Progress in gyrokinetic validation studies using NBI heated L-mode discharge in KSTAR

Progress in the first gyrokinetic validation study using KSTAR NBI heated L-mode discharge is reported in this paper. The energy flux levels simulated from gyrokinetic code, CGYRO[J. Candy et al., J. Comput. Phys. 324, 73–93 (2016)] were compared with experimental levels in this study for validation purposes. The linear stability analysis indicates that trapped electron modes (TEM) are the most unstable ion-scale modes at r/a = 0.5. The simulated energy flux was under-predicted compared to the experimental energy flux level within their uncertainties. We also observed that simulated energy flux levels were sensitive to the input parameters related to impurity density profile such as effective charge, ${\text{Z}}_{\text{eff}}$, and inverse gradient scale length of impurity and main ion, $\text{a}/{\text{L}}_{\text{nc}}$ and $\text{a}/{\text{L}}_{\text{ni}}$, respectively. For the conclusive future validation studies, we identified the ${\text{Z}}_{\text{eff}}$ profile, which can give constraints on not only impurity but also main ion profiles, as necessary input.     

Dynamics of FREI with/without cool flame interaction

The interactions between cool flames and flames with repetitive extinction and ignition (FREI) of stoichiometric n-heptane/air mixture were studied using a micro flow reactor with a controlled temperature profile from 373 to 1300 K. Two different flame dynamics with and without cool flames were observed in reactors with inner diameters $d_{\text{inner}}$ of 1 and 2 mm. Cool flames and FREI are spatially separated at $d_{\text{inner}}=$ 1 mm, whereas interactions between cool flames and FREI are observed at $d_{\text{inner}}=$ 2 mm. At $d_{\text{inner}}=$ 1 mm, the brightness intensity from cool flames depends on the inlet velocity ($u_{\text{inlet}}$). Approximately above $u_{\text{inlet}}=$ 10 cm/s, the brightness intensity from cool flames decreases with increasing inlet velocity, despite a large amount of mixture input. This is because before low temperature ignition occurs under higher inlet velocity conditions, the mixture archives temperature where negative temperature coefficient is dominant. Reaction front propagation speed of FREI decreases monotonically due to heat loss because the extinction points of FREI are located in higher temperatures than the cool flame region. At $d_{\text{inner}}=$ 2 mm, the acceleration of the reaction front in the cool flame region is confirmed experimentally, as predicted in our previous two-dimensional numerical simulations. Additionally, the instantaneous reaction front speed after autoignition is analyzed at $d_{\text{inner}}=$ 1 mm. The instantaneous reaction front speed decreases as the time from extinction to ignition $t_{\text{ex}\_\text{ig}}$ becomes longer because a moderate mixing zone of reactants and products is formed.     

Exchange bias behavior and magnetocaloric effect in Ni2In-type Mn7Sn4 alloy

In this work, the exchange bias behavior and magnetocaloric effect have been studied in Mn₇Sn₄ alloy. The X-ray powder diffraction pattern recorded at room temperature indicates that the sample crystallizes in a single phase with Ni₂In-type hexagonal structure (space group $P{6}_3/\mathit{mmc}$). The maximum magnetic entropy change value across paramagnetic/ferrimagnetic transition is about 3.3 J kg⁻¹ K⁻¹ under the magnetic field change of ${\mu }_0\mathrm{\Delta }H=0\text{-}5\text{T}$. With further cooling, the reentrant spin-glass-like state is obtained below 150 K, for which the exchange bias effect has been observed. The exchange bias field is ∼7.8 mT and ∼6.7 mT at $T=10\text{K}$ when the cooling field is ${\mu }_0{H}_{\mathrm{CF}}=0.1\text{T}$ and 0.5 T, respectively. The magnetic behavior and the origin of exchange bias in Mn₇Sn₄ are discussed.     

