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

Nonlinear transmission lines (NLTLs) have been increasingly studied to produce high power microwaves (HPM) at higher repetition rates than conventional HPM devices without requiring the same auxiliary systems. The ability to array NLTLs to produce higher power and achieve beam steering provides an additional capability. This review summarizes the various NLTL topologies designed to achieve these design objectives. Specifically, we summarize modeling and experimental studies for three primary topologies: the lumped element NLTL, the split ring resonator, and the traditional transmission line geometry using nonlinear materials. The lumped element NLTL and the traditional transmission line constructed with nonlinear materials lend themselves to higher power applications, while the split ring resonator is better suited for applications involving antennas. We provide a detailed summary of past studies for these topologies and conclude by exploring ongoing work and future opportunities for technological development.

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A Review of Nonlinear Transmission Line System Design

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

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/ $\mathrm{cm}^{2}$

A stripline gyromagnetic nonlinear transmission line (NLTL) was constructed out of yttrium iron garnet ferrite and tested at charge voltages of 35 kV–55 kV with bias fields ranging from 10 kA/m to 20 kA/m. Typically, high power gyromagnetic NLTLs are constructed in a coaxial geometry. While this approach has many advantages, including a uniform transverse electromagnetic (TEM) mode, simple interconnection between components, and the ability to use oil or pressurized gas as an insulator, the coaxial implementation suffers from complexity of construction, especially when using a solid insulator. By moving to a simpler transmission line geometry, NLTLs can be constructed more easily and arrayed on a single substrate. This work represents a first step in exploring the suitability of various transmission line structures, such as microstrips and coplanar waveguides. The resulting high power microwave (HPM) source operates in ultra high frequency (UHF) band with an average bandwidth of 40.1% and peak rf power from 2 MW to 12.7 MW.

Investigation of a stripline transmission line structure for gyromagnetic nonlinear transmission line high power microwave sources

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

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

This paper evaluates the performance of a bimodal carbon fiber cathode and a carbon-epoxy multicapillary cathode operating within a reflex-triode sealed-tube virtual cathode oscillator (vircator). The experimental results reveal that both cathodes exhibit similar emission behavior, although with some significant operational differences. An eight-stage 84-J pulse-forming network-based Marx generator serves to drive both cathodes at 250 kV and 3–4 kA with a $\sim 70$ -ns pulsewidth. Both cathodes undergo conditioning over 10 000 pulses to determine gas evolution as well as electrical changes over time. Gas evolution of both cathodes is observed using a residual gas analyzer to determine individual gas constituents. A comparison of diode voltage, diode current, RF output, and outgassing data for both cathodes during vircator operation over 10 000 pulses is presented to quantify cathode performance in a sealed-tube vircator. Changes in cathode surface morphology, from virgin to postmortem, are discussed. Data for various anode–cathode gap distances, from 3 to 15 mm, are presented. The evolution of voltage and current inputs to the vircator is discussed.

Jonathan M. Parson

Papers

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

Investigation of a stripline transmission line structure for gyromagnetic nonlinear transmission line high power microwave sources

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

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

Evaluating the Performance of a Carbon-Epoxy Capillary Cathode and Carbon Fiber Cathode in a Sealed-Tube Vircator Under UHV Conditions