Analyses on Plasma Flow Measurement in Collisional Plasmas with Negative Ions by Laser Induced Fluorescence and Mach Probe

Abstract

In collisional plasma at various plasma devices, plasma flow with supersonic flow plasma and negative ions has been analysed by using a laser induced fluorescence (LIF) method and a Mach probe (MP) for the direct measurement of plasma flow and various electric probes such as single, triple and emissive probes for the measurement of reference plasma. An LIF system composed of a diode laser with Littmann/Metcalf cavity and master oscillator power amplifier (MOPA) was developed to measure ion temperature and drift velocity in plasma material interaction system–II (PLAMIS-II), which is a sputtering device with plasma jet. Due to constrained accessibility of PLAMIS-II, an acute angle optical path with an optical fiber system was devised, and LIF measured ion temperature as 0.2 – 0.3 eV and Mach number as 0.1 or so. To characterize supersonic plasma flow by an MP, the system of plasma flow generation was developed by using a cylindrical ion extraction electrode (CIEE), and supersonic plasma flow has been generated by applying a cylindrical ion extraction electrode within a cylindrical RF plasma. To investigate characteristics of ion velocity distribution along the axial direction, plasma velocities were measured by MP, and compared with those by equation of energy conservation, which is to be provided by the measurement of plasma potential along the axial direction. In steady state conditions, it was observed that the plasma flow (𝑀∞ ~ 0.6) from a plasma source was increased up to supersonic plasma flow (𝑀∞ = ~ 1.1 – 1.2) inside CIEE. Partially, for increasing plasma flow velocity, an acceleration system with a metal meshed grid was tested and checked by measurement of emissive probe on sheath and plasma potentials near negatively biased grid. Potential drops which was from ~ 5 – 8 V (Vp) in bulk plasma to ~ – 45 – 55 V (Vs) at z = 0.3 cm were observed as moved toward a negatively biased grid (– 100 V). Accelerated plasma flow larger than ion acoustic speed toward the grid/wall. Electronegative plasma was generated by adding oxygen gas (0 – 14 %) to argon plasma generated from a lanthanhexaboride (LaB6) DC plasma source of Divertor Plasma Simulator–2 (DiPS-2) to analyse plasma flow in collisional plasma with negative ions by using an LIF method and Mach probe with the other electric probes such as single, triple and emissive probes. Before, to investigate plasma flow in the collisional magnetized plasma which was contained negative ions at DiPS-2, characteristics of plasma parameters in argon pure plasma was analysed as function of discharge current, gas flow rate and magnetic intensity. By using parallel MP, the ratio of upstream to downstream currents was measured with oxygen/argon ratio and compared with results of LIF method. Using the collisionless model for MP in collisional magnetized plasma contained negative ions near 1 mTorr, higher discrepancy for absolute values of Mach numbers between MP (0.3 – 0.5) and LIF method (0.1 – 0.2) was observed with oxygen/argon ratio. To interpret the data of MP, we have introduced a model for calibration factor of Mach probe by considering collisionality between ions and neutrals, which lead to a good agreement of the data by MP with those by LIF within 10 % of uncertainty.