ELSEVIER Journal of Nuclear Materials 228 (1996) 254-256 Letter to the Editors Estimation of Zeff in Novillo Tokamak R. Valencia *, G. Olayo, G. Cruz, R. Ldpez, E. Chfivez, L. Mel~ndez, A. Flores, E. Gaytfin lnstituto Nacional de Investigaciones Nucleares, Plasma Physics Laboratory, Apartado Postal 18-1027, Col. Escanddn C.P. 11801, M~xico D.F., Mexico Received 2 November 1994; accepted 1 December 1995 Abstract We estimated the Zef f in the Novillo Tokamak after having applied a HeGDC process through two different methods: anomaly factor and mass spectrometry. The first one gave a Zcff value of 2.07 for a tokamak discharge of 4350 A plasma current and 3 V of loop voltage. By mass spectrometry 30 s after the discharge had finished a Zcff of 4.19 was obtained for the same discharge. By mass spectrometry we observed that the Z~ff value is a time function. Furthermore this method is helpful for evaluating the level of impurities after many discharges in Novillo Tokamak. 1. Introduction The effective ion charge, Zeff, represents the aver- age molecule mass Z i of gases inside the system, which indicates the level of the impurities in the plasma. The impurities can be chemical elements alone or a wide variety of compounds coming from the walls of the vacuum vessel and from the peripheral systems of the tokamak. As the impurities have Z i >> 1, in order to approximate Zeff to the Z of the working gas, the tokamak chambers have to be adequately conditioned with the aim of avoiding the sputtering of particles in the plasma-wall interaction process. The origin of re- leased impurities, and their effects in tokamak dis- charges, has been an important area of investigation since the early systematic analysis of Simonov in 1961 [1]. Several techniques have been applied to condition vacuum vessels to reduce the impurities in the plasma. Glow Discharge Cleaning (GDC) and radiofrequency (RF) in inert gases [2,3] were pioneering techniques * Corresponding author. which have produced good results. Oren and Taylor [4] implemented an effective method to reduce Zeff nearly to 1 by generating low-temperature < 10 eV hydrogen plasmas. This technique, which prevents water molecules produced by chemical reactions or sputter- ing from breaking up, has had wide acceptance and is still in use. Electron Cyclotron Resonance (ECR) dis- charges [5] are also applied as a conditioning technique with approximately the same Zeff results as those ob- tained by Oren and Taylor. Another technique uses the tokamak discharges themselves to condition the vacuum vessel [6]. Nowadays, the deposition of thin layers of carbon, boro or lithium [7,8] is the best technique for conditioning tokamaks. In the Novilto Tokamak [9], the vacuum vessel was trained with the Helium Glow Discharge Cleaning (HeGDC) technique. With this technique the Novillo vacuum vessel was conditioned by removing hydrogen and low-Z impurities like carbon monoxide, hydrocar- bons and water vapour. In this process, the impurities removed from the walls were studied by monitoring the partial pressure of the residual gases with a mass spectrometer. After HeGDC technique, the Zeff values were calculated through the methods of anomaly factor a [10] and mass spectrometry. 0022-3115/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0022-3115(95)00243-X