Plasma limited by a divertor keeps losing plasma particles, which are removed together with impurities by the divertor pumps.

In a future fusion power plant the "ash" from the fusion process, helium, will also be removed in this way by the divertor.

There are various refuelling methods: gas puffing from the edge of the vessel, neutral particle injection, and pellet injection. In the latter, hydrogen gas is cooled till it freezes and pellets a few millimetres in diameter can be formed. After being accelerated in gas guns or centrifuges, they are injected into the hot plasma, where they again evaporate and the individual atoms are ionised.

View into the plasma of ASDEX Upgrade just as a pellet of frozen deuterium is evaporating (top left)

As the pellets deposit the fuel at any selected location in the plasma, it is thus possible to change the density profile of the plasma.

The pellet centrifuge on ASDEX Upgrade can inject up to 80 pellets per second into the plasma at a maximum velocity of 1200 metres per second – four times the sound velocity. Here it takes just one of the roughly 1 milligram pellets to refuel about one-third of plasma.

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