Characterizing new electric field (E-field) sensors requires knowing the frequency limits of one’s radio-frequency (RF) chamber. A commonly used RF chamber is called a gigahertz transverse-electromagnetic (GTEM) cell that supports transverse-electromagnetic (TEM) fields through the gigahertz spectra. This manuscript proposes a new methodology for determining the range of usable frequencies within a GTEM cell having unknown characteristics (e.g., after moving a GTEM cell from one laboratory to another). Above a certain frequency, all frequencies become overmoded and yield unreliable measurements. We found that comparing international standards to measured data can provide insights into where overmoding occurs. The comparison to international standards was assisted by a derived free-space wave impedance formula in the frequency domain using both flush-plate dipole (D-dot) and magnetic field (B-dot) probes. Overall, this article proposes a new tool for determining the operating frequencies at which GTEM cells can reliably function in accordance with international standards before overmoding occurs.

Experimental results from a study investigating the inactivation of bioaerosols containing Bovine Coronavirus (BCov) under repetitively pulsed radio frequency (RF) electromagnetic exposure will be presented. These experiments were performed in a waveguide containing a flowing aerosol stream and were limited to the use of a single RF waveform: ~2 μs square envelope, 5.6 GHz, 4.8 kHz repetition rate. Aerosol streams were exposed to pulsed RF electric field amplitudes in the range of 41.9 +/- 6.2 kV/m. Compared to the results of the control (no-RF) experiments, RF waveform exposure results in a 74% reduction in mean survival rate of the aerosolized BCov. RF exposure was also demonstrated to have a substantial impact on the variance of the experimental results, with the RF exposure data showing an 800% increase over the control results. Experimental results will then be compared to those from an analytic electromagnetic-heating inactivation model for aerosolized pathogens.

A system capable of exposing a flowing aerosol stream to short duration (2-4 ns), high-power RF waveforms is described. The system utilizes a C-band gyromagnetic nonlinear transmission line source having peak power outputs ranging as high as 80 kW at a center frequency of 4.2 GHz. RF electric field magnitudes of up to 280 kV/m ± 17% are achieved within the aerosol flow region of the RF exposure apparatus.

A set of three apparatus enabling RF exposure of aerosolized pathogens at four chosen frequencies (2.8 GHz, 4.0 GHz, 5.6 GHz, and 7.5 GHz) has been designed, simulated, fabricated, and tested. Each apparatus was intended to operate at high power without leakage of RF into the local environment and to be compact enough to fit within biocontainment enclosures required for elevated biosafety levels. Predictions for the range of RF electric field exposure, represented by the complex electric field vector magnitude, that an aerosol stream would be expected to encounter while passing through the apparatus are calculated for each of the chosen operating frequencies.

A previous computational study of a relativistic magnetron with diffractive output at the University of New Mexico using the particle-in-cell code MAGIC achieved an efficiency of 70%. The research described in this paper aimed to achieve an experimental verification of 70% efficient operation of this design. Changes to the topology were necessary, such as the introduction of a Helmholtz-like magnetic coil pair and the use of a polished stainless steel endcap on the downstream end of the transparent cathode. These structures maintain a high degree of magnetic field uniformity in the anode-cathode gap interaction space and intercept leakage electrons that might otherwise impact the dielectric output window. The use of these structures stands in contrast to the modified magnetic fields used in the work of others to protect the window. An ethanol calorimeter and S-band waveguide detector were placed in the near-field to obtain radio frequency pulse shape and frequency and to infer peak power through a measurement of total energy radiated. A coaxial D-dot probe and self-integrating Rogowski coil were used to measure pulser voltage and total current, respectively, and showed that the tube operated at an efficiency of 63.5% until the endcap started emitting electrons, after which the efficiency was 40.4% in pi-mode. Images of radiated field pattern on a neon-bulb grid and of air breakdown outside of the 21-cm diameter output window were captured to confirm conversion of the pi-mode of operation to a TE31 circular radiated output mode.

