The observation of avalanche-like transport during confinement power degradation in Heliotron J

Abstract

In this study, we observe avalanche-like electron thermal transport in Heliotron J under plasma conditions that exhibit power degradation to the central electron cyclotron heating (ECH). The newly installed GHz sampling electron cyclotron emission diagnostic allows for the observation of the long-distance radial propagation of electron temperature fluctuations. We find that the electron temperature fluctuations are associated with avalanches, because the Te fluctuations (i) propagate from the core to the edge with a speed comparable to the diamagnetic drift velocity, (ii) exhibit a 1/f power-law scaling in the frequency spectrum and a Hurst exponent close to 1, and (iii) dominate in the ECH deposition location and spread to the edge as the heating power increases. Furthermore, the electron heat avalanches can spread to the scrape off layer region when they are enhanced, i.e., the Te fluctuations correlate with the Dα emission, which also has a frequency spectrum that scales with a 1/f power law.

Observation of avalanche-like transport in Heliotron J and JT-60U plasmas

“…The significant information propagates from ρ = 0.13 to 0.95 within 0.2 ms, which accounts for a propagation speed of ≈0.7 km s −1 . This is consistent with the result obtained by the cross-correlation analysis [25]. Note that the time scale of 0.2 ms is shorter than the time constant of the lowpass filter of 3 kHz, however, the transfer entropy calculated with the data filtered by 10 kHz lowpass shows an identical time scale, 0.2 ms.…”

“…As studied in previous works [17,25], electron temperature fluctuation exhibits a long-temporal correlation, which is a signature of avalanches. The long-temporal correlation, or long-term memory, can be revealed when a Hurst exponent takes a value within 0.5 < H < 1 [11,12].…”

“…Especially, the void ( Te / Te < 0) at t ≈ 0.265 s appears in the whole radial region. It was found that the average radial correlation length increases with the heating power [25]. The amplitude of Te / Te is pronounced in the region at ρ < 0.3, which corresponds to the location of the ECH deposition.…”