Internal Diffusion Barrier Research Articles

Drosophila \xdfHeavy-Spectrin is required in polarized ensheathing glia that form a diffusion-barrier around the neuropil

In the central nervous system (CNS), functional tasks are often allocated to distinct compartments. This is also evident in the Drosophila CNS where synapses and dendrites are clustered in distinct neuropil regions. The neuropil is separated from neuronal cell bodies by ensheathing glia, which as we show using dye injection experiments, contribute to the formation of an internal diffusion barrier. We find that ensheathing glia are polarized with a basolateral plasma membrane rich in phosphatidylinositol-(3,4,5)-triphosphate (PIP3) and the Na+/K+-ATPase Nervana2 (Nrv2) that abuts an extracellular matrix formed at neuropil-cortex interface. The apical plasma membrane is facing the neuropil and is rich in phosphatidylinositol-(4,5)-bisphosphate (PIP2) that is supported by a sub-membranous ßHeavy-Spectrin cytoskeleton. ßHeavy-spectrin mutant larvae affect ensheathing glial cell polarity with delocalized PIP2 and Nrv2 and exhibit an abnormal locomotion which is similarly shown by ensheathing glia ablated larvae. Thus, polarized glia compartmentalizes the brain and is essential for proper nervous system function.

Fueling requirements of super-high-density plasmas towards innovative ignition regime

Super-high-density plasma with an internal diffusion barrier which is observed in the Large Helical Device has been extrapolated to a fusion reactor grade plasma to explore an innovative ignition regime and to clarify essential requirements for pellet fueling. The peaked density profiles due to the internal diffusion barrier formation allow reduction in the required minimum temperature to sustain a self-burning plasma down to 10keV. Direct core fueling beyond the internal diffusion barrier is essential to sustain the peaked density profile. Furthermore, the acceptable pellet size becomes small in terms of fusion output perturbation because the effective volume of the burning plasma becomes small with the peaked density profile. In order to sustain a self-burning plasma with an internal diffusion barrier, therefore, extremely high velocity pellet injection beyond 10 km/s is inevitable unless another solution to the core fueling is found.

Electron Bernstein wave heating by electron cyclotron wave injection from the high-field side in LHD

In the Large Helical Device (LHD), evident electron Bernstein wave (EBW) heating was successfully performed. The experiment was carried out using the electron cyclotron heating (ECH) system that was upgraded by installation of high-power, long-pulse 77 GHz gyrotrons. The EBW heating was achieved by a mode conversion from injected EC wave to EBW, by the so-called slow-XB technique where an X-mode wave is injected to the plasma from the high magnetic field side. The specific magnetic configuration of LHD provides a good opportunity to realize the slow-XB technique, which is generally difficult for tokamaks. With the slow-XB technique, increases in kinetically evaluated electron energy Wpe and electron temperature Te were observed in overdense plasmas. An electron heating in the so-called super dense core plasma in LHD, which is characterized with an internal diffusion barrier and a steep density gradient at the plasma core, was successfully demonstrated in the plasma core region where the central electron density ne0 of 17 × 1019 m−3 was about 1.2 times higher, at the beginning of the EC-wave injection, than the left-hand cut-off density of applied 77 GHz EC waves.

Overview of results from the Large Helical Device

The physical understanding of net-current-free helical plasmas has progressed in the Large Helical Device (LHD) since the last Fusion Energy Conference in Geneva, 2008. The experimental results from LHD have promoted detailed physical documentation of features specific to net-current-free 3D helical plasmas as well as complementary to the tokamak approach. The primary heating source is neutral beam injection (NBI) with a heating power of 23 MW, and electron cyclotron heating with 3.7 MW plays an important role in local heating and power modulation in transport studies. The maximum central density has reached 1.2 × 1021 m−3 due to the formation of an internal diffusion barrier (IDB) at a magnetic field of 2.5 T. The IDB is maintained for 3 s by refuelling with repetitive pellet injection. In a different operational regime with moderate density less than 2 × 1019 m−3, a plasma with a central ion temperature reaching 5.6 keV exhibits the formation of an internal transport barrier (ITB). The ion thermal diffusivity decreases to the level predicted by neoclassical transport. In addition to the rotation driven by the momentum input due to tangential NBI, the existence of intrinsic torque to drive toroidal rotation is identified in the plasma with an ITB. This ITB is accompanied by an impurity hole which generates an impurity-free core. The impurity hole is due to a large outward convection of impurities in spite of the negative radial electric field. The magnitude of the impurity hole is enhanced in the magnetic configuration with a large helical ripple and for heavier atoms. Another mechanism for suppressing impurity contamination is identified at the plasma edge with a stochastic magnetic field. A helical system shares common physics issues with tokamaks such as 3D equilibria, transport in a stochastic magnetic field, plasma response to a resonant magnetic perturbation, divertor physics and the role of radial electric field and meso-scale structure.

