Low Confinement Factor Research Articles

Understanding the core confinement in DIII-D super-H experiments by transport modeling

The DIII-D super-H (SH) scenario, which is characterized by a significantly higher pedestal pressure compared to standard high confinement mode (H mode) plasmas, typically exhibits two phases in its temporal dynamics. The early hot ion (SH–HI) phase has higher core ion temperatures and normalized confinement factor (H 98(y,2) ∼ 2) than the later ‘standard’ SH phase, which has similar pedestal pressure characteristics to the SH–HI phase but a lower confinement factor (H 98(y,2) ∼ 1.2) as well as lower pedestal T i/T e ratio. However, beyond the pedestal differences, it is also observed that in the core plasma T i is more peaked and has a significantly larger normalized gradient scale length a/L Ti in the SH–HI phase than in the SH phase. This paper identifies the physics responsible for the different core profiles via gyrokinetic and gyrofluid modeling. It is found that the ion temperature gradient (ITG) mode dominates the core transport for both phases. Absent flow shear effects, the ITG critical gradient (a/L Ti,crit) is shown to be far smaller in the SH–HI phase than the SH phase. The lower a/L Ti,crit in the SH–HI phase is shown to be mainly induced by the hollow carbon (impurity) density profile, which is strongly destabilizing relative to the nearly flat carbon density profile in the SH phase. Differences in the T i/T e ratio between these phases are found to have a minor impact. However, the significantly stronger flow shearing in the SH–HI phase relative to the SH phase enables the achievement of higher core a/L Ti values and is therefore mainly responsible for the higher core T i values observed in the early SH–HI phase. Predictive transport modeling shows that the confinement in the lower-rotation SH phase could be elevated significantly if a peaked impurity density profile can be achieved, and potential applications to the performance improvement of future reactors are discussed.

Surface plasmon polaritons frequency-blue shift in low confinement factor excitation region* *Project supported by the National Key Research and Development Program of China (Grant Nos. 2017YFA0701000, 2018YFF01013001, and 2020YFA0714001) and the National Natural Science Foundation of China (Grant Nos. 61988102, 61921002, and 62071108).

Surface plasmon polaritons’ (SPPs’) frequency blue shift is observed in finite-difference time-domain (FDTD) simulation of parallel electron excitation Au bulk structure. Comparing with cold dispersion of SPPs, an obvious frequency blue shift is obtained in low confinement region excitation simulation results. Then, according to SPPs’ transverse attenuation characteristics, the excited frequency mode instead of cold dispersion corresponding frequency mode matches it. Thence, this excited mode is confirmed to be SPPs’ mode. As is well known the lower the frequency, the smaller the confinement factor is and the lower the excitation efficiency, the wider the bandwidth of excited SPPs is. And considering the attenuation in whole structure, the excited surface field contains attenuation signal. In a low confinement factor region, the higher the SPPs’ frequency, the higher the excitation efficiency is, while broadband frequency information obtained in attenuation signal provides high frequency information in stimulation signal. Thence, in the beam–wave interaction, as the signal oscillation time increases, the frequency of the oscillation field gradually increases. Thus, compared with cold dispersion, the frequency of excited SPP is blueshifted This hypothesis is verified by monitoring the time domain signal of excited field in low and high confinement factor regions and comparing them. Then, this frequency-blue shift is confirmed to have commonality of SPPs, which is independent of SPPs’ material and structure. Finally, this frequency-blue shift is confirmed in an attenuated total reflection (ATR) experiment. Owing to frequency dependence of most of SPPs’ devices, such as coherent enhancement radiation and enhancement transmission devices, the frequency-blue shift presented here is of great influence in the SPPs applications.

Experimental and numerical study on the compressive behavior of micro-expansive ultra-high-performance concrete-filled steel tube columns

In concrete-filled steel tube (CFST) columns, conventional ultra-high-performance concrete (UHPC) has a large shrinkage and could easily be deboned from the steel tube, resulting in a failure of the effective lateral restraint of the steel tube to the core concrete. To solve this problem, a new type of micro-expansive and self-compacting UHPC was adopted to short CFST columns, which were further subjected to push-out and axial compressive tests. Test results indicated that the UHPC with the expansive agent could achieve an effective bonding between the core concrete and the steel tube. Compared with the compressive behavior of conventional UHP-CFST columns with a low confinement factor, the load-shortening curves of micro-expansive UHP-CFST columns do not show any obvious softening section. Furthermore, a finite element model of the micro-expansive UHP-CFST column was developed to predict the relationship of load and displacement by ABAQUS. Finally, the axial capacities of the short micro-expansive UHP-CFST columns obtained by the finite element analyses (FEA) were compared with that calculated by different design codes. It was concluded that the axial capacity of the composite members calculated by current codes is less than the FEA results and the formulas provided by the codes need to be further validated or modified.

