Recently, experiments on basic plasma physics issues for solving future problems in fusion energy have been performed on a Large Helical Device. There are several problems to be solved in future devices for fusion energy. Emerging issues in burning plasma are: alpha-channeling (ion heating by alpha particles), turbulence and transport in electron dominant heating helium ash exhaust, reduction of the divertor heat load. To solve these problems, understanding the basic plasma physics of (1) wave–particle interaction through (inverse) Landau damping, (2) characteristics of electron-scale (high- k ) turbulence, (3) ion mixing and the isotope effect, and (4) turbulence spreading and detachment, is necessary. This overview discusses the experimental studies on these issues and turbulent transport in multi-ion plasma and other issues in the appendix.

The shape of a plasma meniscus is a key factor to determine the beam focusing. The physics model of the meniscus formation for hydrogen negative ion sources has not been established yet. A backward trajectory calculation based on experimental observation is performed in order to derive the particle information at the meniscus. It is observed that the negative ion density is spatially nonuniform in the direction parallel to the magnets for suppression of co-extracted electrons. A nonuniformity of the negative ion density in the vicinity of the meniscus is taken into account in the forward trajectory calculation. It reveals that the nonuniform negative ion distribution leads to degradation of the beam focusing and the beam splitting in phase space. The importance of the spatial distribution of negative ions on meniscus modelling is discussed with a comparison to uniform extraction model.

Improvement of the performance on a hydrogen/deuterium negative ion source for a nuclear fusion device is reported. In particular, the suppression of the co-extracted electron current, I e , is an important issue to ensure the stable beam acceleration. Improvement of the I e has been confirmed by optimizing the magnetic field of the electron deflection magnet in the extraction grid. Two other new methods for reduction of the I e were validated. The first was an electron fence whose rods were set between the rows of apertures on a plasma grid. The electron and negative ion current ratio, approximately I e /I acc , was greatly improved from 0.7 to 0.25 in deuterium. The second was an outer iron yoke which enhanced the magnetic flux density 19% inside the arc discharge chamber. The I e /I acc using the outer yoke decreased by 0.1 compared with using a normal magnetic filter in a deuterium operation. These attempts have improved the total deuterium injection beam power of 8.4 MW by three negative ion based NBIs.

Improvement of deuterium injection power in the negative-ion-based NBIs (n-NBIs) for the Large Helical Device (LHD) are reported. Co-extracted electron current at acceleration of deuterium negative ions (D − ions) limits the injection power. The electron current is reduced by decreasing the extraction gap, and the injected D − current evaluated from the injection power increased from 46 to 55 A. Greater electron reduction was achieved by installing a structure named an ‘electron fence’ (EF), with which D − beam power was successfully improved from 2.0 MW to 3.0 MW. The injection power in three configurations − without EF, with EF of 5 mm and 7 mm distance from the plasma grid (PG) surface − have been compared in both cases of hydrogen and deuterium operations, and it was found that the configuration with the EF of 5 mm distance was the best to satisfy the performance for both of hydrogen and deuterium injections. Although the co-extracted electron current is reduced in the negative ion sources applied for JT-60SA and ITER by utilizing the PG filter, it is possible to achieve more effective electron reduction by combining the PG filter and the EF.

An isotope effect of negative ion motion in plasma near a plasma grid (PG), which is a plasma-beam boundary grid, has been investigated in a negative hydrogen-ion source with the surface production process of the negative ion on the PG surface. Negative deuterium-ion (D - ) density was higher than negative hydrogen-ion (H - ) density in the condition without extraction-grid bias ( V egb ) and approached the H - density in the condition with V egb . Thus, the D - density responded stronger than the H - density to the V egb . The density of the negative hydrogen-ion isotopes and the density response to the V egb have been organized into respective identical trends by momenta of isotopes emitting from the PG. The Larmor motion can be a dominant mechanism of the negative ion transport from the PG to the plasma.

Flows of the charged particles in hydrogen (H) and deuterium (D) plasmas were measured with the single-tip directional Langmuir probe. The flow patterns of positive and negative ions have been constructed from the data by moving the probe near the plasma grid. The positive ion flow from driver to extraction regions showed the different velocity equivalent to the mass difference between H and D. The negative ion flow indicated the surface production of negative ions on the plasma grid, and changed the direction corresponding to the magnitude of the extraction electric field.

