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Schematic diagram of experimental setup. THz transmitter (TX) uses GaAs photoconductive switch to generate monocycle THz pulses which are collected by a silicon ball lens (Si) and focused by off-axis ellipsoidal mirrors (EM) into the vacuum chamber. Imaging of plasma fluorescence through viewport provides a good estimate of axial plasma density (shown in lower inset for 10, 30, and ). Receiver (RX) uses electro-optic sampling in ZnTe to measure the THz electric field. Insets in upper-left and upper-right show THz measured wave form without plasma (input) and with plasma (output).
rms error between measured THz pulse and that predicted using plasma phase shift in Eq. (1) with peak values of density and collision frequency as parameters. Insets show measured (light gray) and calculated wave form (black) at values of and indicated by arrows. The time delay is and the pressure is .
(a) Pulsed argon plasma electron densities measured by THz-TDS for indicated pressures. Dashed curve is fluorescence brightness for measured with a high-speed photodiode. (b) Electron collision rate calculated from the same data.
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