Fast neutral lithium beam for density and its fluctuation measurements at the boundary regions of ETE tokamak

Abstract

A Fast Neutral Lithium Beam Probe (FNLBP) is being developed, in order, to perform measurement of the boundary plasma density in discharges of the Spherical Tokamak ETE, recently built at LAP/INPE. This plasma diagnostic method is adequate for use in fusion devices because it does not perturb the plasma and it provides data with high space and time resolution for the entire discharge lifetime. To obtain reliable measurements, however, FNLB probing depends on high signal-to-noise ratio (S/N) during beam emission spectroscopy of the Li beam injected into the plasma, specially if density fluctuation measurements are sought. Hence, our effort is been focused in the achievement of high intensity Li0 output. Recently, probing of a low temperature (T ∼ 5 eV) and low density target plasma (n ∼ 1010 cm-3) produced in a plasma immersion ion implantation experiment (PIIIE) was performed by a 2-5 keV FNLBP. The method used for the density determination of this glow discharge plasma was based on the comparison of the fluxes of 6708 Å photons emitted from Li0 beam injected into nitrogen gas that filled the PIIIE chamber and from the same beam injected into the plasma discharge. For this case, the attenuation of the beam was neglected. The plasma density measured was ne = 8.5 × 1010 cm-3 for pressure of operation of p = 7.0 × 10-4 mbar and ne = 7.2 × 1010cm-3 for p = 4.6 × 10-4 mbar. These results are in good agreement with Langmuir probe data. The success of this measurement was only possible after a tenfold increase in the Li+ output and the increase of the lifetime of Li source by vitrification of the β-eucryptite source, besides the optimization of the optical detection system and the neutralization of the beam. Presently, a new 10 kev FNLBP is been developed to probe ETE plasma. For this case, where the Li beam will be strongly attenuated by the high density plasma (ne ∼ 1013 cm-3), the method of density reconstruction from the whole photon flux profile will be used. All the improvements performed during the operation of the old FNLBP device, will be implemented on this new FNLBP.