Preliminary Measurements of Low Frequency Fluctuations by a Microwave Interferometer System and a Fast Camera in Pilot-PSI Device

Plasma Phys. Control. Fusion

et al. 2018

We have performed multipoint measurements with segmented electrodes and a microwave interferometer in the linear plasma device NAGDIS-II, in order to reveal cross-field motion and axial localization of the enhanced radial transport in the detached plasma. By changing the neutral pressure successively and applying several statistical analysis techniques, it was clarified that there is axially localized ion flux broadening accompanying an enhanced plasma ejection from the center with radially elongated spiraling structure. The spiraling plasma ejection accompanies the m=0 mode drop near the center with the similar time scale. Further, such behavior composed of f>1 kHz fluctuations is modulated by several-hundred-hertz fluctuation with m=0. This cross-field transport causes non-negligible effect for the reduction of the ion flux peak in the detached plasma.

Section: Microwave Interferometermentioning

confidence: 99%

Localized spiraling plasma ejection contributing the ion-flux broadening in the detached linear plasma

Tanaka

Takeyama

Plasma Phys. Control. Fusion

et al. 2018

“…Stronger oscillations in the ionization zone and/or recombination front region were observed in hydrogen plasmas in Pilot-PSI in earlier studies of linear divertor plasmas [6,7]. In an area that was axially localized and where pure He plasmas in NAGDIS-II experienced high volume recombination, the cross-field transport was enhanced [9][10][11].…”

Section: Introductionmentioning

confidence: 78%

“…The radiation zone, ionization region, recombination region, and momentum loss region are thought to make up the plasma conditions along the magnetic field to the diverter plate [1][2][3][4]. In addition, in some linear plasma simulators, including NAGDIS-II, Pilot-PSI, and Magnum-PSI [5][6][7][8][9][10][11][12][13][14][15], and the SOL of the tokamak plasma of ASDEX Upgrade [16], low-frequency fluctuations around the recombination front region have been observed in the detached plasma conditions. These fluctuations result in radial particle losses across the magnetic field.…”

Section: Introductionmentioning

confidence: 99%

Effects from the Target Plate Geometry on Fluctuations of Helium Plasma in the Linear Divertor Simulator Magnum-PSI

Plasma and Fusion Research

Tanaka

Hayashi

et al. 2022

Self Cite

With the help of the linear divertor simulation device Magnum-PSI, a fluctuation investigation of the impact of the target plate geometry was conducted. We simultaneously quantify coherent low-frequency fluctuations with a newly built 70-GHz microwave reflectometry system, a reciprocating probe, a light emission detector system, and a fast-framing camera system. The strong low-frequency fluctuations were observed at both the electron density and the plasma radiations by moving the target plate along the magnetic field line. Furthermore, a strong peak in fluctuation intensity and the influence of the target plate tilt angle on the fluctuation intensity were noted. c

“…In addition, it is not known where the large amplitude fluctuation occurs in the detached plasma. The knowledge of the location of the fluctuation is able to reveal the mechanism of plasma instability in the detached plasma.In this study, we investigate plasma density fluctuation at around the recombination front, where the volume recombination process is the most enhanced, in NAGDIS interferometer [6], and the location of the recombination front is identified by a spectroscopy. Figure 1 shows the schematic diagrams of this experiment.…”

mentioning

confidence: 99%

“…In this study, we investigate plasma density fluctuation at around the recombination front, where the volume recombination process is the most enhanced, in NAGDIS interferometer [6], and the location of the recombination front is identified by a spectroscopy. Figure 1 shows the schematic diagrams of this experiment.…”

mentioning

confidence: 99%

Localized Density Fluctuation in the Downstream of Detached Plasma

Takeyama

Ohno

Plasma and Fusion Research

et al. 2017

Self Cite

The density fluctuation was measured with a 70-GHz microwave interferometry system in the linear divertor plasma simulator NAGDIS-II while changing from the attached to the detached states. In the detached plasma, a large amplitude fluctuation of ≤ 10 kHz appeared in the peripheral region. The fluctuation has a maximum value at the neutral gas pressure of 30 -34 mTorr. Further, the measurement of the light emission associated with the plasma recombination indicates that the large amplitude fluctuation is located at just downstream of the recombination front, in which the recombination process is the most enhanced. For the success of ITER and future DEMO reactor, reducing plasma heat loads on the divertor plate is an essential issue. In this regard, detached plasma is considered as the most effective method. However, the reduction of the particle and heat loads has not been fully understood in the detached plasma and not simulated perfectly [1]. There are two important mechanisms to reduce peak particle and heat loads onto the divertor plate. One is volume plasma recombination process and the other is enhancement of crossfield transport.In the linear plasma device NAGDIS-II, the enhancement of the blob-like cross-field transport as well as volume plasma recombination have been observed under the detached plasma condition [2,3]. When the ion flux dramatically decreases due to an increase of the neutral gas pressure, a large amplitude ion flux fluctuation with ≤ 10 kHz components was detected by using a circular end-target plate with a diameter of 50 mm [4]. Threedimensional simulation suggested that such axisymmetricmode fluctuation generates intermittent plasma structures in the periphery [5], however, there were a few measurements of this fluctuation. In addition, it is not known where the large amplitude fluctuation occurs in the detached plasma. The knowledge of the location of the fluctuation is able to reveal the mechanism of plasma instability in the detached plasma.In this study, we investigate plasma density fluctuation at around the recombination front, where the volume recombination process is the most enhanced, in NAGDIS interferometer [6], and the location of the recombination front is identified by a spectroscopy. Figure 1 shows the schematic diagrams of this experiment. The interferometer is installed ∼1 m from the end-target and at vertically 3 cm distance from the plasma center. The neutral gas pressure P is simultaneously acquired at ∼0.3 m from the density measurement position (see Fig. 1 (a)). By controlling the gate valve in front of the vacuum pump, the neutral gas pressure is increased [4,7] in order to move the recombination front from near the end-target to the upstream region during the detached plasma condition, as shown in Fig. 1 (b).Figures 2 (a) and (b) show the time evolutions of P and the spectrogram of the density fluctuation. In Fig. 2 (b), a periodic fluctuation of ∼10 kHz is observed at t ∼ 1.5 -