Perturbations of Microwave Cavity Fields by Time-Varying Xenon Laser Discharges

The effects of a time-varying complex dielectric constant upon the resonant characteristics of a cylindrical microwave cavity are presented. The cavity resonance equations are developed to consider the influence of small periodic changes in resonant frequency, bandwidth, and cavity-coupling coefficient upon the microwave power density within the cavity as a function of the probing microwave frequency. Expressions are derived which enable the separate contributions of periodic changes in cavity power density to be extracted from measurements at several microwave frequencies. It is shown that for high-Q cavities, variations in the sampled microwave cavity fields are due predominantly to shifts in the cavity resonant frequency. By repetitively interrupting the mirror cavity of a 3.51-μ xenon laser with a mechanical chopping wheel, very small periodic changes are induced in the electron density and collision frequency of the xenon dc discharge due to changes in the laser field strength. The resultant changes in microwave power level within a cylindrical cavity enclosing the discharge are measured as a function of probing frequency and compared with theoretical values, yielding excellent agreement for high-Q cavity conditions. Where cavity loading severely degrades the Q, good agreement is obtained within the cavity half-power frequencies. A microwave-frequency compensation technique is employed which permits accurate direct measurement of small electron-number density changes (Δne/ne∼3×10-5) along the discharge-tube length.