D. H. Barnett

Optically isolated, 2 kHz repetition rate, 4 kV solid-state pulse trigger generator

This paper presents the design and operation characteristics of a solid-state high voltage pulse generator. Its primary utilization is aimed at triggering a gaseous spark gap with high repeatability. Specifically, the trigger generator is designed to achieve a risetime on the order of 0.1 kV/ns to trigger the first stage, trigatron spark gap of a 10-stage, 500 kV Marx generator. The major design components are comprised of a 60 W constant current DC-DC converter for high voltage charging, a single 4 kV thyristor, a step-up pulse transformer, and magnetic switch for pulse steepening. A risetime of <30 ns and pulse magnitude of 4 kV is achieved matching the simulated performance of the design.

Material selection of a ferrimagnetic loaded coaxial delay line for phasing gyromagnetic nonlinear transmission lines

Implementing nonlinear transmission line (NLTL) technology in the design of a high power microwave source has the benefits of producing a comparatively small and lightweight solid-state system where the emission frequency is easily tuned. Usually, smaller in physical size, single NLTLs may produce significantly less power than its vacuum based counterparts. However, combining individual NLTL outputs electrically or in free-space is an attractive solution to achieve greater output power. This paper discusses a method for aligning a four element NLTL antenna array with coaxial geometry using easily adjustable temporal delay lines. These delay lines, sometimes referred to as pulse shock lines or pulse sharpening lines, are placed serially in front of the main NLTL line. The propagation velocity in each delay line is set by the voltage amplitude of an incident pulse as well as the magnetic field bias. Each is adjustable although for the system described in this paper, the voltage is held constant while the bias is changed through applying an external DC magnetic field of varying magnitude. Three different ferrimagnetic materials are placed in the temporal delay line to evaluate which yields the greatest range of electrical delay with the least amount of variability from consecutive shots.

Frequency tuning a reflex triode vircator from 1.5 to 5.9 GHz

Vircators (virtual cathode oscillators) are easily frequency tunable by simply altering the electron beam current density through either adjusting the accelerating voltage or varying the anode cathode gap (A-K gap). It is noted that vircators typically exhibit frequency hopping, an often undesired feature. Here we address the frequency hopping problem by introducing a rectangular shaped carbon fiber cathode with the width of the cathode less than one quarter of a wavelength in the direction of microwave propagation. Operating the vircator with the rectangular cathode revealed a match with a resonant condition from back wall reflections, which explains the observed dominant microwave frequency selection. That is, the vircator emitted stably during 50 ns operation at 3.89 GHz or 2.07 GHz with a 3 dB bandwidth of 37.5 MHz or 93.7 MHz, at A-K gap spacing of 8 mm, and 12 mm respectively. To further investigate frequency tuning, the A-K gap was swept from 3 mm to 17 mm while keeping a constant 5.1 cm diameter circular cathode. The resulting frequencies of operation were 1.5 GHz, 2.1 GHz, 4.1 GHz, 4.6 GHz, and 5.9 GHz at A-K gap spacing of 17mm, 14 mm, 7 mm, 5.75 mm, and 3 mm respectively. This demonstrates that a tuning range of 4.4 GHz should be achievable for a practical vircator system.

Repetitive operation of a megawatt class reflex-triode vircator

Repetitive operation of high-power microwave (HPM) sources is primarily restricted by thermal properties of anode and cathode materials. Note that even in single shot operation, vircators generally inject undesired plasma from the anode/cathode surfaces under high current densities, leading to excessive heating of electrode materials, vacuum degradation and impedance collapse that may ultimately lead to cut-off of microwave power. Heating of the anode and cathode materials during repetitive operation intensifies these problems and will cause accelerated erosion and/or permanent damage of the anode and cathode surfaces, especially when metallic electrodes are used. Hence, the vircator herein utilizes a carbon fiber cathode and a pyrolytic graphite anode. This paper elucidates the heating of the anode and cathode, and their spectrally resolved black-body radiation properties during repetitive operation.

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.

Compact reflex triode with multi cavity adjustment

Summary form only given. This study focuses on the achieving wide tunabiltiy of a compact reflex triode virtual cathode oscillator (vircator). The cathode is of a bimodal carbon fiber (CF) material paired with a pyrolytic graphite anode. These materials where chosen for their operating characteristics which including but not limited to, long lifetime > 106 shots, high operating temperatures > 1000 K, and large current densities ~200 A/cm3. A 12 stage, 158 J pulse forming network (PFN) based Marx generator serves to drive the Vircator at 350 kV, 4 kA with ~100 ns pulsewidth. The operating frequency of interest is in the range of 1-6 GHz, where the tunability is achieved by varying the size of the anode-cathode (A-K) gap, the length from the back wall to the A-K gap, or/and the distance from the bottom of the cavity to the A-K gap. The primary focus in this experiment was to increase the achievable frequencies by placing a square waveguide within a sealed vacuum tube. This allows the bottom part of the waveguide to be easily adjusted while still maintaining the waveguide integrity. The resulting microwave frequencies are shown along with the systems performance.

