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Optical diffusion is an essential process used to manage photons in a wide range of photoelectric systems. This work proposes an approach to fabricate novel optical diffusers by a plasma-processing technique, using helium plasma to form fiberform nanostructures on tungsten thin films. After an annealing procedure in the air for oxidation, the optical properties and the light-diffusing abilities of these nanostructured thin films were studied. The diffusion efficiency of the optical diffusers was analyzed using a transmitted scatter distribution function (TDF), and it was revealed that the diffusion efficiency of a device with an irradiation time of 30 minutes could reach 97%.

From pulsed plasma experiments focusing on neutral pressure dependence, the impacts of a transition from a low to a high recycling target on the particle load were investigated and discussed in the linear plasma device, Magnum-PSI. Time traces of the target ion flux were mitigated in high neutral pressure cases because of a plasma-neutral interaction. On the other hand, in low neutral-pressure cases, the target ion flux indicated partial suppression in the last part of the pulse. The Langmuir probe, located 200 mm upstream from the target plate, did not exhibit such a suppression. Pulse suppression can be expected from the localized interaction between recycled neutral flux and pulsed plasma in front of the target. The mean-free paths of recycled neutral particles regarding the charge exchange with pulse ions and elastic scattering with background neutral particles were compared. Modeling using a fluid code coupled with a neutral transport code was performed, and it was concluded that dynamic pressure induced by the transient recycled neutral flux caused sufficient momentum loss to stagnate the pulsed plasma toward the target plate.

ITERではタングステン(W)ダイバータ板等で反射した光の混入によるプラズマ発光計測の精度悪化が問題視されており、この反射特性は壁面の表面状態に依存する。本研究では、様々な熱負荷を与えたITERダイバータ用Wモノブロックに対してゴニオフォトメーターを用いて双方向反射率分布関数(BRDF)を計測した。熱負荷を与える前のWモノブロックでは切削痕によって異方的な反射特性を示す一方、熱負荷を与えたWモノブロックでは再結晶化によって表面が粗くなり、拡散的な反射特性を示した。熱負荷による表面構造変化に応じたBRDFの変化について報告する。, 第38回 プラズマ・核融合学会 年会

ITERのダイバータは光の反射率の高いタングステン（W）で製造されるため、プラズマ分光計測器の測定光にダイバータ表面からの反射光が混入し、プラズマ発光量評価の精度が低下する恐れがある。反射光はダイバータ板表面の状態によって特性が変化するため、光の反射特性とダイバータ表面の状態の関係を把握することが重要となる。 本研究では、直線型プラズマ装置NAGDIS-IIで生成したアルゴンプラズマの照射によりスパッタリングしたW試料の双方向反射率分布関数（BRDF）をゴニオフォトメータを用いて調べた。プラズマ照射前のW試料表面には紙やすりで傷を作成し、ITERダイバータのWモノブロックの表面粗さ（Ra）を模擬している。W試料へのイオン入射エネルギーを変化させたときの、RaとBRDFの変化について報告した。, The 30th International Toki Conference on Plasma and Fusion Research

We have investigated detailed plasma structure around the recombination front region (RFR), where volume electron-ion recombination (EIR) strongly occurs in detached plasmas, by using two-dimensional (2D) movable probe system as well as laser Thomson scattering diagnostics. The 2D distribution of electron density, electron temperature, and plasma potential around the RFR, has been revealed for the first time. The spatial distribution of the particle loss rate due to EIR estimated from electron temperature and electron density profiles indicated that the three-dimensional structure of the RFR adopts a cone structure. Furthermore, in the peripheral region of the RFR, the radial profile of the electron density was widened locally with increasing cross-field transport caused by plasma fluctuation. From the heat transport analysis of electrons, it was found that the thermal relaxation process between electrons and ions is important for the cooling of electrons, and that the convection of electrons regulates electron heat transport near the RFR. Keywords: Divertor, Detached plasma, Volume recombination, Recombination front

To increase the accuracy of a particle, momentum, and energy source terms in the detached helium plasma simulation, rate coefficients with the collisional-radiative model were introduced into the fluid code LINear Divertor Analysis (LINDA). Obtained effective rate coefficients and related source terms were compared with those from the conventional empirical databases. It is shown that a high-density condition in future fusion devices causes larger deviation between the effective and the empirical source terms. One-dimensional detached plasma simulation indicated that the peak amplitude of the plasma density during the rollover is sensitive to the source term difference related to the recombination. This study additionally revealed that the heating effect in the three-body recombination process strongly affects the detached plasma formation and downstream plasma parameters., ファイル公開：2021/10/07

Three-dimensional spatiotemporal dynamics of detached helium plasma parameters along time, radius, and magnetic field were revealed in the linear device NAGDIS-II. To measure plasma parameters before and after the radial plasma ejection that was enhanced around the volume-recombining region, the conditional averaging technique was applied. The radial ejection was found to correlate with low-frequency changes of plasma-column parameters, which seemed to suppress the axial movement of the recombining region. Moreover, an azimuthal charge separation inside the ejected structure was observed, similar to the typical edge transport phenomenon: blobby plasma transport. The neutral flow effect was suggested as a candidate of the driving force.

Multipoint measurements were carried out by employing a microwave interferometer (MI) and a Langmuir probe (LP) in steady-state detached plasmas in the linear plasma device NAGDIS-II to reveal the structure of fluctuations along the magnetic field. We changed the LP position along the magnetic field while the MI was fixed at an upstream position. In addition, a fast framing camera was used to identify an azimuthal mode number, and the predominant mode number was identified as m = 1. By analyzing correlations between signals observed by the LP and the MI, it was found that a time delay of 10–20 kHz fluctuations gradually decreased toward the downstream direction. The results indicate a decrease in the rotation velocity in the E × B direction, and suggest that the 10–20 kHz fluctuation forms a spiral shape.