Relativistic electron beam driven instabilities in the presence of an arbitrarily oriented magnetic field

The electromagnetic instabilities driven by a relativistic electron beam, which moves through a magnetized plasma, are analyzed with a cold two-fluid model. It allows for any angle theta(B) between the beam velocity vector and the magnetic field vector and considers any orientation of the wavevector in the two-dimensional plane spanned by these two vectors. If the magnetic field is strong, the two-stream instability dominates if theta(B)=0 and the oblique modes grow faster at larger theta(B). A weaker magnetic field replaces the two-stream modes with oblique modes as the fastest-growing waves. The threshold value separating both magnetic regimes is estimated. A further dimensionless parameter is identified, which determines whether or not the wavevector of the most unstable wave is changed continuously, as theta(B) is varied from 0 to pi/2. The fastest growing modes are always found for a transverse propagation of the beam with theta(B)=pi/2, irrespective of the magnetic field strength. (c) 2008 American Institute of Physics.