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Direct and resonant photoionization of Al-like W61+ ions from their ground state 1 s 2 2 s 2 2 p 6 3 s 2 3 p 2 P 1 / 2 and four lowly excited states 1 s 2 2 s 2 2 p 6 3 s 2 3 p 2 P 3 / 2 , 1 s 2 2 s 2 2 p 6 3 s 3 p 2 4 P 1 / 2 , 3 / 2 , and 1 s 2 2 s 2 2 p 6 3 s 3 p 2 2 D 5 / 2 is studied by using the multiconfigurational Dirac–Fock method and the relativistic R-matrix method. Emphasis is placed upon exploring direct and resonant ionization limits and also upon identifying possible resonance channels of W61+ ions. To do so, the photoionization cross sections are calculated with the use of the Dirac Atomic R-matrix Code. For the ground-state photoionization, the direct and resonant ionization limits are obtained, and the identified resonance peaks are found to be associated with the resonances 1 s 2 2 s 2 2 p 6 3 s 3 l n l ′ and 1 s 2 2 s 2 2 p 6 3 p 3 l n l ′ (l , l ′ = p , d) at a level of fine-structure energy level or configuration. For the excited-state photoionization, however, although the respective direct ionization limits are obtained, the resonant ionization limits and resonance peaks are hardly distinguishable due to much more densely spaced resonance peaks and complex resonance structure. It is expected that the present work could fill the vacancy of studies on the photoionization of W61+ ions and be helpful to the diagnosis and simulation of the thermonuclear fusion plasmas together with other available photoionization data of tungsten ions with other charge states. [ABSTRACT FROM AUTHOR]

The influence of resonant magnetic perturbations (RMPs) on the dynamics of turbulence and flows at the edge of the HL-2A tokamak is analyzed utilizing transfer entropy technique. The results have shown that the RMP damps the poloidal flows as well as the E × B shearing rate, whereas enhances the toroidal flows and leads to a broadened particle spectrum with increased small scale turbulence transport. The causality analysis indicates that the regulation impact of poloidal flow on turbulent fluctuations and particle flux is weakened, while that of the toroidal rotation on the latter is strengthened by the RMP field. The impact of the changes in poloidal flow dominates over that of the modified toroidal flow on turbulent transport in the edge. The magnetic perturbation and the flows generally show predator–prey oscillations, where the causal effect between the former and the toroidal flow transits to a synchronization relation in the presence of RMP. In addition, the RMP field will weaken the causal effect on poloidal Reynolds stress while strengthening the parallel-radial component simultaneously. The present findings provide a possible explanation on the effects of external fields on the edge transport, which is suggested to be dominated by the complex interactions among external perturbations, flows, and ambient microturbulence. [ABSTRACT FROM AUTHOR]

Hot spots induced by lower hybrid wave in experimental advanced superconducting tokamak tokamak have caused high performance experiment disruption and serious damages to the guard limiters. Experimental and theoretical analyses have been carried out to study its physical mechanism. Plasma density scan experiments indicate that the wall temperature within the hot spots enhanced by a factor of 5 and increases with the plasma density near the antenna. A lower hybrid current drive (LHCD)-only density climb experiment shows that the carbon impurity decreases to a minimum value at certain plasma density and then increases with the line averaged plasma density. A model has been developed to explain the mechanism of sputtering of graphite tiles due to hot spots as the plasma density near the LHCD antenna and the time increases. A theoretical scaling of the heat flux driven by LHCD is also presented and is consistent with the experimental scaling in the Tore Supra tokamak. The simulation results show that the total sputtering flux density has a minimum at a certain plasma density and gradually increases as the plasma density increases or decreases away from the minimum value, and the increase in parallel heat flux near the antenna would enhance the sputtering flux density. The sputtering flux density trend is qualitatively consistent with the density scan experiments. The simulated temporal evolution of sputtered flux implies that the chemical sputtering could be a candidate for the carbon impurity explosion. [ABSTRACT FROM AUTHOR]

