| Exhausting the huge amount of power from fusion plasma in the core region is one of the most critical issues for future fusion tokamak devices.For China Fusion Engineering Test Reactor(CFETR),with a designed fusion power of 1 GW,it is an indispensable technique to seed gas impurity in the edge plasma,in order to induce the radiative divertor and exhaust the power that flowed into the scrap-off layer by impurity radiation.One of the most concerned issues for impurity seeding is to combine the low heat flux on the divertor targets with the low impurity concentration in the core plasma.A series of simulation study was performed for the radiative divertor of CFETR with impurity seeding systematically,based on the 2D edge plasma simulation code of SOLPS.The influence of impurity species,inj ection location of impurity and magnetic configuration on the divertor impurity screening was studied,under high fraction of radiated power.The simulation study can be divided into following parts,specifically.Firstly,simulations were performed for the radiative divertor of CFETR with argon seeding under a conventional lower-single-null configuration and a density scan was applied.Results show that both inner and outer divertor plasmas can achieve fully detachment with a radiated power Prad～170 MW,whether the density is high or low.Therefore,the heat flux on divertor targets can be significantly reduced.Also,a relation among the electron density ne,the effective ion charge Zeff and the radiated power Prad is derived according to Matthews’ experimental scaling,and is well fitted to the simulation results with argon seeding.It is considered that the scaling is able to provide proper boundary condition for further optimization of core plasma scenarios.The SOLPS simulations were performed for CFETR with different seeding impurities of nitrogen,neon and argon,which are the most common radiation impurities in experiments,in order to compare the impurity transport and radiation.Simulation results indicate that nitrogen seeding is related to least fuel dilution among the three impurity species with detached divertor plasma of Prad～170 MW and similar electron density.However,the chemical reactions between tungsten and nitrogen will increase the tritium retention,and the formation of ammonia will create additional challenges for the tritium plant.Even though the atomic number of argon is larger between the two noble gases,argon seeding is related to lower upstream Zeff due to its lower ionization potential,easier ionization near targets and correspondingly stronger non-coronal effect.A tentative expression of modified Matthews’scaling law with the ionization potential,in order to evaluate the non-coronal effects of the impurity radiation with ionization potential,which is Zeff=1+C(Z-1)α Pad/ne2-β(EionD/Eionimp)γ,is presented.The simulation results fit pretty well to the modified scaling.Next,simulations were performed for the radiative divertor of CFETR with four different gas puffing schemes:deuterium and argon mixed gas injected from the outer leg;mixed gas injected from the inner leg;mixed gas injected from the top of main chamber and deuterium inj ected from the top while impurity injected from the outer leg,in order to compare the impurity screening with different gas puffing locations.Results indicate that,better impurity screening and lower upstream Zeff can be achieved with deuterium puffed from upstream,due to the increased background plasma flow in the scrap-off layer,while the puffing location of argon has a minor influence on the simulation results since divertor target recycling being the major impurity source in the edge plasma.Lastly,SOLPS simulations were performed for radiative quasi-snowflake(QSF)divertor of CFETR with argon seeding.And the simulation results are compared with the results of a conventional lower-single-null ITER-like divertor(ILD)configuration.For high density cases,both inner and outer divertor plasmas can achieve fully detachment for QSF configuration with lower radiated power,and the impurity screening is much stronger due to higher degree of detachment,when compared with ILD configuration.For low density cases,outer divertor plasma of QSF configuration is firstly detached due to more pronounced increase of outer divertor flux expansion,while the inner divertor detaches at first for the ILD configuration with the increase of the radiated power.And the impurity radiation in the QSF shows noticeable in-out asymmetry.The simulation results imply that the increase of flux expansion and connection length in QSF is of benefit to facilitate the divertor plasma detachment as well to increase the degrees of detachment,as a result,the divertor impurity screening can be improved due to suppression of the reverse flow of impurities. |
