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AbstractTo aid understanding of physics related to fast ions in the Large Helical Device (LHD) with a three-dimensional shape, a comprehensive set of fast-particle diagnostics has been developed. Fast ions (H+) have been created by neutral beam injection and ion cyclotron resonance heating in LHD discharges. Intense fast-ion populations not only heat the bulk plasma but also drive collective instabilities under certain experimental conditions. Fast-ion experiments on LHD have been conducted to investigate neoclassical confinement of fast ions and issues on anomalous transport of fast ions induced by fast-ion-driven instabilities. This paper reviews fast-particle diagnostics that are essential for study of fast-ion physics on LHD.

AbstractVarious ion cyclotron range of frequencies (ICRF) heating experiments have been conducted in the Large Helical Device (LHD) by changing the magnetic field strength and the wave frequency using hydrogen and helium. When the resonance layer of hydrogen was located in the peripheral region on the lower-magnetic field side, efficient electron heating, i.e., mode conversion heating, was realized. When the ion cyclotron resonance layer was located near the “saddle point” of magnetic field strength, where the gradient of the magnetic field strength is zero, hydrogen ions were efficiently heated by the minority ion heating. The second-harmonic ion cyclotron heating experiments were also conducted by decreasing the magnetic field strength, and the plasma was successfully sustained for 1 s. Ion tails were observed in the ion heating modes. High-energy ions were well confined by the inward-shifted magnetic configuration. The ion tail formed by the second-harmonic heating was enhanced by the injection of a perpendicular neutral beam.

AbstractThe Large Helical Device (LHD) is the world’s largest heliotron-type device with l= 2, m= 10 continuous superconducting helical coils and three pairs of superconducting poloidal coils. The major and minor radii of the plasma are 3.6 to 3.9 and 0.6 to 0.65 m, respectively. A plasma with an elliptic cross section confined in the helical magnetic field rotates poloidally along the magnetic axis and has no axial symmetry. For the installation of various kinds of diagnostic instruments, large-sized ports are equipped. The diameter of the largest horizontal ports is 2410 mm, which enables us to easily access the full plasma cross section with multichannel viewing chords aligned parallel to one another. For the precise measurement of plasma quantities in a three-dimensional helical plasma, an extensive set of diagnostics has been developed with national and international collaborators and is routinely operated in LHD. The diagnostic system now consists of [approximately]50 measuring instruments and includes many challenging diagnostics that have been developed and operated for the study of LHD plasma confinement. These are classified as profile diagnostics, fluctuation diagnostics, and advanced diagnostics, some of which are selected for introduction in this article. In addition, diagnostics for the divertor and for energetic particles are discussed, along with topics that are somewhat unique to helical devices such as diagnosing three-dimensional phenomena and flux surface mapping. This large number of diagnostics in LHD rely on a data acquisition system that has broken world records for the amount of information accumulated in one shot. Finally, looking to the near future, countermeasures have been taken to protect diagnostics from the neutrons and gamma fluxes anticipated during deuterium-deuterium experiments, such as placing much of the diagnostic instrumentation behind a 2-m-thick concrete biological shield encompassing the LHD test cell.

AbstractOn the Large Helical Device (LHD), improved confinement of fast ions by moving the magnetic axis location inward is theoretically predicted. This improvement is observed significantly in the fast-ion tail formation during the experiments of ion cyclotron range of frequency heating. The fast-ion loss effect on the tail formation is 10 times more significant in the standard configuration than in the inwardly shifted configurations. The superiority is also confirmed by the comparison of steady-state fast-ion spectra for the neutral beam (NB)–heated plasmas and by the flux decay of fast neutrals after the short-pulse injection of radial NB.

AbstractFusion product diagnostics can be used to determine a fusion reaction rate, which indicates how close the plasma is to the ultimate goal of making a power plant based on nuclear fusion. However, these diagnostics can also provide large amounts of additional information, such as ion temperatures, the thermonuclear fraction in the fusion reaction rate, degree of fast ion confinement, fast ion loss mechanism, etc. Measurement systems for fusion product diagnostics are usually designed and optimized to a specific performance so that they play different roles in the experiment. The neutron emission rate, which is directly related to the fusion output, can be determined by (a) time-resolved emission monitors, which are well calibrated onsite, in combination with (b) activation systems and (c) profile monitors with accuracy up to several percent. The time-resolved neutron profiles also provide useful information for transport analysis. Velocity distributions and confinement properties of fast ions can be obtained from (d) the neutron spectrometers and (e) gamma-ray measurement. The interaction between plasma dynamics and fast ions can be studied with most fusion product diagnostic systems, especially with (f) escaping charged fusion product detectors. Each section of this chapter contains a general explanation of these systems, showing some experimental results obtained on present devices. A lot of interesting and useful information on the behavior of energetic particles and their degree of confinement are provided by them because interaction between thermal and nonthermal energetic ions and that among nonthermal ions contribute dominantly to the fusion reaction rate in present deuterium-deuterium experiments. In future deuterium-tritium fusion experiments on ITER, the contribution of thermonuclear fraction will be increased, and the combination of several neutron measurement systems will provide the absolute fusion output and neutron fluence on the first wall. Together with neutron measurement, alpha particle and gamma-ray measurement play important roles in research on self-heating burning plasma physics and hence in the burning control of the device.

