Runaway electron effects

One of the possible consequences of tokamak operation is the generation of runaway electrons which can impact plasma facing components and cause damage due to high local energy deposition. This problem becomes more serious as the machine size and plasma current increases. Since large size and high currents are characteristics of proposed future machines, control of runaway generation is an important design consideration. The impact of runaway electrons on plasma facing components can be quantitatively studied with Monte Carlo codes which simulate the various processes leading to energy losses of the electrons. Some of these codes (GEANT3 and EGS4) originate from high energy physics where they have been used for many years to model detector responses. The accuracy of the codes is estimated to be better than 20%. Another code package (PTA) was developed specially for fusion purposes, and electron beam experiments were used to validate it. It is important to include the magnetic field in the analysis. It causes bending back of reflected charged particles and reduced penetration depth of the electrons due to gyration of the electrons around the magnetic field lines. Cooling tubes covered with 1 cm of graphite will melt at about 50 (25) MJ/m2 if molybdenum (copper) is used. A bare metallic wall is prone to melting at about 15 MJ/m2. (author). 68 refs, 13 figs, 2 tabs