Measurement of Anomalous High-Frequency Oscillations during Field Emission and Their Possible Significance in Pulsed Field Emission

Trains of evenly spaced 20 ns wide (width at half-height) current pulses with a repetition rate of 200 MHz were measured from a tungsten tip in vacuum when a DC high-voltage supply, a 100 MOmega ballast resistor, a 50 Omega load resistor, and an analog microammeter were connected in series with a field emission tube. These current pulses have peak values as large as 320 mA, whereas the tungsten tip in the tube is rated for a continuous DC field emission current of only 10 microamperes. We assume that the current density is approximately 109 A/m2 at the rated current, and this would suggest that the peak current density would be approximately 3 times 1013 A/m2 in the pulses. As the applied DC voltage is increased the pulse repetition rate increases, but the peak current for each pulse remains nearly constant. Several oscilloscopes and spectrum analyzers were connected across the load resistor in order to characterize these oscillations. The current pulses are caused by relaxation oscillations due to the charging and discharging of the capacitance between two wire pins on the header of the field emission tube. There is no connection to the anode so the anode floats in potential and becomes negatively charged as it receives the electrons that are emitted by the nearby tip. This reduces the potential difference between the tip and the anode, which decreases the field emission current. Thus, the potential difference across the capacitance between the anode pin and the adjacent connected pin increases until breakdown occurs on the outside surface of the header, to cause a sudden increase in the potential difference between the tip and the anode, which causes the observed current burst. The correction for displacement current is essential in all measurements of pulsed field emission