A. Tomioka

J. Morikawa

K. Ibano

K. Natsume

K. Uchijima

N. Yanagi

S." Nose

T. Mito

T. Takemoto

Y. "Ogawa

Y. Terazaki

National Institute for Fusion Science (NIFS-Repository)

It is necessary to deepen our knowledge of plasma turbulence for improving the confinement in fusion plasmas. Nonlinear coupling in multi-scale turbulence is one of the keys in the transport mechanism, and dynamical changes of the mean and fluctuation profiles must be measured with high spatio-temporal resolutions for identifying the mechanism. In this research, development of a new experimental diagnostic for detailed fluctuation measurements is promoted with the integration of theoretical, simulation and experimental approaches. For that purpose, a reflectometer using microwave frequency comb has been developed 1). Turbulence Diagnostic Simulator (TDS) 2), which is an assembly of simulation codes for numerical diagnostics in magnetically confined plasmas, is used as the platform for the development to accelerate the collaboration between the simulation and experiment. This was the first year of the research planned for three years, and the basis for the identification of multi-scale couplings was established both in the experimental and simulation parts. The system of a microwave frequency comb reflectometer has been developed. An experimental test has been begun on the PANTA device 3) in Kyushu University, which will be applied to the LHD experiment 1). The principal of the diagnostic is as follows. The comb signal with the frequency range of 12 - 27 GHz and the frequency interval of 0.5 GHz is injected and reflected in the plasma. Each Fourier component of the microwave has time delay in accordance with its reflected point, so the density profile can be reconstructed from the detected comb signal. The phase of the carrier wave is precisely controlled with respect to the shape of the envelope. The signals are directly transferred to the digital storage oscilloscope (DSO), which has a frequency band of 33 GHz (the sampling frequency is 80 GHz), so the waveforms of the incident and reflected signals are detected in the form of digital signals with high temporal resolution. Our system enables simultaneous monitoring of fluctuation levels at more than 20 distinct spatial locations with high temporal resolution. Figure 1 illustrates the circuit used in the observations. Statistical noise can be eliminated by convoluting microwave pules. The time resolution for constructing the density profile is found to be on the order of 1 Ps. Preliminary date has been also obtained in the LHD experiment by using the existing reflectometry system and the DSO. This experimental method is promising for the physics of plasma turbulence and transport.  For the reconstruction of the profiles, it is necessary to carry out a numerical simulation, so the routine to simulate the microwave frequency comb reflectometer has been developed in the TDS. The module of the ray tracing of the RF wave in the integrated simulation code TASK 4) is introduced for the wave analysis for the first step. The original routine is modified to adjust to the cylindrical magnetic field configuration and the incident comb signal for the linear device. Calculation of the ray and the phase delay gives the evaluated output signal in accordance with the plasma density profile. Figure 2 shows an example of the incident and detected waveforms. Different Fourier components have the different reflected points as in Fig. 2 (c), which gives the detected signal as Fig. 2 (b). This simulation gives the insight to experimental observations, and will be extended to analyze both simulation and experimental data in the same routine. The development of the data analysis method is also important to identify the spatio-temporal structure, so the conditional averaging method has been developed to apply to both experimental 5,6) and simulation data 7). This method is effective to clarify the characteristic responses of the turbulence, and will be applied to the reflectometory data. Fig.1: Block diagram of the microwave frequency comb reflectometer. Fig.2: Simulation result of the (a) incident and (b) detected waveforms of the microwave frequency comb reflectometer. The phase delay in (c) gives the detected signal as (b). The incident comb signal has the frequency range of 12 - 27 GHz and the frequency interval of 0.5 GHz in this case.  1) Inagaki, S. et al.: Plasma Fusion Res. 8 (2013) 1201171. 