Beam instability in the presheath region of negative ion beam extraction is investigated in theoretically and experimentally. The linear stability analysis shows that the beam instability is unstable due to coupling between positive ion flow and negative ion flow. On the other hand, no clear activity can be seen in the experiment in the frequency range predicted by the theory. The beam instability in the presheath region of negative ion beam extraction may not cause the degradation of the beam focusing because of collisional damping and/or Landau damping.

The extraction mechanism of negative ion beams is affected by the behavior of positive and negative ions near the extraction apertures. Flow of the charged particles in a hydrogen discharge were measured with a newly developed directional photodetachment probe which has a single probe tip. Compared with our previous multiple-tip directional probe, the single-tip directional probe was able to measure fine flow structure of the positive and the negative ions without spatial ambiguity associated with electron deflection magnetic field and density gradient. The single-tip probe observed the turning of the flow of the negative hydrogen ion by increasing the bias voltage in the extraction region of the negative ion source.

The difference of the stripping loss between hydrogen and deuterium is examined using two approaches. The first is the measurement of the optical beam emission. The wavelength of beam emission spectrum reflects the energy distribution of beam particles by the Doppler effect. The low-energy stripping peak is observed in the energy band corresponding to the extraction voltage, and also a moderate shoulder is distributed in the lower energy region. Secondly, the spatial and the energy distribution in the accelerator is estimated by the attenuation calculation using the vacuum pressure distribution in the accelerator. Stripping neutrals are concentrated in the low energy region, and a peak is formed at 9 keV in the energy distribution due to stripping neutrals inside the extraction grid aperture. The total stripping loss inside the accelerator is 16% for hydrogen and 24% for deuterium. The calculated Doppler-shifted spectra for hydrogen and deuterium clearly show the peak with the moderate shoulder on the redshift side, which is consistent with the measured results.

Density distributions of negative hydrogen (H-) ions and negative deuterium (D-) ions were measured with the laser photodetachment method in the extraction region of the negative ion source. The distribution of H- ion density peaks at the center of the ion source, while that of the D- ion shows a flatter profile in the direction parallel to the plasma grid. The positive ion densities of hydrogen and deuterium estimated from the positive saturation current indicate similar profiles with different amounts close to the grid. The difference in the H- ion and D- ion distributions can be explained by the difference in the negative ion yield and the survival probability of the ions due to the isotope effect.

Beam focusing is one of the most important elements for the stable and safe operation of high power negative ion beams, such as neutral beam injection into magnetically confined fusion plasmas. In order to investigate impacts of the source plasma fluctuation on beam focusing, a simultaneous measurement of the source plasma fluctuation and the beam current profile has been carried out in the research-and-development negative ion source at the National Institute for Fusion Science. The responses of beam width and of the beam centre deviation are observed for the first time, indicating the importance of the source plasma stability for the negative ion beam focusing. (C) 2020 The Japan Society of Applied Physics

Simultaneous measurement of negative ion source plasma and extracted beam is carried out in order to clarify a key plasma parameter governing the meniscus formation in negative ion sources for fusion. The plasma discharge is performed with various discharge powers at different bias voltages in order to vary the plasma parameters. It is shown that the beam width changes along the same curve with respect to the negative ion density at any bias voltage while it varies along different curves with other plasma parameters depending on the bias voltage. This implies that the mechanism of meniscus formation in negative ion sources could be described along the similar manner as positive ion sources.

The deuterium experiment with various source plasma diagnostics was conducted at the research and development negative ion source at National Institute for Fusion Science, which is a cesium-seeded large-scale negative hydrogen-ion source with half discharge volume and the same type of ion source for large helical device. A change of filling gas from hydrogen to deuterium increased line-averaged negative ion density by a factor of 1.3 in beam extraction region as a result of flattened profile with keeping central density. Positive ion and electron densities increased threefold. This was influenced by the increase of plasma density in the plasma generation region. The plasma space potential varied as equivalent of the collisionless sheath theory in electro-positive plasma expected by the hydrogen isotope gas species, although the negative ion could affect sheath formation. Higher atomic cesium density was observed because of higher sputtering yield due to higher momentum of deuterium.

