Helium Pumping Strategies for D-T Fusion Devices

Abstract

In order for the new generation of long pulse D-T (deuterium-tritium) burning tokamaks to be successful, it is necessary to minimize the helium “ash” that accumulates during the thermonuclear burn. Since tritium is radioactive as well as relatively expensive, the tritium inventory stored at any given facility must also be minimized. It is therefore necessary either to preferentially remove the helium from the plasma, or to recycle all of the gas, extracting the deuterium and tritium for reuse as fuel. This latter approach requires incorporating a high pumping speed and correspondingly large pumping ducts into the fusion device design, as well as an external facility for separating helium from the D-T fuel. Since the He/H ratio in the plasma should not exceed ~5%, the pumping speed must be 20 times higher than required for a system which removes the helium only. The resulting complexity of the pumping system and the need to handle large quantities of tritium imposes a cost penalty of ~100M$ for a typical commercial fusion reactor design. It is therefore desirable to design a pumping system which removes helium only, leaving the D-T fuel in the fusion device. Recall, however, that existing pumping technologies generally provide better pumping of hydrogen, and many pump designs are totally ineffective at pumping helium.Despite the low solubility of helium in many metals, permanent trapping of helium atoms occurs at defect sites, culminating ultimately in the formation of helium bubbles. Retention of trapped inert gas is determined by diffusional processes and is consequently temperature-dependent. Hydrogen has not only a much higher solubility, but also a much higher diffusion coefficient than helium in most metals. Consequently, it is to be expected that for some metals, there is a temperature range in which implanted hydrogen is released much more quickly than implanted helium. Brooks and Mattas proposed using this “self-pumping” effect as the basis for a pump which preferentially removes helium in the presence of hydrogen.