Overview of recent TJ-II stellarator results

Abstract

The main results obtained in the TJ-II stellarator in the last two years are reported. The most important topics investigated have been modelling and validation of impurity transport, validation of gyrokinetic simulations, turbulence characterisation, effect of magnetic configuration on transport, fuelling with pellet injection, fast particles and liquid metal plasma facing components. As regards impurity transport research, a number of working lines exploring several recently discovered effects have been developed: the effect of tangential drifts on stellarator neoclassical transport, the impurity flux driven by electric fields tangent to magnetic surfaces and attempts of experimental validation with Doppler reflectometry of the variation of the radial electric field on the flux surface. Concerning gyrokinetic simulations, two validation activities have been performed, the comparison with measurements of zonal flow relaxation in pellet-induced fast transients and the comparison with experimental poloidal variation of fluctuations amplitude. The impact of radial electric fields on turbulence spreading in the edge and scrape-off layer has been also experimentally characterized using a 2D Langmuir probe array. Another remarkable piece of work has been the investigation of the radial propagation of small temperature perturbations using transfer entropy. Research on the physics and modelling of plasma core fuelling with pellet and tracer-encapsulated solid-pellet injection has produced also relevant results. Neutral beam injection driven Alfvénic activity and its possible control by electron cyclotron current drive has been examined as well in TJ-II. Finally, recent results on alternative plasma facing components based on liquid metals are also presented.

Observation of low frequency electrostatic coherent oscillation in the proximity of the density limit in the TJ-II stellarator

“…Experiments were carried out in the TJ-II [25], a fourperiod helical-axis stellarator, with a mayor radius of R = 1.5 m, a minor radius of a 0.22 m and a magnetic field…”

Plasma diagnostic systems and methods used on the stellarator TJ-II

“…TJ-II is a heliac-type stellarator with major radius of 1.5 m [3]. It was designed to allow exploration of a wide range of rotational transforms (0.9 ι 0 /2π 2.2) in low, negative shear configurations (Δι/ι < 6%), where ι 0 is the central iota value.…”

“…Most recently, research on TJ-II has been focussed on several areas [3,8]. These include transport, confinement, plasma fuelling, magneto-hydrodynamic stability, fast-particle physics, plasma-edge physics and plasma-wall interactions.…”

27th IAEA Fusion Energy Conference: summary of sessions EX/C, EX/S and PPC

“…Further insight into the underlying physics processes determining transport in stellarator plasmas was provided by a series of papers from the TJ-II group [53][54][55]. A specific issue highlighted in these papers was the role of asymmetries in the electrostatic potential, and in the resulting electric field, on the variation of turbulent fluctuations around flux surfaces under different conditions of density, heating and plasma configuration (iota).…”