Observation and Suppression of Radial Beam Blowup in the Maryland ERA

In the Maryland ERA, a rotating electron ring beam is produced by passing a nonrotating hollow beam through a narrow magnetic cusp. As the beam progresses in the acceleration region downstream of the cusp transition, a fast growth in the radial thickness of the beam is observed, resulting in rapid loss of the beam to the conducting boundaries. The effect of different conducting boundary configurations on the radial growth of the beam has been measured using magnetic probes and time-resolved and time-integrated photographs of the beam cross section. Measurements of the microwave radiation produced by the beam as it propagates in the downstream region indicate that a substantial fraction of the total beam power is converted into radiation. This radiation is attributed to the negative mass instability and coupled into the TE and TM waveguide modes of the downstream drift chamber. When the beam is passed through a 6.4mg-cm-2 titanium foil located at the anode plane, microwave radiated power is reduced by a factor of 100 and the radial blowup of the beam is suppressed.