Study Of ELM-induced Tungsten Sputtering And Its Control On EAST Divertor

Edge localized mode(ELM)is a common MHD instability during high performance mode and limits the lifetime of ITER divertor.Although the divertor is made of tungsten(W)which shows a high melting point and high sputtering threshold,the ELM also can cause serious W erosion thus shorten the lifetime of the divertor.Besides,the released W source may also penetrate into the core region and degrade the confinement of plasma.So it’s valuable to investigate the behavior of W source caused by ELM and develop the method to control it.That will be propitious to the operation of ITER the design of future power plant.ELM is a transient in the order of 0.1 ms,so the tracing for the W source during ELM by spectroscopic method is challenging.In this work,we develop a method that combines the photomultiplier tube(high time resolution)and spectroscopic system(high wavelength resolution)to obtain the ELM-resolved W source and use this method to process the spectral data in EAST W divertor.Basing on the systematic research,we find that the features of intra-ELM W sputtering are distinct from the heat deposition and analyze the main influencing factors The W sputtering during ELM is can be divided into two parts,namely the rapid rise part and slow decay part.The rise part is important in heat deposition since it decides the maximum temperature.For the W source,the decay part is a fraction of 1/2-2/3 to the total W sputtering during ELM and can be changed by plasma condition.The W sputtering yield is proportional to the pedestal electron temperature.Besides,the intrinsic carbon impurity and helium can increase the intra-ELM W sputtering.The rise time for the heating load is proportional to the ion parallel transit time.However,the rise time of intra-ELM W source doesn’t show a correlation to the ion transit time during ELM but the delay time of W source to the ELM event.The delay time shows a much correlated relation to the pedestal colisionalities.Intra-ELM W source also shows an in-out divertor asymmetry which can be changed by density,toroidal magnetic field direction,ELM size and plasma type.Usually,the intra-ELM W sputtering is more favorable to the outer divertor.That is different from heat/particle flux on the divertor.The deposition process on the divertor may play an important role according to the initial result of the simulation.The intra-ELM W sputtering under natural ELM,resonant magnetic perturbation(RMP)and lower hybrid wave(LHW)mitigation schemes are compared.The W sputtering per ELM is mitigated under these three schemes.In natural mitigation scheme,the ELM sputtering rate increases with ELM frequency(fELM)at first and rolls over in a certain fELM.In the RMP mitigation scheme,the ELM sputtering rate is larger than the one without RMP and increases with fELM.The characteristic times also are important features for the ELM-induced W sputtering.In LHW mitigation scheme,ELM sputtering rate decreases accompanied by the decrease of fELM.These features relate to different pedestal temperature behaviors under these three schemes.For the characteristic times,the rise time of intra-ELM W source increases and the delay time doesn’t necessarily correlate to the ion triansit time in RMP and LHW mitigation schemes,indicating the change of transport mechanisms during ELM.The partial divertor detachment and intra-ELM W sputtering mitigation are obtained at the same time in a proper phase difference of RMP field.That provides an alternative way to synergistically control the W source in steady-state and during ELM.At last,some other mitigation effects such as neon injection and ion cyclotron radio frequency heating are also investigated.They show a mitigation effect on intra-ELM W source,but their capabilities to the other operational parameters like divertor detachment are needed more researches.