Decay instability of an ion-cyclotron wave in a plasma

An electromagnetic ion-cyclotron wave, near the second harmonic cyclotron frequency of majority ions in the central region of a tokamak, is seen to decay efficiently into two Bernstein modes. The coupling coefficient for this decay vanishes for a dipole pump, hence a rigorous kinetic theory for decay instabilities of a finite wavenumber pump is developed. For parameters characterizing the Princeton Large Torus (PLT) experiment, the uniform medium growth rate at 200 kW turns out to be 3 μsec, whereas, the threshold due to plasma inhomogeneity is 100 kW. The nonresonant decay into ion acoustic waves is also significant, though relatively less important. For a pump wave near the cyclotron frequency of majority ions, nonlinear Landau damping and oscillating two stream instability turn out to be very important.