Summary form only given. The Pulserad electron beam accelerator at the University of New Mexico (UNM) was refurbished and reconfigured to provide a voltage pulse with a very fast risetime, inspired by simulation results that indicated that the magnetron output characteristic was very sensitive to the voltage risetime. The modified Pulserad has a 2 ns risetime after which it maintains a flat-top for 30 ns and is capable of providing 350 kV into a matched load of 20 Ω. The main contributing factor for such a fast risetime is the low inductance oil breakdown switch located between the pulse forming line and the transmission line. Additionally, a low inductance transition region between oil and vacuum was designed to keep the degradation to the risetime minimal.

Summary form only given. The total efficiency of an HPM (High Power Microwave) system can be significantly increased by replacing the pulsed electromagnet system with a solenoidal permanent magnet. Additionally, the total size and weight of the system can be greatly reduced, thus providing a highly compact HPM source. We will test the feasibility of a permanent magnet on an A6 magnetron that uses a simple mode converter for axial extraction of microwaves. A solenoidal neodymium permanent magnet was simulated using FEMM, a finite element method magnetics code, to determine the ideal solenoid dimensions for a field uniformity of ~99% and a magnitude of 0.31 T [1]. The optimized design has dimensions of L=22.5 cm, R i =5.72 cm, and R o =14.3 and provides a field uniformity of 99.2% in the interaction region of the magnetron. The compact A6 magnetron with this optimized permanent magnet will be further studied using MAGIC, a 3-dimensional fully relativistic, fully electromagnetic particle-in-cell code, and the results will be presented.

Summary form only given. A previous computational study of a relativistic magnetron with diffractive output at UNM using MAGIC [1] indicated an electronic efficiency of 70% [2]. The original anode design in this study was tested in experiment. Some subtle changes in the source design were necessary, such as introduction of a Helmholtz-like magnetic coil pair and use of a polished stainless steel endcap on the downstream end of the transparent cathode. The purpose of these changes was to protect the dielectric output window from electron bombardment, an approach that is separate from the work of others to do the same with modified magnetic fields [3]. An ethanol calorimeter and L- and S-band waveguide detectors placed in the near-field were used to obtain RF pulse shape, frequency, and to calculate peak power. A self-integrating Rogowski coil and D-dot probe, which measured total current and voltage respectively, were then used to calculate a total efficiency of 40.4% for the π-mode. This lower efficiency is suggestive of a specific mechanism by which the spherical endcap has poor efficacy. Finally, images of radiated field pattern on a neon-bulb grid and of air breakdown outside of the 21 cm diameter output window were captured.

Summary form only given. This research was inspired by the need for a microwave source powerful enough to lead to enhancement in the air breakdown volume, when synchronized in time and space with a small region of pre-ionized air created with an intense laser pulse. We used the compact and portable RADAN pulser that drives a backward wave oscillator to produce microwaves for this purpose. The microwave pulse duration and power are 5 ns and 2 MW, respectively. The frequency of operation is 35 GHz. It is well known that air breakdown at atmospheric pressure requires an electric field amplitude of 30 kV/cm. Thus, an output power of 2 MW simply radiated in air does not have sufficient electric field to assist in air breakdown. For this reason, a parabolic dish was used to focus the microwaves into a small spot, thereby enhancing the electric field. The 2-dimensional, fully relativistic and fully electromagnetic code MAGIC was used to simulate the amplitude of the electric field and the axial position of the focal point of the parabolic dish. The parabolic dish used for this purpose was 30 cm across and 4.5 cm deep. It was observed in the simulations that the focal spot was located 15 cm from the dish depth and the radial electric field was 20 kV/cm. Despite this promising value of the electric field, experimental verification was necessary due to the following arguments that 5 ns was too short a pulse and that 35 GHz was too high a frequency for breakdown. In order to confirm the fidelity of the simulation results a low pressure chamber transparent to the 35 GHz was manufactured. The parabolic dish was placed opposite the radiating horn antenna with enough distance between them to allow breakdown to occur within the low pressure chamber. Time integrated photographs were captured for every microwave pulse. A very well defined breakdown was observed at 180 Torr of air. These results together with some theoretical development of the breakdown mechanism will be presented.