| In future fusion reactors,the divertor targets will endure massive particle and heat fluxes from the core plasma.Without effective mitigation methods,the heat flux exhausted to divertor targets will far exceed the engineering limit,and the particle flow will erode the plasma-facing material of the target plate.The sputtered impurities will further contaminate the core plasma,leading to a degradation of the confinement performance and even a disruption.Therefore,the high heat load onto the divertor target and the core contamination by the sputtered PFM will be key issues that restrict the realization of high performance steady-state operation of future fusion reactors.In order to explore solutions,it is necessary to find effective control methods based on a sufficient understanding of plasma transport in the scrape-off layer(SOL)of tokamak.The E×B drift plays an important role in boundary plasma transport,so a thorough understanding of the influence of the E×B drift on particle transport in the SOL of tokamak is of great significance for achieving high performance steady-state operation of future fusion reactors.In this paper,combined with the recently observed double emission belts phenomenon and the double-peaked divertor target profile phenomenon on EAST,numerical simulation studies using SOLPS-ITER and DIVIMP codes have been conducted,and physical mechanisms of the influence of the E×B drift on distributions of main ions and the carbon(C)impurity radiation have been analyzed;Based on the typical EAST conditions,the influence of the E×B drift on tungsten(W)impurity transport has been further investigated.Specific contents are as follows:(1)Combined with EAST experiments,the influence of the E×B drift on distributions of main ions in the SOL was studied.In EAST experiments,the doublepeaked divertor target profile phenomenon is observed,i.e.there exist two peaks in the particle deposition profile at the target,which appears at the inner target when the ion B×▽B drift pointing toward the X-point and the outer target when the ion B×▽B drift pointing away from the X-point.Through numerical simulations,the double peaks in the plasma density profile was reproduced.and the physical mechanism of the formation of the double peaks was investigated.It is pointed out that the first peak(located near the strike point)is formed by the strong ionization source near the strike point,and the second peak(located in the far-SOL region)is formed by the synergetic effect of poloidal and radial E×B drifts.Further simulations investigated the effects of the power entering SOL(PSOL),impurity seeding rate,and upstream density(ne,sep)on the double-peaked profile.The parameter space for the formation of the doublepeaked profile and the dependences of the height ratio of two peaks on PSOL and ne.sep were given.The results are qualitatively consistent with the rules observed in EAST experiments.(2)Combined with the visible double emission belts phenomenon observed for the first time in EAST experiments,the effect of E×B drift on the distribution of C impurity radiation in the divertor region was studied by numerical simulations.In EAST experiments,the unique visible light double emission belts are obsenved in the upper divertor region,with one emission belt located near the X-point and the other located in high-field side(HFS)SOL near the upper inner(UI)divertor baffle.Based on numerical simulations,a possible explanation for the double emission belts phenomenon is proposed,i.e.the two belts are formed by line emission of C impurity(rather than W impurity)deposited in the upper divertor region.The emission belt in the channel G is contributed by CⅡ(514 nm),while the emission belt in the channel B is contributed by CⅢ(465 nm).The corresponding relation between the emission peaks and the C sources,as well as the physical mechanism of the formation of the emission peak were further analyzed.It was pointed out that the E×B drift leads to a significant enhancement of the emission peak in HFS SOL,so that the intensities of simulated two peaks are comparable,which is consistent with experimental observation.(3)Based on the typical upper-single null(USN)configuration,the effect of the E×B drift on the transport process of W impurity from the divertor target to the confined plasma region was studied.The simulation not only includes the drift of the background plasma,but also introduces the E×B drift of W impurity.In the cases with different drift options,the plasma near the target plate with stronger sputtering are all in a high-recycling regime with a similar peak electron temperature value.The results show that W source is dominated by the outer target when the ion B×▽B drift pointing toward the X-point,and the inner target when the ion B×▽B drift pointing away from the X-point.The analysis of W particle flux indicates that drifts can reduce the prompt redeposition of W impurity,promote the W escape from the divertor,and the diffusion from the main SOL into the confined plasma region.By affecting the sputtering source intensity and the transport of W impurity.the drift effects can lead to an increase in the W particle flux entering the confined plasma region by more than one order of magnitude,resulting in a significant increase in the averaged concentration of W in the confined region(CWavg).Therefore,the physical design of future fusion reactors should pay attention to the influence of the E×B drift on the W impurity contamination.In this paper,physical mechanisms of influences of the E × B drift on the particle distribution in the SOL,the C impurity radiation distribution,and the W impurity transport are investigated,which improves the understanding of the physics of particle transport in the scrape-off layer plasma.The research results are instructive to the physical design of the divertor in future fusion reactors. |
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