In re-entry simulation experiments at General Motors Defense Research Laboratories, where projectiles are launched into the controlled atmosphere of a free-flight range LI] at hypersonic velocities (up to about 7 km/s), Open Microwave measurements of the electron density distribution in the wake and its decay as a function of distance behind the Resonators for Ionized projectile give valuable information about the processes occurring in the wake. These measurements are also Wake Measurement necessary in order to understand the radar scattering properties of the ionized wake which are observed concurrently by multifrequency CW radars. 1011 e/cm3) for some of the flight conditions of interest, eemLber, with very rapid axial decay behind the projectile. This GMDefenalMotrse R rchpLboratioriesimplies a requirement of high sensitivity and good spatial Santa larbara, Cali o resolution along the axis of the ionized wake, and of a fast transient response (approximately 1 ,s) in the measurement system. In addition, since the flow field of the projectiles is extremely sensitive to any physical obstruc-Abstract tions, it is necessary to have a structure which ensures A method is described for measuring changing ionization levels by minimum disturbance to the wake during the measureobserving the phase shift of a microwave signal which has passed ment interval.through an open resonator while the resonator is being perturbed by For electron densities in the range from 1011 to 1013 the ionization. The measurement range is 109 to 1011 e/cm' with a e/cm3 in a 1-cm-diameter wake, these requirements are met by focused microwave beam transmission probes time resolution of 1 ,us and a spatial resolution of 2.5 cm. Methods of [2] fopri a3 ando 70 GHzm theseihave spatia measuring the phase shift and converting the phase information to resolutions of 1.25 and 0.6 cm, respectively, and 10useful results are detailed. Sources of error are also considered.MHz bandwidths.It is desirable to measure down to at least 109 e/cm3 in a 1-cm-diameter plasma, so it is necessary to use resonator techniques to obtain the added sensitivity.
While conventional closed resonators [3], [4] operatingat UHF frequencies have excellent sensitivity (down to about 106 e/cm3) their spatial resolution of about 39 cm Key Words-Electron density, hypersonle c, ionzation, microwave, is not adequate when the electron density in a wake can perturbation, projectiles, resonator, wakes.decay several orders of magnitude within that distance. They are also limited by the fact that their entrance port interferes with the wake structure. The open resonator with spherical mirrors has been shown [5]-[7] to be particularly suited to meet all the requirements. This paper describes an open resonator system operating at 35 GHz which can measure electron densities as low as 109 e/cm3 in a 1-cm-diameter plasma, and has transient response as fast as 1 ,s and spatial resolution of 2.5 cm (beamwidth to 1/e power point).Since the resonator structure is approximately 25 cm away from the pla...
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