Planar explosive emission diodes operating at moderate pulse energy densities of a few joules per square centimeter conform normally with the Child–Langmuir law in its basic unipolar form. Several recent publications [D. A. Shiffler et al., IEEE Trans. Plasma Sci., 30, 1232, (2002)] suggest, however, that the bipolar flow develops in the diode at comparatively low electron current densities of ∼10 A/cm2 and pulse durations of ∼50 ns. Understanding the nature of a charged particle flow (unipolar or bipolar) in explosive emission diodes is of paramount importance for interpretation of the experimental results. We have therefore investigated the operation of a diode having velvet cathodes at voltages in the 40–170 kV range, electron current densities of 4–16 A/cm2, beam power densities of 0.3–2.2 MW/cm2, and pulse energy densities of 0.5–3.3 J/cm2. Within these latitudes of operating parameters, unipolar flow was always observed and bipolar flow was achieved only when specific measures for generation of anode plasma were introduced. Prolonged operation of the CsI coated cathodes may lead to a deposition of thin CsI films on the anode and, potentially, other elements of the high voltage structure. In interpreting the plasma images, care must therefore be taken to distinguish between the light coming from the anode plasma and the luminescence from CsI films.

Distributions of emission centers (ECs) on planar explosive emission dielectric (velvet) cathodes at moderate electric fields of 30–70 kV/cm and pulse durations of ∼2 μs with the use of a fast framing camera have been investigated. The experimental results suggest a link between the EC distribution and current conduction paths through which the electron current is supplied to the cathode plasma. On bare velvet, the surrounding metal electrode was shown not to be of primary importance for the current conduction, instead, the current is supplied to explosive emission plasma mainly through the base of the velvet fabric. Development of a circle of brighter and larger ECs along the perimeter of the cathode was normally observed shortly after the beginning of the high voltage pulse. These ECs were found to be a major factor in the diode perveance growth and instability. Domination of the ECs on the cathode periphery has been suppressed by adding a pattern of well defined current conduction points, e.g., perforations of the velvet fabric, to the cathode design.

Perveance of a planar diode with explosive emission velvet cathodes of 20–100 mm diameters in a range of diode gaps of 0.5–5 cm is investigated experimentally. An empirical universal curve linking the diode perveance and the cathode radius-to-diode gap ratio is determined thus allowing application of a Child–Langmuir model to diodes of finite dimensions. Comparison of experimental results with two-dimensional (2D) computational models is made and a simple method to translate 2D simulation results to the three-dimensional diode configurations is suggested.

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We have investigated the crossed-field secondary emission (CFSE) electron source which is of a magnetron type with smooth cylindrical electrodes and axial applied magnetic field. It initiates at the negative slope d|U|/dt<0 of the high voltage pulse U∼10–40 kV, but further current production is maintained by a self-sustained secondary electron emission regardless to the voltage pulse shape. The output electron beam is tubular with a thin ∼1 mm wall. This article is concerned mainly with the identification of the mechanisms governing the excitation and generation of the electron beam and with the determination of the principles upon which the “optimal” CFSE electron source should be designed. We have demonstrated that the CFSE diode starts operation in a self-excitation regime (i.e., without application of the primary current) provided there is a partial trapping of the multiplying electrons inside the diode boundaries. The required axial decelerating force can be established with the use of either axial electric or nonuniform magnetic fields. Amongst all of the practical methods tested (shifting of the anode with respect to the cathode, double diode, diodes with ferromagnetic parts, use of the nonuniform external magnetic field), the diode with a ferromagnetic ring insert inside the cathode cylinder has been shown to be the most successful. It has generated an ∼240 A electron beam with a perveance of ∼85 μA/V3/2. The operating range of the CFSE diode is limited by both low and high magnetic fields. The lower limit arises from a necessity to comply with a Hull cutoff condition. The upper limit is determined by the time required for development of an electron avalanche. A secondary electron emission mechanism of current production in the CFSE diode allows the diode to operate in an oscillating regime when the applied magnetic field is higher but close to the Hull cutoff value. It has thus been possible to generate 100% density modulated electron beams at a modulation frequency of ∼107 Hz in our present experiments with the possibility of further increases up to ∼108 Hz. A geometrical scaling law for the CFSE diodes has been deduced empirically. It states that the perveance of the output electron beam is proportional to the geometrical factor X=(Dk/de)(Ld/de−0.8), where Dk is the cathode diameter, de is an effective diode gap, and Ld is the diode length. The scaling law provides a tool for designing the CFSE diodes and predicting the ultimate beam currents. For a practical size of device, this electron current could be as high as ∼1 kA.