Fast ion physics related to cyclotron emission

Collective stability of fusion produced fast ions and its effect on confinement are the intellectually most challenging as well as programmatically most important issues of burning plasma physics. By including an energy dependence on the cyclotron frequency of fast and slow ions, the authors reveal a novel concept, a two-stream in gyrospace even without beams in real space. This leads to a two-energy-stream cyclotron instability for fast ions to drive waves and to slow down anomalously. The instability comes from the coupling of the fast and slow ion Bernstein branches. The growth rate of the cubic instability is proportional to (nf/ns)1/3 (γf-1)1/3 while the efficiency ∝ (nf/ns)1/3 (γf-1)-2/3, where n is the density and γ is the Lorentz factor. The first proton harmonic in a D-D plasma belongs to the cubic instability while the first alpha harmonic in a D-T plasma becomes a coupled quadratic instability, which is much weaker. A quadratic instability with growth rate ∝ (nf/ns)1/2 dominates at high harmonics. Both linear and nonlinear scaling laws have been verified by the full kinetic particle-in-cell simulation with quiet start. The theoretic and simulation studies achieved good agreement with measurements of ion cyclotron emission in D-D and D-T experiments on JET. The simulation results also show that the fast ions are anomalously slowed down while some of them are accelerated to twice of their birth energy, the fast ion density fluctuation and the phase space bunching are large, and the background ions are heated up and become non-Maxwellian