REVIEW OF SCIENTIFIC INSTRUMENTS
VOLUME 71, NUMBER 2
FEBRUARY 2000
Production of highly charged ions in electron cyclotron resonance ion sources using an electrode in two modes S. Biri,a) L. Kene´z, and A. Valek Institute of Nuclear Research (ATOMKI), Bem te´r 18/c, Hungary
T. Nakagawa, M. Kidera, and Y. Yano The Institute of Physical and Chemical Research (RIKEN) Wako, Saitama 351-01, Japan
共Presented on 7 September 1999兲 One of the most known ways to obtain higher beam intensities in electron cyclotron resonance 共ECR兲 ion sources is to install an electrode 共usually disk兲 into the plasma chamber. We found that a majority of the groups observed the beam intensity improvement by supplying a suitable biased voltage to the electrode and an electron current was injected into the plasma. A few groups observed the enhancement, however, when the electrode operated at floating potential—without being an electron donor. In spite of the great success of the ‘‘biased disk’’ method, the mechanism is still not completely clear. In this contribution, as a step toward of understanding, we examined the above mentioned two modes. The experiments were performed at the 18 GHz RIKEN and at the 14.5 GHz ATOMKI ECR ion sources. © 2000 American Institute of Physics. 关S0034-6748共00兲59902-X兴
I. INTRODUCTION
B. The 14.5 GHz ATOMKI-ECRIS
Examining the popular ‘‘biased disk’’ method in detail, we categorized the published experimental data into two groups.1 The majority of the groups reported that improved beam intensities were observed by supplying a negative voltage 共several tens to several hundreds volts兲 to the electrode with respect to the plasma chamber and an electron current in the order of mA was injected into the plasma. A few groups observed the enhancement, however, when the electrode operated at floating potential—without being an electron donor. In spite of the great success of this method, the mechanism is still unclear. In this article, as a step toward understanding, we examine the conditions on how to observe the above mentioned two modes. The secondary goal was to enhance the beam intensity of highly charged ions by using the electrode. Most of the experiments were performed at the 18 GHz RIKEN-electron cyclotron resonance ion source 共ECRIS兲 and some tests and simulations took place at the 14.5 GHz ATOMKI-ECRIS.
Since 1997 the 14.5 GHz 共ECR兲 ion source of ATOMKI3 has produced beams of multiply charged ions in a very reliable and stable way. Highly charged ions with medium intensities can be easily obtained. The ion source is mainly devoted to low energy atomic physics experiments including material research. The latter is one of the future directions at the ATOMKI-ECRIS because this topic has already been promisingly and intensively investigated 共at higher beam energies兲 by the ATOMKI ECRIS users in international cooperation.4–6 The ECR ion source itself is also the object of research in order to obtain information on the elementary processes of the plasma and to improve the parameters of the facility.3 C. Electrode preparation after simulations
To obtain some initial information on the dimension and position of electrodes to be tested we visualized in three dimension 共3D兲 the magnetostatic field line structure by the TrapCAD code.7 The details of this simulation are in Refs. 1 and 8. Based on these results and also fulfilling some technological requirements, we chose 8, 13, and 26 mm diameter disks made of stainless steel. We made two other 13 mm disks of copper and aluminum and one SS ‘‘star-shaped’’ disk for the axial position, where the magnetic field has the injection peak. The thickness of all the electrodes was 1 mm. The axial position of the electrode was remotely controlled and the bias voltage was applied between the electrode and plasma chamber.
II. EXPERIMENTAL APPARATUS A. The 18 GHz RIKEN-ECRIS
In the 18 GHz RIKEN ECRIS2 the plasma chamber wall is covered up with a thin aluminum tube 共of 1 mm thickness兲 to emit secondary electrons which helps to increase the plasma density. The maximum of the axial magnetic field 共1.3 T兲 at the injection side is slightly outside the hexapole volume. For a symmetrical point of view we placed the electrode holder rod and the tested electrodes themselves on the chamber axis. Other details on the ion source 共including the effect of the electrodes on the highly charged ion currents兲 are shown in Ref. 2.
