THz Pulse Spectroscopy of Dynamic Plasmas: A New Diagnostic Tool
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1163-K09-03
THz Pulse Spectroscopy of Dynamic Plasmas: A New Diagnostic Tool Brian H. Kolner Departments of Applied Science and Electrical & Computer Engineering University of California, Davis One Shields Avenue, Davis, CA 95616
ABSTRACT Rapidly evolving plasmas represent a challenging environment for both study and control. Density, collision frequency and temperature fluctuations can change over orders of magnitude on time scales of one ns with spatial features less than one cm and thus are not amenable to conventional continuous-wave diagnostic techniques such as microwave or mm-wave interferometry. We have developed a new technique for studying plasmas undergoing rapid nonequilibrium changes that uses THz time-domain spectroscopy (THz-TDS) in conjunction with optical fluorescence imaging. The advantages of using THz pulses lie in the fact that the broad bandwidth of a THz pulse contains frequency components both above and below the plasma frequency allowing a single ps-duration pulse to carry away information about the complex path-integrated susceptibility. Transverse fluorescence gives us a model of the longitudinal plasma distribution and using a novel rms error-minimization technique we can recover the real and imaginary parts of the susceptibility with 103 . The principle of this new technique will be discussed along with results on a pulsed DC-discharge plasma. We will also present some new ideas such as concurrent molecular spectroscopy and computed tomography. INTRODUCTION Plasmas that evolve rapidly in time can have certain advantages over steady-state plasmas in a manner similar to that of the peak-to-average power ratio gain in other disciplines. Dynamic plasmas evolving on time scales of 1 ns < τ < 1 μs have become useful in diverse areas of science and technology. Applications and important areas of study include: • Enhancing ignition and combustion of propellants [1], • Plasma-assisted combustion for reduction in pollution and improved energy conversion efficiency [2, 3],
• Studies of high-voltage breakdown in spark gaps used for pulsed power switching and ultrawideband radar [4–6], • Studies of formation and propagation of fast ionization fronts [7–9], • Pulsed electron beam-generated plasmas for materials processing [10] and • Pulsed electron beam-driven flash x-ray sources for radiography [11, 12], to name a few. Studying plasma evolution on this time scale presents some unique challenges. In addition to the requirement of increased time resolution, many of the processes of interest also require spatial information that can then give rise to spatio-temporal maps of the plasma behavior. The tried and true diagnostics of the early days of plasma physics such as electric and magnetic probes, microwave interferometry, and optical spectroscopy all have limitations of either space- or time-averaging on the scales of interest [13, 14]. Modern ultrafast laser techniques, on the other hand, have provided us with a new set of tools for studying dynamic plasmas. The high carrier frequency of the near infra
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