Matt Kirley

Study of the effect of surface roughness and skin depth on the conductivity of metals at 650 GHz

A high-quality-factor quasi-optical resonator operating at 650 GHz is used to measure the conductivity of mechanically roughened metal surfaces. The results explore the effective conductivity of metals in the terahertz regime when the surface roughness is on the order of the skin depth.

Examination of field emission from Lanthanum Hexaboride coated knife edge cathodes

We report experiments and analysis of field emission from metallic knife-edge cathodes coated with thin film Lanthanum Hexaboride (LaB6), a low work function material (∼2.5 eV). The emission current density is found to depend sensitively on film properties, particularly the thickness of the LaB6 layer. Films thinner than approximately 10 nm greatly enhance the emitted current. However, cathodes coated with a thicker layer of LaB6 are observed to emit less current than the uncoated metallic cathode even though the bare metal cathode possesses a higher work function (> 4 eV). A hypothesis is proposed to explain this surprising experimental finding, and a computational model is developed. This simulation model incorporates field emission as well as solid state electron transport from the metal substrate through the LaB6 thin film, and is found to support the proposed hypothesis and agree with experimental observations.

11.4: Examination of field emission from copper knife edge cathodes with low-work function coatings

We report experiments and analysis of field emission from copper knife-edge (CKE) cathodes, both bare and coated with low work function (∼2.5 eV) Lanthanum hexaboride (LaB6) thin films. The bare CKE cathode exhibits evidence of space charge limited emission currents at high field strengths. The LaB6 coated cathodes exhibit a nonlinear Fowler-Nordhiem (FN) type emission. An intermediate saturation region is observed from field emission data, which is more prominent at elevated (185 C) temperature. Surprisingly, the LaB6 coated cathodes are observed to emit less current than the higher work function (> 4 eV) bare CKE cathode. A hypothesis and corresponding model including both field emission and solid state electron transport from the Cu substrate, through the LaB6 thin film, is proposed to explain the experimental observations.

P4-10: Distributed discharge limiter studies for X-band high power microwaves

The design, fabrication, initial measurements and analysis of an X-band high power microwave (HPM) limiter are presented. The microwave discharge test chamber is an L-band rectangular waveguide with Lexan or Rexolite microwave windows. The chamber is illuminated by the output of an X-band waveguide pressed against the chamber window. The objective is to study conditions and configurations that enable rapid (< 50 ns) discharge formation above a pre-set power density threshold. A 25 kW X-band magnetron (9.38 GHz) with a 0.8 µs pulse width is used to produce the breakdown. Incident, reflected and transmitted microwave powers and optical emission intensity are measured to observe the discharge breakdown and extinction rates. Pressures used are in the 10s of Torr range, near Paschen minimum pressures for Ar and Ne/Ar mixtures‥ Design modeling, preliminary experimental data and data analyses are presented.

The physics of conductivity at terahertz frequencies

Effective terahertz (THz) component and systems design requires accurate predictive models for the frequency-dependent conductivity of materials. We have measured the THz conductivity of metals (including rough surface effects) and doped silicon at 0.4 - 0.85 THz. After a comprehensive analysis, we conclude that the THz-regime conductivity of highly-doped Si is accurately described by Drude theory, while metal conductivity is accurately described by a combination of measured DC conductivity, Drude theory, and the Hammerstad-Bekkadal formula.

Increased surface resistance of rough copper surfaces in the terahertz regime

Previous study on the surface resistivity of copper surfaces measured at 400 and 650 GHz has been extended to 850 GHz. These measurements were conducted using a high quality factor quasi-optical hemispherical resonator. The measurements of the surface resistance of rough metallic surfaces and the effect of roughness on reflectivity will be discussed. The results will be compared with measurements from theoretical predictions.

Electromagnetic attenuation due to water vapor measured at 400 GHz

We present experimentally measured electromagnetic attenuation losses due to water vapor at 400 GHz. The measurements are made using a hermetically sealed high-Q quasi-optical resonator system, which allows for control of water vapor levels. We compare our measurements to attenuation predictions from two versions of the Millimeter-wave Propagation Model. We find that one model has predictions in close agreement with the data while the other model differs significantly.

Terahertz conductivity of rough metallic surfaces

Summary form only given. The terahertz (THz) regime, commonly designated as the frequency region between 300 GHz and 3 THz, is relatively underutilized compared to other areas of the electromagnetic spectrum. However, THz radiation has many potential applications in molecular spectroscopy, communications, medical imaging, and security imaging1.

Field emission from low-work function cathode coatings

We report experiments and analysis of field emission from copper knife-edge (CKE) cathodes, both bare and coated with low work function (2.6 eV) Lanthanum hexaboride (LaB6) thin films. The bare CKE cathode exhibits evidence of space charge limited emission currents at high field strengths. The LaB6 coated cathodes exhibit a nonlinear Fowler-Nordhiem (FN) type emission. An intermediate saturation region is observed from field emission data, which is more prominent at elevated (185 C) temperature. Surprisingly, the LaB6 coated cathodes are observed to emit less current than the higher work function (> 4 eV) bare CKE cathode. A hypothesis and corresponding model including both field emission and solid state electron transport from the Cu substrate, through the LaB6 thin film, is proposed to explain the experimental observations.

Breakdown lemiter studies for high power X-band microwaves

The design, fabrication and initial measurements and analysis of an X-band high power microwave (HPM) limiter are presented. The microwave discharge test chamber is an L-band rectangular waveguide with Lexan or Rexolite microwave windows. The chamber is illuminated by the output of an X-band waveguide pressed against the chamber window. The objective is to study conditions and configurations that enable rapid (< 50 ns) discharge formation above a pre-set power density threshold. A 25 kW X-band magnetron (9.38 GHz) with a 0. 8 |is pulse width is used to produce the breakdown. Incident, reflected and transmitted microwave powers and optical emission intensities are measured to observe the discharge breakdown and extinction rates near Paschen minimum pressures for Ar and Ar/Ne mixtures in the 10s of Torr range. Design modeling, preliminary experimental data and data analyses are presented.