Download Diagnostic research of highly ionized plasma generated by an ECR

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
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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
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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
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