Recent Fusion Research in the National Institute for Fusion Science

The National Institute for Fusion Science (NIFS), which was established in 1989, promotes academic approaches toward the exploration of fusion science for steady-state helical reactor and realizes the establishment of a comprehensive understanding of toroidal plasmas as an inter-university research organization and a key center of worldwide fusion research. The Large Helical Device (LHD) Project, the Numerical Simulation Science Project, and the Fusion Engineering Project are organized for early realization of net current free fusion reactor, and their recent activities are described in this paper. The LHD has been producing high-performance plasmas comparable to those of large tokamaks, and several new findings with regard to plasma physics have been obtained. The numerical simulation science project contributes understanding and systemization of the physical mechanisms of plasma confinement in fusion plasmas and explores complexity science of a plasma for realization of the numerical test reactor. In the fusion engineering project, the design of the helical fusion reactor has progressed based on the development of superconducting coils, the blanket, fusion materials and tritium handling

Blob/Hole Generation in the Divertor Leg of the Large Helical Device

We have analyzed ion saturation current fluctuation measured by a fast scanning Langmuir probe (FSP) in edge region of the Large Helical Device (LHD). Positive and negative spikes of the ion saturation current were observed in the private region and on the divertor leg, respectively. It was found that the boundary position between these regions corresponds to the low-field side (LFS) edge of the divertor leg where the gradient of the ion saturation current profile was the maximum. Such a positional relationship resembles that near the separatrix in the LFS in tokamaks, where blobs and holes are generated. Statistical analysis indicates similar fluctuation characteristics among different magnetic devices

The Effect of Non-Axisymmetry of Magnetic Configurations on Radial Electric Field Transition Properties in the LHD

Transition property of the radial electric field (Er) in LHD have been theoretically investigated and also applied to explain experimental results. Especially, effects of the helicity of the magnetic configuration on the condition to realize the electron root are examined. Larger helicity makes the threshold collisionality higher. This is attributed to the nonlinear dependence of Γe(Er) in a low collisional regime. This interesting feature predicts that the threshold temperature becomes higher for a case of smaller helicity. The variation of the threshold density anticipated from the analysis for cases with different magnetic axis position is qualitatively verified in the density scan experiment

New Morphological Factor for Predicting Late Proximal Type I Endoleak after Endovascular Aneurysm Repair

Background: Although we have witnessed several cases of late proximal type I endoleak (T1AEL) after endovascular aneurysm repair (EVAR), most patients did not have “hostile neck” preoperatively. We hypothesized that the distance between the lowest renal artery and the neck angulation point and neck length are the 2 most important factors for maintaining long-term proximal sealing. This study evaluated “neck hostility,” which is the product of the distance to the angulation point and the neck length, as a preoperative morphological risk factor for the development of late T1AEL after EVAR. Methods: A retrospective review of a prospectively assembled database was performed for all patients who had undergone EVAR at a single institution from June 2007 to May 2017. Patient demographics and preoperative imaging data were collected, and Cox regression analysis was performed to identify the risk factors for late T1AEL. Results: Of the 655 patients who underwent EVAR during the study period, 115 were excluded due to complex EVAR (n = 14), primary indications for iliac aneurysms (n = 86), primary T1AEL (n = 3), or other reasons (n = 15). Of the remaining 537 patients, twelve patients (2.2%) developed late T1AEL a median of 3.2 (interquartile range [IQR]; 3.0, 5.4) years after EVAR. Receiver operating characteristic (ROC) curve analysis revealed a neck hostility cutoff value of 8. Cox regression analysis revealed that a neck hostility value ≤8 and conical neck anatomy were risk factors for the development of late T1AEL after EVAR. Well-known hostile neck factors such as short neck, severe angulated neck, and severe calcification/thrombus in the proximal neck were not significantly different. Conclusions: The present study demonstrated a correlation between late T1AEL and the product of the angulation distance and the neck length. This factor may be useful for predicting poor late proximal outcomes after EVAR.journal articl

Design of a Closed Helical Divertor in LHD and the Prospect for Helical Fusion Reactors

A new closed helical divertor configuration for efficient particle control and reduction of the heat load on the divertor plates is proposed. The closed divertor configuration practically utilizes an ergodic layer and magnetic field line configuration on divertor legs in helical systems. For optimization of the design of the closed divertor, the distribution of the strike points is calculated in various magnetic configurations in the Large Helical Device (LHD). It suggests that the installation of the closed divertor components in the inboard side of the torus under an inward shift configuration (Rax=3.60m) is the best choice for achieving the above two purposes. This divertor configuration does not interfere with plasma heating and diagnostic systems installed in outer ports. The prospect of the closed divertor configuration to a helical fusion reactor is investigated using a three-dimensional neutral particle transport simulation code with a one-dimensional plasma fluid calculation on the divertor legs. The investigation shows efficient particle pumping from the in board side and reduction of the heat load due to the combined effect of the optimized closed divertor geometry, ergodized divertor legs, and low electron temperature in the ergodic layer. It indicates a promising closed divertor configuration for helical fusion reactors

