Chien-Hao Liu

High-power microwave filters and frequency selective surfaces exploiting electromagnetic wave tunneling through ϵ-negative layers

In this paper, we experimentally investigate the phenomenon of electromagnetic wave tunneling through ϵ-negative (ENG) metamaterial layers surrounded by double-positive layers. Initial experiments are conducted by using a rectangular waveguide, which operates below its cutoff frequency to emulate an ENG layer. This ENG layer is then sandwiched by two dielectric substrates with relatively high dielectric constants and it is shown that the entire setup acts as a classical microwave filter with a second-order bandpass response. The power handling capability of this filter is examined experimentally using a high-power magnetron source with a frequency of 9.382 GHz, a pulse duration of 1 μs, and a peak power of 25 kW. Based on the results of this experiment, two methods for improving the power handling capability of these multi-layer structures are proposed. In particular, it is demonstrated that emulating the ENG layers with thin perforated metallic sheets with sub-wavelength holes significantly enhances thei...

Tunneling and filtering characteristics of cascaded ɛ-negative metamaterial layers sandwiched by double-positive layers

In this paper, we examine the electromagnetic (EM) wave tunneling and filtering characteristics of multi-layer structures composed of an arbitrary number of ɛ-negative (Epsilon-negative or ENG) metamaterial layers sandwiched by very thin double-positive (DPS) layers with high dielectric constant values. We explain the phenomenon of EM wave tunneling through this propagation barrier by drawing an analogy between this problem and a generalized coupled resonator system. Using this analogy, we demonstrate that a multi-layer structure composed of N DPS layers (N ≥ 2 is an integer number) that sandwich N-1 ENG layers can not only be made transparent in a frequency range where the ENG layers are normally opaque but also be designed to provide a desired spectral filtering characteristics. Furthermore, we present an analytical method for synthesizing such multi-layer spectral filters from the characteristics of their desired responses. The proposed synthesis procedure can be used to develop spatial filters operati...

Investigating the Impact of Microwave Breakdown on the Responses of High-Power Microwave Metamaterials

We investigate the effect of microwave-induced breakdown on the frequency responses of a class of metamaterials composed of planar sub-wavelength periodic structures. When breakdown occurs in such a structure, its frequency response changes based on the nature of the plasma created within its unit cell. We examine how the frequency responses of such periodic structures change as a result of creation of microwave-induced discharges within their unit cells. To do this, we examine single-layer metasurfaces composed of miniature LC resonators arranged in a 2-D periodic lattice. These metasurfaces are engineered to be opaque at microwave frequencies when operated at low power levels but can be made transparent if a localized discharge is created within the LC resonators. By measuring their transmission and reflection coefficients under high-power excitation in different conditions, the impact of breakdown on the frequency responses of these devices is determined. Several prototypes of such structures are examined both theoretically and experimentally. It is demonstrated that when breakdown occurs in air and at atmospheric pressure levels, the responses of such periodic structures can be predicted with a reasonable degree of accuracy. Additionally, when the unit cell of the metasurface is composed of two different resonators, breakdown is always observed to occur in both resonators despite their different topologies and local field enhancement factors. In such structures, the discharge in one resonator appears to be mediated by the one in the other.

Investigating the Physics of Simultaneous Breakdown Events in High-Power-Microwave (HPM) Metamaterials With Multiresonant Unit Cells and Discrete Nonlinear Responses

Electromagnetic metamaterials offer a significant potential to enable new capabilities in many applications. Under high-power illumination, metamaterials and periodic structures experience internal breakdown, altering frequency response, and/or yielding thermal damage. Our prior research observed simultaneous breakdown discharges at two separate sites within a multiresonator metamaterial unit cell, even though the electric field intensities at one of the resonator sites should have been well below the threshold intensity required for breakdown. Here, we investigate three candidate mechanisms for the simultaneous breakdown discharges: energetic electrons, ultraviolet (UV) radiation, and vacuum UV (VUV) radiation. Experiments inserting different dielectric barriers between the two resonators of a multiresonator unit cell were able to selectively isolate the coupling influence of the candidate mechanisms. It was established that, VUV radiation from the discharge at the resonator with a lower electric field breakdown threshold causes simultaneous breakdown at the other resonator where the field intensities are otherwise too low to induce breakdown.

Thermoelectric Powered Wireless Sensors for Dry-Cask Storage

This paper describes using thermoelectric generators to power wireless sensors to monitor spent nuclear fuel during dry-cask storage. OrigenArp was used to determine the decay heat of the spent fuel at different times during the service life of the dry-cask. The Engineering Equation Solver computer program modeled the temperatures inside the dry-cask during its service life. The temperature distribution of a thermoelectric generator and heat sink was calculated using the computer program Finite Element Heat Transfer. From these temperature distributions the power produced by the thermoelectric generator was determined as a function of the service life of the dry-cask. To investigate the ability to provide wireless signal transmission using this power source, CST Microwave Studio was used to determine the scattering parameter S2,1 for a horizontal dry-cask. Important parameters that can influence the transmission of the signal are antenna orientation, antenna placement, and transmission frequency. This analysis estimates that a thermoelectric generator can produce enough power for a sensor to function and transmit data from inside the dry-cask throughout its service life.

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.

Investigating the effective range of vacuum ultraviolet-mediated breakdown in high-power microwave metamaterials

Metamaterials and periodic structures operating under high-power excitations are susceptible to breakdown. It was recently demonstrated that a localized breakdown created in a given region of a periodic structure can facilitate breakdown in other regions of the structure where the intensity of the incident electromagnetic fields may not be high enough to cause breakdown under normal circumstances. It was also demonstrated that this phenomenon is due to the generation of vacuum ultraviolet radiation at the location of the initial discharge, which propagates to the neighboring regions (e.g., other unit cells in a periodic structure) and facilitates the generation of a discharge at a lower incident power level. In this paper, we present the results of an experimental study conducted to determine the effective range of this physical phenomenon for periodic structures that operate in air and in pure nitrogen gas at atmospheric pressure levels. It is demonstrated that when breakdown is induced in a periodic structure using a high-power pulse with a frequency of 9.382 GHz, duration of 0.8 μs, and peak power level of 25 kW, this phenomenon is highly likely to happen in radii of approximately 16–17 mm from the location of the initial discharge under these test conditions. The results of this study are significant in designing metamaterials and periodic structures for high-power microwave applications as they suggest that a localized discharge created in such a periodic structure with a periodicity less than 16–17 mm can spread over a large surface and result in a distributed discharge.

Stretchable spring electrodes for cylindrical dielectric elastomer actuators

Recently, the developments of dielectric elastomer actuators (DEAs) increase rapidly for providing large deformations in micro-scale and bio-compatibility. They are widely used in replacing motors in various areas such as micro-robots and artificial muscles. Most DEAs are composed of stretchable materials sandwiched by inextensible compliant electrodes on both sides and convert input electric energy to mechanical deformation. The main challenge of DEAs is that compliant electrodes do not co-extend with sandwiched material and tend to break during formation resulting in low repeatability and difficulty of manufacturing. In this research, we propose spring electrodes inspired by their mechanical extensibility for designing cylindrical DEA. The proposed structure is composed of a hollow VHB cylinder wrapped by copper springs inside and outside and provide axial deformation under high bias voltages. A prototype was designed, fabricated and examined with laser measurements and a high-voltage source. The research results are expected to provide a new guideline for designing DEAs and benefit to biomedical applications.

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.