Investigation of the interaction of high current relativistic electron beams with electromagnetic fields
The results obtained in the investigation of high current relativistic electron beams produced by explosive electron eMission from the flare cathode plasma of a high voltage, cold-cathode, field-emission diode are reported. In addition, the production of high power ( ~100kW) microwave radiation through the interaction of such beams with the natural modes of a resonant cavity is reported. A Marx bank generator was used to excite a hollow cylindrical cathode stalk producing diode currents of the order of 1 kA. A fraction (≤ 5%) of the diode current was transmitted through a thin anode mesh to produce a relativistic electron beam drifting in a low pressure (2 µ torr) region. Diode voltage pulse widths of typically one microsecond were observed with anode-cathode (A-K) gap spacings of 1.5mm to 50mm being employed. The thermal expansion of the cathode plasma resulted in a time dependent diode impedance and measurements of this impedance for varying initial A-K gap settings illustrated a space-charge limited current-voltage characteristic. A quasi-static magnetic field, produced by a pulsed solenoidal coil, was used to magnetise the vacuum drift space in which the beam propagated. The interaction of the relativistic beam with the magnetic field produced high power microwave radiation at X-band and Q-band frequencies. The most prominent feature of this interaction was the existence of sharp "resonant" field values where radiation ocurred. This self-excited radiation is thought to be due to an interaction between the Doppler-shifted electron cyclotron frequency (ECF) of the electron beam and the natural modes of oscillation of the cavity in which the beam drifts. Observation of radiation at Q-band frequencies would indicate that harmonics of the ECF are also present, albeit at weaker intensities. The magnitude of the collected electron beam current was observed to be dependent upon the strength of the applied magnetic field in which the beam drifted.