Xun Xiang

Reduced breakdown delay in high power microwave dielectric window discharges

Summary form only given. Development of high power microwave (HPM) distributed discharge limiters relies critically on minimizing the delay time between HPM incidence and diffuse plasma creation. We present a range of pulsed plasma experiments conducted in neon, argon, helium, and mixtures of these gases, from 50-760 torr. Breakdown is achieved by illuminating a gas cell with a ~25kW, ~2 kV/cm, 800ns-long pulse as well as 41Hz pulse trains. Current results focus on preliminary experiments with metamaterial window coatings that indicate significant improvement opportunities for controlling breakdown thresholds and reducing breakdown delay. New results with gas mixtures in which observed breakdown occurs in <;100ns are also presented.

Observations of Memory Effects and Reduced Breakdown Delay via Penning Gas Mixtures in High-Power Microwave Dielectric Window Discharges

Recent improvements in high-power micro-wave (HPM) source power and portability make protecting sensitive electronics from electronic attack critically important. The research reported in this paper examined basic phenomena associated with a distributed area, highly attenuating gas discharge for HPM attack protection. In particular, the research examined gas breakdown delay since effective protection against HPM attack requires rapid activation, significantly faster than the hundreds of nanoseconds typical of HPM pulses. These studies, conducted in mixtures of neon, argon, helium, and xenon gases from 50 to 150 torr, demonstrate how polycarbonate window precharging metastable-excited atoms and appropriate gas composition enable Penning effects to significantly reduce breakdown delay.

Diagnostics of fast formation of distributed plasma discharges using X-band microwaves

We present measurements of high power (25.7 kW), pulsed (800 ns), X-band (9.382 GHz) microwave breakdown plasmas, including reflected power measurements, mixer reflected amplitude and phase measurements, optical emission spectroscopy (OES) measurements, and an analysis that estimates the average electron density and electron temperature. In addition, a six-region, 1-D model was used to determine plasma parameters and compare with the experimental results. The experimental results show that using a 43 Hz repetition rate with an 800 ns pulse, fast (<300 ns) breakdown occurs in neon measured between 50 Torr and 250 Torr, producing plasma that lasts for over 7 μs. It also leads to large microwave reflections (70%) and an on-axis transmission attenuation of −15 dB. Moreover, a comparison between a 1-D model and mixer measurements shows that at 100 Torr, the neon plasma electron density peaked at 2 × 1012 cm−3, and the electron temperature peaked at 2.5 eV assuming a Maxwellian distribution. The addition of 2% ...

Metamaterials for Rapidly Forming Large-Area Distributed Plasma Discharges for High-Power Microwave Applications

Electromagnetic metamaterials have broad application potential including new high-power microwave (HPM) sources and anti-HPM devices. In the previous work, we demonstrated that an initial breakdown at one location within a multiresonator unit cell of the single-layer metamaterial emitted vacuum ultra-violet (VUV) radiation that induced breakdowns at the neighboring locations even though the electric field intensities were below the breakdown thresholds. In this paper, we report the results of experimental investigations of single-layer metamaterials deliberately designed to exploit this effect. When illuminated by 26-kW, 9.382-GHz, 0.8-μs intense microwave pulses, breakdown was initially induced only in one small location where the radio frequency (RF) electric fields exceeded the breakdown threshold. In experiments with the metamaterials, the initial breakdown locally occurred in 5-10 ns, after which the VUV preionization effect facilitated the rapid spread of the breakdown across the entire surface within 10-20 ns. In contrast, without a metamaterial, the breakdown remained localized and was delayed by 25-30 ns compared with the metamaterial cases. The experimental results are expected to provide a useful guideline for designing metamaterials in HPM systems.

