A multi-channel capacitive probe for electrostatic fluctuation measurement in the Madison Symmetric Torus reversed field pinch

Abstract

A 40-channel capacitive probe has been developed to measure the electrostatic fluctuations associated with the tearing modes deep into Madison Symmetric Torus (MST) reversed field pinch plasma. The capacitive probe measures the ac component of the plasma potential via the voltage induced on stainless steel electrodes capacitively coupled with the plasma through a thin annular layer of boron nitride (BN) dielectric (also serves as the particle shield). When bombarded by the plasma electrons, BN provides a sufficiently large secondary electron emission for the induced voltage to be very close to the plasma potential. The probe consists of four stalks each with ten cylindrical capacitors that are radially separated by 1.5 cm. The four stalks are arranged on a 1.3 cm square grid so that at each radial position, there are four electrodes forming a square grid. Every two adjacent radial sets of four electrodes form a cube. The fluctuating electric field can be calculated by the gradient of the plasma potential fluctuations at the eight corners of the cube. The probe can be inserted up to 15 cm (r/a = 0.7) into the plasma. The capacitive probe has a frequency bandwidth from 13 Hz to 100 kHz, amplifier-circuit limit, sufficient for studying the tearing modes (5-30 kHz) in the MST reversed-field pinch.

Development of a multi-channel capacitive probe for electric field measurements with fine spatial and high time resolution

“…Therefore, a low frequency limit is given by 1/[2π(C 1 + C 2 )R] = 4.4 Hz. While this value is a factor of three smaller than the low frequency limit reported in Ref 11 , MST plasmas with a duration of ≤100 ms still need corrections for low-frequency attenuation. Although this correction can be made in the frequency domain 11 , FFT inherently assumes periodic signals which are not necessarily appropriate, and any window function has finite frequency resolution which introduces errors.…”

“…The internal and external connection to each electrode is about 5 m in length, which also poses a challenge since the capacitance of the cable (∼ 500 pF) allows a significant portion of the input current to flow to the ground, and limits the maximum achievable input impedance of the readout circuit. Whereas a capacitive divider 12 allows for a constant and reasonable high gain over a wide fre- quency range, there is still significant signal attenuation at low frequencies in the gains reported by Refs 11,12 due to insufficient input impedance. The issues associated with a capacitive probe becomes even more challenging as the size of an electrode is reduced to attain high spatial resolution because the capacitance between a plasma and the electrode decreases, resulting in an even larger input impedance.…”

“…Also, this probe remains operational as long as a Boron Nitride (BN) particle shield, which is known for its excellent thermal and chemical stability, survives the environment. A previously developed capacitive probe demonstrated that capacitive coupling with a BN serving as a dielectric provides the measurements of V plasma fluctuations associated with sawtooth crashes, which have a global spatial (MHD) structure and a characteristic frequency of below 30 kHz 11 . Based on the expertise obtained through the previous probe development, the measurement capabilities of a capacitive probe are extended from MHD scales to micro-turbulence scales.…”

Development of a novel plasma probe for the investigation and control of plasma‐enhanced chemical vapor deposition coating processes

“…Several other probe methods that are effective in PECVD environments have been proposed, such as the self-excited electron resonance spectroscopy (SEERS), the surface wave probe (SWP), multipole resonance probe (MRP), and capacitive probe (CP). [19][20][21][22][23][24][25][26][27][28][29][30][31] The compact combined sensor, which is used as a heat flux sensor and a planer LP, has also been developed. [32,33] SEERS, which is a passive plasma probe based on a general hydrodynamical discharge model, can monitor n e and the electron collision rate.…”

“…[19][20][21] SWP can measure n e , and CP can determine time-varying V f . [22][23][24][28][29][30][31] The MRP is a further development based on the SWP. Its advantage is the calibration-free measurement and the fact that it allows local measurements, in which the SWP might be affected due to the grounding of the plasma chamber.…”