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Plasma Density Diagnostic
Two techniques have been developed by ATF experimenters to characterize the time evolution of plasma density in the capillary channels that have routinely been used for studies of electron/laser/plasma interaction. The capillary typically used is a 5-10 mm long, 1 mm ID ceramic channel with end electrodes. Hydrogen gas injected into the capillary is ionized by a longitudinal electrical discharge. Plasma densities up to 1019 cm-3 have been produced.
The first diagnostic allows for precise control of the plasma density at the time of interaction by measuring the rate of decay of density following termination of the electrical discharge. Plasma density evolution is very repeatable and is then adjusted simply by controlling the amount of time between electrical discharge and interaction process. The diagnostic relies on spectral measurement of the Ha emission that experiences Stark broadening in the plasma. The Ha emission is collected from the central portion of the capillary by an optical fiber inserted through the sidewall and transported to photomultiplier tubes through narrowband spectral filters. The ratio of intensity at different wavelengths is then calculated to determine the amount of line broadening. In this way the exponential decay of plasma density is verified for each capillary configuration in order to produce the desired mapping from delay to density.
The second diagnostic enables measurement of plasma wakefields on the picosecond timescale using ultrafast optical probe pulses. Although in this frequency domain interferometry has yet to be completed at ATF, it has been demonstrated experimentally elsewhere. Two optical pulses originate from a Ti:sapphire laser, and are separated in time by several picoseconds. The pulses are synchronized such that the first traverses the plasma channel before the wakefields are generated, and the second immediately after. The pulses are combined downstream in an optical spectrometer, where the different spectral components interfere. The resulting interference fringes can be analyzed and the density profile of the wakefields determined by scanning the time between pulses. This method also allows some spatial information to be recovered transverse to the capillary axis. A single shot extension of this technique known as frequency domain holography requires chirped pulses, and is also possible under ATF conditions.