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Ionization Balance in EBIT and Tokamak Plasmas N Ionization balance in EBIT and tokamak plasmas N. J. Peacock, R. Barnsley, M. G. O’Mullane, M. R. Tarbutt, D. Crosby et al. Citation: Rev. Sci. Instrum. 72, 1250 (2001); doi: 10.1063/1.1324755 View online: http://dx.doi.org/10.1063/1.1324755 View Table of Contents: http://rsi.aip.org/resource/1/RSINAK/v72/i1 Published by the American Institute of Physics. Related Articles Bragg x-ray survey spectrometer for ITER Rev. Sci. Instrum. 83, 10E126 (2012) Novel energy resolving x-ray pinhole camera on Alcator C-Mod Rev. Sci. Instrum. 83, 10E526 (2012) Measurement of electron temperature of imploded capsules at the National Ignition Facility Rev. Sci. Instrum. 83, 10E121 (2012) South pole bang-time diagnostic on the National Ignition Facility (invited) Rev. Sci. Instrum. 83, 10E119 (2012) Temperature diagnostics of ECR plasma by measurement of electron bremsstrahlung Rev. Sci. Instrum. 83, 073111 (2012) Additional information on Rev. Sci. Instrum. Journal Homepage: http://rsi.aip.org Journal Information: http://rsi.aip.org/about/about_the_journal Top downloads: http://rsi.aip.org/features/most_downloaded Information for Authors: http://rsi.aip.org/authors Downloaded 08 Aug 2012 to 194.81.223.66. Redistribution subject to AIP license or copyright; see http://rsi.aip.org/about/rights_and_permissions REVIEW OF SCIENTIFIC INSTRUMENTS VOLUME 72, NUMBER 1 JANUARY 2001 Ionization balance in EBIT and tokamak plasmas N. J. Peacock,a) R. Barnsley,b) and M. G. O’Mullane Euratom/UKAEA Fusion Association, Culham Science Centre, Abingdon, Oxon OX14 3DB, United Kingdom M. R. Tarbutt, D. Crosby, and J. D. Silver The Clarendon Laboratory, University of Oxford, Parks Road, Oxford OX1 3PU, United Kingdom J. A. Rainniec) Department of Pure and Applied Physics, The Queen’s University of Belfast, Belfast BT7 1NN, United Kingdom ͑Presented on 22 June 2000͒ The equilibrium state in tokamak core plasmas has been studied using the relative intensities of resonance x-ray lines, for example Ly␣ ͑H-like͒,‘‘w’’ ͑He-like͒, and ‘‘q’’ ͑Li-like͒ from test ions such as Arϩ15,Arϩ16, and Arϩ17. A full spatial analysis involves comparison of the line intensities with ion diffusion calculations, including relevant atomic rates. A zero-dimensional model using a global ion loss rate approximation has also been demonstrated by comparison with the data collected from a Johann configuration spectrometer with a charged coupled device ͑CCD͒ detector. Since the lines are nearly monoenergetic, their intensities are independent of the instrument sensitivity and are directly proportional to the ion abundances. This method has recently been applied to Ar in the Oxford electron beam ion trap ͑EBIT͒ with a beam energy in the range 3–10 keV. Taking into account the cross sections for monoenergetic electron collisions and polarization effects, model calculations agree with the observed line ratios at 4.1 keV beam energy. This work will be expanded to provide nomograms of ionization state versus line intensity ratios as a function of EBIT beam energy. © 2001 American Institute of Physics. ͓DOI: 10.1063/1.1324755͔ I. INTRODUCTION influx terms, respectively. Given that the local flux of ions of charge state z, atomic number Z, is described by a diffusive In this article, comparison of the x-ray emission spectra ͑D͒ and convective (V) equation from highly ionized argon in the Joint European Torus ͒ ͒ ϭϪ ٌ͒ ͒ϩ ⌫ ͒ ͑ JET , tokamak with the same emission lines in an electron Z D͑r nZ͑r V͑r nZ͑r , beam ion trap,1–3 namely the Oxford EBIT,4 indicates the relative ionization balance in these very different plasmas. It then profiles of the D and V coefficients are selected that is argued that as in tokamaks the relative ion abundances in give computed emission profiles in agreement with the mea- an EBIT are determined not only by atomic collision pro- sured radial distribution of the ions. ␶ It is the practice to measure a restricted range of line and cesses but also by a characteristic loss time Z . The physical interpretation of this loss parameter depends on the operating continuum radiances and fit these data to an ion transport 5 conditions in these two plasmas. code such as SANCO which can encompass a relevant 6,7 Particle loss rates from tokamaks are often established atomic data base such as ADAS. It is common to assign through a study of the time and space dependence of the the convection term the form relative ion abundances of injected or intrinsic test ions. The D͑r͒rS concentrations of the ions are given by a solution of the V͑r͒ϭϪ2 , coupled equations a2 where S is the dimensionless ‘‘pinch’’ parameter,8 and 0 nץ Z ϭϪٌ ⌫ ϩ ͑ Ϫ ϽSϽ3. Uncertainties in the transport and atomic physics Z ne nZϪ1SZϪ1 nZSZ . ץ t modeling as well as errors in the absolute radiances make n these detailed transport simulations a complex, lengthy, and ϩ ␣ Ϫ ␣ ͒Ϫ Z ϩ␥ nZϩ1 Zϩ1 nZ Z ␶ Z , often error-prone procedure. A simpler algorithm is to relate Z the ion balance, through intensities of resonance emission lines from successive ion charges, to a global ion confine- where S and ␣ are the appropriate ionization and recombina- ␶ ment time, Z , tion coefficients while ␶ and ␥ are the nondiffusive loss and ץ nZ nZ ϭϪ ϩ␥ ϩn ͑n Ϫ S Ϫ ␶ Z e Z 1 Z 1 ץ a͒ Electronic mail: [email protected] dt Z b͒Attached to the Department of Physics, University of Leicester, LE1 7RH, Ϫ ϩ ␣ Ϫ ␣ ͒ UK. nZSZ nZϩ1 Zϩ1 nZ Z , c͒Present address: Euratom/UKAEA Fusion Association, Culham Science Center, Abingdon, Oxon, OX14 3DB, UK. where 0034-6748/2001/72(1)/1250/6/$18.001250 © 2001 American Institute of Physics Downloaded 08 Aug 2012 to 194.81.223.66. Redistribution subject to AIP license or copyright; see http://rsi.aip.org/about/rights_and_permissions Rev. Sci. Instrum., Vol. 72, No. 1, January 2001 Plasma diagnostics 1251 TABLE I. Argon emission lines analyzed in this work. Ion Label ␭͑Å͒ ArXVIII Ly␣ 3.7311 1 ArXVIII Ly␣ 3.7365 2 ArXVII ‘‘w’’ 3.9490 ArXVII ‘‘x’’ 3.9657 ArXVII ‘‘y’’ 3.9693 ArXVII ‘‘z’’ 3.9943 ArXVI ‘‘q’’ 3.9810 a2 D ϭ . Z 2Ϫ ͒ϫ␶ ͑2.4 1.4S Z In a tokamak such as JET with the minor plasma radius ϳ1 Ͻ Ͻ 2 Ϫ1 m and with 0.05 DZ 0.5 m s , depending on the con- ␶ ϳ finement mode, Z in the core plasma can vary between 0.2 ␶ ϳ ␣ s and a few seconds. Thus eq 1/ Z , the time to reach ␶ Ͻ␶ ionization-recombination balance, is eq Z , and the rela- tive ion abundance is only temperature Te dependent. For FIG. 1. Energy level diagram of nϭ1!2 transitions in ArXVI and smaller tokamaks like COMPASS the ionization balance in ArXVII. 9 ␶ Cl has been demonstrated to be Z dependent in lossy plas- ␶ Ͼ␶ mas with eq Z . The object of this article is to apply the same principles ⑀ A to an EBIT plasma where again ionization balance is of ϭ ij ϭ ij ␯ 10,11 Iij ␲ neqZRi ͚ h ij , interest and considerable effort has been expended on 4 Aij writing ionization balance codes.12,13 In the present article where Ri , the relative excited state population, can be cal- we deal with a version of the ionization balance code termed 14 culated using the ADAS population structure code. OXGAS. Analogously in the case of the EBIT we can de- Sample spectra of ArXVIII and ArXVII from the JET ␶ fine a loss Z where tokamak are indicated in Figs. 2 and 3, respectively. The 1 1 1 relative abundances of argon ions ͓Fig. 4 ͑top͔͒ and the as- ϭ ϩ ␶ ␶ ␶ , sociated line intensities ͓Fig. 4 ͑bottom͔͒ in JET discharge Z esc eq #33311 are calculated using the SANCO ion transport code ␶ and esc is the escape time from the trap which depends on with the ADAS atomic physics package. The transport coef- 10 such factors as evaporative cooling and gas flow, trap po- ficients D(r) and V(r) shown in Fig. 4 have been derived ␶ tential, and beam energy. However, eq depends only on mo- previously by fitting to the transient spectral signature of noenergetic electron collision rates such as ionization, radia- injected test ions. The ratio of ‘‘q/w’’ assumes considerable tive recombination, and charge exchange with neutral thermal argon atoms. The highly charged ions are assumed to visit all regions in the EBIT trap. In the case of resonances between multiply excited levels and the beam energy, dielec- tronic recombination also has to be admitted. The argon emission features studied here are the nϭ2!1 resonance lines of Li-like, He-like, and H-like ions as indicated in Table I. The term diagram for the He-like ions and the Li- like ion satellites and their labels15 are as indicated in Fig. 1. II. ION ABUNDANCES FROM Ar SPECTRAL LINE EMISSION „MAXWELLIAN EXCITATION… Except in the presence of non-Maxwellian electron beams16 in tokamaks, magnetic confinement emission is iso- tropic and unpolarized. In the x-ray region, line splitting due to induced Lorentz electric fields and Zeeman splitting re- sulting from the main toroidal field is small relative to mass motion spectral shifts. The line radiance from a tokamak in ⑀ terms of the volume emissivity ij and then the ion abun- FIG. 2. JET spectrum of ArXVIII Ly␣ using a Ge͑111͒ double crystal 1,2 ␭ ⌬␭Ϸ dance qZ is given by spectrometer with collimation resulting in / 3500. Downloaded 08 Aug 2012 to 194.81.223.66. Redistribution subject to AIP license or copyright; see http://rsi.aip.org/about/rights_and_permissions 1252 Rev. Sci.
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