A stretched betatron

A betatron of novel geometry is under development at UCI. Electrons are injected into an elongated magnetic mirror. The Larmor radius is 6 cm. The distance between mirrors is 80 cm. We seek to show that the total charge capacity of such a mirror trap is simply proportional to its length. The electron trajectories are helical with an axial period of 40 ns and an azimuthal period of 3 ns. The 40 ns axial period simplifies the use of both electrostatic and magnetic inflectors. A field emission injector (70 kV, 15 amp, 50 ns) allowed us to trap about 10 nC and contain it for about 10 µs. A IS-fold increase in the injected current led to radial loss of electrons in about 100 ns with no significant increase in the amount of charge trapped. Diagnostic methods are varied. A directional X-ray detector permits localization of beam spill. Electrostatic antennas detect radiation associated with collective motion of the electrons. A gate coil allows controlled opening of the magnetic trap. As the electrons are gated out they are collected to yield a direct measurement of trapped charge. Present work seeks to establish the mechanism of electron loss and to study the effect of asymmetries in fields and conducting boundaries.