High Energy Particle Measurements during Long Discharge in LHD

The spatial resolved energy spectra can be observed during a long discharge of NBI plasma bycontinuously scanning the neutral particle analyzer. In these discharges, the plasmas are initiated by the ECH heating, after that NBI#2 (Co-injection) sustains the plasma during 40-60 seconds. The scanned pitch angle is from 44 degrees to 74 degrees. The injected neutral beam (hydrogen) energy of NBI#2 is only 130 keV because the original ion source polarity is negative. The shape of spectra is almost similar from 44 degrees to 53 degrees. However the spectra from 55 degrees are strongly varied. It reflects the injection pitch angle of the beam according to the simulation (53 degrees ot R* = 3.75 m in simulation). The beam keeps the pitch angle at incidence until the beam energy becomes to the energy, which the pitch angle scattering is occurred by the energy loss due to the electron collision. The low flux region can be observed around 10-15 keV, which is 15 times of the electron temperature. The energy region may be equal to the energy at which the pitch angle scattering is occurred. At the energy, the particle is scattered by the collision with the plasma ions and some of particles may run away from the plasma because they have a possibility to enter the loss cone. According to the simulation, the loss cone can be expected at the 10 keV with the small angular dependence. The depth of the loss cone is deep at the small pitch angle. The hollow in the spectrum may be concluded to be the loss cone as the tendency is almost agreed with the experimental result

Spatial resolved high-energy particle diagnostic system using time-of-flight neutral particle analyzer in Large Helical Device

The time-of-flight-type neutral particle analyzer has an ability of horizontal scanning from 40 to 100° of the pitch angle. The information from the spatially resolved energy spectrum gives not only the ion temperature but also the information of the particle confinement and the electric field in plasmas. We have been studying the energy distributions at various magnetic configurations in the neutral beam injection (NBI) plasma. The spatially resolved energy spectra can be observed during long discharges of the NBI plasma by continuous scanning of the neutral particle analyzer. The shape of spectra is almost similar from 44° to 53°. However, the spectra from 55° are strongly varied. They reflect the injection pitch angle of the beam. The pitch angle scanning experiment during the long discharge of NBI plasma has also been made under the reversal of the magnetic field direction. NBI2 becomes counter injected with the reversal. We can easily observe the difference between co- and counter injections of NBI. During the electron cyclotron heating in the low-density plasma for the formation of the internal thermal barrier, large neutral particle increase or decease can be observed. The degree of the increase/decrease depends on the energy and the density. The reason for the variation of the particle flux is that the orbit of the trapped particle changes due to the electric field formed by the strong electron cyclotron heating

In-situ real-time monitoring of spurious modes in HE11_{11} transmission lines using multi-hole couplers in miter bends

Transmission of high-power millimeter waves for ECRH is often realised with oversized corrugated circular waveguides. Coupling from the gyrotron source to the waveguide is typically done via matching mirrors in free space. Small alignment errors of the system lead to the excitation of higher-order modes inside the waveguide beside the main transmission mode HE$_{11}$. Tose modes have comparably higher losses and can in worst case result in local fields exceeding the breakdown limit of the medium inside the waveguide. For alignment control over the whole pulse duration of the gyrotron, a set of hole-array couplers placed into a miter bend mirror probes the field inside the waveguide. The arrays are designed to detect the marker modes for beam offset and tilt (LP$^{(e=0)}$$_{11}$) as well as for beam waist mismatch (LP$_{02}$). In addition, a main mode coupler sensitive mostly for the HE$_{11}$ content is used as a power monitor. By maximizing the signal of the power monitor and minimizing the content of marker modes, a first-order optimization of the coupling from free space to the waveguide can be achieved. Signal processing of the 140 GHz information is done at kHz range after downmixing, using a frequency shifted part of the power monitor signal. As the measurement system is placed in a miter bend mirror, it can also be easily installed at various locations along the transmission line to check for possible misalignments of the waveguide connections between miter bends. Simulation and low power experimental results will be shown

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

Advanced Multi-Step Brazing (AMSB) for fabrication of new type of W/stainless steel first-wall component with ODS-Cu intermediate layer

