Hydrogen control in Alcator C-Mod walls and plasmas

The control of the hydrogen fraction in D plasmas [H/(H+D)] is necessary for efficient ICRF minority heating in Alcator C-Mod. Boronizing the chamber when H/(H+D) ∼ 10% would promptly lower H/(H+D) to ∼5%. However, the reduction was transitory when the accumulated boron layer was thick (>6 μm) and with boron nitride tiles protecting the ICRF antennas, indicating that the H buried under a new B layer diffused back to the surface over time. Prior to the 2005 campaign, all boron on molybdenum tiles and surfaces was removed. Boronization was performed only after H/(H+D) was down to <5% level. Subsequently, the H fraction was found to be nearly constant in the following period of the campaign. The removal of the thick boron layers and better control of H/(H+D) also contributed to a better plasma performance.