Electrostatic trapping of an intense relativistic electron beam in a magnetic mirror

Observations of rapid axial oscillations of an intense relativistic electron beam in a magnetic mirror are reported. The mirror field primarily provides radial confinement of the relativistic electrons. The axial confinement was achieved by placing thin aluminized mylar foils at the conjugate mirror field maxima. The region between these foils was filled with a few Torr air to provide a beam induced plasma for charge and current neutralization. The regions outside these foils were maintained at approximately 10-4 Torr. One foil formed the anode of a space-charge limited relativistic electron diode which launched the beam into the mirror. When the beam passed through the second foil it was no longer charge neutralized. In a manner quite similar to the anode foil oscillations observed by others, a space-charge limited electrostatic well was established which stopped the electrons and reaccelerated them through the foil, thereby reflecting the beam. When the reflected electrons reentered the diode, they were once again ''electrostatically'' reflected. This process continued until the oscillating beam was either lost through the ''virtual cathodes'' outside the foils, dissipated in the drift region or quenched in the diode plasma after gap closure