Laser Induced Fluorescence (LIF) provides measurements of flow speed, temperature, and density of ions or neutrals in a plasma. Traditionally, a LIF measurement requires two ports on a plasma device; one for laser injection and one for emission collection. Proper alignment of LIF optics is time consuming and sensitive to mechanical vibration. We describe a confocal configuration for LIF that requires a single port and requires no alignment. The measurement location is scanned radially by physically moving the entire optical structure. Confocal LIF measurements are compared to traditional LIF measurements over the same radial range.

Over the past decade, a number of studies have demonstrated the spontaneous formation of ion beams in expanding plasmas. The ion beams are identified through measurements of the ion velocity or ion energy distribution function. The two primary diagnostic techniques for the performing these measurements are laser induced fluorescence (LIF) and retarding field energy analyzers. Each measurement method has its strengths and weaknesses. One particular challenge for LIF is the difficulty in obtaining good signal to noise when the measurement location is far from the point of optical access to the plasma chamber, the optical access is through a relatively small window, and the light must be conveyed to the experiment through optical fibers. Here we present LIF measurements performed over 3 meters away from a 2.75” injection port. Laser light is coupled into a polarization preserving, single mode, optical fiber and then focused to a beam radius of a few mm over 3 meters away after the light emanating from the fiber is collimated with a concave mirror to eliminate chromatic distortion as the laser wavelength is scanned during the LIF measurement. Significant gains in signal-to-noise were obtained over the previous, unoptimized, injection optics; expanding the pressure and RF power range over which LIF measurements could be performed. LIF measurements of the argon ivdf in low pressure, less than 1 mTorr, and high pressure, greater than 10 mTorr, expanding helicon plasmas will be presented Original Laser Injection Scheme Expansion Chamber Magnetic Field Scan