Investigation Of The Ion And Electron Thermal Coupling At Plasma Edge In The J-TEXT Tokamak

In recent decades,magnetic confinement fusion has developed unstintingly,many countries have upgraded or built various fusion devices to realize the commercialization of nuclear fusion.Tokamak,due to its many advantages,has been the most widely choice.In magnetically confined plasma,the edge plasma which is a primary factor for determining plasma confinement property has been the focus in the research field.The ion(Ti)and electron(Te)temperatures in the edge plasma are important plasma parameters for understanding the edge plasma physics.The Te can be easily measured by simple Langmuir probes.The Ti,however,is difficult.The retarding field analyzer(RFA)is effective and one of the few Ti diagnostics.The retarding field analyzer will also be affected by external conditions when measure the ion temperature.Such as the presence of capacitance in the transmission line,the current collected by the retarding field analyzer(RFA)becomes asymmetrical(i.e.hysteresis)during the voltage scanning and it further affects the measurement of the ion temperature(Ti).In this manuscript,we firstly analyze the hysteresis of the RFA I-V characteristics measured in the edge plasma of J-TEXT tokamak and find that the ion temperature fitted from the falling side of the collector current is about 14%higher than that fitted from the rising side.An analytic model is built and verified by the experimental results.Based on the model,the influences of the capacitance of transmission lines and the frequency of the scanning voltage on the hysteresis are investigated.It is found that the hysteresis becomes more remarkable with increasing capacitance or scanning frequency.Besides,the impacts of the capacitance and the scanning frequency on the Ti measurement are also studied,which indicates that the Ti is overestimated due to the hysteresis.The temperature error depends linearly on the capacitance and the scanning frequency.The analytical results can be used to correct the ion temperature measured by the retarding field analyze and reduce the measurement error.Furthermore,the analytical results are expected to be used in the prediction of scanning frequency for the RFA application.Meanwhile,the ion temperatures(Ti)measured by a bidirectional RFA and electron(Te)temperatures measured by a four-tip Langmuir probe(FTLP)at the last closed flux surface(LCFS)during density scanning discharges and in the scrape-off layer(SOL)with a constant plasma density have been reported on the J-TEXT tokamak.Both the LCFS Ti and Te are found to reduce with the central line-averaged density(ne)increasing.Meanwhile,the temperature ratio(τ=Ti/Te)is about 2 for a low ne(1.5→2.5×1019m-3),decays in a moderate ne(2.5→3.5×1019m-3)and tends to unity at the high ne(3.5→4.2×1019m-3).It suggests that the ions and electrons at the LCFS(even in the SOL)are thermally decoupled in a low or moderate ne and trend to fully thermal coupling in the high density plasmas.In addition,both Ti and Te decrease with the radius.On the contrary,the τ rises with the radius,which indicates that the ions and electrons are much more thermally decoupled in the deeper SOL.The reason could be that in the SOL,the parallel energy loss of electrons is much larger than that of ions and the electron temperature drops faster than the ion temperature,which will lead to the thermal decoupling between ions and electrons.