Laser Induced Fluorescence Diagnostic for the ASTRAL Plasma Source.

A Laser Induced Fluorescence (LIF) diagnostic is presented in this poster. The ion temperature measurements are made in the ASTRAL (\textbf{A}uburn \textbf{S}teady s\textbf{T}ate \textbf{R}esearch f\textbf{A}ci\textbf{L}ity) helicon plasma source using a diode laser based LIF diagnostic. ASTRAL produces Ar plasmas with the following parameters: n$_{e}$ = 10$^{10}$ to 10$^{13}$ cm$^{-3}$, T$_{e}$ = 2 to 10 eV and T$_{i}$ = 0.03 to 0.5 eV. A series of 7 large coils produce an axial magnetic field up to 1.3 kGauss. Operating pressure varies from 0.1 to 100 mTorr and any gas can be used for the discharge. A fractional helix antenna is used to introduce rf power up to 2 kWatt. A number of diagnostics are presently installed on the plasma device (Langmuir Probe, Spectrometer, LIF system). The LIF diagnostic makes use of a diode laser with the following characteristics: 1.5 MHz bandwidth, Littrow external cavity, mode-hop free tuning range up to 16 GHz, total power output of about 15 mW. The wavelength is measured by a precision wavemeter and frequent monitoring prevents wavelength drift. For Ar plasma, a new LIF scheme has been developed. The laser tuned at 686.354 nm, is used to pump the 3d$^{4}$F$_{5/2}$ Ar II metastable level to the 4p$^{4}$D$_{5/2}$ state. The fluorescence radiation between the 4p$^{4}$D$_{5/2 }$and the 4s$^{4}$P$_{3/2 }$terms (442.6 nm) is monitored by a PMT.