Helium ash

Ash is the name given to the energetic alpha-particles or helium nuclei produced by fusion reactions in a deuterium-tritium plasma, even though helium bears no physical resemblance to ash from a fire. On a more figurative level, the alpha-particles are termed ash because once they have shared their energy with the rest of the plasma they have no further use; their removal and replacement by deuterium-tritium fuel is required to prevent dilution of the plasma.