EAST Lower Divertor Assembly Accuracy Control And The Effect Of Geometric Accuracy On Heat Flux

With the development of magnetic confinement fusion research,high power and long pulse operation has become one of the key points of experimental research.EAST,as a fully superconducting tokamak device,is equipped with double divertors and the active water-cooled first wall.During the 15 years stable operation,it has achieved plasma operation of 160 million degrees and 1056 seconds respectively.The divertor is one of the key plasma facing components(PFCs)of the EAST device with the function of removing the heat flux and impurities from the plasma.The assembly accuracy is necessary to ensure the geometric and physical configuration of the divertor,which is closely related to the stability and reliability of the divertor.With the continuous improvement of the EAST experimental parameters,the analysis and control of assembly accuracy have become the focus of the divertor design process.In this paper,taking the upgrade of the EAST lower divertor in 2021 as an opportunity,the accuracy analysis,design,control and detection of the key regions(high heat flux regions)of the lower diverrtor were carried out.Firstly,CETOL(a 3D tolerance analysis software)was used to carry out the simulation in the accuracy design stage.The design tolerance of the vertical surface of the inner and outer target to the EAST central axis was± 0.39 mm and ± 0.50 mm,and the design tolerance of the horizontal surface of the outer target to the EAST mid-plane was ± 0.41 mm.Then,a laser tracker device was used to measure the accuracy of the assembled lower divertor surface.The measuring points of 95%of the vertical surface of the inner target,91%of the horizontal surface of the outer target and 85%of the vertical surface of the outer target are within the tolerance zone of±0.5 mm,respectively.Finally,for the small probability of excessive deviation during the assembly process,the repair and assembly method was introduced to control the assembly accuracy of the divertor.The aim of accuracy control of the lower divertor is to reduce the non-uniformity of the annular heat flux distribution of the targets.In the EAST high-power and longpulse discharge experiment,the small deviation of the divertor targets may lead to the uneven distribution of circumferential heat flux,or even the "hot spot" phenomenon due to the local heat flux concentration,resulting in the sputtering,melting,ablation and other serious consequences of the target materials.This paper mainly uses infrared thermograph diagnostic system and water cooling calorimetry system to carry on the related research since the two methods can measure the heat flux or temperature distribution of the divertor locally or as a whole.The conclusions obtained by comparing the measured results with the divertor error data are as follows.Through IR camera,it is found that the assembly deviation difference of the target is positively nonlinearly correlated with the heat flux distribution,that is,the larger the deviation difference is,the greater the heat flux non-uniformity is,and the greater the peak fluctuation of the surface temperature distribution is.While for the water cooling calorimetry system,the heat flux distribution of eight water cooling submodules of the divertor was measured.The fluctuation range was-3.5%～+4.0%,and the variation trend was similar to the mean value of the potential difference of each water cooling submodule,which has the characteristics of positive correlation.In addition,after the EAST experiments,the W/Cu interface of the W/Cu monoblock may appear micro errors(also called micro defects).High heat flux test was carried out to evaluate the heat removing ability and thermal fatigue stability of W/Cu monoblocks with defects.Three large,small and non-defect W/Cu monoblocks were selected to perform high heat flux test on 100 kW electron gun equipment of Institute of Plasma Physics,Chinese Academy of Sciences(ASIPP).All the W/Cu monoblocks have passed the high heat flux screening and fatigue test according to the standards.In the fatigue test,the tungsten surface temperature fluctuation is less than ±7.5%.Under the same experimental conditions,the tungsten surface temperature of the W/Cu monoblock with large defect is 12%～17%higher than that of the W/Cu monoblock with small defect,and the W/Cu monoblock with small defect is 6%～8%higher than that of the W/Cu monoblock with no defects.Besides,the thermal decay time constants of the W/Cu monoblock with large,small and no defects are 11.1 s,8.5 s and 7.4 s respectively.This can quantitatively evaluate the heat transfer performance of the three kinds of W/Cu monoblocks,namely,the smaller the constant is,the better the heat transfer performance is.The above test results will be helpful to understand the relationship between the micro accuracy of W/Cu monoblock and the heat flux removing capability.