Augustine Y. Cheung

The Effects of Microwaves on Cell Survival at Elevated Temperatures

Since microwaves are used in human cancer therapy, information on specific biological effects of microwaves at elevated temperatures is important. To help supply this information, we exposed mammalian cells (CHO) and bacteria (Serratia marcescens) to hyperthermal temperatures (43, 44, and 45/sup o/C for CHO and 48, 49, and 50/sup o/C for the bacteria) with and without microwave irradiation. Temperature control was maintained by a refrigeration-reheat system and high-velocity water recirculation. The 2450-MHz microwave source was operated in a pulsed mode with power density up to 500 mW/cm/sup 2/. As expected, the survival curve slopes for both cell types increased rapidly with temperature, doubling for each degree Celsius. Microwave irradiation produced no significant change in extrapolation number for either cell type. However, survival curves of CHO cells which received microwaves were steeper by a factor of 1.25 than their sham-irradiated controls. No significant effect on slope was seen with the bacteria. Liquid crystal thermometry revealed a microwave-induced temperature elevation of 0.3/sup o/C in the glass microcapillary exposure tubes. This temperature elevation closely corresponded to the observed difference in survival curve slopes for the CHO cells and suggests a simple thermal origin for that difference.

A Lecher Wire Microwave Interferometer for Measurements of Electron Density and Electron Temperature in a Flowing Transient Plasma

A microwave diagnostic system for measurement of electron density and temperature in a transient plasma flow, generated by a laser pulse focused on a solid target, has been constructed using a twin conductor (Lecher wire) probing element coupled to X‐band waveguide components by tapered parallel planes. The analysis of the operation of the probing section has required the solution of a symmetric seven layer dielectric problem, which models the transmission, reflection, and attenuation of the microwave signal by plasma located around Lecher wires. Simultaneous measurements of the phase and amplitude of the transmitted signal, together with an experimental configuration which accurately defines the interaction of the plasma with the Lecher wire section, have made possible the reliable determination of plasma properties with a spatial resolution of ∼0.5 cm and a temporal resolution of ∼5×10−8 sec.

Single wire microwave conductors for density diagnostics of flowing anisotropic transient plasmas

With a single wire conductor, microwave radiation can be guided through a plasma so that high spatial resolution (~0.5 cm) can be realized at X-band frequencies (λ0≃3 cm). Dispersion relations for a single wire surrounded by a longitudinally magnetized plasma have been calculated. A Fabry-Perot single wire interferometer for measurements in magnetized flowing plasmas (U~107 cm/sec) with a fast time resolution (~50 nanoseconds) has been built and calibrated, and used for electron density measurements.

Microwave Studies of Magnetically Induced Counterstreaming Interactions in Laser-Produced Plasma

With a single wire conductor, microwave radiation can be guided through a plasma so that high spatial resolution (~0.5 cm) can be realized at X-band frequencies (λ ≃ 3 cm). Dispersion relations governing a longitudinally magnetized single wire cavity filled with a plasma have been calculated. A Fabry-Perot single wire cavity capable of giving fast time resolution (~50 nanoseconds) was used to study momentum couplings between a flowing laser-produced plasma (U ~ 107 cm/sec) and a photoionized background. The results indicate a collisionless coupling occurs when a 700 gauss transverse external magnetic field is present. Such couplings may be due to the onset of magnetized two-stream instabilities.