Several unexplored physics issues will be studied in burning plasma experiments, planned on ITER in the near future. To carry out these studies, it is essential to develop novel diagnostic systems, such as fusion product measurement systems and high-resolution, high-reliability profile measurement of various plasma parameters. Scientific research on “Advanced Diagnostics for Burning Plasma Experiment” was assigned as a priority area by MEXT in 2004, and more than 20 studies are currently underway under this program. The feasibility of several new diagnostic concepts applicable to ITER is examined and diagnostic components for these systems are now under development.

AbstractElectrode biasing experiments under electrode current control were carried out in the Tohoku University Heliac and the Compact Helical System to examine the role of an ion viscosity on a transition to a high-confinement regime and to investigate the dependence of the ion viscosity on magnetic structure. Observations included (a) an increase of electron density, (b) an increase of electron stored energy, (c) a formation of the steep gradient of electron density, and (d) a formation of a negative electric field in both devices during electrode biasing negatively. The dependence of the ion viscosity normalized by the ion pressure on the poloidal Mach number qualitatively agreed with the neoclassical theory based on the Shaing model. This result supported the transition mechanism of the neoclassical theory based on ion viscosity, which advocates that the transition to a high-confinement mode is the bifurcation phenomenon resulting from the existence of local maximum in ion viscosity.

AbstractIn the Tohoku University Heliac, a high-density plasma is produced by a vanadium electrode. The vanadium electrode is pretreated for hydrogen storage. In biasing experiments using the vanadium electrode, a high-density plasma is observed in not only argon plasmas but also helium plasmas. When the vanadium electrode is biased negatively, the radial distribution of the electron density steepens at the electrode position, and a strong negative radial electric field is formed between the electrode and the last closed flux surface. The E× Bdrift velocity is 30 km/s, and the estimated poloidal Mach number Mpis -20. The measured beta value exceeded 0.5% in the low-field discharges.

AbstractThe influence of the low-order rational surface (n,m) = (5, 3) on ion viscosity and radial electric field formation were investigated by electrode current sweep biasing with a hot cathode at the Tohoku University Heliac. In the improved mode, the position of the maximum electric field remained on the n/m= 5/3 rational surface. After the H-L transition, the local maxima shifted outward to the plasma periphery of ρ ~ 0.8. The low-order magnetic islands were formed resonating the magnetic Fourier components of (n,m) = (5, 3) by external perturbation coils. As the widths of the magnetic islands were increased, the biasing electrode current required for the improved mode transition increased. It was suggested that the ion viscosity increased according to the increase in magnetic island width. The increase in the biasing electrode current is equivalent to the increase in the driving force for the poloidal rotation. This suggests the possibility of an active viscosity control assisted by externally controlled island width and magnetic island rotation.

AbstractRemarkable progress to access the reactor-relevant regime has been made in a recent experiment in the Large Helical Device. Optimizing the rotational transform, the average beta value of 4.3%, which is the highest record among helical devices, was achieved. The high-performance plasma with a fusion triple product up to ~2.2 × 1019m−3·keV·s was sustained for >7 s by repetitive hydrogen pellet injection. With regard to steady-state operation, which is one of the key issues to realize a fusion reactor, discharges for >30 min were successfully sustained by ion cyclotron range of frequency heating with the aid of the magnetic axis swing technique to reduce the heat load to the plasma-facing component. In the discharge, the total input energy to the plasma reached 1.3 GJ, which also established a new record.

AbstractConfinement of energetic ions from neutral beam injection heating is investigated by changing the magnetic field configuration of the Large Helical Device from a classical heliotron configuration to an optimized neoclassical transport configuration to a level typical of “advanced stellarators.” The experimental results show the highest count rate of fast neutral particles not in the optimized configuration but in the inward-shifted one. The GNET simulation results show a relatively good agreement with the experimental results, and they also show a lower energy loss rate in the optimized configuration. This contradiction can be explained by the radial profile of the energetic ions. The relatively good agreement between experimental and simulation results suggest that ripple transport (neoclassical) dominates the energetic ion confinement and that the optimization process is effective in improving confinement in helical systems.

AbstractRecent progress in the Large Helical Device (LHD) experiment during the last 2 yr is reviewed. The LHD has been extending its operational regime toward fusion-relevant conditions while taking advantage of the net-current-free heliotron concept employing a superconducting coil system. Heating capability has exceeded 10 MW, and the central ion and electron temperatures have reached 7 and 10 keV, respectively. The maximum values of beta and pulse length have been extended to 3.2% and 150 s, respectively. Several encouraging physics observations have been obtained, i.e., simultaneous achievement of the mitigation of the magnetohydrodynamic instability criteria and good confinement, and formation of an internal transport barrier. The initial results have been obtained using a local island divertor, which shows the possibility of particle control at the plasma edge.

In the Tohoku University Heliac, transition to the improved mode has been observed when the plasma is biased with a hot cathode inserted into the plasma. Ion temperature measurement using high-resolution spectroscopy demonstrated that when the plasma was biased and transited to the improved mode, the ion temperature increased or was kept almost constant, and the ion density also increased. The total stored energy (the products of temperature and density, neTe, niTi) increased in the improved mode. In neoclassical theory, nonlinearity in ion viscosity plays a key role in the transition. The ion viscosity estimated from the measured ion temperature was consistent with the predictions of the neoclassical theory.