Impurity emission characteristics of long pulse discharges in Large Helical Device

Line spectra from intrinsic impurity ions have been monitored during the three kinds of long-pulse discharges (ICH, ECH, NBI). Constant emission from the iron impurity shows no preferential accumulation of iron ion during the long-pulse operations. Stable Doppler ion temperature has been also measured from Fe XX, C V and C III spectra

Self-efficacy is an independent predictor for postoperative six-minute walk distance after elective open repair of abdominal aortic aneurysm

Objectives: Open surgery is performed to treat abdominal aortic aneurysm (AAA), although the subsequent surgical stress leads to worse physical status. Preoperative self-efficacy has been reported to predict postoperative physical status after orthopedic surgery; however, it has not been sufficiently investigated in patients undergoing abdominal surgery. The purpose of the present study is to investigate the correlation between preoperative self-efficacy and postoperative six-minute walk distance (6MWD) in open AAA surgery. Methods: Seventy patients who underwent open AAA surgery were included. Functional exercise capacity was measured using preoperative and 1 week postoperative 6MWD. Self-efficacy was preoperatively measured using self-efficacy for physical activity (SEPA). The correlations of postoperative 6MWD with age, height, BMI, preoperative 6MWD, SEPA, Hospital Anxiety and Depression Scale (HADS) score, operative time, and blood loss were investigated using multivariate analysis. Results: Single regression analysis showed that postoperative 6MWD was significantly correlated with age (r = −0.553, p ≤ 0.001), height (r = 0.292, p = 0.014), Charlson’s comorbidity index (r = −0.268, p = 0.025), preoperative 6MWD (r = 0.572, p ≤ 0.001), SEPA (r = 0.586, p ≤ 0.001), and HADS-depression (r = −0.296, p = 0.013). Multiple regression analysis showed that age (p = 0.002), preoperative 6MWD (p = 0.013), and SEPA (p = 0.043) score were significantly correlated with postoperative 6MWD. Conclusions: Self-efficacy was an independent predictor for postoperative 6MWD after elective open AAA surgery. This suggests the importance of assessing not only physical status but also psychological factors such as self-efficacy.Published online: 21 Feb 2017journal articl

Study of the effect of a closed divertor configuration on neutral particle control in the LHD plasma periphery

An optimized closed divertor configuration for effective particle control in LHD is proposed from the viewpoints of the distribution of the strike points and neutral particle transport. Calculations of the distribution of the strike points indicate that 50% of the strike points locate in the inboard side of the torus in a standard magnetic configuration (Rax = 3.60 m). The ratio increases to 80% by installing target plates near lower/upper ports. A three-dimensional neutral particle transport simulation shows that installation of closed divertor components with the target plates raise the neutral pressure in the inboard side by more than one order of magnitude compared to that in the present open divertor case. The analysis of the neutral particle transport predicts that enhancement of the neutral pressure becomes moderate in outward shift configurations (Rax > 3.75 m)

Recent Results from LHD Experiment with Emphasis on Relation to Theory from Experimentalist’s View

he Large Helical Device (LHD) has been extending an operational regime of net-current free plasmas towardsthe fusion relevant condition with taking advantage of a net current-free heliotron concept and employing a superconducting coil system. Heating capability has exceeded 10 MW and the central ion and electron temperatureshave reached 7 and 10 keV, respectively. The maximum value of β and pulse length have been extended to 3.2% and 150 s, respectively. Many encouraging physical findings have been obtained. Topics from recent experiments, which should be emphasized from the aspect of theoretical approaches, are reviewed. Those are (1) Prominent features in the inward shifted configuration, i.e., mitigation of an ideal interchange mode in the configuration with magnetic hill, and confinement improvement due to suppression of both anomalous and neoclassical transport, (2) Demonstration ofbifurcation of radial electric field and associated formation of an internal transport barrier, and (3) Dynamics of magnetic islands and clarification of the role of separatrix

Three-dimensional neutral particle transport simulation for analyzing polarization resolved H-alpha spectra in the large helical device

Change of Hα intensity profiles depending on magnetic configurations is observed in the divertor plasma. It can be explained by the magnetic field line structures in the ergodic layer and the divertor legs. The behavior of neutral particles in the plasma periphery is investigated by a three-dimensional neutral particle transport simulation code which assumes that the distribution of the plasma flow onto the divertor plates corresponds to that of the strike points calculated by magnetic field line traces. Vertical Hα intensity profiles and polarization resolved Hα spectra are calculated by the simulation code including the effect of Doppler broadening, fine structure splitting and polarization of the Hα emission, which agree well with the measurements in various magnetic configurations. It shows spontaneous formation of high neutral density in inboard side of the torus, which is independent of the magnetic configurations in LHD