Mini-RT. Plasma Production and Levitation Experiments of a High-temperature Superconductor Coil in a Mini-RT Internal Coil Device

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1 Mini-RT Plasma Production and Levitation Experiments of a High-temperature Superconductor Coil in a Mini-RT Internal Coil Device Junji MORIKAWA, Kotaro OHKUNI, Dan HORI, Shigeo YAMAKOSHI, Takuya GOTO, Yuichi OGAWA Nagato YANAGI* and Toshiyuki MITO* Synopsis: Plasma is produced using a 2.45 GHz microwave in a Mini-RT internal coil device, where a high-temperature superconductor is employed. The radius of the internal coil is.15 m and its weight is 16.8 kg. The maximum coil current is 5 ka, and the typical magnetic field strength at the internal coil is.1 T. First plasma experiments were carried out using a mechanically supported coil, and a typical plasma density in the range of 1 16 m -3 was produced. When the internal coil current is decreased to less than 1 ka, plasma cannot be produced because of the disappearance of the electron cyclotron resonance layer of the 2.45 GHz microwave. Concerning to the levitation experiments of the internal coil, the coil position is monitored with laser sensors, and the levitation coil current is feedback-controlled. The HTS coil has been levitated for a period of one hour with an accuracy of 1 micrometers. Plasma production with a floating coil has been successfully initiated. Keywords: internal coil device, HTS conductor, plasma production, magnetic levitation, feedback control Mini-RT 2 K 5 ka Mini-RT Received April 16, 24 The University of Tokyo, Yayoi, Bunkyo-ku, Tokyo , Japan National Institute for Fusion Science, Oroshi-cho, Toki, Gifu , Japan morikawa@ppl.k.u-tokyo.ac.jp Mini-RT 2.8 kw 2.45 GHz 2.45 GHz Mini-RT 3 Fig. 1 Mini-RT 2.45 GHz 875 G.875 T 5 ka 1 ka 514 ka Fig. 2.1 T 2.45 GHz

2 Fig.1 Magnetic configuration of the Mini-RT device. (a) Icoil = 5 ka (b) Icoil = 14 ka Fig. 2 Magnetic configuration for (a) Icoil =5 ka and (b) Icoil = 14 ka. The magnetic field strength is also depicted. 875 G 5 mm 5 ka 875 G 14 ka 875 G 1 ka 875 G 2.45 GHz 43 ka 428 Fig. 3 1 A 7 1 ka 23 A Mini-RT 7 Fig. 4 Mini-RT 12 kw Mini-RT ECH.5.5 Pa.3 Pa.3 Pa m -3.1% Mini-RT Mini-RT 3 Mini-RT 2 K Fig. 3 Fig. 3 Decay of the coil current and increase in coil temperature. 21 J. Cryo. Soc. Jpn. Vol. 39 No. 5 (24)

Fig.4 Plasma produced by a 2.45 GHz microwave. The internal coil is mechanically supported. 2.6 kw 51 16 m -3 2.45 GHz 71 16 m -3 Fig. 5 36 ka.2 Pa 12 W 24 mm Mini-RT n e (1 16 m -3 ) 1 5 2 1 -(56) s.

3 Fig.4 Plasma produced by a 2.45 GHz microwave. The internal coil is mechanically supported. 2.6 kw m GHz m -3 Fig ka.2 Pa 12 W 24 mm Mini-RT n e (1 16 m -3 ) (56) s.1% 1-6 s.1 Pa 1-4 s Mini-RT mm 3-4) LDX 6) Mini-RT 197 Fig. 6 6 X YZ XYZ Z 5 Z XY XY Tilt( x, y) Vertical(Z-axis) Sliding(Sx, Sy) Internal Coil Major Radius (mm) 1 Fig. 5 Electron density and temperature profile. T e (ev) Fig. 6 Six freedoms of the floating coil

5 1 5 Fig. 7 3 2 5 5 LK- 5 (LK- 25) 35 mm1 mm 69 nm 1 µm 3 P1-P3 Fig. 8 3 P 3 P 2 P 1 Levitation coil Laser sensor Vacuum vessel Fig. 8 Top view of the Mini-RT vacuum vessel.

