Excitation of broadband electrostatic noise and of hydrogen cyclotron waves by a perpendicular ion beam in a multi-ion plasma

Abstract

Results from the PORCUPINE experiment show that a perpendicular heavy ion beam, injected into an O+ dominated plasma which contains a small concentration of H+, induces a broadband electrostatic noise near the lower hybrid frequency and also discrete electrostatic emissions at frequencies close to multiples of the hydrogen gyrofrequency. The dependence of these instabilities on the parameters characteristic of the beam-background plasma system is studied. It is shown that, provided the beam is of sufficiently high density and low temperature, the frequency range of the broadband noise extends continuously from zero frequency up to the lower hybrid frequency. In this case the harmonics of the hydrogen gyrofrequency are also excited but their growth rates are much lower than that of the broadband emission, up to two or three orders of magnitude for the first harmonics.