In a burning plasma, 3.5-MeV alpha particles are generated, and as they cool down they transfer their energy to the plasma, keeping it hot. Before they become thermalized, however, the alphas are in the form of beams streaming along the magnetic field lines, and beams can excite instabilities. To do this, the velocity of the beam has to coincide with the velocity of a wave in the plasma; and the syn­chronism causes the beam energy to be transferred to the wave. The wave can become

so strong that it disrupts the plasma. There is a plasma wave called the Alfven wave that travels along the B-field and can have just the right velocity to match that of the alpha-particle beam. The danger that this can happen can be predicted precisely by theory [19], but whether it will actually happen or not depends on the details. ITER will be the first machine that can test for Alfven wave instabilities in a D-T plasma. If these turn out to be important, their avoidance is a physics problem that needs to be solved.