ORCID　0000-0002-3744-2481The novel method “Advanced Multi-Step Brazing (AMSB)” has been developed to fabricate a new type of “divertor” and “first-wall” heat removal component in a fusion reactor. This study is focused on the latter component, in which a tungsten (W) sheet is jointed through AMSB to a stainless steel (SUS) substrate via an oxide dispersion strengthened copper (ODS-Cu) intermediate layer. The principle of AMSB is a repetitive application of the advanced brazing technique (ABT). The initial purpose of the ABT was to braze W to ODS-Cu (GlidCop®) with the Ni-11 %P filler material. Later, we confirmed that the ABT is able to produce a very tough GlidCop® and SUS (GlidCop®/SUS) joint. One of the major advantages of GlidCop®/SUS joints is physically strong tolerance against the repetitive brazing heat-cycle. Thus, a repetitive application of the ABT does not cause any negative effects against post-brazed GlidCop®/SUS joints, and hence AMSB can be applied for fabricating a single heat-removal component with multiple joint interfaces. A small-scale sample of the new type of first-wall component was fabricated through two-step brazing. At first, the GlidCop® plate was jointed to the SUS by the ABT, and then a thin W-sheet with the thickness of 0.254 mm was jointed to GlidCop®/SUS by the ABT. If a large area of the first-wall surface in the fusion reactor can be covered with such a thin W-sheet, the amount of hydrogen isotopes trapped on the first-wall surface could be significantly reduced, compared to other fabrication methods such as a vacuum plasma spray W (VPS-W). The VPS-W is theoretically less dense than a W-sheet and often contains pore structures, which could act as effective trapping sites for hydrogen isotopes. A small-scale sample of the new type of first-wall component with a W-sheet (W/GlidCop®/SUS) was successfully fabricated to overcome the above disadvantages. In addition, the W surface of the component showed low retention characteristics of hydrogen isotopes compared with other W surfaces, e.g., atmospheric plasma-sprayed W (APS-W).journal articl

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

Effects of toroidally-distributed-divertor biasing on scrape-off-layer (SOL) current drive, divertor particle flux and fast electron confinement in the QUEST spherical tokamak

ORCID　0000-0002-8244-8305A novel divertor biasing by four biasing plates distributed toroidally (TDDB) on the upper divertor target plate is applied to low density tokamak plasmas started-up by 28 GHz 2nd harmonic electron cyclotron current drive (ECCD) in a quasi-double null configuration of the QUEST spherical tokamak (ST). In the ST plasmas of line averaged electron density ∼0.7–1 × 1018 m−3, about 20%–40% of the current ${I_{{\text{bias}}}}$ driven by ∼85 V sawtooth bias voltage reaches the lower divertor plate along the biased scrape-off-layer (SOL) flux tube as the SOL current ${I_{{\text{SOL}}}}$. The fact ${I_{{\text{SOL}}}}$ is noticeably lower than ${I_{\text{bias}}}{\text{ }}$ indicates an appreciable leakage of parallel current from the biased SOL flux tube. The leakage currents in the toroidal and radial directions are confirmed by detection of them using the unbiased plates in the TDDB experiments. From the ion saturation current density profile obtained by a divertor Langmuir probe array, the fall-off lengths of divertor particle flux are estimated together with the strike line position. Total particle flux to the upper divertor, evaluated by the integrated ion saturation current density profile is reduced by up to 45% during positive biasing of the TDDB, depending on the position of the strike line to the biased plate. In addition, the TDDB also induces a noticeable loss of fast electrons produced by ECCD, leading to an ∼2% reduction in the maximum toroidal current of the ST plasma compared to a shot without the TDDB. Reduction of the divertor particle flux and enhancement of the fast electron losses are thought to be dominantly caused by E$\times B$ drift induced by the TDDB. In the present experimental conditions, the effects of magnetic perturbations produced by the SOL currents on the fast electron losses can be neglected because of a too small SOL current.journal articl

High Harmonic ECH Experiment for Extension of Heating Parameter Regime in LHD

High harmonic electron cyclotron resonance heating (ECH) can extend the plasma heating region to higher density and higher β compared to the normal heating scenario. In this study, the heating characteristics of the second-harmonic ordinary (O2) and third-harmonic extraordinary (X3) modes and the possible extension of heating regime are experimentally confirmed. At the same time, a comparative study using ray-tracing calculation was performed in the realistic three-dimensional configuration of the Large Helical Device. The O2 mode heating showed a 40% absorption rate even above the X2 mode cut-off density. The X3 mode heating using powerful 77 GHz gyrotrons demonstrated an increase of about 40% in the central electron temperature in the plasmas at β-value of about 1%. These results were quantitatively explained to some extent by ray-tracing calculations

Experimental Results for Electron Bernstein Wave Heating in the Large Helical Device

Electron cyclotron heating (ECH) using electron Bernstein waves (EBWs) was studied in the large helical device (LHD). Oblique launching of the slow extraordinary (SX-) mode from the high field side and oblique launching of the ordinary (O-) mode from the low field side were adopted to excite EBWs in the LHD by using electron cyclotron (EC) wave antennas installed apart from the plasma surface. Increases in the stored energy and electron temperature were observed for both cases of launching. These launching methods for ECH using EBWs (EBWH) is promising for high-density operation in future helical fusion devices instead of conventional ECH by normal electromagnetic modes