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Vacuum 61 (2001) 391}396 Diagnostic research of highly ionized plasma generated by an ECR ion source Cs. SzaboH *, S. Biri, L. KeneH z , T. Suta , A. Valek University of Debrecen, DEBRECEN, Egyetem te& r 1, H-4031, Hungary Institute of Nuclear Research (ATOMKI), DEBRECEN, Bem te& r 18/C, H-4026, Hungary Babes-Bolyai University Cluj-Napoca, 3400, Str. Kogalniceanu Nr. 1, Romania Abstract One of the research directions of the ATOMKI-ECRIS is the systematic investigation of the con"ned plasma. In our laboratory we have the instrumentation background for two plasma research methods: X-ray and visible light diagnostics is possible outside the plasma chamber and the plasma can be investigated directly by movable Langmuir probes. Recently, a new research direction was started in order to produce plasmas and beams from high vapour pressure solid state samples. A special crucible was designed for the evaporation of the solid Zn, which can be moved along the axis of the plasma chamber. The position of the crucible in the plasma de"nes its temperature and thus the amount of the material injected into the plasma. In this paper we investigate the connection between the amount of the injected material into the plasma and the ion currents of the extracted ion beam. Further investigation of the plasma was carried out by taking numerous X-ray spectra and Langmuir probe characteristics. 2001 Elsevier Science Ltd. All rights reserved. Keywords: ECRIS; ECR plasma; Metallic plasma; Fullerene; Ion source; HCI 1. Introduction Research of metallic ECR plasmas is important for di!erent reasons. Our investigations of Zn plasmas prepare the way for further metallic plasma investigations and later fullerene experiments, where gigantic molecules of 60 or more carbon atoms can be ionised and observed in the ECR plasma. Our aim was to "nd experimental methods for monitoring the appearance of massive particles in the plasma. A second goal was to compare the * Corresponding author. Tel.: #36-52-417266/1088; fax: #36-52-416181. E-mail address: [email protected] (C. SzaboH ). composition of the extracted beam with the composition of the plasma. Zinc can be regarded as a massive particle in a pure nitrogen plasma. The injection of Zn into the plasma can be performed from a crucible heated by the plasma itself. This is the "rst time we have used this method in the ATOMKI-ECRIS. Earlier metallic plasmas were produced using a ferrocene container [1]. Among the new directions of applications of heavy ion sources, the investigation of fullerene (e.g. C , C 2 and other) ions is one of the most promising "elds. ECR ion sources can play an important role in the research of the properties of these big molecules and by possible production of endohedral fullerenes (X C where the fullerene ] 0042-207X/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 1 ) 0 0 1 5 3 - 1 392 Cs. Szabo& et al. / Vacuum 61 (2001) 391}396 molecule encapsulates an atom e.g. nitrogen: N C ). ] Our current investigation of the Zn plasma can support the development of methods for production and investigation of plasmas from solid samples and powders (e.g. other metals and fullerenes). Very heavy low charged ions (e.g. C>) are very di$cult to analyse because of their big mass to charge ratio. Zinc is an optimal test material for later fullerene experiments because it has similar thermal properties to C (solid at room temper ature, sublimates, similar vapour pressures) but its ions (from Zn>) can be analysed as they have not too high a mass-to-charge ratio. In Fig. 1, the layout of the ECR ion source is shown. The extracted ion beam can be analysed by charge-to-mass ratio by a 90-deg analysing magnet. The ATOMKI-ECRIS is planned to serve simultaneously as: (1) a tool for atomic physics experiments at low energy (up to 30q keV) [3]: ion}atom collisions, ion}surface interactions; (2) a tool for plasma physics research: global plasma diagnostics (visible, UV, X-ray spectroscopy) [4], local plasma diagnostics (Langmuir probe); (3) investigation of plasma}surface interactions; and, (4) for practical applications: production and study of endohedral fullerenes and other large molecules, plasma chemical vapour deposition. The most important parameters of the ATOMKI-ECRIS are summarised in Table 1. 