Second deuterium operation of the negative ion based neutral beam injector was performed in 2018 in the large helical device. The electron and ion current ratio improves to Ie/Iacc(D) = 0.31 using the short extraction gap distance of 7 mm between the plasma grid (PG) and the extraction grid (EG). The strength of the magnetic field by the electron deflection magnet installed in the EG increases by 17% at the PG ingress surface, which effectively reduces the electron component in the negative ion rich plasma in the vicinity of PG apertures. The reduction of the electron current made it possible to operate at a high power arc discharge and beam extraction. Then, the deuterium negative ion current increases to 55.4 A with the averaged current density of 233 A/m2. The thermal load on the EG using 7 mm gap distance is 0.6 times smaller than the thermal load using a 8 mm gap caused by the reduction of coextracted electron current. The injection beam power increases to 2.9 MW in the beam line BL3, and the total beam injection power increases to 7 MW by three beam lines in the second deuterium campaign.

The achievements of the deuterium beam operation of a negative-ion-based neutral beam injector (N-NBI) in the large helical device (LHD) are reported. In beam operation in LHD-NBIs, both hydrogen (H) and deuterium (D) neutral beams were generated by changing the operation gas using the same accelerator. The maximum accelerated deuterium negative-ion current (I-acc(D)) reaches 46.2 A from two beam sources with the averaged current density being 190 A m(-2) for 2 s, and the extracted electron to accelerated ion current ratio (I-e/I-acc(D)) increases to 0.39 using 5.6V high bias voltage in the first deuterium operation in 2017. An increase of electron density in the vicinity of the plasma grid (PG) surface, which is considered the main reason for the increase of co-extracted electrons in a beam, is confirmed by the half-size research negative-ion source in the neutral beam test stand at the National Institute for Fusion Science (NIPS). The deuterium negative-ion density is also larger than the hydrogen negative-ion density in the vicinity of the PG surface using the same discharge conditions. In the latest experimental campaign in 2018, I-acc(D) increases to 55.4 A with the averaged current density being 233 A M(-2 )for 1.5 s using the shot extraction gap length. The low I-e/I-acc(D) of 0.31 can be maintained by using high discharge power. The various parameters mentioned above are defined in detail below.

We report on the characteristics of the beam-generated plasma in the multibeamlet case of a hydrogen negative ion beam at NIPS. The plasma potential, and the energy of secondary particles in the drift region of an ion beam, offer an insight into the mechanisms that allow beam transport in low pressure gasses. The first measurements reported here were made by means of a four-gridded retarding field energy analyzer, combined with the measurement of the drain current at the beam dump, and with infrared beamlet monitoring technique. The retarding field analyzer measures the energy distributions of particles emitted radially from the beam-generated plasma [1]. The beam is dumped onto a graphite calorimeter, which is electrically insulated, so that it can be electrically biased and the collected current can be measured. The thermal image of the calorimeter is acquired during the beam pulse, thus offering a quantitative estimation of the single beamlet optics and multibeamlet focusing. Finally, the influence of the neutral gas density is studied by puffing hydrogen gas in this drift region. The measured ion and electron energy distribution functions are strongly affected by the electric bias of the calorimeter. For a 47kV, 0.3A beam, an electron temperature of 3eV at the plasma edge was found. The energy distribution of the ions for a pressure of about 0.03Pa is presented, from which the plasma potential can he inferred.

x-ray magnetic circular dichroism K. Ikeda,1 T. Seki,2 G. Shibata,1 T. Kadono,1 K. Ishigami,1 Y. Takahashi,1 M. Horio,1 S. Sakamoto,1 Y. Nonaka,1 M. Sakamaki,3 K. Amemiya,3 N. Kawamura,4 M. Suzuki,4 K. Takanashi,2 and A. Fujimori1 1)Department of Physics, University of Tokyo, Hongo 113-0033, Japana) 2)Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan 3)Institute of Materials Structure Science, KEK, Tsukuba 305-0801, Japan 4)JASRI, 1-1-1 Kouto, Sayo, Hyogo 679-5198, Japan