High density and high temperature plasmas in Large Helical Device

Recently a new confinement regime called Super Dense Core (SDC) mode was discovered in Large Helical Device (LHD). An extremely high density core region with more than ∼ 1 × 1021 m−3 is obtained with the formation of an Internal Diffusion Barrier (IDB). The density gradient at the IDB is very high and the particle confinement in the core region is ∼ 0.2 s. It is expected, for the future reactor, that the IDB-SDC mode has a possibility to achieve the self-ignition condition with lower temperature than expected before. Conventional approaches to increase the temperature have also been tried in LHD. For the ion heating, the perpendicular neutral beam injection effectively increased the ion temperature up to 5.6 keV with the formation of the internal transport barrier (ITB). In the electron heating experiments with 77 GHz gyrotrons, the highest electron temperature more than 15 keV was achieved, where plasmas are in the neoclassical regime.

Advanced Operational Regime with Internal Diffusion Barrier on LHD

An interesting high-density operational regime with an internal diffusion barrier (IDB) has been extended to the helical divertor configuration in the Large Helical Device. The IDB is characterized by steep density gradient in core plasma and the attainable central density exceeds 1 × 1021 m−3 at the moderate magnetic field ~2.5 T while keeping relatively low density mantle plasma surrounding the core. In the IDB discharge, significant central pressure rise is observed, and the maximum central pressure attains 150 kPa by optimizing magnetic configuration. Such a high central pressure causes very large Shafranov shift, more than half radius, even at high magnetic field. Core fueling is absolutely essential for the IDB formation, and the IDB is reproducibly obtained by employing intensive multiple-pellet injections. The attainable density is restricted by lack of heat deposition at core plasma due to strong attenuation of a neutral beam in the high-density plasma.

Density Collapse Events Observed in the Large Helical Device

AbstractA core density collapse (CDC) phenomenon is a rapid collapse events observed in super dense core (SDC) plasma with internal diffusion barrier (IDB) in the Large Helical Device (LHD). By CDC, the central beta is decreased by up to 50%. The collapse starts from the edge region of the plasma. CDCs appear with plasma parameters where the high–n ballooning modes are unstable at ϱ ∼ 0.8. With less collisional conditions, m = 1 type oscillations are observed with similar beta profile. The origin of the m = 1 oscillations is not clarified (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Shafranov shift measurements by a soft x-ray CCD camera for internal diffusion barrier discharges in the Large Helical Device

Magnetic axis shift (Shafranov shift) in a high density operational regime with an internal diffusion barrier (IDB) formation in Large Helical Device experiments has been studied using a two-dimensional imaging system composed of a soft x-ray CCD camera and beryllium filters. Magnetic axis positions are derived from the fitting of pre-calculated equilibria to measured (line-integrated) soft x-ray profiles assuming that the emissivity profile could be expressed by Fourier–Bessel series expansion. It has been clearly observed that the Shafranov shift in a plasma with IDB is much larger than that without IDB at a similar average beta value measured by a diamagnetic loop because of the strongly peaked pressure profile in this regime. According to systematic analyses of the Shafranov shift for various high density plasmas in this regime, the dependence of the Shafranov shift on the central beta value is summarized and compared with that in discharges without IDB.

Development of net-current free heliotron plasmas in the Large Helical Device

Remarkable progress in the physical parameters of net-current free plasmas has been made in the Large Helical Device (LHD) since the last Fusion Energy Conference in Chengdu, 2006 (Motojima et al 2007 Nucl. Fusion 47 S668). The beta value reached 5% and a high-beta state beyond 4.5% from the diamagnetic measurement has been maintained for longer than 100 times the energy confinement time. The density and temperature regimes have also been extended. The central density has exceeded 1 × 1021 m−3 due to the formation of an internal diffusion barrier. The ion temperature has reached 5.2 keV at the density of 1.6 × 1019 m−3, which is associated with the suppression of ion heat conduction loss. Although these parameters have been obtained in separated discharges, each fusion-reactor relevant parameter has elucidated the potential of net-current free heliotron plasmas. Diversified studies in recent LHD experiments are reviewed in this paper.