2. Experimental set-up 2.1. Method for production of Zn plasma The electron cyclotron resonance ion source (ECRIS) of the Institute of Nuclear Research (ATOMKI) of the Hungarian Academy of Sciences has been operational since 1997 [2]. The ion source can produce highly ionised plasma and ion beams at low energies from almost all elements of the periodic table. In our measurements we used a Zn-"lled crucible heated by the plasma itself. The small * (ca. 160 mm) capsule could be moved on-line along the axis of the ion source forward into the hotter plasma regions and back to the cold wall. To avoid sudden and excessive heating of the crucible a Cu wire was used as a heat bridge. At position zero the Fig. 1. Layout of the 14.5 GHz ATOMKI-ECRI: 1 * microwave coupling, 2 * waveguide windows, 3 * iron (5 cm), 4 * coils, 5 * plasma chamber, 6 * extraction optics, 7 * pumping, 8 * cooling, 9 * gas inlet, 10 * crucible, 11 * Langmuir probe, 12 * NdFeB hexapole, 13 * insulation, 14 * Einzel lens. Cs. Szabo& et al. / Vacuum 61 (2001) 391}396 Table 1 The main technical features of the ATOMKI-ECRIS Electromagnetic wave frequency Resonant magnetic "eld, B CAP Axial magnetic induction peak (coils) Max. radial magnetic induction in chamber (hexapole) Hexapole material and structure Plasma chamber diameter Plasma chamber length (variable) Extraction (acceleration) voltage Vacuum system (turbo pumps) Vacuum in the ion source without gas Analysing magnet Coils DC power consumption Ion source total AC power consumption 14.5 GHz 0.52 T 0.95}1.2 T 0.95 T NdFeB, 24 pieces 5.8 cm 15}25 cm 2}30 kV 250 and 500 l/s 2;10\ mbar 90 deg, r"24 cm 80 kW 120 kW 393 analysed ion beam. It can be clearly noticed that the N> currents change roughly opposite to the Zn> currents. When the crucible was positioned near the wall of the chamber (in colder plasma regions). This opposite change is more clear compared to hotter regions where cavity e!ects of the crucible in#uence the plasma as well. (The position of the metallic crucible changes the waveguide mode of the plasma chamber cavity.) Similar changes of the beam current were observed in case of other nitrogen peaks (N>, N>) in the analysed spectrum. The monitoring of the extracted beam is suitable for the detection of the appearance of a di!erent material in a stable plasma made of a good analysable support gas. 3.2. Zn in the X-ray spectra crucible was pushed back to the cooled wall of the plasma chamber. At this position the temperature of the crucible was not high enough for the sublimation of the Zn. When the crucible was pushed forward deeper into the hotter regions, the sublimation began when the Zn reached the proper temperature (&4003C). The support gas in the experiments was nitrogen. To analyse the process of Zn injection into the plasma we needed systematic measurements. The crucible was moved millimetre by millimetre from position zero into the plasma until it was 20 mm distant from the wall. Other parameters (microwave power, magnetic "eld, extraction, etc.) of the ion source were not changed, which means that all changes in the plasma were caused by the crucible and the sublimation of the Zn from it. The ion source was optimised for N> at a microwave power of 100 W. At each position (after waiting a few minutes to reach thermal equilibrium between the plasma and the crucible) we recorded the extracted beam spectrum and an X-ray spectrum at the same time. 3. Results 3.1. Ewect of the Zn on the support gas Fig. 2a and b shows the results of the measurements of the N> and the Zn> currents in the Fig. 3 shows a typical X-ray spectrum recorded during our Zn experiments. Three components of the spectrum can be observed: the radiation of the energetic electrons (background: exponential at high energy), the characteristic lines of the working gas or element (Zn in this case) and the characteristic lines of the excited wall materials (Cr, Fe, Ni). X-ray spectra were recorded with a solid state X-ray detector of PIN diode type. X-ray diagnostics of ECR plasmas can provide an experimental method for investigation of global plasma parameters [5]. The exponential background can give us information about the electron temperature and the electron density in the plasma. The characteristic peaks give information about the amount and the charge state distribution of the ions of the observed element. By the Gaussian "t of the Zn K peaks in the X-ray spectra the change of the intensity of this peak can be drawn depending on the position of the crucible (Fig. 2b and c). We can observe that the curve of the Zn> ion beam current and the intensity of the Zn K peak in the X-ray spectra correlate with each other. This result shows that, at least for low charge state plasmas, for the &Zn#N' case the extracted beam re#ects the components of the plasma. We concentrated on the most intensive Zn