ELECTRON-DENSITY MEASUREMENTS COMBINING ATOMIC-BEAM PROBES AND LIF DIAGNOSTICS OF A RAPIDLY-EXPANDING PLASMA

A method combining laser-induced fluorescence and injection of a neutral beam was employed to determine a time-varying electron-density in a conical theta-pinch plasma. The method consists of measuring the electron excitation rate relative to the spontaneous emission coefficient. An expression for the electron density in terms of the measured fluorescence and collision-induced emission signals has been formulated, taking into account the appreciable population of the excited level, as well as incomplete laser saturation. Neutral atomic beams (Li and Mg) were produced using laser ablation. A low-density hydrogen plasma was produced using a conical theta-pinch to simulate tokamak edge-plasma conditions. Using a Langmuir double-probe and emission spectroscopic methods, a time-varying electron density rising to 2 x 10(13) cm-3 and an electron temperature between 4 and 10 eV were measured. The expansion velocity of the plasma determined from the hydrogen emission and continuum emission at 520 nm. Radial emission profiles were obtained through Abel inversion techniques. Using collisional radiative model calculations, the absolutely calibrated H-alpha emission could be attributed to a neutral background gas of 10(12) cm-3. Measuring the electron excitation of the atomic beams with laser-induced fluorescence (LIF), the rapidly varying electron density could be deduced using either the Li 670 nm or the Mg 285 nm transition. The measured attenuation of the Li and Mg beams by electron-impact ionization compares well with the predicted attenuation based on independent Langmuir-probe temperature measurements, provided that ionization from the excited levels is included in the attenuation model.