Plasma performance and confinement in the TJ-II stellarator with lithium-coated walls

Abstract

In the last campaign, the TJ-II heliac has been operated under lithium-coated walls, representing the first stellarator ever working under these boundary conditions. Enhanced density control and discharge reproducibility, leading to the drastic enlargement of the operational window, have been obtained. A strong decrease in recycling together with changes in the shot by shot fuelling characteristics and in the wall particle inventory have been recorded. These changes, associated with the new wall scenario, had led to a long-lasting good density control. The new conditions were also mirrored in the plasma profiles under NBI heating scenarios with increased peaking of the electron density profiles. Fuelling rates corresponding just to the nominal beam current were obtained for the first time, and transitions from bell to dome-type plasma profiles, with different collapsing limits, were observed and tentatively ascribed to changes in the local edge power balance. ELM-type activity was observed in concomitance to reduced fluctuation levels and confinement improvement. Record values of plasma energy content were measured at central densities up to 8 × 10 19 m −3 under Li-coated walls.

Dynamic Simulation of the Electron Bernstein Wave Heating Under NBI Conditions in TJ–II Plasmas

“…2, with a continuous line for the high Z eff calculation and a dashed line for the low Z eff one. As a warning note, we must inform that two distinct profiles have been clearly identified in TJ-II NBI discharges: bell-like and dome-like [20]. The latter precedes the radiation collapse process and this is the kind of radiation profile we are reproducing in accordance with the experimental data in which we base the simulations.…”

Probing the edge ion temperature by passive Doppler spectroscopy in the TJ-II stellarator

“…These time estimations are shown in the figure 2(a). These times should also be compared with the global particle confinement time, which has been estimated to be about 5-6 ms for lithium-coated walls in TJ-II [21]. Similar results have been obtained for impurity confinement time in TJ-II as determined by measuring the time relaxation of global radiation signals after injecting a small amount of either a light impurity such as Boron (B) or a heavy impurity like Iron (Fe) by laser blow-off technique.…”

Energy and Particle Balance Studies Under Full Boron and Lithium‐Coated Walls in TJ‐II

“…Comparison of the plasma performance under boron and lithium walls allows us to distinguish between particle balance (i.e. recycling) and energy balance-related effects concerning the specific characteristics that have been found under fully lithiated walls in TJ-II [1]. Example of these characteristics are the spontaneous development of peaked plasma profiles and their transition to broader, cleaner ones and the onset of plasma collapse as defining the operational density limit [2].…”