Development of Li beam probing and laser-induced fluorescence technique for electric field measurement in a plasma

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Abstract
To measure local electric field in an rf‐plugged sheet plasma in the RFC‐XX‐M open‐ended machine, a spectroscopic system using a combined technique of Li‐beam probing (laser blow‐off method) and laser‐induced fluorescence has been developed. A Li atom in the rf field can be populated to 42F by a stepwise laser pumping (22S→22P→42F) because of the Stark mixing between 42F and 42D. A cascade fluorescence of 610.4 nm (32D→22P) subsequent to the direct transition (42F→32D) was observed to estimate the rf field strength (rms) Erms. To correct the effect of electron collisions in the plasma, we observed a blue fluorescence consisting of two lines: a forbidden line of 460.2 nm (42F→22P) and an allowed line of 460.3 nm (42D→22P) induced by electron collisional transfer (42F→42D). From these intensities, we obtained Erms and electron density ne by solving numerically the rate equations, including the relevant radiative and collisional processes under the experimental conditions. Erms and ne were estimated to be 2.4 kV/cm and 3.1×1011 cm−3 near the center of the sheet plasma, respectively. It was found that the obtained Erms was enhanced by about 2.5 times with respect to the vacuum field (1 kV/cm). The obtained ne agreed well with ones by other methods. A spatial distribution of ne was also obtained from that of the Li0 resonance line (22P→22S) due to electron impact excitation. The present spectroscopic system is applicable to plasmas of ne≲1013 cm−3 and static or quasi‐static electric fields of E≳400 V/cm.