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Unwanted particles present in a magnetically confined plasma. These particles can remove energy from the plasma, complicate heating, increase the probability of disruption and thus make fusion more difficult to achieve. They may be atmospheric remnants (oxygen, nitrogen, carbon), metal particles blown off the chamber walls, dust particles, or even fusion products such as helium. The biggest problem is partially ionised impurities with high atomic numbers. These use the energy gained to excite their remaining electrons, which subsequently de-excite by emitting electromagnetic radiation, thus cooling the plasma. The purity of the plasma is taken care of by vacuum pumps to which the impurities are directed, e.g. by means of a divertor.