A Study on Low-Frequency Fluctuations Using New Azimuthal Probe Array in High-Density Helicon Linear Plasma

Abstract

In a high-density plasma produced by helicon waves in a linear device, a change from the coherent to broadband density fluctuations was observed on increasing the magnetic field. The coherent mode exhibited the drift wave features, and a steady oscillation state was observed. When the magnetic field was increased, the fluctuation level increased, and the higher azimuthal Fourier components were excited. When the spectrum became broad at the higher magnetic field, the behavior of the azimuthal structure became complex and changed rapidly with time. The analysis performed using the movable probe array (48ch.-probe) decomposed the short-lived structure into poloidal mode-frequency space and indicated mode-mode coupling in the broad-band spectrum. In fusion plasmas, turbulence, particularly the drift wave turbulence, has been investigated actively. Recent works indicate that the mutual interaction between fluctuations, which have different temporal and spatial scales, plays an important role in turbulence dynamics. Thus, many studies have been conducted with torus and linear devices (e.g. [1][2][3][4]), but the detailed spatiotemporal structure and the interactions between fluctuations in turbulence have not been well understood. The electrostatic probe array can directly determine the turbulence structure, and multiple probe arrays [5,6] have been used in linear and torus devices. However, conventional arrays are fixed in space and cannot cover a wide measuring region. We have recently developed a new probe array consisting of 48 probes extended in the poloidal direction. Since this array is movable in the radial direction, it can measure plasma parameters in a wide radial-poloidal region, and thus, the poloidal mode-frequency space spectra, S (m, f ), can be analyzed. In this Rapid Communication, we report the first result from the proposed 48ch.-probe array.The 48ch.-probe has 16 modules using an individual 1.5 inch port. In addition, each module has three tungsten tips (the diameter and length are 0.8 mm and 4.0 mm, respectively), with the separation of 5.2 mm between two tips in the poloidal direction. In this experiment, all movable modules were set at r = 40 mm (hence 5.2 mm apart between all adjacent tips) and 1625 mm axially away from author's e-mail: tera-225@aees.kyushu-u.ac.jp the end of a helicon source tube. Drift wave measurements with the new probe array were performed in Large Mirror Device-Upgrade (LMD-U), which is a modification of the previous device [7]. The LMD-U consists of a cylindrical vacuum vessel, which is 3740 mm long and has a 445 mm inner diameter. The high-density helicon plasma is produced using a double-loop antenna with a width of 40 mm and axial central distance of 100 mm. The antenna is wound around a quartz tube, which is 400 mm long and has a 95 mm inner diameter. A magnetic field of up to 1500 G is generated by the coils around the vacuum vessel. In this experiment, argon gas was used with a pressure of 2 mTorr, and the RF power (frequency) was 3 kW (7 MHz). The typical electr...

Spatiotemporal Behavior of Drift Waves in LMD-U

“…Drift wave modes driven by steep radial density gradient were identified, and a drift wave turbulence regime was achieved [17]. The poloidal mode numbers and frequencies of the drift wave modes were compared with the calculated linear dispersion relation of drift wave [18], and they were in a good agreement.…”

“…The probe tips are arranged at an equal distance when the measuring radius is set to 40 mm (the probe tips are 5.2 mm apart). This probe array can measure the poloidal mode number and radial profile of the plasma fluctuation [17]. With these two poloidal probe arrays, the details of the broadband and peak fluctuations, such as poloidal mode numbers, poloidal correlation lengths, and axial correlations, which are functions of mode number and frequency, are measurable.…”

Experimental Study of Drift Wave Turbulence in Linear Plasmas

“…Figure 4 shows the results of the same discharge conditions measured with a single probe. Figure 4 (a) shows the radial profile of the power spectrum S (r, f ) measured with a radially movable probe [15], and Fig. 4 (b) shows the power spectrum S ( f ) at r = 40 mm measured with one probe of the 64-channel probe array.…”

“…This fluctuation peak has a strong intensity at a steep gradient region in the plasma electron density profile (see Fig. 4 (a)), and is considered a drift wave mode [15]. The peaks at (m, f ) = (2, 5.6 kHz) and (3, 8.4 kHz) must be harmonics of the peak (m, f ) = (1, 2.8 kHz) produced by self-nonlinear couplings, since these satisfy the relationship of f [kHz] = 2.8 m. Other peaks are excited instability modes or peaks excited by nonlinear couplings.…”

“…In a Large Mirror Device-Upgrade (LMD-U) linear plasma [14], drift waves and the route to drift wave turbulence were observed [15]. A 64-channel poloidal Langmuir probe array, which can measure the precise poloidal wave numbers of the fluctuations [16], obtained a highquality, two-dimensional (poloidal wave number k θ and frequency ω) power spectrum of drift wave turbulence.…”