Overview of TJ-II experiments

Abstract

This paper presents an overview of experimental results and progress made in investigating the link between magnetic topology, electric fields and transport in the TJ-II stellarator. The smooth change from positive to negative electric field observed in the core region as the density is raised is correlated with global and local transport data. A statistical description of transport is emerging as a new way to describe the coupling between profiles, plasma flows and turbulence. TJ-II experiments show that the location of rational surfaces inside the plasma can, in some circumstances, provide a trigger for the development of core transitions, providing a critical test for the various models that have been proposed to explain the appearance of transport barriers in relation to magnetic topology. In the plasma core, perpendicular rotation is strongly coupled to plasma density, showing a reversal consistent with neoclassical expectations. In contrast, spontaneous sheared flows in the plasma edge appear to be coupled strongly to plasma turbulence, consistent with the expectation for turbulent driven flows. The local injection of hydrocarbons through a mobile limiter and the erosion produced by plasmas with well-known edge parameters opens the possibility of performing carbon transport studies, relevant for understanding co-deposit formation in fusion devices.

Multiple interactions between fishbone instabilities and internal transport barriers in EAST plasmas

“…Note that the minimum value of the weak reversed shear safety factor q is slightly below 1. It can be observed that the q-profile is flat in the plasma core with q 0 ≈ 1; such a core-flat q-profile with integer q rational surface has been confirmed to be favorable for ITB triggering and maintenance by a series of experimental reports, such as on ASDEX Upgrade [47], JET [9,10,22], JT-60U [48] and DIII-D [49] devices, and even on a stellarator [50]. The main parameters are as follows: inverse aspect ratio ε = a/R = 0.237, triangularity δ = 0.43, elongation κ = 1.6, on-axis magnetic field B 0 = 1.58 T, electron central density In (e), the blue curve and blue dotted line represent the safety factor q = 1 and its corresponding radial location, respectively, and the shaded area indicates the ITB region.…”

First dynamic magnetic configuration scans in ECRH plasmas of the TJ-II Heliac

“…Rotation velocities in the TJ-II stellarator have been associated with E × B plasma rotation with maxima around 4 km s −1 in typical ECH plasmas [24,25]. Using the times at which the (vacuum) 8/5 and 5/3 resonances occupy the region ρ ≈ 0.7, and based on discharges with similar density ( n ≈ 0.7 × 10 19 m −3 ) at those times, we find frequencies around 40 kHz (8/5) and 27 kHz (5/3).…”

Transport studies using laser blow-off injection of low-Z trace impurities injected into the TJ-II stellarator

“…In the TJ-II stellarator [9], previous studies of impurity confinement using LBO have focused on heavy (iron) impurities [10][11][12] where moderate core confinement times (τ r ∼ 10 ms) were observed for low plasma densities, n e 0.6 × 10 19 m −3 , when the core radial electric field, E r , is positive. However, in the vicinity of the density value where a transition to negative E r occurs a significant rise in impurity confinement time, τ r , to ∼20 ms was observed, this being most appreciable with neutral beam injection (NBI) heating [12].…”