High-density plasma with internal diffusion barrier in the Large Helical Device

An attractive high-density operational regime which is a so-called internal diffusion barrier (IDB) has been discovered in a helical divertor configuration on the Large Helical Device (LHD). The IDB is characterized by steep density gradients and the plasma profile is divided by the IDB into a high-density core plasma and a low density mantle plasma. The IDB enables the core plasma to access the high-density/high-pressure regime. The attainable central density exceeds 1 × 1021 m−3 and the central pressure reaches ≈1.5 times atmospheric pressure. Core pellet fuelling is absolutely essential for the IDB formation and it is reproducibly obtained by employing intensive multiple-pellet injection. In the IDB core plasma, the particle diffusion coefficient is kept at a considerably low level, 0.05 m2 s−1, in spite of high-density and steep-density gradients whereas an inward particle convection velocity is not observed.

Edge plasma physics/plasma–wall interactions during high density operation in LHD

Superdense plasma with a highly peaked electron density profile was obtained in the reduced recycling discharge in the Large Helical Device (LHD). During the discharge, a core region with a density as high as ∼5 × 10 20 m −3 and a temperature of ∼0.8 keV is maintained by an Internal Diffusion Barrier (IDB) with a steep density gradient. In spite of such a high density at the core region, no serious impurity accumulation has been observed. According to the numerical calculation with the EMC3–EIRENE code, downstream parallel flow in the ergodic region plays an important role to screen the influx of impurities with its strong friction force. The divertor flux distribution and global recycling properties do not change so much as against the change in the dense core plasma.

Progress of design studies on an LHD-type steady state reactor

Progress of design studies on a heliotron reactor is reported. Recent experimental achievements in large helical device (LHD) are building up the physical basement. Integrated designs on LHD-type energy reactors have improved the base design of force free helical reactor and revealed economical feasibility. Large helical device is exploring a new horizon of net current free helical plasmas. Inherent characteristics demonstrate reactor relevant performances individually; steady state (1 h), high beta (5%), high density (1 × 10 21 m −3), etc. In particular, discovery of internal diffusion barrier provides a novel attractive scenario of super high density operation. It greatly facilitates physical requirements as well as mitigates engineering demands. New methods have been proposed to access and operate ignited plasmas; a long rise-up over 300 s to reduce the heating power to 30 MW and a new proportional-integration-derivative (PID) control of the fueling to handle the thermally unstable plasma at high-density operations.

Stability and Confinement Studies of High-Performance NBI Plasmas in the Large Helical Device Toward a Steady-State Helical Fusion Reactor

Recent progress in plasma performance and the understanding of the related physics in the Large Helical Device is overviewed. The volume-averaged beta value is increased with an increase in the neutral beam injection (NBI) heating power, and it reached 5.0% of the reactor-relevant value. In high-β plasmas, the plasma aspect ratio should be controlled so that the Shafranov shift would be reduced, mainly to suppress transport degradation and the deterioration of the NBI heating efficiency. The operational regime of a high-density plasma with an internal diffusion barrier (IDB) has been extended, and the IDB, which was originally found using the local island divertor, has been realized in the helical divertor configuration. The central density was recorded as high as 1 × 1021 m-3, and the central pressure reached 130 kPa. Based on these high-density plasmas with the IDB, a new ignition scenario has been proposed. This should be a scenario specific to the helical fusion reactor, in which the helical ripple transport would be mitigated. A low-energy positive-NBI system was newly installed for an increase in the direct ion heating power. As a result, the ion temperature (Ti) exceeded 5.2 keV at a density of 1.2 × 1019 m-3 in a hydrogen plasma. Transport analysis shows improvement of ion transport, and the Ti-increase tends to be accompanied by a large toroidal rotation velocity of the order of 50 km/s in the core region. The plasma properties in the extended operational regime are discussed from the perspective of a steady-state helical fusion reactor.

Abrupt Flushing of High-Density Core in Internal Diffusion Barrier Plasmas and its Suppression by Plasma Shape Control in LHD

20 m -3 is formed inside the internal diffusion barrier (IDB) during the reheat phase of the central pressure after pellet injection in LHD. Large Shafranov shift of the plasma center is observed in IDB plasmas and it reaches roughly 50 % of the plasma minor radius. In some cases, abrupt flushing of the central density takes place and this event is called as “core density collapse (CDC)”. CDC must be suppressed since it inhibits further increase of the central pressure. CDC is always accompanied by large Shafranov shift of the plasma center exceeding a critical position. Theoretically, it is predicted that vertical elongation of the plasma shape (κ > 1) is effective to mitigate the Shafranov shift and the κ is controllable with the quadrupole magnetic field, BQ. According to this prediction, BQ scan experiment has been performed in LHD. Shafranov shift is indeed mitigated by increasing κ. As a result, CDC is suppressed and high central β values reaching ~ 7 % have been achieved in vertically elongated plasmas.