近年我々が開発し,PF BL-16Aにおいて運用を行っているベクトルマグネット型X線磁気円二色性(XMCD)装置を用 いて,SrTiO3 (STO)および LaAlO3 (LAO)基板上に成長させた強磁性 La1-xSrxMnO3 (LSMO)薄膜の角度依存 XMCD測定を行 い,薄膜中のスピン分布異方性,すなわちスピン分極した電子の軌道占有状態の直接観測を試みた。XMCDスペクトルの 磁場方向依存性の測定により,基板応力に由来するスピン密度分布の異方性の変化を観測することができた。得られた結 果と先行研究のX線直線二色性との比較から,スピン分極した電子とそうでない電子との間で軌道占有状態に差が見られ ることが示唆された。 場合は,LSMO薄膜と基板との格子定数の差に応じて薄膜 が一軸性の応力を受けるため,それによっても物理的特性 が大きく変化する。例えば Konishiらは [2],LSMO (x=0.30.5)薄膜を格子定数の異なる複数の基板の上に堆積させ, それらの輸送特性・磁気特性を調べることにより,面内方 向伸張性応力の時には Aタイプ反強磁性相に,面内方向 圧縮性応力の時には Cタイプ反強磁性絶縁体相に変化す る傾向があることを提案した。また第一原理計算の結果 との比較から,伸張性応力の場合にはMnの dx2-y2軌道が, 圧縮性応力の場合は d3z2-r2軌道がそれぞれ優先的に占有さ れることも予言している [2]。この他に,強磁性 LSMO薄 膜の磁化容易軸が,基板応力の正負に応じて面内または面 直に変化することも知られており [3-4],磁気異方性とMn 3d電子の軌道占有との間に関係性があることを示唆する ものとなっている。 しかしながら,過去に実験的に観測されたMn 3d電子 の軌道占有状態は必ずしも第一原理計算の結果どおりには なっていない。X線直線二色性(XLD)の先行研究 [5-7] によると,伸張性基板の SrTiO3 (001) (STO)と圧縮性基板 の LaAlO3 (001) (LAO)のどちらを用いた場合でも,面直方

The consumption rate of cesium (Cs) for negative hydrogen (H-) ion source increases when the source operation gas is changed from hydrogen to deuterium. There was observed a clear indication that a deuterium discharge erodes Cs atoms on the plasma grid (PG) surface to increase work function and the co-extracted electron current. We have proposed a model, that the enhanced sputtering yield of Cs from the PG due to deuterium ions, which carries more kinetic energy to Cs adsorbed on the surface directly or indirectly, is the main reason for this fast dissipation of Cs. Introduction of helium (He) into discharge can verify the enhanced sputtering effect due to the lager mass ions in the source discharge. Comparing the effect due to seeded Cs before and after the He injection into discharge through Cs OES signals as well as the H- density measured with cavity ring-down method, the sputtering/evaporation enhancement due to He is estimated. Neutral atoms and positive ions in the He discharge should cause enhanced sputtering like deuterium, while the system does not generate neutron under the induction of acceleration voltage to diagnose the extracted negative ion beam. Plasma parameters of the H2 and He plasmas are investigated by diagnostics tools installed on NIFS-RNIS (National Institute for Fusion Science, Research and development Negative Ion Source) together with H- density measurement by cavity ring down. Enhanced consumption rate of Cs is compared with proposed sputtering yield data to predict the rate for deuterium operation of negative ion sources.The consumption rate of cesium (Cs) for negative hydrogen (H-) ion source increases when the source operation gas is changed from hydrogen to deuterium. There was observed a clear indication that a deuterium discharge erodes Cs atoms on the plasma grid (PG) surface to increase work function and the co-extracted electron current. We have proposed a model, that the enhanced sputtering yield of Cs from the PG due to deuterium ions, which carries more kinetic energy to Cs adsorbed on the surface directly or indirectly, is the main reason for this fast dissipation of Cs. Introduction of helium (He) into discharge can verify the enhanced sputtering effect due to the lager mass ions in the source discharge. Comparing the effect due to seeded Cs before and after the He injection into discharge through Cs OES signals as well as the H- density measured with cavity ring-down method, the sputtering/evaporation enhancement due to He is estimated. Neutral atoms and positive ions in the He discharge should cause enhance...

To evaluate negative ion beam properties, a dual beamlet monitor system has been developed. The dual beamlet monitor system has two diagnostics in one hexagonal box. One diagnostic is a "fast beamlet monitor" for measuring the time evolution of beamlet current profiles with the time resolution of up to 25 MHz. The other diagnostic is a "pepper-pot-type phase space analyzer," which is applied for the evaluation of a phase space structure of the negative ion beamlet. The dual beamlet monitor system is applied to the measurement of the beamlet in the Neutral Beam Test Stand at National Institute for Fusion Science (NIFS-NBTS), in which the beam accelerator is almost identical to those of working beam injectors in the large helical device. It is demonstrated that the overlapping components from the neighboring beamlet can be eliminated, and the phase space structure can be obtained for the single beamlet.