NONLINEAR COUPLING OF LOW N-MODES IN PBX-M

In many of the medium and high beta discharges in PBX-M, low n modes (n = 1-4) with different n numbers are observed. The prevalence of low n modes decreases with increasing n number. Furthermore, if two modes of different frequency and n number (omega(1) and omega(2); k(1) and k(2)) are simultaneously present in the plasma, these modes interact non-linearly and create sidebands in frequency (omega(2) +/- omega(1)) and wavenumber (k(2) +/- k(1) or n(2) +/- n(1) and m(2) +/- m(2)). It has also been observed that if these fundamental modes, omega(1)/k(1) and omega(2)/k(2), contain strong harmonics, the harmonics also interact non-linearly, creating more non-linear products: k omega(2) +/- l omega(1) and kk(2) +/- lk(1), where k and l are integers describing the harmonies. These multiple three wave couplings on a given rational surface can create several modes of the same m and n numbers but of different phase velocities. The multiple perturbations of the rational surfaces can increase the plasma losses. In fact, a decrease in neutron rate and an enhanced loss of medium energy ions are observed during this three wave coupling interaction. Although the three wave coupling has been observed primarily in medium or high beta discharges, it is a more general plasma phenomenon because it also occurs occasionally at low beta.