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Correction for Neutral Pressure-Driven Signal in Radiated Power Measurements on Proto-MPEX

by Seungsup Lee, Matthew L Reinke, Theodore M Biewer, David C Donovan, Jamie Coble
Publication Type
Conference Paper
Journal Name
IEEE Transactions on Plasma Science
Publication Date
Page Numbers
1649 to 1654
Conference Name
IEEE Symposium on Fusion Energy (SOFE)
Conference Location
Jacksonville, Florida, United States of America
Conference Sponsor
Conference Date

A pinhole camera using 4-channel resistive bolometers was designed and implemented on the Prototype Material Plasma Exposure eXperiment (Proto-MPEX) to measure the plasma radiated power. Quantification of the radiated power is required for comprehensive power balance assessments but faces unique challenges on linear plasma devices due to weak absolute signal, compared to typical use in toroidal fusion plasmas, and high neutral pressure changes. Around 1 mTorr, the pressure-driven signal contribution was shown to overwhelm the radiated power signal in Proto-MPEX, found to be ~ 3 μW. Since the pressure-driven signals could not be predicted from first principle arguments, the signals were corrected using empirical models that take two experimental pressure measurements and the raw bolometer signal to reveal the radiated power signals. Four ‘gas-only’ Proto-MPEX shots were taken, without the rf plasma discharge, to train, optimize, and validate empirical models. The data included two pressure measurements at different locations along with the bolometer signal. Multiple models were tested; partial least square regression with a cross-correlation method to account for the time offset in the pressure measurements provided the best combination of accuracy and stability. The model was validated to have a modeling error of 0.0036 V, which was lower than 10% of the expected radiated power signal. The model was tested on a different “gas-only” shot with a complete time-series data. For plasma operation shots, the corrected bolometer measurements were compared with an absolute extreme ultraviolet photodiode measurement, and similarities and differences are discussed.