Massive runaway positrons are generated by runaway electrons in tokamaks. Thefate of these positrons encodes valuable information about the runawaydynamics. The phase space dynamics of a runaway position is investigated usinga Lagrangian that incorporates the tokamak geometry, loop voltage, radiationand collisional effects. It is found numerically that runaway positrons willdrift out of the plasma to annihilate on the first wall, with an in-plasmaannihilation possibility less than 0.1\%. The dynamics of runaway positronsprovides signatures that can be observed as diagnostic tools.
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