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In order to understand the atomic hydrogen distribution in different kinds of plasma and its influence on the recycling, two kinds of plasmas were used: non-confined annular electron cyclotron resonance (ECR) and confined long duration plasmas. The permeation probes are used to measure directly the atomic hydrogen flux at several poloidal positions. The permeation through metals due to the ion and atom component of the hydrogen flux to the wall is indistinguishable. To estimate the contribution of the ions directly, Langmuir probes were used. The Гinc profile behind the plasma facing components (PFCs) is almost constant, ∼2 ×1018 H/s/m2.

In order to confirm the formation of a tungsten fiberform nanostructure (W-fuzz) by helium plasma exposure in the large-sized plasma confinement device, the bulk tungsten with the size of 80 × 20 × 1.5 mm3 was inserted into the divertor leg position in the Large Helical Device (LHD). Then, it was exposed to the divertor plasma during the ultra-long pulse helium discharges with 10,190s in total. The width of the divertor plasma, incident ion energy and total fluence were expected to be ∼2cm, 100–200eV and ∼5 ×1025 He/m2 (strike position), respectively. The surface temperature of the tungsten specimen was monitored by IR camera. The typical surface temperature of the divertor strike point was estimated to be around 1900∼2300K. After the exposure, an initial growth phase of tungsten fiberform nanostructure (W-fuzz) was able to be identified on the tungsten surface. The finest initial growth phase of the W-fuzz structure was able to be identified on the central region of the divertor strike point, where retention amount of helium was estimated to be ∼8 ×1021 He/m2. This study is the first simultaneous evaluation of the W-fuzz growth and quantification of the helium retention in the large-sized plasma confinement device. Keywords: W-fuzz, TEM observation, Ion beam analysis, LHD

The ion cyclotron range of frequencies (ICRF) heating power injection in the hydrogen experiment in LHD was demonstrated after the upgrade of ICRF antennas. The ICRF wave couples and accelerates the energetic particles injected by perpendicular-NBIs with 40 keV. The simulation by the MORH code shows the existence of energetic particles around the ICRF third harmonic resonance layers. As the result of ICRF heating power deposition, the beta value increased by 0.2% in absolute beta mainly due to the increased energetic particle content. The increase of energetic ions particularly around 60 keV, which should be accelerated by the ICRF heating, is observed. The ICRF heating efficiency was approximately 30%–50%, estimated from the break-in-slope analysis at the turn off timing of ICRF power from the stored energy measured by diamagnetic loops. This increase of the stored energy is mostly the contribution of the increased energetic particles. The heating efficiency increases as the density increases. [ABSTRACT FROM AUTHOR]

Megawatt-class gyrotrons covering a wide frequency range (14 GHz–300 GHz) are in increasing demand for nuclear fusion. Recent electron cyclotron heating and electron cyclotron current drive experiments highlight a requirement of megawatt-scale gyrotrons at a relatively lower frequency (14–35 GHz) range of some plasma devices, like GAMMA 10/PDX of the University of Tsukuba, QUEST of Kyushu University, NSTX-U of Princeton Plasma Physics Laboratory, and Heliotron J of Kyoto University. Collaborative studies for designing a new 28 GHz/35 GHz dual-frequency gyrotron and a 14 GHz gyrotron have commenced. Operation above 1 MW of 28 GHz/35 GHz dual oscillation was demonstrated experimentally. Further in the design of dual-frequency gyrotron, operations with 2 MW 3 s and 0.4 MW CW (continuous wave) at 28 GHz, and power exceeding 1 MW for 3 s at 34.8 GHz have been shown to be feasible. The 14 GHz gyrotron is expected to operate above 1 MW. We are also developing higher frequency gyrotrons (77–300 GHz). The joint program of National Institute for Fusion Science and the University of Tsukuba developed two new 154 GHz gyrotrons for the large helical device after the demonstration of three 77 GHz gyrotrons. The 154 GHz gyrotrons achieved a maximum output power of 1.25 MW and quasi-CW operation of 0.35 MW for 30 min. [ABSTRACT FROM AUTHOR]

Non-inductive plasma current start-up and sustainment by waves in the lower-hybrid frequency range (200 MHz) have been studied on the TST-2 spherical tokamak (R0 ⩽ 0.38 m, a ⩽ 0.25 m, Bt0 ⩽ 0.3 T, Ip ⩽ 0.14 MA) using three types of antenna: the 11-element inductively-coupled combline antenna, the dielectric loaded 4-waveguide array antenna, and the 13-element capacitively-coupled combline (CCC) antenna. The maximum plasma currents of 15 kA, 10 kA and 16 kA were achieved, respectively. The highest current drive figure of merit was achieved by the CCC antenna. The efficiency of current drive should improve by reducing prompt orbit losses of high energy electrons by operating at higher plasma current (to improve orbit confinement) and higher toroidal magnetic field (to improve wave accessibility to the plasma core), while keeping the density high enough (to avoid excessive acceleration of electrons), but under the ‘density limit’. [ABSTRACT FROM AUTHOR]

Effectiveness of high harmonic electron cyclotron resonance heating (ECRH) was investigated by both experiments and ray-trace analyses. The conditions of both the EC wave injection and the magnetic field configuration were optimized in the large helical device. In the case of the second harmonic ordinary mode injection with a frequency of 77 GHz and with the optimized injection angle, about 30–40% absorption could be kept beyond the cut-off density of the second harmonic extraordinary (X2) mode, which is 3.7 × 1019 m−3. In the third harmonic X (X3) mode heating experiment, the dependence of the absorption rate on plasma density and temperature of the target plasma was precisely investigated and compared with the ray-trace (TRAVIS code) calculation. The calculation results of the absorption rate show fairly good agreement with the experimentally obtained ones on the plasma-parameter dependences. The maximum absorption rate in the X3 heating experiment attained approximately 40% around the electron density of 1.5 × 1019 m−3 and the electron temperature of 1.2 keV. Superposed stepwise injection from three gyrotrons with a total of 3 MW increased the central electron temperature to about 3.5 times of the initial target plasma temperature of 0.6 keV. This shows that the temperature increase improves the absorption rate of the subsequent injection. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]