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 × 1019 m−3 under Li-coated walls.

http://iopscience.iop.org/article/10.1088/0741-3335/50/12/124051/pdf

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

Design and analyses of a one-dimensional CFC calorimeter for SPIDER beam characterisation

The power balance measurements in a 1 kW argon helicon plasma discharge are demonstrated The quantitative wall heat flux measurements using nonintrusive infrared imaging of the glass tube surface, along with data from bolometric probes and calibrated photodiodes to measure heat flux from the rest of the plasma, are shown For the cylindrical discharge, with radius much less than the axial length, approximately 15% of 1 kW input power is lost in the transmission lines and matching network, 54% is lost through radial particle transport to the walls, 16% is lost through radiation, and 8% is lost through ion impact in the vicinity of the antenna Wall erosion of the confining glass tube occurs due to sputter etching by ions accelerated in the electrostatic fields produced by the antenna Wall etch rate measurements are used to quantify particle fluxes to the wall, which are in rough agreement with particle fluxes calculated from thermal flux

Power balance and wall erosion measurements in a helicon plasma

Beamline duct monitoring of the TJ-II neutral beam injectors

A new beam diagnostic based on infrared thermography has been developed for the neutral beam injectors of the stellarator TJ-II A highly anisotropic movable target intercepts the beam at its entrance into the stellarator The thermal print of the beam is captured with a high resolution infrared camera The infrared images of the target can be translated, with the appropriate analysis, into power density patterns of the beam The system is calibrated in situ with two thermocouples adiabatically mounted in the target The two-dimensional beam power density distribution can be accurately characterized allowing beam optimization with respect to the different parameters involved in the beam formation and transport

Thermographic calorimetry of the neutral beam injectors heating beams at TJ-II

Neutral beam injection (NBI) heating has been used on the TJ-II stellarator for the first time. The beam has a port-through power between 200 and 400 kW and injection energy 28 kV. Beam transmission is limited by beam interception at the injection port and the first toroidal field coil, therefore, beam steering optimization is of critical importance. The beam interaction areas inside TJ-II vacuum chamber are surveyed by infrared thermography. Beam reionization can be a problem due to the presence of residual gas in the duct region. Halpha emission is used to monitor the reionization at the duct. A careful optimization of the injected gas has been carried out.

Neutral beam injection optimization at TJ-II

A carbon fiber composite (CFC) target calorimeter has been installed at TJ-II to study in situ the power density distribution of the neutral beams. The thermographic print of the beam can be recorded and analysed in a reliable way due to the highly anisotropic thermal conductivity of the target material. Direct conversion from temperature to power density is possible due to the independent thermocouple measurements on two adiabatically mounted copper buttons. With the combined thermographic and calorimetric measurements it has been possible to determine the power density distribution of the beams. It has been found that a large beam halo is present, which can be explained by the extreme misalignment of the grids. This kind of halo has a deleterious effect on beam transport and must be minimized in order to improve the plasma heating capability of the beams.

Power transmission of the neutral beam heating beams at TJ-II

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Neutral beam injection optimization at TJ-II

Thermographic calorimetry of the neutral beam injectors heating beams at TJ-II

Power transmission of the neutral beam heating beams at TJ-II