Experimental study of cusp injected electron beam

Critical parameters were measured for the axis-encircling electron beam of a cusptron, a high-power microwave tube for several different magnetic field profiles. The tube utilizes the negative mass instability (NMI) of the beam either in a circular or multivane circuit. Thus, microwave radiation can be produced at the fundamental or high harmonics of the electron cyclotron frequency. If the beam orbit does not coincide with the peak wave amplitude, the NMI is damped. The off-centering stems mainly from a nonideal magnetic field. A series of experiments was set up to measure the beam envelope radius and velocity ratio as a function of axial distance for several unique magnetic field profiles. Each field profile maximized one of the off-centering sources from which measurements were taken. Thus, insight to the magnitude of off-centering and spread in axial velocity for each source led to a final field profile set up to reduce the undesirable effects of off-centering.<>