Positron Creation and Annihilation in Tokamak Plasmas with Runaway Electrons

It is shown that electron-positron pair production is expected to occur in post-disruption plasmas in large tokamaks, including JET and JT-60U, where up to about ${10}^{14}$ positrons may be created in collisions between multi-MeV runaway electrons and thermal particles. If the loop voltage is large enough, they are accelerated and form a beam of long-lived runaway positrons in the direction opposite to that of the electrons; if the loop voltage is smaller, the positrons have a lifetime of a few hundred ms, in which they are slowed down to energies comparable to that of the cool ($\lesssim 10\mathrm{e}\mathrm{V}$) background plasma before being annihilated.

Figure 1

Current ($I_p$) history in JET discharge 13340. In the disruption, which occurs at $t\simeq 50\mathrm{s}$, a large fraction of the plasma current is converted into a current of runaway electrons, which persists and decays gently for about 6.5 s. The runaway column remained vertically stable in this pulse, which predates the pumped divertor installation.

Figure 2

Fraction of positrons that are annihilated before thermalization as a function of energy at birth.