The tritium burn-fraction in DT fusion

TL;DR

This paper discusses the importance of tritium burn fraction in DT fusion, analyzing how confinement times and plasma purity affect fusion efficiency and energy requirements.

Contribution

It provides a detailed analysis of the relationships between tritium burn fraction, confinement times, and plasma purity, highlighting areas needing further study for fusion optimization.

Findings

Burn fraction increases as tritium confinement improves.

Energy confinement time scales with gyro-Bohm transport.

The ratio of tritium to energy confinement time impacts burn efficiency.

Abstract

The fraction of the tritium that is burned during one pass through a DT fusion system, $f_{tb}$, is a central issue for success of fusion energy. Reducing the tritium fraction, $f_t$, in a $DT$ burning plasma below a half increases the burn fraction, $f_{tb}\propto1/f_t$ but also the required confinement to achieve a burn $nT\tau_E\propto1/f_t(1-f_t)$. A doubling of the fractional burn entails only a 4/3 enhancement of the required $nT\tau_E$. The energy confinement time $\tau_E$ in tokamaks and stellarators is empirically gyro-Bohm with an approximate factor of two between the best and worse results used to construct scaling laws. Gyro-Bohm is also the approximate level of transport needed in a power plant. What has received little study are $\tau_t/\tau_E$, the ratio of the tritium to the energy confinement time, and $\tau_\alpha/\tau_E$, the ratio of the alpha particle to the energy confinement time. The tritium burn fraction is proportional to $\tau_t/\tau_E$, so the larger the better. The contamination of the plasma by helium ash is proportional to $\tau_\alpha/\tau_E$, so the smaller the better.