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.
There are various refuelling methods: gas puffing from the edge of the vessel, neutral particle injection, and pellet injection. In the latter, the 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.
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 Wendelstein 7-X stellarator will feature a pellet accelerator, which will propel the pellets by means of helium or hydrogen gas like an air gun. The ice for the pellets will be produced in a refrigeration unit, which is cooled by liquid helium. Up to 60 pellets can be produced at a stretch and "fired".