F.L. Tabarés

Suppression of hydrogenated carbon film deposition by scavenger techniques and their application to the tritium inventory control of fusion devices

The well-known radical and ion scavenger techniques of application in amorphous hydrogenated carbon film deposition studies are investigated in relation to the mechanism of tritium and deuterium co-deposition in carbon-dominated fusion devices. A particularly successful scheme results from the injection of nitrogen into methane/hydrogen plasmas for conditions close to those prevailing in the divertor region of present fusion devices. A complete suppression of the a-C : H film deposition has been achieved for N2/CH4 ratios close to one in methane (5%)/hydrogen DC plasma. The implications of these findings in the tritium retention control in future fusion reactors are addressed.

Present status of liquid metal research for a fusion reactor

Although the use of solid materials as targets of divertor plasmas in magnetic fusion research is accepted as the standard solution for the very challenging issue of power and particle handling in a fusion reactor, a generalized feeling that the present options chosen for ITER will not represent the best choice for a reactor is growing up. The problems found for tungsten, the present selection for the divertor target of ITER, in laboratory tests and in hot plasma fusion devices suggest so. Even in the absence of the strong neutron irradiation expected in a reactor, issues like surface melting, droplet ejection, surface cracking, dust generation, etc., call for alternative solutions in a long pulse, high efficient fusion energy-producing continuous machine. Fortunately enough, decades of research on plasma facing materials based on liquid metals (LMs) have produced a wealth of appealing ideas that could find practical application in the route to the realization of a commercial fusion power plant. The options presently available, although in a different degree of maturity, range from full coverage of the inner wall of the device with liquid metals, so that power and particle exhaust together with neutron shielding could be provided, to more conservative combinations of liquid metal films and conventional solid targets basically representing a sort of high performance, evaporative coating for the alleviation of the surface degradation issues found so far. In this work, an updated review of worldwide activities on LM research is presented, together with some open issues still remaining and some proposals based on simple physical considerations leading to the optimization of the most conservative alternatives.

Spatially resolved Hα-emission simulation with EIRENE in TJ-II to study hydrogen atomic and molecular physics in low density, high temperature fusion edge plasmas

This paper is a continuation of previous studies (de la Cal et al 2007 J. Nucl. Mater. 363–365 764, de la Cal et al 2007 Proc. 34th EPS Conf. (Poland, Warsaw) P 2.029), where hydrogen recycling under ionizing plasma conditions was analysed by spatially resolved Hα-emission spectroscopy with a tangentially viewing camera looking at a poloidal limiter of the TJ-II stellarator operated with a low density, high temperature plasma edge (ne = (1–10) × 1012 cm−3 and Te = 40–400 eV). In this study, the first objective is to validate for hydrogen the recently implemented EIRENE code* by comparing simulated Hα-emission chords with that obtained experimentally. The second objective is to analyse the atom and molecular neutral distributions in front of the limiter and at other plasma and chamber locations. The third one is to study the atomic and molecular reactions involved in the dissociation, ionization and excitation reactions, as calculated from the EIRENE code, in order to study hydrogen atomic and molecular physics in a low density, high temperature fusion plasma edge, with special focus on interpretation of Hα-emission. The contribution of the different reactions to the emission is analysed as a function of plasma radius. A relevant result obtained from EIRENE under this item is that the main precursor of the molecular Hα-emission is not H2 as proposed in many previous studies, but .

Characterization of He/CH4 dc glow discharge plasmas by optical emission spectroscopy, mass spectrometry, and actinometry

The gas‐phase kinetics responsible for the formation of some electronically excited radicals (CH) and atoms (H, He, Ar) in glow discharge plasmas of He‐methane admixtures has been investigated under several conditions of gas composition and discharge current at a total initial pressure of 0.01 Torr. Actinometry has been used to characterize the microscopic plasma parameters and, in combination with mass spectrometry and optical emission spectroscopy, to establish the formation mechanism of excited species. A very good correlation between CH emission intensity and carbon deposition rate has been found under all conditions. The effective cracking kinetic constant for methane molecules depends on plasma conditions, its value ranging from ≊1 to 5×10−10 cm3 s−1 as plasma current is increased, in good agreement with the expected value according to the actinometric results. A simplified kinetic model, accounting for all the observations reported in this work, is proposed.

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.

Plasma processing techniques for tritium inventory control in fusion research

Some techniques with a long tradition in the plasma technology field have already been successfully applied to research in plasma–wall interactions of fusion devices. They have produced important advances in the control of particle and energy exhaust. In this paper, the possible application of these techniques to the problem of tritium inventory control in fusion reactors with carbon-based plasma facing materials, as in ITER, is proposed. It is based on a critical analysis of relevant information obtained in the field of hard CN film deposition and consists of the use of chemical scavengers for the inhibition of tritium-rich carbon-film formation in hidden areas of the divertor. The practical implementation of the technique, however, requires a detailed knowledge of the physio-chemical processes involved, and, to date, experiments in cold and divertor plasmas have been performed. Very recent experiments in the ASDEX Upgrade device have shown that the injection of nitrogen in the sub-divertor region can lead to a drastic decrease in the level of deposited material with no significant effects in the performance of the main plasma. This and other findings are interpreted in the light of recent results from laboratory and divertor plasma experiments and the extrapolation to new divertor scenarios is discussed.

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.

Overview of TJ-II flexible heliac results

The TJ-II is a four period, low magnetic shear stellarator, with high degree of configuration flexibility (rotational transform from 0.9 to 2.5) which has been operating in Madrid since 1998 (R = 1.5 m, a 0.22 m, B0 = 1T , PECRH600 kW, PNBI3 MW under installation). This paper reviews the main technical aspects of the TJ-II heliac as well as the principal physics results obtained in the most recent TJ-II experimental campaign carried out in 2000.

Pulsed supersonic helium beams for plasma edge diagnosis

An experimental setup for the production of pulsed supersonic He beams to be used for plasma edge diagnosis in fusion devices is described. A compromise between compact design, low cost, and good quality of the probe beams has been met. The main characteristics of the generated beams, such as pulse shape, absolute flux intensity, and velocity distribution, differ in general from those expected for ideal beam performance and have been determined and optimized experimentally. A first test of this He beam source at the TJ-I UP Torsatron in Madrid is also reported.

Interaction of high flux deuterium/nitrogen plasmas with beryllium

Before nitrogen can be used as a radiator for edge plasma temperature control in experiments with beryllium as the wall material, the compatibility of nitrogen-containing plasma with beryllium has to be tested. Therefore beryllium samples were exposed to a variety of mixed N2/D2 plasmas in PISCES-B and codeposits from the sputtered material were collected. It was found that introducing N2 to a D2 plasma reduces Be erosion significantly but recovery to the pre-N2 levels is possible in pure D2 plasma. Berylliated vessel walls can be a reservoir for N2 and chemical processes probably play a significant role during nitriding and N2 removal. Nitrided target samples remain conductive and do not lead to additional arcing, but codeposits are insulating. Thermal desorption measurements of nitrided and un-nitrided target samples are comparable, while codeposits show a slightly reduced D retention. However, D release for both target and codeposits is shifted ≈100 K to higher temperatures, above 510 K.