10 research outputs found
Mitigation of plasma-wall interactions with low-Z powders in DIII-D high confinement plasmas
Experiments with low-Z powder injection in DIII-D high confinement discharges
demonstrated increased divertor dissipation and detachment while maintaining
good core energy confinement. Lithium (Li), boron (B), and boron nitride (BN)
powders were injected in high-confinement mode plasmas (1 MA, 2 T,
6 MW, m) into the
upper small-angle slot (SAS) divertor for 2-s intervals at constant rates of
3-204 mg/s. The multi-species BN powders at a rate of 54 mg/s showed the most
substantial increase in divertor neutral compression by more than an order of
magnitude and lasting detachment with minor degradation of the stored magnetic
energy by 5%. Rates of 204 mg/s of boron nitride powder further
reduce ELM-fluxes on the divertor but also cause a drop in confinement
performance by 24% due to the onset of an tearing mode. The application
of powders also showed a substantial improvement of wall conditions manifesting
in reduced wall fueling source and intrinsic carbon and oxygen content in
response to the cumulative injection of non-recycling materials. The results
suggest that low-Z powder injection, including mixed element compounds, is a
promising new core-edge compatible technique that simultaneously enables
divertor detachment and improves wall conditions during high confinement
operation
Growth and characterization of A_{1-x}K_xFe_2As_2 (A = Ba, Sr) single crystals with x=0 - 0.4
Single crystals of AKFeAs (A=Ba, Sr) with high quality
have been grown successfully by FeAs self-flux method. The samples have sizes
up to 4 mm with flat and shiny surfaces. The X-ray diffraction patterns suggest
that they have high crystalline quality and c-axis orientation. The
non-superconducting crystals show a spin-density-wave (SDW) instability at
about 173 K and 135 K for Sr-based and Ba-based compound, respectively. After
doping K as the hole dopant into the BaFeAs system, the SDW transition
is smeared, and superconducting samples with the compound of
BaKFeAs (0 0.4) are obtained. The
superconductors characterized by AC susceptibility and resistivity measurements
exhibit very sharp superconducting transition at about 36 K, 32 K, 27 K and 23
K for x= 0.40,0.28,0.25 and 0.23, respectively.Comment: 9 pages, 6 figures, 1 table. This paper together with new data are
modified into a new pape
Mitigation of plasma-wall interactions with low-Z powders in DIII-D high confinement plasmas
International audienceExperiments with low-Z powder injection in DIII-D high confinement discharges demonstrated increased divertor dissipation and detachment while maintaining good core energy confinement. Lithium (Li), boron (B), and boron nitride (BN) powders were injected in high-confinement mode plasmas (1 MA, 2 T, 6 MW, m) into the upper small-angle slot (SAS) divertor for 2-s intervals at constant rates of 3-204 mg/s. The multi-species BN powders at a rate of 54 mg/s showed the most substantial increase in divertor neutral compression by more than an order of magnitude and lasting detachment with minor degradation of the stored magnetic energy by 5%. Rates of 204 mg/s of boron nitride powder further reduce ELM-fluxes on the divertor but also cause a drop in confinement performance by 24% due to the onset of an tearing mode. The application of powders also showed a substantial improvement of wall conditions manifesting in reduced wall fueling source and intrinsic carbon and oxygen content in response to the cumulative injection of non-recycling materials. The results suggest that low-Z powder injection, including mixed element compounds, is a promising new core-edge compatible technique that simultaneously enables divertor detachment and improves wall conditions during high confinement operation
First Evidence of Local E× B Drift in the Divertor Influencing the Structure and Stability of Confined Plasma near the Edge of Fusion Devices
The structure of the edge plasma in a magnetic confinement system has a strong impact on the overall plasma performance. We uncover for the first time a magnetic-field-direction dependent density shelf, i.e., local flattening of the density radial profile near the magnetic separatrix, in high confinement plasmas with low edge collisionality in the DIII-D tokamak. The density shelf is correlated with a doubly peaked density profile near the divertor target plate, which tends to occur for operation with the ion B×â‡B drift direction away from the X-point, as currently employed for DIII-D advanced tokamak scenarios. This double-peaked divertor plasma profile is connected via the E×B drifts, arising from a strong radial electric field induced by the radial electron temperature gradient near the divertor target. The drifts lead to the reversal of the poloidal flow above the divertor target, resulting in the formation of the density shelf. The edge density shelf can be further enhanced at higher heating power, preventing large, periodic bursts of the plasma, i.e., edge-localized modes, in the edge region, consistent with ideal magnetohydrodynamics calculations.Peer reviewe
Nature of magnetic excitations in superconducting BaFe1.9Ni0.1As2
Since the discovery of the metallic antiferromagnetic (AF) ground state near superconductivity in iron pnictide superconductors1–3, a central question has been whether magnetism in these materials arises from weakly correlated electrons4,5, as in the case of spin density wave in pure chromium6, requires strong electron correlations7, or can even be described in terms of localized electrons8,9 such as the AF insulating state of copper oxides10. Here we use inelastic neutron scattering to determine the absolute intensity of the magnetic excitations throughout the Brillouin zone in electron-doped superconducting BaFe1.9Ni0.1As2 (Tc = 20 K), which allows us to obtain the size of the fluctuating magnetic moment 〈m2〉, and its energy distribution11,12. We find that superconducting BaFe1.9Ni0.1As2 and AF BaFe2As2 (ref. 13) both have fluctuating magnetic moments 〈m2 〉 ≈ 3.2 µ2B pe