Multi-functional Diagnostic Method with Tracer-encapsulated Pellet Injection

In order to obtain a better understanding of impurity transport in magnetically confined plasmas, a Tracer-Encapsulated Soild PELlet (TESPEL) has been developed. The essential points of the TESPEL are as follows: the TESPEL has a double-layered structure, and a tracer impurity, the amount of which can be known precisely, is embedded as an inner core. This structure enables us to deposit the tracer impurity locally inside the plasma. From experiences of developing the TESPEL production technique and its injection experiments, it became clear that various plasma properties can be studied by the TESPEL injection. There are not only impurity transport in the plasma but also transport both outside and inside of the magnetic island O-point, heat transport and high-energy neutral particle flux. Therefore, the TESPEL injection has a favorable multi-functional diagnostic capability. Furthermore a Tracer-Encapsulated Cryogenic PELlet (TECPEL) has been also developed. The TECPEL has an advantage over the TESPEL in terms of no existence of carbons in the outer layer. The TECPEL injector was installed at LHD in December 2005, and the preliminary injection experiments have been carried out

Devlopment of Fast Neutral Particle Diagnostics and Study of Suprathermal Ion Behaviors in LHD Plasmas

　　　Analysis of energy-resolved spectra of neutral particles escaped from plasma can provide important knowledge about ion confinement and ion distribution function during different types of plasma heating such as neutral beam injection (NBI), ion cyclotron heating (ICH) or electron cyclotron heating (ECH). Such knowledge is very important for the successful development of a fusion reactor. Effective ion heating and good fast ion confinement are essential for ignition. Compared with tokamaks, studying the fast particle confinement properties in heliotrons is more complex mainly due to more complex magnetic configuration. Such a complex 3D geometry of the Large Helical Device (LHD) may lead to appearance of additional types of confined particles (such as helically trapped particles), additional confinement effects (presence of loss-cones) and may result in more complicated drift motions. For studying fast ion confinement properties in plasma, a variety of neutral particle analyzing diagnostics have been developed on modern fusion devices. In LHD these are one-chord Compact Neutral Particle Analyzer (CNPA) and six-chord Silicon-Detector NPA (SDNPA), etc. E∥B type CNPA utilizes one array of 40 detectors and measures energy and time resolved neutral particle fluxes in the energy range 1-170 keV. CNPA can be used in a combination with Tracer Encapsulated Solid Pellet (TESPEL) injector. Sightline of CNPA is very close to the nominal TESPEL trajectory, and thus active localized measurements can be made by the pellet charge exchange method. SDNPA can provide the measurements in the energy range 25-4000 keV and the aim of it is to make angle-resolved passive measurements of fast particles. 　　　According to some theoretical estimations, in heliotron devices the transition particles may be lost from the confinement region through loss cones. The loss cones of fast particles in LHD plasmas could not be measured so far by the existing diagnostics mainly due to the poor angular resolution (to make angle resolved measurements either a long time discharge or several shots with exactly similar parameters are required). To clarify the situation with loss-cones and to improve the angular resolution versus the existing SDNPA analyzer, a novel diagnostic with a much better angular resolution is required. The overview of currently used NPA diagnostics in tokamaks and heliotrons will be made in the Introductory chapter I . Among the preceding multi-sightline NPA systems used in magnetic confinement fusion devices the maximum sightline NPA systems with 6 chords of view are on LHD (currently operating SDNPA) and on former TFTR (discontinued). Thus a novel 20-sightline diagnostic based on an Angular Resolved Multi-Sightline NPA (ARMS-NPA) described in this thesis may become a new powerfu1 tool in fast ion physics studies. It can provide energy-, angle-, and time-resolved spectra of escaped fast neutral particles from the plasma. In addition, a precise radial scan of the plasma column can be realized. The data obtained by this new ARMS-NPA diagnostic in addition to CNPA and SDNPA experiments will help in understanding of fast particle physics in helical systems. 　　　First measurements by ARMS-NPA diagnostic have been made on LHD for a variety of plasma heating conditions. Angularly resolved measurements were made for co-, counter- and perpendicularly directed NBI, for ICRH and ECH regimes. Measurements were made for a wide range of plasma parameters such as electron density, magnetic axis position, positive and negative magnetic field directions, and magnetic field strength. Obtained data demonstrate angular dependence of fast particle distribution for the type of heating and plasma parameters. Magnetic axis shift effect on loss-cones has been noticed. Inward shifted magnetic axis configuration lead to improved fast ion confinement and uniform angular distribution (reduction of the loss-cone).　Since the naturally occurring charge exchange neutral particle source is not localized in contrast to the diagnostic neutral beam or pellet charge exchange methods, the correct interpretation of such measurements in a complex toroidal asymmetric geometry requires a careful numerical modeling of the neutral flux formation and the knowledge of the charge-exchange target distributions, relevant cross sections and the magnetic surface structure. The measured chord integral neutral flux calculation scheme for the LHD magnetic surface geometry is given. Calculation results are shown for measurable atomic energy spectra corresponding to heating-induced fast ion distributions from simplified Fokker-Planck models. The behavior of calculated and experimental suprathermal particle distributions in NBI and ICRF heated plasmas is discussed in the context of the experimenta1 data interpretation. The geometry effect on the measured spectra interpretation is discussed. Results of experimental measurements are also compared with simulation results made by different codes with taking fast particle orbits into account

Effect of wall light reflection in ITER diagnostics

The reflection of light from walls will result in parasitic signals for various optical diagnostics and can be a serious issue in ITER. In this study, we show recent progress in the assessment of the effects of wall reflections in ITER based on ray tracing simulation results. Four different diagnostics in ITER were chosen for the simulation, i.e. visible spectroscopy, infrared thermography, edge laser Thomson scattering, and charge exchange recombination spectroscopy

Helium Ion Observation during 3rd Harmonic Ion Cyclotron Heating in Large Helical Device

In higher harmonic ion cyclotron resonance heating using the fast wave, a helium resonance layer appears near the plasma core. It is very important to detect the helium ions to investigate the confinement of α particles, which are produced by a nuclear reaction in ITER or a fusion reactor. In Large Helical Device (LHD), we attempt to observe the charge-exchange helium particles using a compact neutral particle analyzer (CNPA). Helium acceleration below 5 keV can be confirmed by comparing the signal ratio of helium in adjusted plate voltages of the CNPA to that of hydrogen. Successful helium measurement in LHD will lead to the development of α particle measurement