Investigating the impact metamaterials have on breakdown delay in plasma formation in high power microwave experiments

The efficacy of protecting electronics from high power microwaves (HPM) through plasma creation depends on how quickly the plasma forms in the protective gas chamber when illuminated with microwave radiation. A cylindrical chamber with polycarbonate windows was filled with a neon, krypton gas mixture and illuminated with a train of ~25kW, 800ns long pulses at 43Hz repetition rate as a simulation of the HPM attack scenario. In order to facilitate the formation of a plasma, a set of metamaterial, stainless steel masks for the incident polycarbonate window were created to increase the effective electric field. In addition to the masks, we investigated the effect chamber pressure had on the breakdown delay.

Rapid X-band microwave breakdown in Ne

Observations of rapidly formed (<;50-400 ns) distributed plasma discharges using X-band microwaves in Neon with 1 mTorr residual air are presented. A stainless steel cylindrical discharge test chamber is used to observe microwave breakdown at 10 to 760 torr pressures. The chamber is enclosed with polycarbonate windows on both ends and has two side ports. The magnetron illuminates the chamber using 25 kW, 9.382 GHz and 0.8 μs pulse-width power through an X-band waveguide pressed against the polycarbonate window. Microwave diodes are used to measure incident, reflected, and transmitted microwave power to a moveable monopole antenna located beyond the discharge chamber. They provide information to determine the discharge reflection and attenuation characteristics as the pressure is varied. Observations of localized transmission power reduction measurements of -20 dB that occur within 50-400 ns caused by the plasma under different conditions have been made. Optical emission spectra experiments allow one to determine the gas temperature of the plasma at different pressures. Microwave mixers are used to compare both the amplitude and phase of the reflected signals in phase and in quadrature (90 degrees) relative to a fixed phase reference signal. Together with a six region 1-D plasma modeling code, the effective plasma density, collision frequency and electron temperature are estimated. An ICCD provides fast (<;50 ns) time-scale optical images to estimate the plasma size, also revealing the plasma formation and decay processes.

Characterization of breakdown delay and memory effects in high power microwave dielectric window discharges

Summary form only given. Development of high power microwave (HPM) distributed discharge limiters relies critically on minimizing the delay time between HPM incidence and diffuse plasma creation. We present a series of pulsed plasma experiments conducted in neon, argon, and a 9∶1 mixture of the two gases from 50–760 torr. Breakdown is achieved by illuminating a gas cell with a train of ∼25kW, ∼2 kV/cm, 800ns-long pulses at 41Hz repetition rate. The results suggest that surface charge accumulation on the gas cells polycarbonate window from many low density precursor discharges eventually results in rapid (<30ns), high density, discharge formation. The number of precursor pulses required to form these high density discharges is significantly reduced following a previous high-density discharge.

Rapid formation of distributed plasma discharges using X-band microwaves

Summary form only given. Observations of rapidly formed (<;50-300 ns) distributed plasma discharges using X-band microwaves in neon with 1 mTorr residual air are presented. A stainless steel cylindrical discharge test chamber, which is enclosed with polycarbonate windows on both ends, is used to observe microwave breakdown in neon gas from 10 to 760 torr. The chamber is illuminated by the output of 25 kW, 0.8 μs pulse-width, 9.382 GHz magnetron through an X-band waveguide pressed against the polycarbonate window. Measured incident, reflected, and transmitted microwave power to a moveable monopole antenna located beyond the discharge chamber are used to detect the discharge and attenuation characteristics as the pressure is varied. Observations of localized transmission power reduction measurements of -20 dB that occur within 50-400 ns caused by the plasma under different conditions have been made. Additionally, an ICCD provides fast (<;50 ns) time-scale optical images of the plasma size, revealing the plasma formation and decay processes. An optical emission spectrum experiment where a small amount of nitrogen is added allows one to determine the gas temperature of the Ne plasma. Mixers are used to compare both the amplitude and phase of the reflected signals before and after 90 degrees shift. Together with a plasma modeling code, plasma parameters such as plasma density, collision frequency and electron temperature are estimated.