B. Terreault

Blistering of silicon crystals by low keV hydrogen and helium ions

The blistering of Si wafers with (100), (110), and (111) orientations, induced by implantation of low energy (5–8 keV) H and/or He ions, was studied. Our earlier work had used either pure H implants at high dose (5×1016 cm−2) or low dose coimplantation (1×1016 H cm−2+1×1016 He cm−2). Here we study pure He implantation, and coimplantation with variable He/H dose ratio, in order to understand the roles of H and He, in the hope of improving the blistering efficiency (as used, e.g., in the Smart-cut® process). After room temperature implantation, the samples were vacuum annealed in three steps at 500, 550, and 600u200a°C. At each step, the development of blistering and exfoliation was quantified by scanning electron microscopy (“exfoliation” is here used to mean blister explosion). We found that the thermal budget required for blistering is higher than for comparable doses of high energy (>30 keV) ions. On the Si(110) surface, H blistering is very weak, He blistering is the strongest, but exfoliation absent. On S...

Biasing of closed double null poloidal divertor plates on TdeV

Abstract Bias voltages applied to the divertor plates in TdeV (Tokamak de Varennes) permit fine control of several main plasma parameters, including the poloidal rotation velocity near the separatrix, microturbulence, the sawtooth period and the heat pulse propagation speed. Biasing also improves the divertor efficiency in either the top or bottom closed divertor chamber depending on polarity, in agreement with E × B drifts. Negative biasing reduces carbon and CO fluxes from the wall, the loop voltage and the X-ray emissivity, all indicative of decreasing impurity contamination.

Effects of divertor plate biasing on radial and poloidal edge fluxes in the TdeV

The divertor plates of TdeV, a tokamak with a double-null divertor and closed divertor chambers, have been electrically biased with respect to the walls. The authors discuss the resulting effects on the edge electron density profile, on the neutral pressures and impurity fluxes in the main vacuum chamber and the divertor chambers, and on the plasma flow to the divertors. As a function of the bias voltage, which was varied between-180 V and +160 V, the electron density scrape-off width and the wall impurity influxes increase monotonically; the flows to the top and bottom divertors vary strongly, in qualitative agreement with an E*B/B2 rotation, but not symmetrically. With negative biasing, the electrostatic barrier and the rotation combine to give a strong improvement of the divertor efficiency

Range and backscattering of hydrogen ions below ∼ 2 keV: Fits of theory to data and application to plasma-materials interactions

Abstract We have analyzed the recent data of our group and others for H and D ions of ∼ 2 keV and less in Be, C and Si. By comparison with our Monte-Carlo code BABOUM we have determined the adjustable parameters which describe elastic and inelastic energy loss processes (and so updated the well-known compilation of Andersen and Ziegler of 1977). Using these parameters we have completed the data base on low-energy interaction of H and D in these materials, and also computed range and backscattering quantities for D ions issuing from a Maxwellian isotropic plasma; this should be useful in the interpretation of controlled fusion experiments.

Measured plasma parameters in TdeV's closed poloidal divertors

Abstract Measurements in TdeV in the double null configuration show strong asymmetries between the upper and lower divertors. The divertor to which the ion ∇ B drift is directed has a higher density, which varies little with the central line-average density. The electron temperature also exhibits a poloidal variation which depends on the direction of the toroidal magnetic field. The divertor plasma shows a double peak structure which evolves as a function of the central density, with the outer peak varying more rapidly. We have measured flow reversal near the separatrix in the bottom divertor when the ion ∇ B drift is directed away from the X -point. Divertor plate biasing allows us to control the flux of plasma in the SOL; under negative biasing there is a strong increase of the electron density in the active divertor, the one favoured by the E × B flow. Calculations of the energy deposition derived by flush-mounted probes agree well with measurements of the heat load derived from the temperature increase of the divertor tiles.

