J.-W. B. Bragg

All solid-state high power microwave source with high repetition frequency.

An all solid-state, megawatt-class high power microwave system featuring a silicon carbide (SiC) photoconductive semiconductor switch (PCSS) and a ferrimagnetic-based, coaxial nonlinear transmission line (NLTL) is presented. A 1.62 cm(2), 50 kV 4H-SiC PCSS is hard-switched to produce electrical pulses with 7 ns full width-half max (FWHM) pulse widths at 2 ns risetimes in single shot and burst-mode operation. The PCSS resistance drops to sub-ohm when illuminated with approximately 3 mJ of laser energy at 355 nm (tripled Nd:YAG) in a single pulse. Utilizing a fiber optic based optical delivery system, a laser pulse train of four 7 ns (FWHM) signals was generated at 65 MHz repetition frequency. The resulting electrical pulse train from the PCSS closely follows the optical input and is utilized to feed the NLTL generating microwave pulses with a base microwave-frequency of about 2.1 GHz at 65 MHz pulse repetition frequency (prf). Under typical experimental conditions, the NLTL produces sharpened output risetimes of 120 ps and microwave oscillations at 2-4 GHz that are generated due to damped gyromagnetic precession of the ferrimagnetic materials axially pre-biased magnetic moments. The complete system is discussed in detail with its output matched into 50 Ω, and results covering MHz-prf in burst-mode operation as well as frequency agility in single shot operation are discussed.

Material selection considerations for coaxial, ferrimagnetic-based nonlinear transmission lines

The growing need for solid-state high power microwave sources has renewed interest in nonlinear transmission lines (NLTLs). This article focuses specifically on ferrimagnetic-based NLTLs in a coaxial geometry. Achieved peak powers exceed 30 MW at 30 kV incident voltage with rf power reaching 4.8 MW peak and pulse lengths ranging from 1–5 ns. The presented NLTL operates in S-band with the capability to tune the center frequency of oscillation over the entire 2–4 GHz band and bandwidths of approximately 30%, placing the NLTL into the ultra-wideband–mesoband category of microwave sources. Several nonlinear materials were tested and the relationship between NLTL performance and material parameters is discussed. In particular, the importance of the materials ferromagnetic resonance linewidth and its relationship to microwave generation is highlighted. For a specific nonlinear material, it is shown that an optimum relation between incident pulse magnitude and static bias magnitude exists. By varying the nonlinear materials bias magnetic field, active delay control was demonstrated.

Bias-field controlled phasing and power combination of gyromagnetic nonlinear transmission lines

Gyromagnetic Nonlinear Transmission Lines (NLTLs) generate microwaves through the damped gyromagnetic precession of the magnetic moments in ferrimagnetic material, and are thus utilized as compact, solid-state, frequency agile, high power microwave (HPM) sources. The output frequency of a NLTL can be adjusted by control of the externally applied bias field and incident voltage pulse without physical alteration to the structure of the device. This property provides a frequency tuning capability not seen in many conventional e-beam based HPM sources. The NLTLs developed and tested are mesoband sources capable of generating MW power levels in the L, S, and C bands of the microwave spectrum. For an individual NLTL the output power at a given frequency is determined by several factors including the intrinsic properties of the ferrimagnetic material and the transmission line structure. Hence, if higher power levels are to be achieved, it is necessary to combine the outputs of multiple NLTLs. This can be accomplished in free space using antennas or in a transmission line via a power combiner. Using a bias-field controlled delay, a transient, high voltage, coaxial, three port, power combiner was designed and tested. Experimental results are compared with the results of a transient COMSOL simulation to evaluate combiner performance.

Ferrimagnetic Nonlinear Transmission Lines as High-Power Microwave Sources

Ferrimagnetic nonlinear transmission lines (NLTLs) have the potential to fill a high-power microwave niche where compact cost-effective sources are lacking. NLTLs utilize nonlinear ferrimagnetic properties and magnetization dynamics to provide ultrafast pulse rise times (100 ps or less) and microwave signals with peak power ranging from kilowatts to hundreds of megawatts. The frequency of operation has been shown to range from 900 MHz up to 5 GHz depending on geometry and external magnetic fields. NLTLs, theoretically, can be pulsed to tens of kilohertz with little to no variance in microwave signal between shots. This paper covers recent advances in ferrimagnetic-based NLTLs, specifically effects of applied and bias magnetic fields on peak power and frequency, as well as temperature dependence.

Investigation Into the Temperature Dependence of Ferrimagnetic Nonlinear Transmission Lines

In pulsed power systems, coaxial based nonlinear transmission lines (NLTLs) loaded with ferrimagnetic materials act as pulse sharpeners or high power microwave sources. Microwave generation comes by way of nonlinearities present in the ferrimagnetic material as well as excitation of damped gyromagnetic precession at large incident power levels. Ferrimagnetic properties highly depend on operating temperature; therefore, there exists a need to understand operational performance of ferrite loaded NLTLs under different temperature environments. Ferrite samples are chilled or heated to temperatures between -20°C to 150 °C, providing a wide range of possible operating temperatures. The Curie temperature of the tested samples is approximately 120 °C; therefore, this study allows observation of precession performance in possible operating temperatures as well as a brief look at the consequences of exceeding the Curie temperature. The design, testing, and results for an NLTL measuring 0.3 m in length with ferrite inner and outer diameters of 3 mm and 6 mm, respectively, are detailed. Results reveal precessional performance, both peak power and frequency of oscillations, versus temperature.