charge state (Zn>) in the beam spectrum and on the Zn K peak in the X-ray spectrum. The K peak 394 Cs. Szabo& et al. / Vacuum 61 (2001) 391}396 Fig. 2. Beam intensities of N>(a) and Zn> (b) ion beams and the intensity of the Zn K peak (c) in the X-ray spectrum depending on the position of the crucible. The error bars on section c) are drawn by the Gaussian "tting. in our X-ray spectra is the sum of the K peaks of all di!erent charge states present in the plasma. We plan to extend our measurements by using higher resolution X-ray spectrometers, to separate the components of the K peak. 3.3. Ewect of the Zn on the Langmuir characteristics The Langmuir probe is a special tool that gives information about the local properties of the studied plasma [6]. Fitting the measured data and applying the proper theoretical model the parallel component of the electron temperature, the electron density and the plasma potential can be obtained [7,8]. The probe itself is a small-size cylindrical electrode, which can be biased on positive and negative voltage and positioned inside the plasma chamber. At a certain position the probe current is measured as a function of applied voltages (I}U characteristics). In Fig. 4, the two curves belong to two di!erent plasmas: the "rst one is a pure nitrogen plasma, while the second one is the mixture of N and Zn. Cs. Szabo& et al. / Vacuum 61 (2001) 391}396 395 Fig. 3. Typical X-ray spectrum emitted by the Zn#N plasma. Fig. 4. Langmuir characteristics with and without Zn in the plasma. During the measurements the Langmuir probe was positioned near the edge of the plasma in a less hot and less dense region of the plasma chamber. In the "gure we can see that the electron and ion currents do not saturate, which indicates that the plasma was not Maxwellian at this point. Thus, the determination of the above-mentioned plasma parameters is not possible with high precision. However, we can see that their evolution in the course of external e!ects such as the appearance of 396 Cs. Szabo& et al. / Vacuum 61 (2001) 391}396 heavy particles in the plasma changes the characteristics considerably. The curves in Fig. 4 are considerably di!erent from each other and the second one is less steep. This di!erence between the two curves clearly shows that Langmuir probe technique is capable of monitoring the appearance of heavy elements in the plasma. research presented in this paper was supported also by the OTKA (T26553) and partly by a Hungarian}German intergovernmental scienti"c-technological co-operation between the National Technical Development Committee (OMFB), and the Bundesministerium fuK r Forschung und Technologie (BMFT) (TeH T D-5/97). One of the authors (B.S.) was supported by the Bolyai JaH nos Research Fellowship. 4. Conclusion With our measurements we wanted to "nd methods for the detection of the appearance of heavy, hardly analysable particles in the ECR plasma. As a result we can say that the change of the intensity of original plasma elements in the beam spectrum and the di!erence between Langmuir characteristics can be methods for monitoring heavy elements in the plasma. These methods can be used in experiments with large molecules e.g. with fullerenes. X-ray spectroscopy has shown us that, at least for low charged ion plasmas, the composition of the extracted beam re#ects the composition of the plasma itself. Acknowledgements The ECR Ion Source facility was built with the support of the National Scienti"c Research Fund (OTKA) under the contract no.: A077/1992. The References [1] Stiebing KE, Biri S, Arje J, DitroH i F, Koivisto H, PaH linkaH s J, Schmidt L, Valek A. Phys Scripta 1999;T80:509}10. [2] Biri S, VaH mosi J. Rev Sci Instrum 1998;69:646-648. See also: http://www.atomki.hu/atomki/ECR/. [3] Suta T, TakaH cs E, Biri S, SzaboH Cs, KeneH z L, Valek A. Poster on 8th Joint Vacuum Conference. Pula, Croatia, 4}9 June 2000. [4] VaH mosi J, Biri S, TakaH cs E, Koncz Cs, Suta T, Veibel E, Szegedi S, Raics P. Proceedings of the 13th International Workshop on ECRIS. College Station, TX, USA, 26}28 February 1997. [5] Shirkov GD, Zschornack G. Electron impact ion sources for charged heavy ions. Braunschweig/Wiesbaden: Friedr.Vieweg & Sohn Verlagsgesellschaft mbH, 1996. p. 185}234. [6] Shirkov GD, Zschornack G. Electron impact ion sources for charged heavy ions. Braunschweig/Wiesbaden: Friedr.Vieweg & Sohn Verlagsgesellschaft mbH, 1996. p. 163}7. [7] Stangeby PC, Mccracken CM. Nucl. Fusion 1990;30:1225}31. [8] Chen FF. Electric probes. In: Plasma diagnostic techniques. New York, London: Academic Press, 1965. p. 113}200.