H. Nakano

K. Terasaka

M.Y. Tanaka

S. Yoshimura

T. Morisaki

Y. Kato

Y. Yoshida

National Institute for Fusion Science (NIFS-Repository)

Intermittent phenomena have been frequently ob-served in magnetized plasmas. For example, a solar flarereleases a large amount of plasma particles and energyfrom sun’s surface to interstellar space through the in-termittently occurring magnetic reconnection. Anotherexample is a plasma blob. The particle transport acrossthe magnetic field due to the blobs is significant for theplasma confinement in torus plasmas.Recently, the generation of intermittent high-energyelectron flux (IHEF) has been found in a linear electroncyclotron resonance (ECR) plasma.1) From our previousexperiments using a local Langmuir probe, the durationof the IHEF is typically 10 µs, and the IHEF has finitespatial size perpendicular to the magnetic field. Theseresults show that 2 dimensional measurement on theplasma cross section is required to clarify the generationmechanism and the characteristics of the IHEF. Then, wehave studied the 2D characteristics of the IHEF, such asthe spatial size and occurrence position, by developing amulti-channel probe.Experiments have been performed in the HYPER-I device at National Institute for Fusion Science. TheHYPER-I device has a cylindrical vacuum chamber (2.0m in axial length and 0.3 m in inner radius). A heliumplasma was generated by ECR heating with a 2.45 GHzmicrowave, and the typical electron density and temper-ature are 1017 m−3 and 10 eV, respectively.To measure the 2 dimensional profile of the IHEF,we used a modified wire-grid probe (WP) shown in Fig.1. The WP consists of eight horizontal and eight verticaltungsten wires with 0.7 mm in diameter, and there are64 lattice points with the grid interval of 10 mm on thelattice plane. Each wire is connected to the independentvoltage-follower circuit, which is compensated the float-ing potential fluctuation with the frequency less than 200kHz. The spatial distribution of the IHEF is estimatedby calculatingFi,j =√V˜f,i × V˜f,j (1)at each lattice point, where V˜f denotes the perturbedquantity of floating potential, and the subscripts i (=1-8) and j (= 1-8) show the horizontal and vertical wires,respectively. The quantity Fi,j becomes large only whenthe IHEF through the lattice point at (xj, yi).Figure 1(b) shows the spatial size of the IHEF eval-uated for 30 events with various magnitude. Since all thedata lie on a solid line showing the relation of Xw = Yw,the spatial profile of IHEF on the plasma cross sectionis circular form with the diameter of 30 ± 10 mm. Thequantities Xw and Yw show the full width at half max-imum of the Fi,j in x and y direction, respectively. Wealso evaluated the characteristic of the occurrence posi-tion of IHEF. Figure 1(c) shows the probability of occur-rence as a function of the distance between occurrencepositions of consecutive two IHEFs. The probability isalmost constant value of 1/6. This shows the IHEF oc-curs randomly.We have studied the 2 dimensional characteristics ofthe IHEF with a WP. It is shown that the WP can easilydetermine both the spatial size and the occurrence posi-tion of IHEF. The IHEF randomly occurs in space, andhas the characteristic diameter of 30 mm. These resultsclearly show that the WP has a capability to study 2Dcharacteristics of IHEF, and then further studies utiliz-ing the conditional averaging method will be performed.1) S. Yoshimura et al. : Bull. Am. Phys. Soc. 56 (2011)209.(b)Y    (mm)wX    (mm)wX    = Yw w(c)probabilitydistance of the next occurrence position (mm)10 mm10 mm(a)Fig. 1: (a) A picture of wire-grid probe. (b) Spatial size of the IHEF. (c) Probability of occurence position as afunction of distance of the next occurrence position of IHEF.430§2. 2D Characteristics of Intermittently Generated High-energy Electrons in an ECR PlasmaTerasaka, K., Kato, Y., Yoshida, Y., Tanaka, M.Y. (Kyushu Univ.), Yoshimura, S., Nakano, H., Morisaki, T.

2D Characteristics of Intermittently Generated High-energy Electrons in an ECR Plasma

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