Development of a High CMRR Magnetic Probe for the Biased Plasma in TU-Heliac

Abstract

The plasma in Tohoku University Heliac (TU-Heliac) is biased by a hot cathode that drives the J × B poloidal rotation. In the biased plasma, a density collapse accompanied by high-frequency bursting density and potential fluctuations was observed. To eliminate the effect of potential fluctuations and to measure the magnetic fluctuation, a high common mode rejection ratio (CMRR) magnetic probe with a preamplifier at the probe head in a vacuum vessel was designed and installed. Owing to the absolute calibration, the magnetic probe has a high magnetic sensitivity of 10 4 V/T (@100 kHz), high CMRR of −75 dB (@500 kHz), and frequency bandwidth DC −1 MHz. In the biased plasma, the power spectrum of the normal mode signal was 10 4 times larger than that of the common mode signal in all frequency ranges, which indicates that the probe can successfully minimize the capacitive pickup noise relative to the actual magnetic fluctuation signal. In addition, we successfully observed the broad spectrum (100 kHz < f < 300 kHz) of the magnetic fluctuation and MHD instability in the biased plasma in TU-Heliac.

Electro-optic Bdot probe measurement of magnetic fluctuations in plasma

“…The measurement of magnetic fluctuations is important in numerous plasma experiments. [1][2][3][4][5][6] Magnetic fluctuation probes (Bdot probes) 7 have been extensively used as a standard diagnostic tool for this purpose. The fundamental principle of the Bdot probe is to measure the induced voltage on a pickup coil loop placed in a temporary changing magnetic field according to Faraday's law.…”

“…In general, the existence of measurement instruments or large volume metallic components close to the experiment is not preferable because they could disturb ambient electromagnetic field lines. To meet experimental requirements under these restrictions, numerous schemes, such as shielding of the transmission line with appropriate grounding, differential measurement technique, 5 DC cut using a transformer, and impedance conversion using amplifiers, 1 have been developed for the reliable operation of Bdot probes for plasma studies.…”