III. OBSERVATION OF TWO WORKING MODES
First we studied the effect of some electrode parameters on the current of highly charged ions. The axial position, the electrode dimension 共diameter兲 and shape, and the material
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Rev. Sci. Instrum., Vol. 71, No. 2, February 2000
FIG. 1. Operation in electron donor 共ED兲 mode. 共a兲 Beam intensity of Ar11⫹ ions 共closed circles兲 and current of electrode 共open circles兲 as a function of negative bias voltage. 共b兲 Charge state distribution of Ar ions when applying voltage of 0 and ⫺160 V.
were investigated and optimized at both ion sources.1,8,9 At the RIKEN source the disk-shape 13-mm-diam stainless steel electrode proved to be the best. Then we studied the effect of the electrode using different bias voltages and electrode currents. At the ATOMKI-ECRIS we found the well-known biased behavior of the electrode. Depending on the plasma conditions we clearly found, however, two operation modes of the electrode at the RIKEN-ECRIS. A. The biased or electron donor „ED… mode
Figure 1共a兲 shows the beam intensity of Ar11⫹ ions and current of electrode as a function of bias voltage. The applied magnetic field was far below the maximum 共it was about 1.0–1.1 T兲 and the RIKEN-ECRIS was tuned to obtain the maximum beam intensity of the Ar11⫹ ions at 10 kV source potential. The electrode was moved slightly closer to the plasma than in the next experiment 共see below兲. The gas pressure was 8⫻10⫺7 mbar and the injected microwave power was 550 W. At this magnetic field the argon beam intensity increased with increasing the bias voltage up to ⫺160 V and then gradually decreased. The current of electrode also increased with increasing the bias voltage up to ⫺160 V and then nearly saturated. Figure 1共b兲 shows the charge state distribution of argon ions when the ion source was tuned for producing the Ar11⫹ ions. We note the charge state distribution 共CSD兲 seems not to have shifted! All of the ion currents increased by approx. the same factor. The total beam current increased from 2.7 to 4.0 mA when adding 1.2 mA of disk current. The same result was also obtained in pulsed mode in RIKEN and in continuous mode 共CW兲 in the ATOMKI 共at any magnetic field in this ECRIS兲.
FIG. 2. Operation in potential tuner 共PT兲 mode. 共a兲 Beam intensity of Ar11⫹ ions 共closed circles兲 and current of electrode 共open circles兲 as a function of negative bias voltage. 共b兲 Charge state distribution of Ar ions without using the electrode, with using it at ⫺160 V, and at floating potential.
The best result was obtained when the electrode operated at several hundred volts negatively biased potential and the plasma seemed to require electrons from the electrode. We call this arrangement the electron donor 共ED兲 mode. B. The floating or potential tuner „PT… mode
Searching for the highest Ar11⫹ current at the RIKEN source, the best result was obtained when the magnetic field was high 共about 1.3 T兲 and the electrode was placed around the maximum magnetic field in an axial direction. Figure 2共a兲 shows the beam intensity of Ar11⫹ ions and the current of the electrode as a function of bias voltage. We obtained the best results at electrode voltage of 0 V without any current through the electrode. Between 0 and ⫺100 V the ion current output was unchanged. Then the beam intensity of Ar11⫹ ions decreased with increasing the negative bias voltage. Figure 2共b兲 shows the charge state distributions of argon ions at different electrode voltages. The mean charge state seems to be shifted to higher charge state when using the electrode at floating potential! The total beam current did not change by tuning the disk voltage! The beam intensity of Ar11⫹ ions was 165 A, almost a 50% increase compared to that without using the electrode ever before at this ECRIS 共at 10 kV ion source potential兲. To check the beam intensity of Ar11⫹ ions without using the electrode again, it was removed from the plasma chamber without changing any other parameters. We got only 105 A current. This strange mode was also observed in pulsed mode at the RIKEN-ECRIS; it was not found, however, at the ATOMKI-ECRIS. Because the best result was obtained when the electrode operated at zero 共floating兲 potential without emitting any
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Rev. Sci. Instrum., Vol. 71, No. 2, February 2000
Ion sources
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TABLE I. Comparison of the two modes of electrode at the 18 GHz RIKEN-ECRIS.
