Experimental evaluation of Langmuir probe sheath potential coefficient on the HL-2A tokamak. (1st February 2018)

• Record Type: Journal Article
• Authors: Nie, L.; Xu, M.; Ke, R.; Yuan, B.D.; Wu, Y.F.; Cheng, J.; Lan, T.; Yu, Y.; Hong, R.J.; Guo, D.; Ting, L.; Dong, Y.B.; Zhang, Y.P.; Song, X.M.; Zhong, W.L.; Wang, Z.H.; Sun, A.P.; Xu, J.Q.; Chen, W.; Yan, L.W.; Zou, X.L.; Duan, X.R.
• Other Names: collab.
• Is Part Of: Nuclear fusion. Volume 58:Number 3(2018:Mar.)
• Publication Date: 2018-02-01
• Subjects: Langmuir probe -- sheath potential coefficient -- floating potential -- plasma potential; Nuclear fusion -- Periodicals; 621.48405
• DOI: 10.1088/1741-4326/aaa4d1
• Languages: English
• ISSNs: 0029-5515

Abstract

Abstract: Systematic calibration experiment of Langmuir probe sheath potential coefficient Λ, which is a critical coefficient for estimating plasma sheath potential, has been carried out in the HL-2A tokamak deuterium plasmas. The electron energy probability function (EEPF) shows that electron outside last-closed-flux-surface (LCFS) is Maxwell distribution, but inside LCFS it changes to bi-Maxwell. Two kinds of plasma potential measuring method and three kinds Λ estmating method were compared. It is found that the estimated Λ coefficient is in the region of 2–3 outside LCFS and then increases to ~5 inside LCFS due to the high temperature electron effect. Fortunately, the results show that the commonly used value Λ = 2.8 is still available to calculate plasma potential when we use the overestimated electron temperature measured by three-tip probe in bi-Maxwell case. Further analysis indicated this value should be corrected. Or it may lead to a error when we calculate the the electric field E r and its shear d E r / d r . The corrected value monotonically increased from ~2.2 to ~2.9 while Langmuir probe moved from 40 mm outside LCFS to 20 mm inside LCFS.