TBOR-shaped electro-optic modulator with dual output based on double-layer graphene

We proposed an electro-optic modulator with two-bus one-ring (TBOR) structure to improve the extinction ratio and reduce insert loss. It has a dual output compared with one-bus one-ring structure. In addition, double-layer graphene makes it possible for the modulation in the visible to mid-infrared wavelength range. It shows that this new electro-optic modulator can present two switching states well with low insertion loss, high absorption and high extinction ratio. At $\lambda =1550\text{ nm}$, when the switching states are based on the chemical potential, ${\mu }_{\mathrm{c}}=0.38\text{ eV}$ and ${\mu }_{\mathrm{c}}=0.4\text{ eV}$, the insertion losses of both output ports are less than 2 dB, the absorption of the output port coupled via a micro-ring reaches 45 dB and the extinction ratio reaches 14 dB. When the refractive index of the dielectric material is 4.2, the applied voltage will be less than 1.2 V, thus can be used in low-voltage CMOS technology.     

Spin switch effect in multiply connected superconductor-ferromagnet hybrid geometry

We consider a superconducting spin valve in multiply connected superconductor-ferromagnet hybrid geometry such as a superconducting ring enclosed a ferromagnetic metal, in the framework of linearized Usadel equations. We simplify our model by considering the presence of the exchange field in the superconducting ring which allows us to manipulate magnetization orientations in parallel or antiparallel configurations by switching the weaker exchange field. In such geometry the superconducting ground state is activated to higher orbital states characterized by the nonvanishing vorticity parameters L which will be the energetically favorable superconducting state in some ranges of the proximity superconductor-ferromagnet region. The competing effects caused by the exchange interactions and the orbital effect, are analyzed through the nonmonotonic dependence of the superconducting critical temperature $T_c$ on the radius $d_f$ of the ferromagnetic core. The analytic $T_c(d_f)$ formula is obtained within the single mode approach and the analysis of the spin switch effect is given.     

Moment of inertia of superconductors

We find that the bulk moment of inertia per unit volume of a metal becoming superconducting increases by the amount $m_e/(\pi r_c)$, with $m_e$ the bare electron mass and $r_c=e^2/m_e{c}^2$ the classical electron radius. This is because superfluid electrons acquire an intrinsic moment of inertia $m_e{(2{\lambda }_L)}^2$, with ${\lambda }_L$ the London penetration depth. As a consequence, we predict that when a rotating long cylinder becomes superconducting its angular velocity does not change, contrary to the prediction of conventional BCS-London theory that it will rotate faster. We explain the dynamics of magnetic field generation when a rotating normal metal becomes superconducting.     

The single-polarization filter composed of gold-coated photonic crystal fiber

A single-polarization filter comprising a gold-coated photonic crystal fiber based on surface plasmon resonance is designed and investigated. The pattern matching and coupled polarization characteristics analyzed by the full-vector finite element method (FEM) and losses at 1,540 nm are achieved to 1,016.01739 dB/cm (x-pol core mode) and 33.81917 dB/cm (y-pol core mode). The crosstalk (CT) value of the 1,540 nm band is ?853.12653 dB for fiber length $L=1,000\mathrm{\mu }\text{m}$ and the bandwidth is 850 nm. The working wavelength of the filter ranges from 1,280 nm to 1,540 nm by varying the diameter of outer air holes ($d_1$), the diameter of inner air holes ($d_4$), the metal film thickness (t), as well as the liquid refractive index (n).     

Self-organization of ultra-thin uranium film

We have studied the adsorption behaviors of uranium (U) atoms coated graphene at different coverage ratios using first-principles calculations. Isolated U atom is demonstrated to be more likely to stay on the hollow site of graphene with a large adsorption energy of 2.80 eV and high magnetic moment of 5.07 ${\mu }_{\text{B}}$. It seems that two U atoms tend to adsorb on the nearest neighbor hollow sites of graphene. As the concentration of U atom increases, no U-dimer or U-cluster appears with increasing coverage ratio. In the case of coverage ratio of 2/3, U atoms on graphene is the most stable configuration, where each U atoms are uniformly-distributed in U network with honeycomb lattice. The adsorption energy is as large as 2.57 eV per U atom. Moreover, the spin–orbit coupling effect on electronic band structures is outstanding, which induces the degeneracy bands splitting. Our calculations will provide profound background to understand the adsorption behaviors of U atoms on graphene.