This paper presents the design and development of a trigger with a high repetition rate, low jitter, and compact structure for the pseudospark switch (PSS), which includes an improved Marx generator based on avalanche transistors and a corona-plasma trigger unit. The generator adopted a novel 3 × 12-stage Marx circuit based on avalanche transistors in which the failure rate of transistors in the first and second stages was significantly reduced by connecting the parallel capacitors compared to the previous similar generator. The reason for the improved performance was also discussed. The main parameters of output pulses were an amplitude of -7 kV, rise time of 6 ns, jitter of 0.2 ns, and repetition rate of 2 kHz. The corona-plasma trigger unit adopted BaTiO3 ceramics with high er as the dielectric and was arranged in the hollow cathode of the PSS. The experiments of triggering a PSS prototype were conducted. The influence of anode voltage and pressure on the trigger delay and jitter was studied, and the minimum trigger jitter achieved <1 ns. This trigger worked for 107 shots at the repetition rate of 2 kHz continuously without obvious performance degradation and any failure of the generator. The main advantage of this trigger is the simultaneous combination of the high repetition rate, low jitter, long lifetime, and great simplicity in a compact structure.

A novel trigger for pseudospark switch with high repetition rate, low jitter, and compact structure

The operation features and the main physical parameters of different modifications of hollow anode (HA) plasma sources based either on multi-arc, or on magnetron-like, or on ferroelectric plasma source (FPS) ignition are described. It was found that these HA sources produce in the vicinity of the HA output grid a satisfactory unifonn plasma with a density n≈ 5×1012 cm-3. It was found that during the HA discharge the plasma acquires a positive potential of several te ns Volts. The HA with incorporated FPS demonstrated the best operation characteristics. It was found that the FPS allows reliable ignition and sustaining of the HA discharge with current amplitude Id≤1.2 kA and pulse duration ≤2×10-3 s at N2 gas pressure of ~ 10-5 Torr. During the HA discharge the density of the FPS surface plasma was found to be 8×1014 cm-3 that allows one to consider FPS as a practically unlimited electron source. In addition, the FPS allows one to decrease signiticantly the overall HA dimensions, thus providing a very compact design. All three HA modifications were used as a cathode in a diode powered by a 200 kV, 400 ns pulse. The parameters of the generated electron beam with an amplitude ≤1.5 kA and the diode characteristics are presented. It was found that the applied accelerating pulse causes a significant up to 6 kV increase of the plasma potential (φpl) inside the HA. A model that explains the electron emission from the positively charged plasma is proposed.

Low-pressure hollow-anode plasma sources for high-current electron beam generation

The results of experimental studies of different types of cathodes—carbon-epoxy rods, carbon-epoxy capillary, edged graphite, and metal-dielectric—under the application of high-voltage pulses with an amplitude of several hundreds of kV and pulse duration of several nanoseconds are presented. The best diode performance was achieved with the edged graphite and carbon-epoxy-based cathodes characterized by uniform and fast (<1 ns) formation of explosive emission plasma spots and quasi-constant diode impedance. This result was achieved for both annular cathodes in a strong magnetic field and planar cathodes of a similar diameter (∼2 cm) with no external magnetic field. The cathodes based on carbon-epoxy rods and carbon-epoxy capillaries operating with an average current density up to 1 kA/cm2 showed insignificant erosion along 106 pulses of the generator and the generated electron beam current showed excellent reproducibility in terms of the amplitude and waveform.

Experimental research of different plasma cathodes for generation of high-current electron beams

Explosive electron emission and exploding wires

Having the device simplicity and high output-power capacity, the capability of the virtual cathode oscillator (vircator) for emitting high power microwaves is demonstrated. We have investigated the axial-typed vircator in our pulsed power generator, “Chundoong” (Max 600 kV, 88 kA, and 60 ns), and simulated the vircator using the 3-D particle-in-cell simulation code called “MAGIC.” We try to find out the position of the virtual cathode inside the drift tube and enhance the power of microwaves by focusing them at the focus point using the ring-typed zone plate with a focal length of 18.8 cm. The dominant frequency is obtained as 3.5 GHz measured by fast Fourier transform, which is in good agreement with simulation frequency. It is found that the mean position of the virtual cathode is almost the same as the A-K gap distance of 10 mm, in which the virtual cathode oscillates from 7.9 mm to 12.1 mm behind the meshed anode, as verified by the simulation results. The maximum output power without the zone plate is obtained as 0.66 GW with the efficiency of 27%, which is maximized up to 1.22 GW with the efficiency of 51% at the focus point by using the zone plate. The microwave emission mode from the vircator is the TM01 mode based on the simulation results. The zone plate contributes significantly in enhancing the power efficiency and determination of the position of the virtual cathode. These observations might be used in developing a high efficient microwave source by using a zone plate under the decision of the position of the virtual cathode.

