This is a beneficial but baffling effect that is still unexplained. In comparing the confinement times of tokamak discharges using hydrogen, deuterium, and helium, it has been carefully documented [5] that the confinement time increases with the mass of the ion, contrary to all neoclassical and instability theories. Heavier ions have larger Larmor radii, so their step size in diffusion across the magnetic field should be larger, leading to shorter rather than longer confinement times. If ions cross the field not by collisions but by instability, most theories predict one of two scalings with atomic number A. (Here A is not the aspect ratio that we used before but the more familiar A used in chemistry. A is 1 for hydrogen, 2 for deuterium, 3 for tritium, and 4 for helium.) The crudest estimate is Bohm diffusion, which we discussed in Chap. 6. That diffusion rate is independent of A. More refined theories predict gyro-Bohm scaling, which takes into account the Larmor radii of ions, which vary as the square root of A or A1/2. That leads to confinement times that vary as 1/A1/2, shorter for heavier ions. What is observed, however, is that confinement varies more like A1/2, improving by a factor between 1.4 and 2 between hydrogen and deuterium. This means that confinement will be even better with tritium, which is not normally used in small experiments because it is radioactive.

The isotope effect seems to be universal, occurring in many different types of tokamak discharges. At first it was proposed that it is caused by impurities in the gas, but very clean discharges also exhibit this effect. There have been several theo­ries on specific instabilities that could have nonlinear behavior that depends on A in this fashion, but so far these have not been confirmed in tokamak experiments. A factor of 1.5 or 2 may be trivial in an experiment but would have great commer­cial benefits in a power plant.