4.18.2.1 Plasma Impurities and the Need for Graphite Materials

The fusion plasma is maintained through a combina­tion of internal heating, (i. e., the 3.5 MeV helium nucleus from the D+T reaction) and externally, by means ofinduction, radio frequency waves, or neutral particle injection. Plasma heating is balanced by plasma-cooling mechanisms among which electro­magnetic radiation dominates. In fully ionized plasma, the radiative cooling comes from the Bremsstrahlung that occurs when the energetic ions interact with the plasma electrons. A fraction of the electromagnetic radiation released from this interaction is lost from the plasma. The energy lost in this manner is signifi­cantly increased by low concentrations of impurities. The plasma power loss in the Bremsstrahlung channel, Pbrem, is determined through:

Pbrem(MWm~3) « 4.8 x 10-43 Z2NiNe TlJ2 / Z? N [2]

where Zi, Ni, Ne, and Tare the atomic number of the radiating species, their density, the electron density, and the plasma temperature, respectively. Clearly, from the linear dependence on the plasma impurity concentration, and the square dependence on the atomic mass of the impurity, the ideal PFMs com­prise light elements that have a low tendency to erode and migrate into the plasma. Carbon and beryl­lium are two low atomic number elements commonly used in tokamaks. The next suitable element is alu­minum, which would have almost a factor of five higher radiative loss on an atom-per-atom basis com­pared to carbon. On the same basis, molybdenum, which has been used in many tokamak experiments, has a radiative loss 49 times that of carbon, and tung­sten 150 times the radiative loss of carbon. However,

this is based on the assumption that the same number of impurity atoms find their way into the plasma (i. e., Ni), which, as discussed later, is not the case.