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En este trabajo se describen los plasmas fríos moleculares y los procesos clave que intervienen en su formación, cómo obtenerlos en laboratorio y construir modelos que nos ayuden a entenderlos mejor. Se explican también los mecanismos que proporcionan a estos plasmas una de sus características más notables: su gran reactividad química a baja temperatura, debida principalmente a las especies inestables que se forman en su seno. Asimismo se aportan ejemplos de líneas de investigación seguidas en este campo por algunos grupos españoles representativos del estado de la ciencia en esta materia

The removal of hard amorphous hydrogenated carbon (a-C: H) films from narrow gaps simulating the macro-brush structures present in controlled fusion devices has been investigated. Films with thickness of 50-150 nm were generated through plasma assisted chemical vapor deposition (PACVD) in glow discharges of CH4/He on Si and stainless steel plates. The deposited plates were then arranged to form sandwich structures building narrow gaps and were subject to erosion by exposure to O2/He plasmas and to thermal oxidation by O2 and by a NO2/N2 (1:1) mixture. In the plasma etching experiments, the deposited layers were only partially removed by the plasma at the side wall gap surfaces, but were efficiently removed at the bottom of the gap. In the thermo-oxidation experiments, the deposited films were effectively and homogeneously removed with oxigen at 670 K and with the NO2/N2 mixture at T > 570 K.

Complete suppression of amorphous C:H film deposition has been reported for N2/CH4 ratios ∼1, in low-pressure technical plasmas. This finding has been recently used by the authors for the proposal of a possible technique for the inhibition of re-deposited T-containing carbon films at the divertor region of fusion devices. Although several works aiming to the understanding of the underlying physicochemical processes have been published, the complexity of the system is far from being properly described by the proposed models. In the present work, experiments in DC glow discharges at low pressure of H2/CH4/N2 admixtures (90:0–5:0–5) are described. Mass spectrometry of neutral species as well as plasma mass spectrometry for ion detection have been used as the main diagnostics. Several plasma conditions (plasma current, gas composition, etc.) as well as isotopic exchange (H/D) have been investigated. Also, the effect of progressive film growing on the metal walls of the reactor in the composition of gas-phase species has been investigated. It is concluded that wall carbonisation is required to trigger the inhibition process. Ethylene and acetylene are found as the main reaction products. © 2004 Elsevier Ltd

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.© 2002 IOP Publishing Ltd

A new, compact atomic beam source based on the supersonic expansion of He has been developed for application as a plasma edge diagnostic. The beam is produced from a pulsed valve with a duration between 0.2 to 2 ms and a nominal repetition rate < 500 Hz. A terminal speed ratio > 10 and a divergence of ± 1° have been achieved at stagnation pressures below 2 bar. The diagnostic has been tested in ECRH plasmas on the TJ-IU torsatron, representing the first application of a supersonic beam to plasma characterization, to our knowledge. Operational conditions which minimized the total amount of He injected into the plasma were chosen. Non-perturbative injection conditions in the low density plasmas could be obtained at local He densities of ⋍ 1 × 1011 cm−3 and a beam diameter < 1 cm. Due to the relatively low electron density of the ECRH plasmas, and to the good penetration characteristics of the supersonic He beam, the diagnostic could be used up to fairly low values of the normalized plasma minor radius, View the MathML source. Details of the optimization of the atomic beam diagnostics and typical results for steady state conditions in the TJ-IU plasmas are presented. © 1997 Elsevier Science B.V.

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. © 1997 American Institute of Physics