Mode Magnetic field Electrode mode Electrode voltage 共V兲 Electrode current 共mA兲 Total beam current 共mA兲 Ar11⫹ current 共A兲
Potential tuner 共PT兲
Basic High B No — — 5.6 110
High B Floating 0 0 5.7 165
electrons into the plasma, the electrode seems to affect by changing or tuning by the plasma potential. We call this arrangement the potential tuner 共PT兲 mode. IV. DISCUSSION
Table I summarizes the measured currents and the difference between the two modes. Most published data seem to fit the ED mode, in other words most ECR plasmas require external electrons in order to increase the ion currents. The biased disk effect is frequently explained by an increase of the electron density. However, as can be seen from Fig. 1共b兲, the CSD did not change in our case. The selected Ar ion intensity simply increased together with the increase of the total current. One of the most likely explanations is that the electron injection in the ED mode improves the extraction conditions for all the ion components. It is much more difficult to explain the PT mode found only at the 18 GHz RIKEN-ECRIS. Here we give only one possible qualitative explanation. To obtain higher HCI beam intensities it is crucial to increase the density of ionizing electrons and to prolong the confinement time of the ions in the electron cloud. For an effective extraction, however, often we need to shorten the ion confinement time 共keeping its value, of course, higher than the ionization time兲. As a result, there exists an optimal confinement time for every ion species. We assume in PT mode the metallic electrode regulates the plasma potential dip that traps the positive ions. By means of moving it axially an optimal confinement time is generated. The plasma potential dip changes but it still remains strong enough to trap the highly charged ions and it becomes optimal for extraction at the same time. A similar conclusion was found in Ref. 9 based on x-ray bremsstrahlung measurements on Xe plasma in the RIKEN-ECRIS. V. CONCLUSION
We successfully observed the enhancement of highly charged ions using an electrode in the ECRIS plasma cham-
High B Biased ⫺160 1.5 5.7 105
Electron donor 共ED兲 Low B Floating 0 0 2.7 5
Low B Biased ⫺160 1.2 4 17
ber. We found that the effect of the electrode is strongly dependent on the plasma parameters and on the position of the electrode. At certain parameters we need a few tens or hundreds of volts and few mA of electron current to increase the beam intensity of highly charged ions. The extraction current increases with increasing the current of the electrode. The electrode works as an electron source 共electron donor or ED mode兲. At higher axial magnetic field the best results are obtained at floating electrode potential. The electrode works to tune the plasma potential dip 共plasma tuner or PT mode兲. We continue these investigations to understand the exact mechanism of electrode-plasma interaction in the ECR ion sources and traps. ACKNOWLEDGMENTS
This work was partly supported by the National Scientific Research Foundation 共OTKA, Hungary, Contract No. T26553兲 and by a Hungarian-Japanese Intergovernmental Scientific-Technological Cooperation 共Ref. No.: Te´T J-6/98兲 between the National Technical Development Committee 共OMFB兲 and the Science and Technology Agency 共STA兲. S. Biri et al., Nucl. Instrum. Methods Phys. Res. B 152, 386 共1999兲. T. Nakagawa et al., Proceedings of the 14th International Workshop on ECRIS, CERN, Geneva, Switzerland, 3–6 May 1999, pp. 1–4. 3 S. Biri and J. Va´mosi, Rev. Sci. Instrum. 69, 646 共1998兲; See also: http:// www.atomki.hu/atomki/ECR 4 F. Ditro´i, J. D. Meyer, R. W. Michelmann, and K. Bethge, Nucl. Instrum. Methods Phys. Res. B 99, 182 共1995兲. 5 J. D. Meyer, R. W. Michelmann, F. Ditro´i, and K. Bethge, Nucl. Instrum. Methods Phys. Res. B 99, 440 共1995兲. 6 F. Ditro´i, J. D. Meyer, R. W. Michelmann, and K. Bethge, Radiat. Eff. Defects Solids 145, 57 共1998兲. 7 J. Va´mosi and S. Biri, Comput. Phys. Commun. 98, 215 共1996兲; see also: http://www.atmoki.hu/atomki/ECR/trapcad.htm 8 S. Biri, T. Nakagawa, M. Kidera, L. Kenez, A. Valek, and Y. Yano, in Ref. 2, pp. 81–85. 9 M. Kidera et al., in Ref. 2, pp. 86–89. 1 2
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