EAST has been equipped with two high power lower hybrid current drive (LHCD) systems with operating frequencies of 2.45 GHz and 4.6 GHz. Comparative LHCD experiments with the two different frequencies were performed in the same conditions of plasma for the first time. It was found that current drive (CD) efficiency and plasma heating effect are much better for 4.6 GHz LH waves than for the one with 2.45 GHz. High confinement mode (H-mode) discharges with 4.6 GHz LHCD as the sole auxiliary heating source have been obtained in EAST and the confinement is higher with respect to that produced previously by 2.45 GHz. A combination of ray-tracing and Fokker-Planck calculations by using the C3PO/LUKE codes was performed in order to explain the different experimental observations between the two waves. In addition, the frequency spectral broadening of the two LH wave operating frequencies was surveyed by using a radio frequency probe. [ABSTRACT FROM AUTHOR]

Significant progress on both L- and H-mode long-pulse discharges has been made recently in Experimental Advanced Superconducting Tokamak (EAST) with lower hybrid current drive (LHCD) [J. Li et al., Nature Phys. 9, 817 (2013) And B. N. Wan et al., Nucl. Fusion 53, 104006(2013).]. In this paper, LHCD experiments at high density in L-mode plasmas have been investigated in order to explore possible methods of improving current drive (CD) efficiency, thus to extend the operational space in long-pulse and high performance plasma regime. It is observed that the normalized bremsstrahlung emission falls much more steeply than 1/ne_av (line-averaged density) above ne_av=2.2x1019m-3 indicating anomalous loss of CD efficiency. A large broadening of the operating line frequency (f=2.45GHz), measured by a radio frequency (RF) probe located outside the EAST vacuum vessel, is generally observed during high density cases, which is found to be one of the physical mechanisms resulting in the unfavorable CD efficiency. Collisional absorption of lower hybrid wave in the scrape off layer (SOL) may be another cause, but this assertion needs more experimental evidence and numerical analysis. It is found that plasmas with strong lithiation can improve CD efficiency largely, which should be benefited from the changes of edge parameters. In addition, several possible methods are proposed to recover good efficiency in future experiments for EAST [ABSTRACT FROM AUTHOR]

A triple Langmuir probe is mounted on the top of the Lower Hybrid (LH) antenna to measure the electron density near the LH grills in Experimental Advanced Superconducting Tokamak. In this work, the LH power density ranges from 2.3 MWm-2 to 10.3 MWm-2 and the rate of puffing gas varies from 1.7 × 1020 el/s to 14 × 1020 el/s. The relation between the edge density (from 0.3 × ne_cutoff to 20 × ne_cutoff, where ne_cutoff is the cutoff density, ne_cutoff = 0.74 × 1017 m-3 for 2.45 GHz lower hybrid current drive) near the LH grill and the LH power reflection coefficients is investigated. The factors, including the gap between the LH grills and the last closed magnetic flux surface, line-averaged density, LH power, edge safety factor, and gas puffing, are analyzed. The experiments show that injection of LH power is beneficial for increasing edge density. Gas puffing is beneficial for increasing grill density but excess gas puffing is unfavorable for coupling and current drive. [ABSTRACT FROM AUTHOR]

A new set of actively cooled toroidal double-ring graphite limiters has been developed in the Hefei Tokamak-7 (HT-7) [X. Gao et al., Phys. Plasmas 7, 2933 (2000)] for long pulse operation. The extension of operational region and density behavior with graphite (C) limiters have been studied in this paper. Extended high-density region at the high plasma current low-qa was obtained. The density profile with the C limiter was studied to compare with the previous molybdenum (Mo) limiter. The critical density of multifaceted asymmetric radiation from the edge (MARFE) onset is observed in the region of Zeff½fGW=0.9∼1.2, where fGW=ne/nGW. (Here ne is the maximum line average electron density and nGW is the Greenwald density.) Under the same injected power, the critical density of MARFE onset with the new C limiter is much higher than the previous Mo limiter. [ABSTRACT FROM AUTHOR]