Closed corner divertor with B×▽B away from the divertor:a promising divertor scenario for tokamak power exhaust

A major challenge facing the steady-state operation of tokamak fusion reactors is to develop a viable divertor solution with order-of-magnitude increase in power handling capability as compared with present experience.A recently developed divertor concept for this end has been tested recently on EAST tokamak through combining the effects of a closed divertor corner and E×B drifts.The E×B drifts in the divertor move particles towards the outer divertor corner area in the scrape-off layer for B×▽B directed away from the divertor,which can significantly enhance the particle concentration there,facilitating divertor detachment.In recent EAST experiments,the effects have been demonstrated where the lowest electron temperature at the divertor plate is obtained with strike point located close to the corner in the horizontal target and with B×▽B away from the divertor.These experimental results are in reasonable agreement with SOLPS-ITER simulations including drift effects,suggesting that the new divertor concept potentially provides a promising divertor solution for long-pulse operations of future tokamak fusion reactors with much higher power fluxes.

Parametric decay instabilities of lower hybrid waves on CFETR

The lower hybrid current drive is a potential candidate for sustaining plasma current in tokamak steady-state oper-ations,which could be used in China Fusion Engineering Test Reactor(CFETR)with input power up to a few tens of megawatts.Such high input power could trigger the well-known parametric instabilities(PIs)at the plasma edge affect-ing the propagation and absorption of the lower hybrid pump waves.By analytically solving the nonlinear dispersion relation describing PIs,an explicit expression of the PI growth rate is obtained and analyzed in detail.It is found that pressure is the key parameter determining the PI characteristics.Ion sound quasi-mode is the dominant decay channel in the low-pressure regime,while the ion cyclotron quasi-mode(ICQM),as well as its harmonics,becomes dominant in the intermediate regime.In the high-pressure regime,only one mixed channel is found,which is related to Landau damping by free-streaming ions.Analytical expressions of growth rates of these decay channels are also obtained to show the parameter dependence at different pressure limits.The above analytical results are used to estimate the PIs on a typical profile of CFETR,and verified by corresponding numerical calculations.ICQM is found to be the strongest decay channel with a considerable growth rate for CFETR.