Laser induced fluorescence in Ar and He plasmas with a tunable diode laser

A diode laser based laser induced fluorescence (LIF) diagnostic that uses an inexpensive diode laser system is described. This LIF diagnostic has been developed on the hot helicon experiment (HELIX) plasma device. The same diode laser is used to alternatively pump Ar II and He I transitions to obtain argon ion and atomic helium temperatures, respectively.. The 1.5 MHz bandwidth diode laser has a Littrow external cavity with a mode-hop free tuning range up to 14 GHz (approximate to 0.021 nm) and a total power output of about 12 mW. Wavelength scanning is achieved by varying the voltage on a piezoelectric controlled grating located within the laser cavity. The fluorescence radiation is monitored with a photomultiplier detector. A narrow band interference filter is used to eliminate all but the. plasma radiation in the immediate vicinity of the. fluorescence wavelength. Lock-in amplification is used to isolate the fluorescence signal from noise and electron-impact induced radiation. For the Ar ion, the laser tuned at 668.43 nm is used to pump the 3 d F-4(7/2) Ar II metastable level to the 4p D-4(5/2) level. The 442.60 nm fluorescence radiation between the 4p D-4(5/2) and the 4s P-4(3/2) levels is captured by the photomultiplier tube. For atomic He, the laser is tuned at 667.82 nm to pump a fraction of the electron population from the 2(1)P state to the 3(1)D upper level. Although the 2(1)P level is not a metastable, the close proximity of 2(1)S metastable makes this new He I LIF scheme possible. In this scheme, a fraction of the laser-excited electrons undergo collisional excitation transfer from the 3(1)D to the 3(1)P level. In turn, the 3(1)P state decays to the metastable 2(1)S by emitting 501.57 nm fluorescence photons. (C) 2003 American Institute of Physics.