@misc{oai:ir.soken.ac.jp:00000485,
 author = {近藤, 隆 and コンドウ, タカシ and KONDO, Takashi},
 month = {2016-02-17, 2016-02-17},
 note = {In magnetic confinement devices, it is required that fast ions, such as heating ions and alpha particles, are well confined until they transfer their energy to plasmas. In order to understand loss processes and loss mechanisms of fast ions in magnetic confinement devices, direct measurements of escaping fast ions are essential. These studies have been advanced on large tokamaks experimentally and theoretically. On heliotron/torsatron devices which are regarded as alternative devices of tokamaks, it is especially important to study loss processes and loss mechanisms because fast ion orbits in heliotron/torsatron devices are complicated due to the presence of helical ripples in addition to toroidal ripples.<br />　　In this study, a scintillator-type lost-fast-ion probe has been used for the first time to measure escaping fast ions originated from a tangentially injected neutral beam (NB) on the Compact Helical System (CHS) heliotron/torsatron. The probe detects escaping fast ions directly in the vacuum vessel and can provide the information on the pitch angle χ and the gyroradius ρi corresponding to the energy of detected ions. The probe mainly consists of a scintillator plate (ZnS(Ag)), an image-intensified CCD camera and a nine channel fiber array coupled each with a photomultiplier tube(PMT). The probe head (detection point) is placed at the distance of a few cm away from the last closed magnetic surface. Positions on the scintillator plate struck by incident escaping fast ions have the information on χ and ρi. The time behaviors of escaping fast ions with different ranges of χ and ρi are obtained by using the PMT array with the time resolution of 50 μs. A χ-ρi profile, light spots on the scintillator,are obtained by the CCD camera as a 2D image with the time resolution of 33.3ms (30 Hz). The probe has a Faraday cup structure. The total absolute flux of ions incident on the scintillator can be measured as ion current. Prior to measurements, calibration works for the gain characteristic of instruments were made to compare the signal intensity among each of plasma discharges measured with different gain of these instruments or each of PMTs. <br />　　Loss orbits are identified by using an orbit following code with measured information on the pitch angle and the energy. Orbits are simply derived by solving the equation of motion with Lorentz force. Orbits start from the detector position and are calculated backwards in time.<br />　　The measurement was applied to NB heated plasmas on CHS. First of all, it was inspected that the probe signal does not arise from X-rays, the leakage of plasma lights and other noises, but from escaping beam ions. As a feature of the signal, the signal was observed just during NB injection and the signal intensity increased as the magnetic axis position Rax moved outward (plasmas come close to the probe). Light spots on the scintillator moved to the position predicted from the magnetic field strength at the probe position with changes of the toroidal magnetic field strength BT.<br />　　The fast measurement is essential for the investigation of time behaviors of escaping fast ions. Three kinds of losses were observed and were identified as losses from passing, transition and trapped orbits. The different time behaviors of escaping ions, which suggest different classical loss processes, were observed on signals corresponding to passing ions and trapped ions. The signal of passing ions started synchronously at the moment of NB injection, while the signal intensity of trapped ions rose gradually in time. To reveal these loss processes, the dependence of the rise time behavior for both signals on the electron density ne was investigated. The signal intensity of passing ions always rapidly increased with NB injection, independently of ne. On the other hand, the rise time of trapped ion signal was inversely proportional to ne and showed similar tendency to that of the calculated deflection time. Therefore, it is thought that the former is due to the prompt loss and the latter is due to the collisional pitch angle scattering loss depending on ne.<br />　　The fast measurement was applied to NB-heated plasmas with fishbone-like instabilities and toroidicity-induced Alfven eigenmode (TAE). In CHS, two types of the fishbone-like instabilities were observed on Mirnov coil array, soft-X ray array and a heavy ion beam probe. One is the m/n = 3/2(m/n = poloidal/toroidal mode number) fishbone-like instability, which is often observed in a discharge with an outward-shifted plasma (Rax〓0.949 m). The other is the m/n = 2/1 fishbone-like instability, which is often observed in a discharge with an inward-shifted plasma (Rax = 0.921 m). The low magnetic field (BT～0.9 T), the co-injected NB and the low ne (ne～1.0x1019 m-3) are characteristics of these two burst modes. No such instabilities were observed on electron cyclotron resonance heated plasmas without NB injection. The periodic beam ion losses correlating with MHD oscillation were observed on the signals of co-going passing boundary ions and counter (ctr.)-going trapped ions with the energy lower than injection energy during the m/n = 3/2 fishbone like instability. There were thresholds in the mode amplitude for the ejection of fast ions. Above the threshold, losses were enhanced with increase of the mode amplitude. The excitation of the fishbone-like instability and the enhancement of MHD-induced losses strongly depend on the degree of the accumulation of beam ions in plasmas. Orbit calculations show that passing boundary ions which have large pitch angle and the toroidal velocity close to zero or close to the propagating velocity of magnetic fluctuation near the m/n = 3/2 surface are ejected by the instability. No MHD-induced losses were seen in plasmas with the m/n = 2/1 fishbone-like instability or TAE.<br /><br />　　In conclusion, direct measurements of escaping fast ions have been carried out in CHS and the classical loss processes (the prompt loss and the collisional pitch angle scattering loss) and the loss mechanism of MHD-induced loss were studied. In <br />classical losses, the signal intensity of passing boundary ions was most intense. With regard to the loss mechanism of MHD-induced losses, it was found that passing boundary ions moving excursively across magnetic surfaces between the outside and the inside of a plasma have resonance points near the m/n = 3/2 surface, where their toroidal velocity is close to zero or close to the propagating velocity of magnetic fluctuation. Therefore, it is necessary for future large devices to control passing boundary ions., application/pdf, 総研大甲第458号},
 title = {Experimental Study on Fast Ion Losses in the Compact Helical System},
 year = {}
}