Characteristics of Gas Mixture Supply/Pressure Control Using Non-Evaporable Getters in a Discharge-Type Fusion Neutron Source

The first tritium burning experiments of the discharge type fusion neutron source were conducted in January 2015, using a 93% deuterium and 7% tritium gas mixture. In order to conduct the experiment in a closed environment, a gas feed and exhaust system using non-evaporable getter material was prepared. This system was designed to minimize tritium usage and produce measurable changes in the neutron production rate on the basis of the dependence of the equilibrium pressure on getter temperature as included in the manufacturer’s data sheet. However, the present experiments revealed that the gas supply was insufficient and that the discharge duration was limited to about 2 minutes by the pressure drop during discharge.

To determine the cause, verification experiments using hydrogen and deuterium gas were performed. It was found that the pressure variation with getter temperature could be mimicked by exploiting isotope effects and adjusting the hydrogen/deuterium concentration in the getter material according to the gas released into the vacuum chamber. Moreover, prolonged maintenance of a discharge was demonstrated by roughly tripling the amount of gas.

The tritium concentration in the gas mixture, estimated on the basis of the present results, varied between 1.5% and 6.7% according to the assumptions used.