  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.
