Characteristics of diode perveance and vircator output under various anode-cathode (A-K) gap distances

Abstract

When the electron beam is generated from the cathode, the electrode sheath plasma is also generated, and hence the perveance of diode is being vaned to change the diode output characteristics. In our pulsed power system "Chundoong", the perveance characteristics have been experimentally investigated, from which the sheath plasma expansion speed has been found to he an about 3 c d~s inside the diode. It is shown inside the diode that the sheath plasma expansion gives the influence on the electron beam current density.~ The Vircator microwave output has been shown to he dependent especially on A-K gap distance of which initial 4 mm in A-K gap distance' is found to he optimized for maximum power.

Characteristics of Diode Perveance and Output Microwave Power in Coaxial Virtual Cathode Oscillator

“…It is noted that the peak diode voltages V d are 250 kV and 340 kV for A-K gap distance of 4 mm and 10 mm, respectively, in this experiment. The sheath plasma expansion speed in the cylindrical diode has been generally more than two times than that of about 3cm/µs for planar diodes (11) , in this experiment. Figure.…”

“…Figure 4 shows the observed initial static perveances from cylindrical diodes, measured by A/V 3/2 various A-K gap distances (a) and their log-log plots obtained from least-squared-error-fitting method (b), in to 3.0×10 −5 A/V 3/2 as the initial A-K gap spacing d is varied from 4 mm to 10 mm in our system. It is noted that these perveance for cylindrical diodes are quite less than those for planar diodes (11) . It is also noted from Fig.…”

“…However, the cathode and anode surfaces are fully covered by sheath plasmas as time goes by. The sheath plasma formed on the cathode and anode surfaces expand toward each other, reducing the effective gap distance, d ef f , between the cathode and anode (11) , and therefore decreasing the diode impedance. It is noted here that the peak diode voltage has been increased as the initial A-K gap distance, d, has been increased from 4 mm to 10 mm, while the diode currents are shown to be vice versa to the diode voltage in this experiment.…”

“…From this, the diode perveance P (11) for infinite planar geometry could be written as where I d and A are diode current and cross sectional area of the electron beam, respectively. The perveance usually have a constant value if the diode A-K gap distance d and electron beam cross section area A are maintained to be constant value.…”