Houshmandyar Saeid, Sears Stephanie H, Thakur Saikat Chakraborty, Carr Jerry, Galante Matthew E, Scime Earl E
Department of Physics, West Virginia University, Morgantown, West Virginia 26506-6315, USA.
Rev Sci Instrum. 2010 Oct;81(10):10D704. doi: 10.1063/1.3466796.
Laser-induced-fluorescence (LIF) is used to measure the density of helium atoms in a helicon plasma source. For a pump wavelength of 587.725 nm (vacuum) and laser injection along the magnetic field, the LIF signal exhibits a signal decrease at the Doppler shifted central wavelength. The drop in signal results from the finite optical depth of the plasma and the magnitude of the decrease is proportional to the density of excited state neutral atoms. Using Langmuir probe measurements of plasma density and electron temperature and a collisional-radiative model, the absolute ground state neutral density is calculated from the optical depth measurements. Optimal plasma performance, i.e., the largest neutral depletion on the axis of the system, is observed for antenna frequencies of 13.0 and 13.5 MHz and magnetic field strengths of 550-600 G.