Enhancing the power of high power microwaves by using zone plate and investigations for the position of virtual cathode inside the drift tube

When subjected to high electric fields in vacuum, carbon fiber cathodes produce intense electron beams suitable for high-power microwave (HPM) generation at very high current densities. However, the production mechanisms of these intense electron beams are not fully understood. This paper presents the postmortem examination of three conditioned carbon fiber cathode types. The three cathode types consist of an uncoated, bare unimodal fiber structure, a bare bimodal fiber structure, and a cesium-iodide (CsI)-coated bimodal fiber structure, all with identical fiber coverage of 2% by area. Each cathode was conditioned prior to testing by single pulse operation driven by an 80 J Marx generator for 10 000 pulses. HPM, voltage, and current waveforms of each cathode are presented. The bare bimodal cathode radiated more microwave power than the CsI-coated cathode and bare unimodal cathode. Scanning electron microscopy imagery presents evidence of two emission mechanisms: 1) explosive electron emission and 2) surface flashover, which both were found on the CsI-coated cathode. In addition, no evidence of surface flashover was found on either uncoated cathode.

Emission Behavior of Three Conditioned Carbon Fiber Cathode Types in UHV-Sealed Tubes at 200 A/ $\mathrm{cm}^{2}$

This paper presents a study on outgassing and electrical conditioning for three carbon fiber cathode types in a vacuum-sealed, high-power microwave virtual-cathode-oscillator (vircator) that operates in the low 10
 -9
 torr pressure regime. The three cathode types consist of a bare bimodal fiber structure, a bare unimodal fiber structure, and a cesium-iodide coated bimodal fiber structure with identical fiber coverage of 2% by area with a surface area of ~20 cm
 2
. The electrodes are cleaned by a 1.2 kW, argon/oxygen microwave plasma prior to complete vircator assembly, followed by a 300 °C bake-out for 72 h. Each cathode was pulsed in a single pulse operation by an 80 J, low inductance Marx generator with an approximate pulsewidth of 100 ns full-width at half-maximum for 10000 current pulses. The data presented includes individual gas constituents, high-speed intensified charge coupled device (ICCD) imaging, and voltage and current waveforms. The conditioning process resulted in a gas load reduction of ~80% overall, with the indication that the bare bimodal fiber structure performed the best as diode power increased throughout the experiment, while the power decreased for the other tested cathode types.

Conditioning of Carbon Fiber Cathodes in UHV-Sealed Tubes at 200 A/cm 2

The high-power-microwave (HPM) sources currently under development typically require constant pumping to maintain the high vacuum levels required for operation. This pumping is often done with either a cryo- or turbopumping system, either of which would be difficult to deploy in a compact portable system. A compact sealed-tube virtual cathode oscillator (vircator) source has been developed at Texas Tech University (TTU) that does not require a bulky external vacuum pump for operation. This device has a base vacuum pressure in the low range compared to the majority of laboratory HPM sources having vacuum levels in the - range. The reduced amount of trapped gasses in the sealed-tube ultrahigh-vacuum environment has the potential to greatly impact device performance. The TTU sealed-tube vircator is useful as a testbed for studying HPM source optimization under UHV conditions. Measured operational characteristics of the tube utilizing a carbon fiber cathode and a nickel anode are presented, along with radiated microwave measurements.

Operation of a Sealed-Tube-Vircator High-Power-Microwave Source

Operation of repetitive high-power microwave (HPM) sources is predominantly limited by thermal properties of anode and cathode materials This letter presents a reflex-triode virtual cathode oscillator (vircator) capable of operating at 500 Hz at current densities between 100–200 A/cm $^{2}$ for multiple burst durations of 1–2 s Stable vircator operation under such a thermally punishing environment is facilitated by the use of a thin pyrolytic graphite anode The results presented focus on two anode–cathode (A–K) gap spacings: 11 and 21 mm, which produce stable microwave radiation at 46 and 16 GHz, respectively Characteristic voltage, current, and microwave waveforms in conjunction with short-time Fourier transforms, frequency spectrographs, and HPM power density data for 1000 and 500 pulses at 16 and 46 GHz, respectively, are presented

A Frequency Stable Vacuum-Sealed Tube High-Power Microwave Vircator Operated at 500 Hz

Many high-power electron devices utilize cold-cathode diodes to generate intense electron beams. These cold cathodes have the advantage of being capable of supplying several kiloamperes of current spread over a large cross section without the need for auxiliary components such as a heater supply. However, they suffer from many known problems such as nonuniform emission that can result in small areas of high current density on the anode and, thus, excessive anode heating. As a consequence, outgassing and vaporization of bulk material frequently leads to premature impedance collapse. Hence, minimizing nonuniform anode heating due to beam nonuniformity is paramount. As previously demonstrated, the use of a CsI-coated carbon velvet cathode improved beam uniformity, reduced outgassing, and mitigated early impedance collapse. To quantify the uniformity, temporal and spatially resolved images of the cathode plasma were taken for a CsI-coated carbon fiber cathode, operated at an average current density of ~150 A/cm2 under various conditions, i.e., without a field shaping ring, before and after discharge cleaning, and with a field shaping ring. All cathodes were operated in a sealed tube with a small integrated sputter ion pump to restore vacuum levels to 10-9 torr levels between subsequent shots.

C. Lynn

Papers

Emission Behavior of Three Conditioned Carbon Fiber Cathode Types in UHV-Sealed Tubes at 200 A/ $\mathrm{cm}^{2}$

Conditioning of Carbon Fiber Cathodes in UHV-Sealed Tubes at 200 A/cm 2

Operation of a Sealed-Tube-Vircator High-Power-Microwave Source

A Frequency Stable Vacuum-Sealed Tube High-Power Microwave Vircator Operated at 500 Hz

Light Emission From CsI-Coated Carbon Velvet Cathodes Under Varied Conditions