Measurements of Micro-Turbulence in High Beta and High Density Regimes of LHD and Comparison with Resistive G-Mode Scaling

Density fluctuations are analyzed in high volume average beta and high core density discharges in the Large Helical Device (LHD) using a 2D phase contrast imaging system and far infra-red interferometer. Both these regimes share similarly high beta gradients and evidence of pressure driven MHD modes is presented. In high volume average beta plasmas, both large and ion-gyro scale density fluctuation levels increase with beta and, in the edge, compare favorably with growth rate of resistive interchange modes, showing additional dependence on density at fixed β. In high core density plasmas with internal diffusion barrier, intermittent fluctuation bursts around mid radius are observed which are triggered when the normalized density gradient exceeds a certain threshold. The intermittent character is stronger for outward shifted plasmas and there appears to be a fluctuation suppression mechanism, possibly related to temperature gradient.

Transport Analysis of Dynamics of Plasma Profiles in Helical Devices

Plasma dynamics and structure are studied using a one-dimensional theoretical model for the anomalous transportdiffusivities. Inthisanalysis, thehighcollisionalPfirsch-Schluterregimeisexaminedandthe anomalous particle diffusivity is employed. The reduction of the anomalous particle diffusivity and a steep gradient in the density profile can be obtained. This prediction may be the theoretical explanation for the internal diffusion barrier observed in super dense core plasmas of large helical device. c

Characterization and operational regime of high density plasmas with internal diffusion barrier observed in the Large Helical Device

A high density regime with an internal diffusion barrier (IDB) has been extended to the helical divertor (HD) configuration in the Large Helical Device (LHD). Avoidance of the local enhancement of neutral pressure is necessary to enable IDB formation, which is consistent with earlier works by using the Local Island Divertor (LID) with efficient active pumping. The central pressure reached 1.3 times atmospheric pressure, where ne(0) = 6 × 1020 m−3 and Te(0) = 660 eV. The plasmas with an IDB are located in the plateau collisionality regime. The significant impurity effect has not been observed throughout the discharges in spite of the existence of a negative radial electric field. A central pressure limiting event is observed in the plasmas with an IDB using the HD. During this event which is referred to as the core density collapse (CDC), particles are flushed out from the core on the time scale of a few hundreds of microseconds. The suppression of the Shafranov shift by vertical elongation (κ) is effective to mitigate CDC. At κ = 1.2, the central β value is increased up to 6.6% at 1 T.

Extended steady-state and high-beta regimes of net-current free heliotron plasmas in the Large Helical Device

The performance of net-current free heliotron plasmas has been developed by findings of innovative operational scenarios in conjunction with an upgrade of the heating power and the pumping/fuelling capability in the Large Helical Device (LHD). Consequently, the operational regime has been extended, in particular, with regard to high density, long pulse length and high beta. Diversified studies in LHD have elucidated the advantages of net-current free heliotron plasmas. In particular, an internal diffusion barrier (IDB) by a combination of efficient pumping of the local island divertor function and core fuelling by pellet injection has realized a super dense core as high as 5 × 1020 m−3, which stimulates an attractive super dense core reactor. Achievements of a volume averaged beta of 4.5% and a discharge duration of 54 min with a total input energy of 1.6 GJ (490 kW on average) are also highlighted. The progress of LHD experiments in these two years is overviewed by highlighting IDB, high β and long pulse.

Superdense core mode in the Large Helical Device with an internal diffusion barrier

In reduced recycling discharges using a local island divertor in the Large Helical Device [O. Motojima, H. Yamada, A. Komori et al., Phys. Plasmas 6, 1843 (1999)], a stable high-density plasma develops in the core region when a series of pellets is injected. A core region with ∼5×1020m−3 and temperature of 0.85keV is maintained by an internal diffusion barrier (IDB). The density gradient at the IDB (r∕a∼0.6) is very high, and the particle confinement time in the core region is ∼0.4s. Because of the increase in the central pressure, a large Shafranov shift up to ∼0.3m is observed. The critical ingredients for IDB formation are a strongly pumped divertor to reduce edge recycling, and multiple pellet injection to ensure efficient central fueling. No serious magnetohydrodynamics activity and impurity accumulation have been observed so far in this improved discharge.

Pellet Injection and Internal Diffusion Barrier Formation in Large Helical Device

Ryuichi SAKAMOTO, Hiroshi YAMADA, Nobuyoshi OHYABU, Masahiro KOBAYASHI,Junichi MIYAZAWA, Satoshi OHDACHI, Tomohiro MORISAKI, Suguru MASUZAKI,Ichihiro YAMADA, Kazumichi NARIHARA, Motoshi GOTO, Shigeru MORITA,Satoru SAKAKIBARA, Kenji TANAKA, Kazuo KAWAHATA, Akio KOMORI,Osamu MOTOJIMA and LHD experimental group