Master thesis at PWI-Lab CIEMAT
During my master internship I worked with getter-films: thin depositions of boron, lithium, titanium, beryllium, carbon or silicon used across industries to complete and maintain vacuum. In magnetic confinement fusion, B and Li films are commonly used to capture oxygen-rich impurities (e.g. H2O, CO, CO2, O2) and reduce the recycling of hydrogenic species. This technique typically leads to improved fuelling control and a better energy return on investment by reducing the effective atomic number of the plasma, in direct connection to radiative losses such as Bremsstrahlung and line-radiation. In devices such as NSTX or COMPASS, boronization provides routine access to high-confinement mode operations. In LTX and TFTR, a low-recycling lithium boundary has been correlated to large improvements in confinement.
In recent years, the inner walls of the TJ-II stellarator were coated with boron in order to extend the lifetime of the very reactive lithium films. My work consisted in reproducing the plasma-wall conditions of TJ-II in a conceptual setup (PWX) designed to study the boron-lithium synergistic effects. The main goal of PWX was to provide a performance-based qualification of B(C)/Li films with respect to pure Li walls. In fusion devices, boronization is most often performed by plasma-assisted chemical vapour deposition (PACVD) of boron-rich carboranes; our experiments used o-carborane (C2B10H12). The lithiation process is open to more creativity. With aims to obtain reproducible depositions, we developed an approach based on the evaporation of Li-infused capillary porous systems. We were able to control the thickness, homogeneity and lithium content of the films. The time between conditioning and oxygen or deuterium irradiations was also varied. A quadrupole mass-spectrometer, calibrated against a mass-flow meter, allowed us to quantify the oxygen and deuterium pumped by the getter walls. The performance of a conditioning was ultimately gauged in terms of the total oxygen or deuterium uptake with respect to the number of lithium atoms in the film.
This project taught me a great deal about vacuum systems, surface science and plasmas. I also learned the practicalities of wall-conditioning, key to high performance operations in present day fusion experiments. I am most grateful to Paco Tabarés, Eider Oyarzabal, David Tafalla, Iván Fernandez and the LNF-CIEMAT team for sharing their great knowledge and enthusiasm throughout ¡Olé!
- Alexis Devitre