Abstract: PSFC/JA-07-11

Mirror Langmuir probe: A technique for real-time measurement of magnetized plasma conditions using a single Langmuir electrode

B. LaBombard and L. Lyons

A new method for the real-time evaluation of the conditions in a magnetized plasma is described. The technique employs an electronic ‘mirror Langmuir probe’ (MLP), constructed from bipolar rf transistors and associated high-bandwidth electronics. Utilizing a three-state bias waveform and active feedback control, the mirror probe’s I-V characteristic is continuously adjusted to be a scaled replica of the ‘actual’ Langmuir electrode immersed in a plasma. Realtime high-bandwidth measurements of the plasma’s electron temperature, ion saturation current and floating potential can thereby be obtained using only a single electrode. Initial tests of a prototype MLP system are reported, proving the concept. Fast-switching MOSFET transistors produce the required three-state voltage bias waveform, completing a full cycle in under 1 μs. Real-time outputs of electron temperature, ion saturation current and floating potential are demonstrated, which accurately track an independent computation of these values from digitally stored I-V characteristics. The MLP technique represents a significant improvement over existing real-time methods, eliminating the need for multiple electrodes and sampling all three plasma parameters at a single spatial location

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