Reflex triode vircator design for extended frequency tuning capabilities

A stand-alone vircator based high power microwave system has been designed as an effects test source. The goal of this system is to cover the frequencies from 4.0 GHz to 6.0 GHz. To date a working prototype has been manufactured and field tested. The current system is capable of producing microwaves at frequencies of 4.06, 4.27, 4.45, 5.83, 5.95 and 6.06 GHz at field levels in excess of 100 kV/m, measured at 3 meters. Additionally, the source and all subsystems are capable of burst mode operation for a duration of 2 s at 500 Hz pulse repetition frequency (PRF). The wide range of tunability was achieved by varying the A-K gap as well as the location of the A-K gap inside the cavity. The A-K gap is varied by a bellows sealed linear actuator which is attached to the cathode. The position of the A-K gap within the cavity is altered by moving a liner and microwave reflector (which form a cavity) within the main vacuum tube. However, the experimentally observed frequencies obtained with the current system leaves a gap from 4.45 to 5.6 GHz. In order for the test system to produce microwaves within this frequency range, a new tube is under development. The cavity of the new tube has a smaller cavity which should push the resonant frequencies into the desired range (from 4.45 GHz to 5.6 GHz). This presentation discusses the experimental results obtained with the new tube and compares that with the previously built and tested source.

A 160 J, 100 Hz rep rate, compact Marx generator for driving and HPM source

This paper presents the electrical and mechanical design considerations of a compact, 160 J modular pulse forming network (PFN) based Marx generator used to drive a high-power microwave (HPM) source with a time variant load at a PRF of 100 Hz. The modular Marx generator is designed to produce an open circuit output voltage of 600 kV from a 50 kV source using twelve stages. Each stage of the Marx was constructed from a PFN created with five, 2.1 nF, high voltage capacitors in parallel. Each Marx module was machined out of acetyl copolymer or Delrin to provide optimal strength, rigidity, and a dielectric constant that closely matches transformer oil. These Marx modules include air supply lines that are machined directly into each block of Delrin allowing airlines to connect to each module chamber rather than every spark gap. The spark gaps are comprised of two electrode inserts placed into the sealed pressure vessel contained within the Marx modules. The electrode inserts are of a sleeve-electrode design, which allows the user to insert the electrode into the sleeve, then the sleeve into the Marx module. The benefit of this design is the ability to adjust the electrode gap spacing without compromising the high pressure seal. Two continuous charging inductors run between each PFN and underneath the Marx modules. Due to high voltages generated by the Marx, outer field shaping rings are used to reduce the field stress across the induction coils, resulting in longer lifetime. The inductors are also of modular design allowing for individual coils to be replaced in the event of failure. Output voltage and current waveforms from a 60 Ohm water load are presented.

Initial testing of a reflex triode vircator with adjustable A-K gap and cavity reflector

Virtual cathode oscillators (vircators) can be easily tuned by altering the current density of the electron beam. This can be accomplished by changing the voltage applied to the a-k gap, or by physically changing the gap separation. Previous results with an adjustable a-k gap vircator achieved discrete tuning at frequencies of 1.5 GHz, 2.1 GHz, 4.1 GHz, 4.6 GHz, and 5.9 GHz. To achieve better tunability an adjustable cavity reflector was incorporated into the vircator design. Initial testing of the new vircator design has shown increased output power, as well as the ability to greatly increase the number of frequencies that can be tuned. Sweeping the A-K gap with the cavity reflector in two positions yielded 10 different frequencies of operation. Also sweeping the cavity reflector with the a-k gap set at 8.0mm yielded another unique frequency. Currently 11 unique frequencies have been identified, and the peak field measured from this vircator increased from ~40 kV/m to ~69 kV/m (> 70% increase in field). This paper illustrates the design of the vircator and presents some of the microwave data obtained. Additionally, a summary of all of the frequencies obtained and the peak field measured are included.

1 kHz rep-rate operation of a spark-gap switched gyromagnetic nonlinear transmission line array

This paper details the design, fabrication and performance of a coaxial ferrimagnetic nonlinear transmission line (NLTL), four element array, high power microwave (HPM) system operated at a 1 kHz repetition rate. Prime power is delivered from an 802L TDK Lambda power supply which charges a 5.2 nF capacitor bank up to -40 kV. The capacitors are discharged through a center pin trigatron spark gap. The trigger generator is optically isolated and battery powered for noise immunity and portability. It produces a 20 kV positive polarity pulse with a 20 ns risetime. The high dV/dt (1 kV/ns) is desirable to reduce jitter inherent to spark-gap switching. After the spark-gap switch has closed, the pulse is split four ways. The four pulses propagate through four adjustable delay lines for synchronization of the individual outputs. The four delay lines connect directly into four 76 cm NLTLs with NiZn ferrites where SF6 is the insulating dielectric. Each NLTL is terminated into a custom fabricated, Rexolite-filled, TEM horn antenna via a zipper balun. Lastly, a LabVIEW based control system automates the whole system using a National Instruments cRIO controller. Experimental observations will include in-line D-dot measurements of voltage waveforms and radiated D-dot field measurements.