Crossed-field Secondary Emission Research Articles

Characterization of sheet electron beams from planar crossed-field secondary emission diodes

Sheet electron beams of 9 cm width having linear current densities of 1.5–3.0 A/cm have been generated from a planar crossed-field secondary emission (CFSE) electron source operated at ∼20 kV diode voltage. The output electron beam consists of two parallel closely spaced sheets of electron flow originated from each side of the cathode. The full width at half maximum of each electron sheet is 1–2 mm depending on the magnetic field strength. The output current depends strongly on the diode voltage as I∝Un where n=5.0±0.5. At a given magnetic field strength B, there is a minimal diode voltage Umin at which the diode is able to maintain a self-sustained operation. The values of Umin for a range of magnetic fields B have been linked to the potential of the diode electron flow V0=0.42 kV which is approximately ten times greater than the first cross-over voltage in the secondary emission coefficient function. The planar CFSE electron source is capable of operating in a long ∼2 ms regime. The generation of low-voltage (<10 kV) electron beams at an elevated background gas pressure of 10−2 mbar has been also demonstrated.

Production of sheet electron beams with crossed-field secondary emission diodes

This paper is concerned with the development of a crossed-field secondary emission plane diode that is capable of producing sheet electron beams. Several diode designs having cathode widths of 45-80 mm have been tested and characteristics of output electron beams were investigated. For diode voltages in the 25-40 kV range, the electron beams were generated with currents of 15-45 A and linear current densities of 1-8 A/cm depending on the diode voltage, magnetic field strength, and the diode design. Cathode edge effects were found to be major contributory factors to nonuniformity of the linear current density distributions j(x) but their influence has been greatly reduced by incorporation of axial edge electrodes and downstream reflectors into the diode design and, as a result, /spl plusmn/12% uniformity of j(x) has been achieved. It has been shown that generation of thin (a few mm) sheet electron beams of /spl sim/1 m wide with /spl sim/1-5 A/cm current density is feasible.

Spatial characteristics of electron beams from symmetric and nonsymmetric crossed-field secondary emission sources

The crossed-field secondary emission (CFSE) diode is a cold electron source based on a self-sustained secondary electron emission. The output electron beams are tubular and could be generated in a wide range of currents up to several hundred amperes. In this study, radial and azimuthal current density distributions of electron beams produced by symmetric and nonsymmetric CFSE diodes have been investigated. The electron beams are characterized by extremely high temporal stability. The wall thickness of the tubular beam with a current of &amp;gt;102 A from the diode with a 5 mm anode–cathode gap was measured to be as small as 1.4±0.2 mm. In axisymmetric diodes, the azimuthal current distribution j(φ) is uniform but this is only achieved by careful adjustment of the cathode–anode assembly. In nonaxisymmetric diodes, the j(φ) distributions are strongly nonuniform and depend not only on the magnitude but also on the direction of the magnetic field. Results of the present research show that the CFSE electron sources are potent candidates for incorporation into medium and high power microwave devices.

Characterisation of Electron Beams Produced by Crossed-Field Secondary Emission Diodes

The crossed-field secondary emission (CFSE) diode is a compact, simple and cheap source of tubular electron beams. The physical mechanism of current production is a self-sustained secondary electron emission. In this study, voltage-current, temporal and spatial characteristics of electron beams produced by axisymmetric and non-axisymmetric CFSE diodes have been investigated. The highest current obtained at the present stage of research is ∼240 A generated with a diode having a cathode diameter of 64 mm and operated with an applied voltage of ∼20 kV. The electron beams are characterised by extremely high temporal stability and narrow (∼1.4 mm) radial current density distributions. In axisymmetric diodes, the azimuthal current density distribution j(φ) is uniform but this is only achieved by careful adjustment of the cathode-anode assembly. In non-axisymmetric diodes, the j(φ) distributions are strongly non-uniform and depend not only on the magnitude but also on the direction of the magnetic field.

Self-excitation and operational characteristics of the crossed-field secondary emission electron source

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&amp;lt;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.

Crossed-field secondary emission electron source

A novel crossed-field secondary-emission (CFSE) electron source that is capable of producing high-current tubular electron beams is described. This new electron source is based on the mechanism of secondary-emission multiplication of electron current in a magnetron-like device having smooth cylindrical electrodes. The input electron current may be as low as a few mA. The multiplication process starts at the negative slope of an applied voltage pulse. After initiation, the current is extracted from the diode region with no regard to the voltage pulse shape and as a consequence, the CFSE electron source can operate in a long pulse mode. At the diode voltage of ∼40 kV for a diode gap of ∼6 mm, the output current reaches a value of more than 100 A.