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Optical emission spectroscopy technique with Bayesian probability theory is used to infer electron temperatures and densities of low density helium plasmas. Helium plasmas are generated in a newly developed DC multi-dipole chamber, MAXIMUS (Magnetic X-point sIMUlator System). Plasma parameters are varied by adjusting neutral pressure, bias voltage and heating current. Likelihoods are calculated based on a helium collisional-radiative model consistent with the measured signal and noise level. With a uniform prior probability, we obtain the posterior which are compared with the simultaneous and independent measurements of the electron temperature and density by Langmuir probes.

A multidipole chamber with a DC discharge system capable of generating magnetic X-point configuration, MAXIMUS (MAgnetic X-point sIMUlator System), is constructed. The vacuum chamber of MAXIMUS is composed of two cylindrical chambers with the total length of 2 m (2 x 1 m) and the diameter of 60cm. An LaB6 hot cathode is installed on one end of the chamber, emitting energetic electrons whose energy can be varied up to approximately 200eV. The base pressure is ~ 1 0–6 Torr, and Ar, He, or mixed H2 DC glow discharge plasmas are generated with working pressure of between 0.1 and 10.0mTorr. Langmuir probes and emissive probes are used to measure electron temperature T e, plasma density ne, and plasma potential $\phi_{\mathrm{p}}$ . Magnetic fields are measured with Hall probes. Typical plasma parameters generated in MAXIMUS are found to be $\mathrm{T}_{\mathrm{e}}\sim 1\text{eV}, \mathrm{n}_{\mathrm{e}}\sim 10^{9}$ cm-3, and $\phi_{\mathrm{p}}\sim 4\mathrm{V}$ . Plasma parameters can be controlled by neutral gas pressure, discharge voltage and discharge current. Magnetic X-point is generated by two axial copper tubes inside the MAXIMUS, which can carry DC currents up to 1kA each. The magnetic field configuration can be adjusted by changing the position of the copper tubes and by changing the driving current of each tube independently. Spatial profiles of plasma parameters are investigated in the presence of the multidipole permanent magnets and various configurations of the magnetic X-point. MAXIMUS is designed to explore fundamental properties of plasmas, effects of neutral particles around the magnetic X -point to the core plasmas, and the plasma behavior due to magnetic perturbations in magnetic X-point configuration.

Summary form only given. Weakly collisional electronegative plasmas were investigated in a dc multidipole chamber. The plasmas were generated by primary electrons emitted from hot thoriated tungsten filaments. The working gases consisted of combinations of Ar and neutral feedgases that easily form negative ions. Measurements were made near a negatively biased stainless steel plate located in the center of the chamber. A planar Langmuir probe indicated the electron density ne and the effective electron temperature Te. Laser-induced fluorescence (LIF), using a tunable diode laser (668.614 nm), was used to determine the profile of Ar+ drift velocity. An emissive probe using the inflection point method in the limit of zero emission measured the plasma potential. In an Ar-O2 plasma with Ar neutral pressure = 0.10 mTorr and 02 at 0.30, ne ~ 3.8 X 109 cm"3 and the Te ~ 0.69 eV, an electronegative core and an electropositive halo were observed. The density ratio of negative ions to electrons (a) in the non-drifting bulk was found to be 0.43 from the phase velocity of an ion acoustic wave (IAW) using CW excitation at 100 kHz. The profile of a was found using both the phase velocity of the IAW and the drift velocity of positive ions determined by LIF. These measurements showed that the negative ions were in Boltzmann equilibrium with a temperature of 0.06 Acircplusmn 0.02 eV. Consistent with the small value of a, double layers were not found separating the electronegative core and the electropositive halo in this plasma. Measurements with other feed gases, which should have larger values of a, are now underway and will be reported.