Magnetic biasing of ferrite filled nonlinear transmission lines

Ferrite filled coaxial lines have been shown to sharpen input pulses due to their nonlinear magnetic properties. This effect can be amplified by applying an axial magnetic biasing field. In addition to a steeper leading edge, oscillations in the microwave bands have been observed. These oscillations arise from damped gyromagnetic precession occurring in the ferrite. The magnetic bias necessary to generate damped oscillations and steepened pulse sharpening effects is produced by two separate means; a current carrying solenoid or, the less investigated case, permanent magnets. Multiple NLTLs are tested at lengths of 0.3 meters and 1 meter with an outer diameter of 13 mm. Permanent magnets with varying strengths were placed in different locations for multiple tests. Experiments and setups for these designs are detailed and their results are discussed.

Serial arrangement of ferrimagnetic nonlinear transmission lines

Nonlinear transmission lines (NLTLs) utilizing ferrimagnetic materials for microwave generation have been realized as a possible solid-state replacement to traditional high power microwave (HPM) sources. The nonlinearities present in the material, along with interaction between pulsed, azimuthal magnetic fields and static, axial-biasing magnetic fields provide microwave (mesoband) generation with peak powers exceeding 30 MW at 2-4 GHz center frequency with 25 kV incident pulse magnitude. Additionally, an incident pulse of several nanoseconds is sharpened to hundreds of picoseconds. This study focuses on a serial arrangement of two NLTLs with 5 ns electrical length separation. Tests with 25 kV incident voltage are performed with varying bias schemes for each NLTL structure. The lines are terminated into a 50 Ω matched load. Measurements taken before and after each NLTL provide insight to the behavior of the traveling pulse. Results regarding peak power, frequency of operation, and system delay are discussed.

Rep-rate operation of a 300 kV, high-power microwave sealed-tube vircator

Thermal limitations of anode materials are known to impose limits on rep-rate operation of cold-cathode high-power microwave (HPM) sources. This study focuses on performance of pyrolytic graphite (PG) anodes at a 500 Hz burst-mode operation in a reflex-triode virtual-cathode-oscillator (vircator). In most experiments, a 42 J, 300 kV pulse forming network (PFN) based Marx generator with an approximate pulse width of 50 ns full-width-half-max (FWHM), was utilized to drive the vircator. Rep-rated operation of the vircator exacerbates the problems already experienced in single-pulsed mode where vircators are plagued by plasma formation on the anode and cathode followed by plasma/gas expansion that causes degradation of anode materials. Hence, for frequency-stable, repetitive operation, vircators require the use of thermally robust electrode materials and ultra-clean vacuum surfaces, leading to repeatable diode operation. This contribution presents thermal modeling of anode heating and experimental electrical behavior of vircator rep-rate operation.

Operation of a 500 kV, 4 kA Marx generator at 500 Hz rep-rate

A 42 J, 10-stage pulse forming network (PFN) Marx generator capable of producing a 500 kV, 50 ns full-width-half-max (FWHM), ~5 ns rise time pulse into an open load at a rep-rate of 500 Hz has been designed for use as a pulsed power source for a reflex triode virtual cathode oscillator (vircator). Rayleigh PFNs are used in place of discrete capacitors for each stage of the 10-stage Marx generator. Effort was taken to minimize parasitic inductance such that the quality of the pulse shape is maintained as much as possible. In order to rep-rate the Marx generator, a trigatron-based triggering scheme is used to initiate erection of the Marx generator. A 20 ns risetime, 24 kV solid-state pulse trigger generator capable of operating at high repetition rates is used to drive the trigatron. The required charge rate for a 500 Hz pulse repetition frequency (PRF) for the Marx generator is 24 kW. Repetitive operation requires additional design considerations that would be irrelevant to single pulse firing. Pressurized air is jetted across the spark gaps by means of built-in gas manifolds to remove remaining ionized gas between each pulse and prevent premature erection during the subsequent charging cycle. The built-in gas manifolds were designed using a hydrodynamic simulation to ensure equal flow rate across each of the spark gaps and equal pressure along the length of the tube chamber.

REP-rate operation of a ∼200 KV sealed-tube reflex-triode vircator at ∼200 A/cm 2

Summary form only given. Thermal limitations of anode and cathode materials have shown to negatively impact operation of cold-cathode high-power microwave (HPM) sources. High pulse-repetition-frequency (PRF) operation of these devices exacerbates the problems already experienced in single shot mode where cold-cathode devices, specifically carbon fiber cathodes, are plagued by plasma formation on the anode and cathode followed by plasma/gas expansion that causes impedance collapse of the anode-cathode (A-K) gap. Hence, for frequency stable, repetitive operation, cold-cathode HPM devices require the use of thermally robust electrode materials and ultra-clean surfaces, leading to repeatable tube operation. This study focuses on burst-mode operation of an HPM sealed tube reflex-triode virtual-cathode-oscillator (vircator) for PRFs greater than 100 Hz. The vircator is driven by a 54 J, ~200 kV Marx generator with an approximate pulse width of 50 ns FWHM, and the vircator chamber has an empty volume of approximately 5 L with background pressures in the low 10-9 Torr. The anode materials studied include grade-1 titanium (TiG1), nickel 201L (Ni201L), and stainless steel 316L (SS316L); all in combination with a carbon fiber cathode. Empirically observed outgassing characteristics in conjunction with anode thermal modeling are presented under single-shot and rep-rate conditions. In addition, scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) techniques were employed to investigate anode and cathode surface integrities before and after vircator operation.