Shuji Kamio

Integrated discharge scenario for high-temperature helical plasma in LHD

The discharge scenario of high temperature plasma with a helical configuration has significantly progressed. The increase of central ion temperature due to the reduction of wall recycling was clearly observed. The peaking of the ion heating profile and the reduction of charge exchange loss of energetic ions play an important role for further improvement of ion heat transport in the ion internal transport barrier (ITB) core. The ion ITB and electron ITB have been successfully integrated due to the superposition of centrally focused electron cyclotron heating to the ion ITB plasma, and the high temperature regime of the ion temperature comparable to the electron temperature (Ti ~ Te) has been significantly extended. The width of the ion ITB formed with electron ITB is wider than the width of electron ITB. The positive radial electric field was observed in the integrated ITB plasma by a heavy ion beam probe, while the negative radial electric field was observed in ion ITB plasmas. The ion temperature gradient decreases with the increase of the temperature ratio (Te /Ti).

Studies of dust transport in long pulse plasma discharges in the large helical device

Three-dimensional trajectories of incandescent dust particles in plasmas were observed with stereoscopic fast framing cameras in a large helical device. It proved that the dust is located in the peripheral plasma and most of the dust moves along the magnetic field lines with acceleration in the direction that corresponds to the plasma flow. ICRF heated long pulse plasma discharges were terminated with the release of large amounts of dust from a closed divertor region. After the experimental campaign, the traces of exfoliation of carbon rich mixed-material deposition layers were found in the divertor region. Transport of carbon dust is investigated using a modified dust transport simulation code, which can explain the observed dust trajectories. It also shows that controlling the radius of the dust particles to less than 1 mm is necessary to prevent the plasma termination by penetration of dust for the long pulse discharges. Dust transport simulation including heavy metal dust particles demonstrates that high heating power operation is effective for shielding the main plasma from dust penetration by an enhanced plasma flow effect and a high heat load onto the dust particles in the peripheral plasma. It shows a more powerful penetration characteristic of tungsten dust particles compared to that of carbon and iron dust particles.

Development of multi-channel Doppler spectroscopic measurement system using 8 × 8 multianode photomultiplier tube assembly.

Using an 8 × 8 channel photomultiplier tube assembly and a single Czerny-Turner monochromator, we have developed a novel Doppler spectroscopic system which can measure the time evolutions of spectral distribution of plasma emission from eight different lines of sight simultaneously. An optical lens system is employed to couple the output of the monochromator with the detector assembly, resulting in small cross-talks less than 5% in spatial distribution together with large magnification of up to 50 in wavelength direction. The suggested system yields cost-effective polychromatic measurements of eight spatial channels with uniform optical and electrical characteristics.

Laboratory study of diffusion region with electron energization during high guide field reconnection

Floating potential profile was measured around the X-point during high guide field reconnection in UTST merging experiment where the ratio of guide field ( Bg) to reconnecting magnetic field ( Brec) is Bg/Brec>10. Floating potential measurement revealed that a quadrupole structure of electric potential is formed around the X-point during the fast reconnection phase due to the polarization by inductive electric field. Also, our floating potential measurement revealed the existence of parallel electric field in the vicinity of the X-point. While field-aligned components of inductive electric field ( E∥ind) and electrostatic electric field ( E∥es) cancel out with each other away from the X-point, E∥ind exceeds E∥es around the X-point, indicating the deviation from ideal MHD criterion within the region. The diffusion region extends in the outflow region and the scale length of region is an order of ion skin depth, which is quite different from the VTF experiment result. Based on the measured magnetic field an...

Development of effective power supply using electric double layer capacitor for static magnetic field coils in fusion plasma experiments

A cost-effective power supply for static magnetic field coils used in fusion plasma experiments has been developed by application of an electric double layer capacitor (EDLC). A prototype EDLC power supply system was constructed in the form of a series LCR circuit. Coil current of 100 A with flat-top longer than 1 s was successfully supplied to an equilibrium field coil of a fusion plasma experimental apparatus by a single EDLC module with capacitance of 30 F. The present EDLC power supply has revealed sufficient performance for plasma confinement experiments whose discharge duration times are an order of several seconds.

Electron cyclotron beam measurement system in the Large Helical Device.

In order to evaluate the electron cyclotron (EC) heating power inside the Large Helical Device vacuum vessel and to investigate the physics of the interaction between the EC beam and the plasma, a direct measurement system for the EC beam transmitted through the plasma column was developed. The system consists of an EC beam target plate, which is made of isotropic graphite and faces against the EC beam through the plasma, and an IR camera for measuring the target plate temperature increase by the transmitted EC beam. This system is applicable to the high magnetic field (up to 2.75 T) and plasma density (up to 0.8 × 10(19) m(-3)). This system successfully evaluated the transmitted EC beam profile and the refraction.

Magnetic island dynamics in magnetic reconnection in UTST experiments

The dynamics of the magnetic island formed inside the reconnection current layer was investigated under the strong guide field in the UTST spherical tokamak merging experiment. A pair of proximately located O- and X-points was generated at ∼5 cm away from the other X-point. The formed O- and X-points immediately started to move toward the downstream region inside the current layer, but the O-point had larger velocity and caught up the preceding X-point within 0.5 μs. The results from Doppler spectroscopy indicated that the ion flow velocity had the intermediate value between the O- and X-point velocities, suggesting that the ions flowed with approximately the same velocity with the magnetic island.

Separated Double-Current Layers in a High-Guide-Field Reconnection Experiment

Magnetic reconnection in the presence of a high-guide-field is utilized to heat the plasma in the merging startup of a spherical tokamak configuration. The reconnection current layer between two spherical tokamaks was gradually compressed, and a quasi-steady current-sheet thickness was obtained. During the compression phase, the current layer split transiently into two separated layers, which provided a flattened magnetic-field-region near the X-point. This modification may affect the electron-energization mechanism in the merging start-up of a spherical tokamak in a high-guide-field.

Performance of Impedance Transformer for High-Power ICRF Heating in LHD

There are two types of ion cyclotron range of frequencies antennas in the Large Helical Device. The handshake form (HAS) antenna has high heating efficiency. However, its loading resistance is small, and injection power is limited by the voltage of the transmission line. On the other hand, the field-aligned-impedance-transforming (FAIT) antenna has higher loading resistance than the HAS antenna despite having a smaller antenna head. However, the high voltage on the transmission line is again the bottleneck for high-power injection, as with the HAS antenna. We developed an ex-vessel impedance transformer for the HAS and FAIT antennas to decrease the voltage on the transmission lines by increasing loading resistance. The estimated enhancement factors of loading resistance were 1.65 and 2.50 for the HAS and the FAIT antennas, respectively, and the experimental result for the HAS antenna was consistent with the estimation. Therefore, higher power injection will be possible.

Measurements of ICRF wave-induced density fluctuations in LHD by a microwave reflectometer

An O-mode microwave reflectometer has been developed to measure ICRF wave induced electron density fluctuations in LHD plasmas. The system has two probing frequencies (28.8 and 30.1 GHz) to measure two spatial points simultaneously. The rms density fluctuation levels are typically 0.01%. The linearity between the measured density fluctuation amplitude and the square root of the RF power is discussed. The decay length of the RF field was estimated to be 1 to 7 m under the operational condition investigated. A typical spatial distance between the two measurement points corresponding to the two probing frequencies is a few centimeters, and the fluctuation amplitudes at the two points are similar in amplitude. The phase difference between the two fluctuations show in-phase relationship on average. Out-of phase relationships, which implies a standing wave structure, are often observed when the wave absorption is expected to be poor.