J.A. Ferreira

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

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.

Cryotrapping assisted mass spectrometry for the analysis of complex gas mixtures

A simple method is described for the unambiguous identification of the individual components in a gas mixture showing strong overlapping of their mass spectrometric cracking patterns. The method, herein referred to as cryotrapping assisted mass spectrometry, takes advantage of the different vapor pressure values of the individual components at low temperature (78K for liquid nitrogen traps), and thus of the different depletion efficiencies and outgassing patterns during the fast cooling and slow warming up of the trap, respectively. Examples of the use of this technique for gas mixtures with application to plasma enhanced chemical vapor deposition of carbon and carbon-nitrogen hard films are shown. Detection of traces of specific C3 hydrocarbons (<50ppm of initial methane) in methane/hydrogen plasmas and the possible trapping of thermally unstable C–N compounds in N2 containing deposition plasmas are addressed as representative examples of specific applications of the technique.

Removal of carbon deposits in narrow gaps by oxygen plasmas at low pressure

Under a carbon-based divertor operation of the International Tokamak Experimental Reactor, tritium retention by codeposits in hard-to-reach areas represents an important concern, and an urgent need exists for cleaning techniques suitable for the in situ removal of these codeposits. The present design of divertor modules includes castellated structures, with narrow gaps prone to accumulate material. On the other hand, glow-discharge cleaning in He∕O2 mixtures is today one of the main candidates for suppressing tritiated deposits in plasma facing components. In the present work, the effectiveness of this technique for removing a‐C:H films in gaps is addressed. The authors show that a higher-than-expected efficiency, as compared to the penetration ability of plasma ions, can be obtained. Optimization of the plasma parameters for complete oxidation of the films has also been performed. Erosion rates above 3nm∕min at room temperature can easily be achieved in fully exposed films deposited in laboratory plasmas.

Surface effects on the diagnostic of carbon/nitrogen low-pressure plasmas studied by differentially pumped mass spectrometry.

In this work, the characterization of the species produced in reactive plasmas by differentially pumped mass spectrometry is addressed. A H2/CH4/N2 mixture (90 : 5 : 5) was fed into a direct current glow discharge and analysed by conventional and cryo-trap assisted mass spectrometry. The gaseous mixture was chosen because of its particular relevance in the inhibition of tritium-rich carbon film deposition in fusion plasmas (scavenger technique) and in the deposition of amorphous hydrogenated carbon films by plasma-assisted chemical vapour deposition. Important changes in the composition of the detected species upon surface modification of the reactor walls (stainless steel or covered by an amorphous hydrogenated carbon layer) or in the way they are sampled (length and spatial configuration of the stainless steel duct) were detected. They are analysed in terms of radical formation and recombination on the reactor walls or into the sampling duct, thus providing some insight into the underlying chemistry. In general, when the reactor walls are covered by an amorphous hydrogenated carbon layer, more hydrocarbons are produced, but the radical production is lower and seem to be less reactive than in stainless steel. Also, two sources of oxygen contamination in the plasma have been identified, from the native oxide layer in stainless steel and from unintended water contamination in the chamber, which modify considerably the detected species.

Toward a full exploitation of the helium beam edge diagnostic.

In this article, extension of the He beam diagnostic, which is routinely applied in fusion devices for the characterization of the edge electronic parameters-ne and Te-and their profile, to the measurement of ion temperature and plasma rotation is proposed. The theoretical background for these new applications is reviewed and examples of their use in TJ-II are provided. Also, the shortcomings and their possible solutions are addressed. Finally, the field of application of the technique to other beam-based diagnostics is discussed.