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A scenario for non-inductive current ramp-up has been demonstrated using electron cyclotron waves in the spherical tokamak QUEST. The configuration was characterized by a high toroidal magnetic mirror ratio of 2 and a steady vertical magnetic field of more than 10% of the toroidal magnetic field. The generation and confinement of energetic electrons having energy greater than 10 keV were studied using hard x-rays. Because of the energetic electron pressure, a natural divertor formed with an inboard poloidal field null at the high poloidal beta (approximately 3–4). [ABSTRACT FROM AUTHOR]

A study of radial propagation and electric fields induced by charge separation in blob-like structures has been performed in a non-confined cylindrical electron cyclotron resonance heating plasma on Q-shu University Experiment with a Steady-State Spherical Tokamak using a fast-speed camera and a Langmuir probe. The radial propagation of the blob-like structures is found to be driven by E × B drift. Moreover, these blob-like structures were found to have been accelerated, and the property of the measured radial velocities agrees with the previously proposed model [C. Theiler et al., Phys. Rev. Lett. 103, 065001 (2009)]. Although the dependence of the radial velocity on the connection length of the magnetic field appeared to be different, a plausible explanation based on enhanced short-circuiting of the current path can be proposed. [ABSTRACT FROM AUTHOR]

Initial results from the recently implemented transient coaxial helicity injection (CHI) system on QUEST are reported. QUEST uses a new CHI electrode configuration in which the CHI insulator is not part of the vacuum boundary, making this configuration easier to implement in fusion reactors. Experimental results show that transient CHI startup in this alternate electrode configuration is indeed possible. Reliable gas breakdown was achieved, and toroidal currents up to 45 kA were generated. [ABSTRACT FROM AUTHOR]

Abstract: Long time scale variation of plasma-wall interactions and its impact on particle balance, main plasma performance and particle behavior have been investigated in ELMy H-mode plasmas by extending the discharge pulse and the neutral beam heating pulse to 65s and 30s, respectively. The wall pumping rate starts to decrease in the latter phase by repeating the long-pulse discharges with 60% of Greenwald density sustained by gas-puffing. After several discharges, the wall inventory is saturated in the latter phase and, consequently, the density increases with neutral beam fuelling only. The edge pressure in the main plasma is reduced and ELMs are close to the type III regime under conditions of wall saturation. The intensities of C II emission near the X-point and CD band emission in the inner divertor start to increase before the wall saturates and continue to increase after the wall is saturated. [Copyright &y& Elsevier]

Abstract: CCS (Cauchy Condition Surface) method is a numerical approach to reproduce plasma shape, which has good precision in conventional tokamaks. In order to apply it in plasma shape reproduction of ST (Spherical Tokamak), the calculation precision of the CCS method in a spherical tokamak CPD (Compact PWI experimental Device) (B t =0.25T, R =0.3m, a =0.2m) has been analyzed. The precision was confirmed also in ST and decided to be applied to a spherical tokamak QUEST (B t =0.25T, R =0.68m, a =0.40m).. In present stage from the magnetic measurement, it is known that the eddy current effect is large in QUEST experiment, and there are no special magnetic measurements for eddy current now, so some proper model should be selected to evaluate the eddy current effect. The eddy current density by not only CS (Center Solenoid) coil but also plasma current is calculated using EDDYCAL (JAEA). The eddy current magnitudes are taken as unknown variables and solved together with plasma shape reconstruction in ohmic discharge and ECCD (Electron Cyclotron Current Drive) discharge. [Copyright &y& Elsevier]

Abstract: The spectral lines of and emitted around the X-point in the detached plasma with MARFE are measured with a VUV spectrometer and a two-dimensional visible spectrometer in order to compare the generation flux of (ionization of and recombination of ) and the loss flux of (ionization and recombination of ). It has been found that is produced by the volume recombination of and the ionization of comparably. In contrast, the volume recombination of is not detected, and the ionization flux of is less than 1% of the generation flux. Thus, the generation flux is higher by two orders of magnitude than the loss flux. This result suggests that another loss mechanism of such as transport loss from the X-point is significant. [Copyright &y& Elsevier]

Abstract: In JT-60U, Xe gas has been injected into reversed shear plasmas with internal transport barriers. During these discharges, spectra from highly ionized Xe atoms have been observed in a wavelength range of 4–16nm. The observed spectra have been analyzed by calculating theoretical spectra for Xe XXXVII–XLV 3s–3p and 3p–3d transitions with relativistic atomic structure codes, a collisional radiative model, and an impurity transport code. Several lines for Xe XLI and XLII have been newly identified. Soft X-ray emission profiles have also been analyzed, and the results showed that the Xe ions accumulated inside the internal transport barrier. [Copyright &y& Elsevier]

Abstract: In a collaboration of eight experiments the dependence of the chemical erosion yield of carbon on the ion flux, Φ, was established to Φ −0.54 at high ion fluxes. With this flux dependence a comprehensive description for chemical erosion is available as function of energy, temperature and flux. With this description the erosion and re-deposition of carbon in the ITER divertor can be calculated using the ITER steady-state plasma scenario and the ERO code. The resulting gross and net erosion rates during steady-state phase of discharges are compared to previous estimates using a constant erosion yield of 1.5%. The use of the complete parameter dependence results in an order of magnitude lower erosion, and, accordingly, of the T codeposition inventory. [Copyright &y& Elsevier]

Abstract: Hydrogen particle behavior, especially H2 molecule behavior, has been studied spectroscopically in attached and detached divertor plasmas of JT-60U. The decay lengths of the H2 Fulcher line intensity in attached and detached divertor plasmas were roughly 1cm and 4cm, respectively. The fall in intensity of the H2 Fulcher lines with distance from the divertor plates was reproduced by calculation using a neutral transport and a collisional radiative model code. Molecular assisted recombination was estimated to be as important as H+–e recombination in a detached divertor plasma. [Copyright &y& Elsevier]

Global particle balance in non-inductive long-duration plasma on QUEST has been investigated. Approximately 70% of the fuel hydrogen (H) was retained in the wall and then was almost exhausted just after the discharge. The global recycling ratio ( R g ), defined as the ratio of the evacuated H 2 flux to that injected, was found to gradually increase during discharges and subsequently rose rapidly. To study the growth of R g , the thermal desorption spectra after deuterium implantation in a specimen exposed to QUEST plasma was analyzed with a model which includes reflection, diffusion, solution, recombination, trapping, and plasma-induced desorption in the re-deposition layer. The model reconstructs the growth of R g during a long-duration plasma and indicates solution plays a dominant role in the growth. [ABSTRACT FROM AUTHOR]

Abstract: Blob-like structures are observed in an open magnetic field line configuration of electron cyclotron resonance (ECR) heating plasma in QUEST. The combined measurement with a fast camera and a movable Langmuir probe show that visible filamentary structures extended along field lines do correspond to over-dense plasma structures. Ion saturation current time trace shows a front-like asymmetric burst when a visible structure reaches the probe during its outward propagation. The typical radial velocity of the blob-like structure, V b, was measured as ∼1km/s with the fast camera at the intermediate area. V b increased as the blob-like structure moved along major radius, R (outward). The average velocity of E × B drift derived from the probe measurement, slightly larger than V b, was also increased as the blob-like structure propagated to larger R. [Copyright &y& Elsevier]