Electron density in low-pressure Na-Ne discharges deduced from laser absorption experiments

Laser absorption experiments are described to measure the radial sodium ground-state distribution in the positive column of a low-pressure Na-Ne discharge. The radial electron density distribution is obtained by combining the results with a particle balance. The electron densities at the tube axis ne(0) derived in this way are approximately a factor of 2.5 higher than the ones derived from a direct determination with Langmuir probes. Measuring the sodium profiles as a function of the distance to a probe reveals quantitatively the perturbative effect on the plasma of the glass capillary that supports the probe. This partly explains the observed discrepancy in ne(0). The electron temperature Te is deduced from the laser absorption data using the measured electrical conductivity of the plasma. This value of Te agrees very well with the value as measured with the probe.