Jason S. Hummelt

Sub-wavelength waveguide loaded by a complementary electric metamaterial for vacuum electron devices

We report the electromagnetic properties of a waveguide loaded by complementary electric split ring resonators (CeSRRs) and the application of the waveguide in vacuum electronics. The S-parameters of the CeSRRs in free space are calculated using the HFSS code and are used to retrieve the effective permittivity and permeability in an effective medium theory. The dispersion relation of a waveguide loaded with the CeSRRs is calculated by two approaches: by direct calculation with HFSS and by calculation with the effective medium theory; the results are in good agreement. An improved agreement is obtained using a fitting procedure for the permittivity tensor in the effective medium theory. The gain of a backward wave mode of the CeSRR-loaded waveguide interacting with an electron beam is calculated by two methods: by using the HFSS model and traveling wave tube theory; and by using a dispersion relation derived in the effective medium model. Results of the two methods are in very good agreement. The proposed all-metal structure may be useful in miniaturized vacuum electron devices.

Design of a Metamaterial-Based Backward-Wave Oscillator

In this paper, we present the design of a microwave generator using metamaterials (MTMs) in a negative index waveguide interacting with a high-power electron beam. The microwave structure is formed by inserting two MTM plates loaded with complementary split-ring-resonators (CSRRs) into a rectangular waveguide. Electromagnetic simulations using the high-frequency structure simulator code confirm the presence of a negative index TM-like mode suitable for use in a backward-wave oscillator (BWO). Particle-in-cell (PIC) simulations using the computer simulation technology (CST) Particle Studio code are performed to evaluate the efficiency of an S-Band MTM-based BWO (MTMBWO) excited by a 500 keV, 80-A electron beam. After about 250 ns, the MTMBWO reaches a saturated output power of 5.75 MW with an efficiency of 14% at a frequency near 2.6 GHz. The MTMBWO is also modeled by representing the MTM plates, which consist of CSRRs, as dielectric slabs whose effective permittivity is given by a Lorentzian model. The dielectric slab model is also simulated with the CST PIC code and shows good qualitative agreement with the simulations including the CSRR loaded plates. A cold test structure was fabricated from brass to test the theoretical predictions of the microwave transmission versus frequency of the negative index waveguide. Test results using a vector network analyzer showed very good agreement with the simulations for the excitation of the negative index TM-like mode near 2.6 GHz. The proposed structure appears to be promising for use in a MTMBWO high-power microwave generator.

Observation of plasma array dynamics in 110 GHz millimeter-wave air breakdown

We present dynamical measurements of self-organizing arrays of plasma structures in air induced by a 110 GHz millimeter-wave beam with linear or circular polarization. The formation of the individual plasmas and the growth of the array pattern are studied using a fast-gated (5–10 ns) intensified camera. We measure the time-dependent speed at which the array pattern propagates in discrete steps toward the millimeter-wave source, observing a peak speed greater than 100 km/s. We observe the expansion of an initially spherical plasma into a disk or an elongated filament, depending on the polarization of the incident beam. The results show good agreement with one-dimensional ionization-diffusion theory and two-dimensional simulations.

Measurements of electron avalanche formation time in W-band microwave air breakdown

We present measurements of formation times of electron avalanche ionization discharges induced by a focused 110 GHz millimeter-wave beam in atmospheric air. Discharges take place in a free volume of gas, with no nearby surfaces or objects. When the incident field amplitude is near the breakdown threshold for pulsed conditions, measured formation times are ∼0.1–2 μs over the pressure range 5–700 Torr. Combined with electric field breakdown threshold measurements, the formation time data shows the agreement of 110 GHz air breakdown with the similarity laws of gas discharges.We present measurements of formation times of electron avalanche ionization discharges induced by a focused 110 GHz millimeter-wave beam in atmospheric air. Discharges take place in a free volume of gas, with no nearby surfaces or objects. When the incident field amplitude is near the breakdown threshold for pulsed conditions, measured formation times are ∼0.1–2 μs over the pressure range 5–700 Torr. Combined with electric field breakdown threshold measurements, the formation time data shows the agreement of 110 GHz air breakdown with the similarity laws of gas discharges.