4 5 1 5 Fig LK- 5 (LK- 25) 35 mm1 mm 69 nm 1 µm 3 P1-P3 Fig. 8 3 P 3 P 2 P 1 Levitation coil Laser sensor Vacuum vessel Fig. 8 Top view of the Mini-RT vacuum vessel. A levitation coil and a laser sensor are located at the top of the vacuum vessel. Fig kg 5 ka Levitation coil current (ka) S 9 S Fig. 7 Laser sensors for detecting five freedoms of the floating coil. Fig. 9 Levitation coil current as a function of the distance between two coils (i.e., floating and levitation coils) for various levitation coil radii. The major radius, the coil current and the weight of the floating coil are,.15m, 1kA and 16.8 kg, respectively. The marks circle, squareand triangleare denoted to the levitation coil radius of 25 cm, 2 cm and 15 cm, respectively. 212 J. Cryo. Soc. Jpn. Vol. 39 No. 5 (24)

Z(a.u.) 2.5 2 1.5 1 2 1.5 tilt instability floating coil : R1,z Unstable Stable Lcoil Fcoil Hz PID Proportional-Integral-Differential 5 5 XY PID.5 1 1.5 2 2.5 1 2 R(a.u.) Fig.

5 Z(a.u.) tilt instability floating coil : R1,z Unstable Stable Lcoil Fcoil Hz PID Proportional-Integral-Differential 5 5 XY PID R(a.u.) Fig. 1 Position between floating (F) and levitation (L) coils. Fig. 1 Fcoil Z= Lcoil 2.8 cm Z=28 cm Fig cm 5 ka 15.3 ka 1.54 Hz 4 FB-RT FB-RT Fig. 11 FB-RT Table 1 Field-Cooling FB-RT 3 4 Fig. 11 Set-up of a feedback control experiment with a miniature HTS coil. Table 1 Parameters of FB-RT miniature HTS coil. Inner / Outer Diameter 77 / 94.5 mm Number of Turns 44 Winding Method Double Pancake Inductance.272 mh Cable Type Ag-sheathed Bi-2223 Coolant Liquid Nitrogen Weight (including LN2) 296 g (321 g)

6 Position (m) 2 3m Fig. 12 t=.1 sec 2 d ~ z m 2 dt d dt d i i dm ( z) dz F L z ~ z L i M ( z) i F F L mg L i M z) i R i e (1) (2) L L ( (3) F L L dt mg irl FL M(z) e O z z ~ z ~ ~ if if if il il il PID e K ~ z K v~ K ~ ~ zdt K i (4) z v I Time (sec) Fig. 12 A floating coil position. The dynamic response of the coil position is studied by changing the reference position of the floating coil at t =.1 sec. L L Mini-RT 3 Fig. 13 Fig. 14 Coil support Floating coil Center stack Fig. 13 A floating HTS coil. The coil position is detected with three laser sensors, and the levitation coil current is feedback-controlled. Fig. 14 Position and current of a floating coil and a levitation coil current. The floating coil position is artificially changed at t = 5 min. The levitation coil current is gradually increased because the floating coil current is decreasing. 214 J. Cryo. Soc. Jpn. Vol. 39 No. 5 (24)

Fig. 14 t = 5 min 5 mm 1 1 m 5 g HTS HTS 69 nm H-alpha Fig. 15 Fig. 1 Fig. 14 Mini-RT 2.45 GHz 1 16 m -3 1-2 ev 1 m 1 Mini-RT m 1 1) N. Yanagi, et al.

7 Fig. 14 t = 5 min 5 mm 1 1 m 5 g HTS HTS 69 nm H-alpha Fig. 15 Fig. 1 Fig. 14 Mini-RT 2.45 GHz 1 16 m ev 1 m 1 Mini-RT m 1 1) N. Yanagi, et al. : Excitation test results of the HTS floating coil for the Mini-RT project, IEEE Trans. Appl. Supercond. 13 (23) ) Y. Ogawa, et al.: Construction and operation of an internal coil device with a high temperature superconductor, J. Plasma Fusion Res. 79 (23) ) J. File, et al.: J. Appl. Phys. 39 (1968) ) S. Skellett : CLM-P427 (1975) 5) S. Yoshikawa: Experiments on plasma confinement in internal-ring devices, Nucl. Fusion 13 (1973) ) J.H. Schultz, et al.: The Levitated Dipole Experiment (LDX) magnet system, IEEE Trans. Appl. Supercond. 9 (1999) ) J. Morikawa, et al. : Levitation experiment using a high-temperature superconductor coil for a plasma confinement device, Jpn. J. Appl. Phys. 4 (21) L129-L131 Fig. 15 Plasma produced by 2.45 GHz microwave at the condition that the HTS coil is floating

8 216 J. Cryo. Soc. Jpn. Vol. 39 No. 5 (24)

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