2) Kasuya, N. et al.: Plasma Sci. Tech. 13 (2011) 326. 3) Oldenbürger, S. et al.: Plasma Phys. Control. Fusion 54(2012) 055002. 4) Fukuyama, A.: http://bpsi.nucleng.kyoto-u.ac.jp/task/. 5) Inagaki, S. et. al.: Nucl. Fusion 53 (2013) 113006. 6) Kobayashi, T. et al.: Phys. Rev. Lett. 111 (2013) 035002. 7) Kasuya, N. et al.: Plasma Fusion Res. 8 (2013) 2403070. In 2003, an HTS coil fabricated from BSCCO was fabricated as the internal floating coil of the plasma experimental device Mini-RT, which has been extensively used in high beta plasma experiments. The decay time constant of the persistent current in this coil was 20–30hours, which was considerably shorter than expected, and possibly due to some defects during the fabrication process. During 10 years of operation, the current decay of the persistent current has gradually fallen to ~2.7 h. Such deterioration is problematic for plasma experiments, where a constant magnetic field is preferable.The performance of REBCO tapes has advanced sufficiently for us to consider replacing the BSCCO coil with a new REBCO coil in the Mini-RT device. We employed the REBCO tape of 4.3 mm wide. As for the countermeasure on the quench, a 0.1 mm-thick copper laminate was attached, and the coil was electrically insulated by a polyimide sheet. The final thickness of the REBCO tape was 0.27–0.28 mm. The total current of the newly fabricated REBCO coil resulted in 55.2 kA. If a few meters of the REBCO tape would be transited into the normal conducting state at the quench, the temperature of the REBCO tape should not exceed 200 K. Thus, to improve the heat dispersion at hot spots, an aluminum sheet was inserted between the winding pack and the coil frame. A persistent current switch was fabricated from the REBCO tape, and several improvements were introduced. As a uniform heating source, a stainless steel sheet was wound together with the REBCO tape. In addition, the cooling system was reinforced by an additional cooling channel. The size of the PCS was determined from its role as a heat sink for the stored magnetic energy during the demagnetization phase. The REBCO tapes were joined by a wrap-joint soldered at the copper laminate sides. Based on R&D results,the joint length was decided to be 30 mm.Using the PCS and an external power supply, the current of the main coil was excited, and a persistent current was established, as shown in Fig. 1[1]. Under operation at 25 K, the decay time of the persistent current was improved to 320 h. The corresponding resistance was 125 n:, which is comparable with the total resistance of the seven joint sections in the persistent current circuit. The decay time was observed to be prolonged with decreasing operation temperature, as shown in Fig. 2. Based on these observations, we infer that the decay constant of the persistent current is attributable to joint resistance, and that the REBCO tape was not seriously degraded during the fabrication and cooling process.Fig. 1 Excitation experiment of the REBCO coil. Fig. 2 Decay time constant of the persistent current for various coil currents as a function of the coil temperature.Immediately following plasma experiments, the main coil must be demagnetized. Demagnetization of the Mini-RT coil was achieved via the PCS, which was designed to safely consume the stored magnetic energy of the main coil.As shown in Fig. 3, the PCS heater was switched on, and the temperature gradually increased. When the PCS temperature reached the critical temperature of the REBCO tape (~85 K), the persistent current was decayed by the emergent PCS resistance. The time constant of the persistent current decay was around 7–8 s, and the induced voltage across the circuit was at most 0.4 V. Fig. 3 Temporal behaviors of the coil parameters during thedemagnetization phase.[1] Y. Ogawa, et al., Plasma and Fusion Research: Regular Articles Volume 9, 1405014 (2014).251§16. Fabrication of REBCO Coil and Application for Fusion Plasma Experimental Device Mini-RTOgawa, Y., Morikawa, J., Uchijima, K., Takemoto, T.,  Ibano, K. (Univ. Tokyo), Mito, T., Yanagi, N., Natsume, K., Terazaki, Y. (Grad. Univ. Advanced Studies), Tomioka, A., Nose, S. (Fuji Co.)

Fabrication of REBCO Coil and Application for Fusion Plasma Experimental Device Mini-RT

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Fabrication of REBCO Coil and Application for Fusion Plasma Experimental Device Mini-RT

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