An Estimate of the Absolute Measurement Error of the Electric Potential in Plasma of the T-15MD Tokamak

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Abstract
An absolute error in measuring the plasma electric potential using the heavy ion beam probe (HIBP) is estimated for the T-15MD tokamak. The main sources of error are analyzed, including the instability of voltage and the inaccuracy of its setting in the high-voltage power supplies of the accelerator and the energy analyzer, the uncertainty in the entrance angle of the secondary ion beam into the analyzer, the noise in the measurement path and the shot noise. The error is calculated for various modes of tokamak operation with the toroidal magnetic field of BТ = 1.2–2.0 T and the chord-averaged electron density of ne = (3–5) × 1019 m–3 for different profiles of electron temperature and density. The radial plasma regions accessible to reliable potential measurement are determined. Two-dimensional distributions of the absolute error for both the average potential and its fluctuations are obtained. It is demonstrated that optimizing the analyzer plate voltage, which reduces the normalized difference in partial currents at the detector, reduces the overall measurement error by approximately a factor of four and substantially expands the sample volume.

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