Single-mode porous silicon waveguide interferometers with unity confinement factors for ultra-sensitive surface adlayer sensing.

Guided wave-optics has emerged as a promising platform for label free biosensing. However, device sensitivity toward surface-bound small molecules is directly limited by the evanescent interaction and low confinement factor with the active sensing region. Here, we report a mesoporous silicon waveguide design and inverse fabrication technique that resolves the evanescent field interaction limitation while achieving maximal transverse confinement factors and preserving single-mode operation. The waveguide sensors are characterized in a Fabry-Perot interferometer configuration and the ultra-high sensitivity to small molecule adlayers is demonstrated. We also identify dispersion to be a promising degree of freedom for exceeding the sensitivity limits predicted by the conventional non-dispersive effective medium theory.

Strongly asymmetric waveguide laser diodes for high brightness picosecond optical pulses generation by gain switching at GHz repetition rates

Rate equations analysis, supported by travelling wave simulations, is used to show that a diode laser design with a very low (<1%) confinement factor is optimal for generating streams of high energy optical pulses for nonlinear applications by large-signal modulation with both existing and potentially improved ac current generators. An asymmetric waveguide laser design is proposed to realize this low confinement factor while simultaneously maintaining good beam properties in a single transverse mode. The rate equations model complemented by transport equations is used to quantify the effects of transport in the broad optical confinement layer(s) on the laser dynamics, and it is shown that in the proposed laser construction the detrimental effects of transport are weak.

17-W Near-Diffraction-Limited 970-nm Output From a Tapered Semiconductor Optical Amplifier

High power, high beam quality and narrow, stable spectra are achieved simultaneously using a truncated-tapered optical amplifier in a master-oscillator power amplifier-system. We compare the influence of lateral geometric design on amplifier performance, by using devices with super large optical waveguide (4.8 μm) and relative low confinement factor (Γ = 1%) . We find that the use of an amplifier with a larger active region results in both high power and high beam quality. An abandonment of cavity spoiling grooves leads to strongly improved beam characteristics.

250 mW, 15µm monolithic passively mode-locked slab-coupled optical waveguide laser

We report the demonstration of a 1.5 microm InGaAsP mode-locked slab-coupled optical waveguide laser (SCOWL) producing 10 ps pulses with energies of 58 pJ and average output powers of 250 mW at a repetition rate of 4.29 GHz. To the best of our knowledge, this is the first passively mode-locked slab-coupled optical waveguide laser. The large mode and low confinement factor of the SCOWL architecture allows the realization of monolithic mode-locked lasers with high output power and pulse energy. The laser output is nearly diffraction limited with M2 values less than 1.2 in both directions.

Plasma Current Rampup by the Outer Vertical Field Coils in a Spherical Tokamak Reactor

To find a solution of the plasma current rampup problem in a low aspect ratio spherical tokamak (ST) reactor, the effect of the outer vertical field coils on plasma current rampup is studied with noninductively driven current and bootstrap current but without the OH transformer during the fusion power rampup phase. As a lower elongation of [kappa] = 2 does not allow a large poloidal beta, a low density discharge with high heating/current drive power is necessary to increase the noninductive plasma current up to 30 MA, and then the vertical field can ramp the plasma current up to 48 MA just before reaching the steady state fusion burn phase. A higher elongation of [kappa] = 3 can reach a higher βp value, in which case the plasma current of 48 MA can be achieved by the vertical field without powerful heating/current drive. As the level of the vertical field current drive depends on the plasma energy, which is determined by the confinement time, a lower confinement factor can only produce a lower plasma current. The current rampup time can be chosen arbitrarily, shorter or longer, facilitating a flexible ST reactor operation.

Analysis of 6-nm AlGaAs SQW low-confinement laser structures for very high-power operation

This paper reports experimental results on single quantum-well separate confinement heterostructures (SQW SCH) with low-confinement factor, designed for very high-power operation. The maximum power output for AR/HR coated 3-mm-long devices, measured in very short pulsed conditions (100 ns/1 kHz), from 10-/spl mu/m-wide stripes was as high as 6.4 W before catastrophic optical degradation. If scaled to continuous-wave (CW) conditions, this value would be 800-1100 MW, which would mean a factor of 22.7 times more than reported for the best devices with normal design for threshold minimization. The absorption coefficient for the symmetrical structure is as low as 1.1 cm/sup -1/, in spite of the low trapping efficiency of carriers in the quantum well (QW). The maximum differential efficiency is 40% (both faces, uncoated devices) for symmetrical structure and 33% for the asymmetrical one (all measurements in pulsed conditions). Threshold current densities were 800 A/cm/sup 2/ for 5-mm-long devices in the symmetrical case and 2200 A/cm/sup 2/ in the asymmetrical one. The effects of inefficient carrier trapping in the QW on the threshold current densities and differential efficiency are discussed.