence, we find the remarkable presence of three regimes of dispersion: superdiffusive, diffusive, and subdiffusive, depending on the energy of the suprathermal ions and the amplitude of the turbulent fluctuations. Results from the source on TORPEX are consistent with the theoretical predictions

Investigation of suprathermal ion transport in TORPEX

In burning plasmas, fast ions may be generated by ion cyclotron resonance heating, neutral beam injection and fusion reactions. Fast ions will be responsible for a significant fraction of plasma heating and, in some scenarios, non-inductive current drive. Fast ions are also present in natural plasmas, such as the solar corona or the magnetosphere, where they are presumably accelerated by wave-particle interactions. The high temperatures of tokamak plasmas and the huge spatial scale of astrophysical plasmas make measurements of the fast ion transport very challenging. Basic plasma devices offer the possibility to study the interaction between fast ions and plasma turbulence with easy access for diagnostics and well-establish plasma scenarios. Experiments in the linear plasma device LAPD have shown that fast ion transport is increased in presence of turbulent or coherent electrostatic waves and that it is generally nondiffusive [1]. Basic aspects of fast ion transport in ideal interchange-mode unstable plasmas are investigated in the simple toroidal plasma device TORPEX. The magnetic field configuration of TORPEX consists of a toroidal component (Bt = 75 mT) and a smaller vertical component (Bv = 2 mT), resulting in helical open field lines, with grad-B and curvature. With this magnetic geometry, the fast ion motion without plasma is a combination of the gyromotion along the field lines and the vertical grad-B and curvature drifts

Sheath boundary conditions for plasma fluid models

A new definition of the sheath edge is rigorously derived taking into account the kinetic properties of the plasma, and a consistent set of local sheath edge conditions is presented for the case of a magnetic field perpendicular to the wall. These local boundary conditions give explicit expressions for the ion velocity, the electron velocity, and the electron heat flux at the sheath edge, which can be easily implemented in a fluid code. It is shown that in the case of positive current to the wall, the commonly used Bohm's relations well aproximate the proposed boundary conditions, while large discrepancies are observed for negative currents. A fully kinetic PIC code simulating the plasma wall transition has been developed to validate these local relations, showing an excellent agreement with the theory. This work represents a first step towards a complete formulation of the sheath edge local boundary conditions for a general magnetic geometry