Electron density and gas density measurements in a millimeter-wave discharge

Electron density and neutral gas density have been measured in a non-equilibrium air breakdown plasma using optical emission spectroscopy and two-dimensional laser interferometry, respectively. A plasma was created with a focused high frequency microwave beam in air. Experiments were run with 110 GHz and 124.5 GHz microwaves at powers up to 1.2 MW. Microwave pulses were 3 μs long at 110 GHz and 2.2 μs long at 124.5 GHz. Electron density was measured over a pressure range of 25 to 700 Torr as the input microwave power was varied. Electron density was found to be close to the critical density, where the collisional plasma frequency is equal to the microwave frequency, over the pressure range studied and to vary weakly with input power. Neutral gas density was measured over a pressure range from 150 to 750 Torr at power levels high above the threshold for initiating breakdown. The two-dimensional structure of the neutral gas density was resolved. Intense, localized heating was found to occur hundreds of nano...

Spectroscopic temperature measurements of air breakdown plasma using a 110 GHz megawatt gyrotron beam

Temperature measurements are presented of a non-equilibrium air breakdown plasma using optical emission spectroscopy. A plasma is created with a focused 110 GHz 3 μs pulse gyrotron beam in air that produces power fluxes exceeding 1 MW/cm2. Rotational and vibrational temperatures are spectroscopically measured over a pressure range of 1–100 Torr as the gyrotron power is varied above threshold. The temperature dependence on microwave field as well as pressure is examined. Rotational temperature measurements of the plasma reveal gas temperatures in the range of 300–500 K and vibrational temperatures in the range of 4200–6200 K. The vibrational and rotational temperatures increase slowly with increasing applied microwave field over the range of microwave fields investigated.

Millimeter wave scattering and diffraction in 110 GHz air breakdown plasma

We present measurements of the scattering, reflection, absorption, and transmission of a 1.5 MW, 110 GHz quasioptical gyrotron beam by a self-induced air breakdown plasma. The breakdown forms a periodic array of plasma filaments, oriented parallel to the incident electric field polarization that propagates toward the microwave source. For incident intensity of 3 MW/cm2, calorimetric measurements show that as much as 45% of the full beam power is absorbed by the plasma, averaged over the pulse, 1% is reflected backward, and the remainder is transmitted and also scattered into a wide angular spread. We observe that approximately 10 times more power is scattered in the direction perpendicular to the filaments than parallel. The far-field angular distribution of transmitted power exhibits a diffraction pattern that changes throughout the 2-μs life of the plasma.

1.5 MW, 110 GHz gyrotron breakdown in air

We present experimental measurements of breakdown in atmospheric pressure air created by a 3 μs pulse length, 1.5 MW, 110 GHz gyrotron producing a quasioptical beam. The linearly polarized focused beam creates a plasma that exhibits a filamentary array of streamers. We use a fast gating camera to measure the time evolution of the plasma filaments and their free space expansion velocity. Microwave measurements show that the scattering of incident microwaves by the plasma through an angular distribution agrees with what is predicted by diffraction of the microwave beam after being absorbed by the plasma. We use a fast gating, high-resolution spectrometer and a broadband spectrometer to measure breakdown plasma temperatures and compare with previous microwave breakdown experiments.

Design of a high power S-band metamaterial backward-wave oscillator

This paper presents the design of a novel backward-wave oscillator (BWO) that uses a negative index mode in a waveguide filled with a metamaterial (MTM). The MTMBWO is designed to operate at 2.6 GHz with electromagnetic simulations using the HFSS code. CST PIC simulations were performed of the device and predict a saturated output power of 5.75 MW using a 500 keV, 80 A electron beam (14% efficiency). In addition, a brass cold test structure of the proposed MTMBWO was built and tested with a vector network analyzer and showed very good agreement with simulation.

Experimental investigation of air breakdown utilizing a 1.5-MW, 110 GHz gyrotron

We present experimental results from air breakdown utilizing a 1.5 MW, 110 GHz 3 µs pulse length gyrotron beam in atmospheric pressure air. The beam is focused to a peak intensity of 5 MW/cm2 and the plasma formed is a two-dimensional array of filaments oriented along the electric field lines with spacing one quarter of the microwave wavelength (∼0.68 mm) that propagate back toward the microwave source.1 The effect of beam polarization on air breakdown structures is examined with a slow and fast gating camera. The periodic filament arrays that are repeatedly observed with the linearly polarized beam disappear when the gyrotron beam is given a circular polarization. This discovery fits with the explanation that array development arises from the result of diffraction of the beam on plasma filaments, and filament formation therefore requires the beam to have linear polarization. A fast gating, high-resolution spectrometer and a broadband spectrometer are used to study breakdown plasma temperature and electron density. Furthermore, diodes are used to measure power reflection and transmission through the plasma. Breakdown field/intensity threshold, power transmission/reflection, and plasma temperature and density measurements are all important in predicting the transmission of high-power millimeter-waves through atmospheric air at various altitudes.