Long-pulse high βN operation scenario with internal transport barrier (ITB) and central flat q profile has been achieved on EAST. Analysis shows that higher heating power is needed to obtain ITBs in ...higher density discharges. Analysis of EAST high βN experiments shows that fishbone activities are accompanied with formation ITB and high performance. Experimental and simulation analysis demonstrates that the increasing E x B shear flow by the neutral beam injection (NBI) is important for the formation of ion temperature ITB (Ti-ITB). And, Ti-ITB disappears with the reduction of plasma rotation. While ITBs in channels of electron density (ne) and temperature (Te) are not achieved until fishbone activities are observed, which means that E x B shear flow mainly affects the formation of Ti-ITB in EAST high βN experiments. Experimental analysis shows that the fishbone is accompanied by the increasing E x B shear flow, leading to the formation of Ti-ITBs. A possible explanation for the formation of ITBs in all channels is that fishbone activities in high βN discharges with central flat q-profiles (closed to unity) induce the feedback process of the formation of ITBs, and leading to the increase of E x B shear flow as well as the formation and sustainment of ITBs.
•Formation and sustainment of ITBs in high βN discharges with different electron density on EAST tokamak are discussed.•Relationship between fishbone activities and formation of ITBs are demonstrated.•Experimental and simulation analysis are carried out to study the effect of E x B shear flow on the formation of Ti-ITBs.•Role of fishbone activities on the formation of ITBs are discussed.
Abstract
To address the needs for a fusion pilot plan design, DIII-D/EAST joint experiments on DIII-D have demonstrated high normalized beta βN~4.2, toroidal beta βT~3.3% with qmin>2, q95≤8 sustained ...for more than six energy confinement times in poloidal beta regime. The excellent energy confinement quality (H98y2~1.8) is achieved with an internal transport barrier (ITB) at high line-averaged Greenwald density fraction fGr > 0.9. Gyrofluid (TGLF) modeling of the transport characteristics shows that the beam-driven rotation does not play an important role in the high confinement quality. The modeling also captures very well several transport features, giving us confidence in using integrated modeling to project these experimental results to future machines. The high-performance phase is terminated by fast-growing modes triggered near the n=1 ideal-wall kink stability limit. New RF capabilities for off-axis current drive could remove the residual ohmic current to achieve a fully non-inductive state, and improve the mode-wall coupling to increase the ideal-wall βN limit, enabling sustainment of the fully non-inductive high performance plasma in stationary conditions.
The validation of a computational fluid dynamics model for a proton exchange membrane fuel cell (PEMFC) is normally conducted by the experimental I–V performance curve. However, it seems this method ...is not solid enough. In the meantime, it's difficult to conduct the item-to-item quantitative comparisons between the internal distributions acquired from numerical and testing results. Therefore, in this paper, as the first attempt, qualitative comparisons between the modeling and experimental results are conducted based on the three important parameters of an air-cooled PEMFC (air stoichiometric ratio, air relative humidity, cathode flow field design) to explore the trends of the related fuel cell I–V performance curves and internal resistances. The internal resistances are tested using the EIS technique and differentiated by an equivalent circuit model. Conclusions show that the qualitative comparisons between the numerical and testing results support each other well and new results are found based on the comparisons. Finally, discussions on the sensitivity based on the experimental EIS results are conducted to explore the response degree of the total resistance to the air stoichiometric ratio, the air relative humidity and the cathode flow field design.
•Qualitative comparisons between modeling and testing results support each other.•Proton transport is the dominant factor affecting the reaction rate and resistance.•Sensitivity is defined to compare different parametric series.•Air relative humidity could lead to “Strong” sensitivity in the case of dry air.•Air stoichiometric ratio has the strongest effect most of the time.
Abstract
The formation of Internal Transport Barrier (ITB) is studied in HL-2A plasmas by means of nonlinear gyrokinetic simulations. A new paradigm for the ITB formation is proposed in which ...different physics mechanisms play a different role depending on the ITB formation stage. In the early stage, fast ions, introduced by Neutral Beam Injection ion system, are found to stabilize the thermal-ion-driven instability by dilution, thus reducing the ion heat fluxes and finally triggering the ITB. Such dilution effects, however, play a minor role after the ITB is triggered as electromagnetic (EM) effects are dominant in the presence of established high pressure gradients. We define the concept of ITB self-sustainment, as the low turbulence levels found within the fully formed ITB are consequences of large scale zonal flows, which in turn are fed by a non-linear interplay with large scale high frequency EM perturbations destabilized by the ITB itself.
Abstract
Local nonlinear gyrokinetic simulations of tokamak plasmas demonstrate that turbulent eddies can extend along magnetic field lines for hundreds of poloidal turns when the magnetic shear is ...very small. By accurately modeling different field line topologies (e.g. low-order rational, almost rational, or irrational value of the safety factor), we show that the self-interaction of such ‘ultra long’ eddies can dramatically reduce heat transport. This extreme sensitivity of turbulent transport to the safety factor reveals novel strategies to improve confinement, constitutes experimentally testable predictions, and illuminates past observations of internal transport barriers.
Abstract
Fishbone instabilities and internal transport barriers (ITBs) are frequently and sequentially observed in tokamak plasmas. Recently, the relationship between fishbone instabilities and ITBs ...was numerically studied, mainly on the basis of experimental results (Liu
et al
2020
Nucl. Fusion
60
122001). It was identified that a radial electric field can be generated by the fishbone itself, which may act as a trigger for ITB formation. To gain a deeper understanding of this subject, in this work we further demonstrate the multiple interactions between fishbone instability and ITBs in Experimental Advanced Superconducting Tokamak (EAST) experiments (discharge #56933) using the hybrid kinetic-MHD code M3D-K. In multiple-
n
simulations, it is found that a zonal electric field can be induced in the nonlinear fishbone stage, leading to a relatively large
E
×
B
zonal flow that is sufficient to suppress the dominant microinstability before ITB formation; this should account for ITB triggering. After the ITB is triggered, the equilibrium pressure gradient increases and fast ions from the neutral beam injection accumulate in the ITB region. Linear simulations are performed to analyze the effect of ITB formation on fishbone instability. It is shown that due to the change of the pressure gradient during ITB expansion, the change in the bootstrap current density profile modifies the
q
-profile and then stabilizes the fishbone mode. Additionally, the accumulation of the fast ions leads to a broadening of fast ion distribution around the ITB region, which also has a stabilizing effect on the fishbone mode.
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