Advanced Probe Measurement System in TU-Heliac

Abstract

An advanced probe measurement system consisting of high-speed Langmuir probes with a preamplifier and a Copper shield was designed and installed in TU-Heliac. Potential and density fluctuations were measured in the hot-cathode biased plasma, and power spectra were calculated using the complex Fourier transform. There were low frequency fluctuations ( f < 10 kHz), high frequency fluctuations (70 < f < 200 kHz) and sharp spectra applied for the plasma production (10 < f < 50 kHz). In the region above 200 kHz, the power decreased by a factor of 1/ f 2 for the potential fluctuation and 1/ f 2∼3 for the density fluctuation. The phase shift between potential and density fluctuations was almost 0 rad at ρ = 0.54. On the other hand, the phase shift was not 0 rad at ρ = 0.21, especially in the 100 ∼ 200 kHz region. The high frequency fluctuation at ρ = 0.21 grew on the time scale of 10 −5 s, which was obtained from the time dependent signals of the high frequency fluctuation.

Density Collapse and Fluctuation Observed in Poloidally Rotating Plasma on TU-Heliac

“…The E × B poloidal rotation frequency exceeded 100 kHz. At the same time, density and potential fluctuations were observed in low (1-10 kHz) and high (100-1000 kHz) frequency ranges [6,9,11]. High frequency fluctuation appeared and disappeared periodically.…”

“…In the hot cathode biased plasma, the E × B poloidal rotation frequency exceeded 100 kHz and the fluctuation frequency reached about 400 kHz by E × B poloidal rotation. A high-speed fluctuation measurement system (cutoff frequency ∼ 1 MHz) was required [11]. In this study, highspeed fluctuation measurement and the triple probe measurement were integrated in the high-speed triple probe (HSTP), which was installed in TU-Heliac.…”

“…Electrode biasing can drive E × B rotation in a small device resulting in an improved confinement mode. In Tohoku University Heliac (TU-Heliac), negative biasing experiments have been carried out to study the transition to the improved mode [3][4][5][6][7][8][9][10][11]. A hot cathode made of LaB 6 was used for electron injection into the plasma and a negative potential profile was formed.…”

Bursting high-frequency fluctuation observed in biased plasma in TU-Heliac

“…In the biased plasma, we have observed the bursting turbulent fluctuation and density collapse in the ion saturation current with Langmuir probes [13][14][15][16][17]. Figure 3 shows typical time evolutions of electrode current I E , electron density n e , electron temperature T e and ion saturation current I s at ρ = 0.2, measured with the Langmuir probe at φ = 0 • .…”

“…Observed Lissajous diagrams of turbulence and gradients demonstrated clockwise and counterclockwise hysteresis loops [5][6][7][8][9][10][11][12]. In Tohoku University Heliac (TU-Heliac), we have carried out electrode biasing experiments and observed the density collapse and high-frequency bursting fluctuation [13][14][15][16][17]. Because of the low temperature plasma in TU-Heliac, we have the advantages of measuring high-frequency fluctuations in ion saturation currents and floating potential with high spaceand time-resolution using a Langmuir probe.…”