Improvements in recycling and impurity control obtained by divertor biasing

Experiments in which the divertor plates are biased with respect to the grounded vacuum chamber were conducted in the Tokamak de Varennes, with ohmic discharges and boronized walls. A comprehensive set of diagnostics was used and those effects relating to plasma-surface interactions and their consequences, namely recycling and impurity production, are reported here. The main conclusion is that the bias voltage actively controls the balance between the particle fluxes to the walls and the diverters, with a weak influence on the confinement. The quantities associated with plasma surface interactions, i.e. the impurity influx, the plasma contamination and the global recycling coefficient, increase as a function of the voltage applied between the plates and the wall. Typically, the quantities of interest vary by a factor of two or more in the range from -240 to 290 V. The poloidal fluxes to the upper or lower divertor are enhanced by negative or positive biasing, respectively, in agreement with the sign of the E*B drift. For negative biasing the reduction in the wall flux and increase in the poloidal flux combine to improve greatly the divertor efficiency: the divertor pressure and the impurity retention time are increased by factors of up to 5 and 8, respectively. The above effects, together with the SOL density profiles and the biasing current-voltage characteristic, are consistent with a model in which the radial transport in the SOL is limited by the ion mobility; a mobility of 0.04 m2/V.s is found. Besides allowing active density and impurity control, it is suggested that biasing could facilitate the attainment of a radiative divertor by injection of well retained impurities in the divertor

Comparison of three boronization techniques in TdeV

Preparation of the internal walls of tokamaks by plasma enhanced chemical vapour deposition (PECVD) of boron containing films has now been implemented on several machines since its development on TEXTOR. More recently, such films were deposited on the internal walls of TdeV using not only this procedure but also two new approaches: solid target boronization (STB) which consisted in inserting a low-density boronized carbon-carbon (C-C) composite into the tokamak plasma and TMB fuelling where trimethylboron was used as fuelling gas during the plasma discharge. These approaches resulted in a rapid shot to shot improvement of important parameters such as the volume averaged resistivity and radiated power over the first dozen shots when the boron source is present. Typically, the resistivity is reduced from ∼4.0×10 −7 to ∼2.5×10 −7 Ωm, comparable to the resistivity obtained with PECVD. The radiated power relative to the ohmic power is reduced by a factor of 2 from 20 to 10%. When the boron source, present during STB or TMB fuelling, is removed however, these plasma parameters start increasing. Within a few tens of shots, they have reverted to their preconditioning values, a situation which requires hundreds of shots after PECVD.

Laser desorption study of beryllium's hydrogen recycling properties at high temperature

Abstract We have investigated the reemission at > 1000 K of 1.5 keV H ions implanted in beryllium, using a novel method of laser flash desorption. Numerical modelling of the hydrogen evolution as a function of the laser energy indicates that the process is, unexpectedly, detrapping-limited rather than diffusion-limited, with a trap energy of 1.6 ± 0.15 eV and an apparent diffusivity > 10−3 cm2/s at 1200 K. It could imply faster than anticipated H isotope recycling at high temperature. A comparison with other data is made.

Chemical Impurity Production in the Tokamak de Varennes

Plasma contamination due to the generation of impurity molecules has been studied by mass spectrometry and by visible emission spectroscopy in the Tokamak de Varennes. The dominant effects are carbon monoxide formation, which is correlated with the residual water vapor pressure in the vacuum chamber, and the formation of C{sub 1}, C{sub 2}, and C{sub 3} hydrocarbons. The measured molecular fluxes are sufficient to account for a large part of the plasma impurity content. Visible spectroscopy indicates that the plasma is significantly affected by these chemical impurity sources. The molecules appear to originate mainly from the stainless steel walls rather than from the graphite limiters.

A method of pulsed‐laser desorption of hydrogen

An apparatus and an experimental procedure, mainly designed to investigate the thermodynamics and kinetics of hydrogen evolution in materials, are described. Employing a 30 ns ruby laser pulse as a heat source, this method probes H behavior at higher temperatures and on a much shorter time scale than classic thermal desorption. Precise calibration techniques for the laser fluence and the desorption yield have been developed. The absolute calibration agrees well with a measurement of the H content by elastic recoil detection. Particular attention has been paid to lateral uniformization of the laser intensity, to allow valid one‐dimensional modeling of laser heating and H evolution, and extraction of the activation energies and kinetic factors. As an example, a cursory study of intrinsic H in Be is presented. The method is also applicable to other volatile dopants.