Measurement of neutral flow velocity in an ECR plasma using tunable diode laser LIF spectroscopy combined with saturated absorption spectroscopy

The radial flow of neutral particles in an electron-cyclotron-resonance (ECR) argon plasma has been measured by using a newly developed high resolution laser induced fluorescence (LIF) measurement system. The flow velocity is determined by the Doppler shift of the LIF spectrum. A very high accuracy calibration of an excitation laser has been achieved by installing a saturated absorption spectroscopy unit into the LIF system. We utilized the Lamb dip, which is obtained by the saturated absorption spectroscopy, as a frequency standard for the determination of the flow velocity. The use of Lamb dip is particularly appropriate for the flow velocity measurement, since the position of Lamb dip in the frequency scale is Doppler-shift free and is not disturbed by the motion of reference medium. By utilizing the Lamb dip as the frequency standard, the reliability and stability of the laser frequency calibration is increased. From the radial measurements of LIF spectra of metastable argon atoms at the microwave power of 250 W and 5 kW, it is found that there exists an inward flow of neutral particles in the both plasmas. Both of the radial flow velocity profiles peak around 4 cm from the center, which is comparable with the radius of the boundary of the E×B rotation and anti-E×B rotation in the 5 kW discharge. The maximum flow velocity increases with the microwave power.