A process in which particles move outside the defined region in which we are trying to achieve the conditions for thermonuclear fusion. We usually talk about it in the context of magnetic confinement. There are many causes of particle loss. It can be particles with too high energy, uncharged particles or particles flying at an inappropriate angle that escape the magnetic trap. Very common is particle loss from so-called magnetic mirrors, devices with open ends. A mechanism called particle transport, which allows particles to move from one magnetic surface to another, can lead to particle loss when a particle finds itself on a magnetic line that ends at the wall of the device. In certain cases, particle loss may be desirable. For example, a stream of impurities directed at the divertor is needed to ensure the purity of the plasma. In general, however, particle loss leads to the loss of not only the particle but also the energy it possesses, thus cooling the plasma and shortening the confinement time.