Investigation on Operating Parameters of the Homemade Penning-Type Ion Source for
Cyclotron
Silakhuddin, S. Santosa
Center for Accelerator Science and Technology ,National Nuclear Energy Agency Jl. Babarsari, Yogyakarta 55281, Indonesia
A R T I C L E I N F O A B S T R A C T AIJ use only:
Received date Revised date Accepted date Keywords:
Ion source Cyclotron Parameters Puller voltage Magnetic field Cathode current
A Penning-type ion source for a cyclotron producing PET radioisotopes has been made. To determine the performance of the ion source for further developments, the investigation has been conducted on the operating parameters. The investigation was carried out by experiments on an ion source test device. The investigated operating parameters are the puller voltage, the magnetic field, the hydrogen gas flow rate and the cathode current. The results showed that the puller voltage is the most sensitive parameter to change ion beam current, and at a voltage of 8 kV an ion beam current of 35 µA was obtained, and this parameter is still likely to be raised if the current beam will enlarge.Change in the magnetic field showed saturation the ion beam current after the magnitude of magnetic field is approximately 450 gauss. It was obtained a moderate range of gas flow rate on the value of 5-10 cc/min, producing a high beam current without effect to the significant decrease of vacuum. The increase of the cathode current gives significant effect until 1 mA and then does not affect to the increase of ion beam current.
© 2014 Atom Indonesia. All rights reserved
INTRODUCTION
Development activities to make a cyclotron are currently being done in the Center for Accelerator Science and Technology BATAN, and one of the activities is the development of a Penning-type ion source. Currently, an ion source development has already been up on the end stage of the experimental phase and will be continued on the construction phase to create an ion source is to be installed at the
Corresponding author.
E-mail address:[email protected]
cyclotron. With the recent testing of the ion source resulting an H- ion beam current of 35 µA, it was tested on a homemade ion source testing device with a puller voltage of 8 kV DC.
Testing on a similar ion source was done at the Institute of Fluid Pysics, China Academy of Engineering Physics China, with result of ion beam current was 200 µA at a puller voltage of 2 kV [1].
Another testing in Sungkyunkwan University of Korea obtained a H- ion beam current of 1 mA at a
puller voltage of 1 kV [2]. Compared with other experimental results, the homemade ion source has not been on a standard performance. It is important to investigate the characteristics of operating parameters in order to get methods to increase the ion source performance.
Operating parameters in a Penning type ion source consist of cathode voltage, puller voltage, gas flow rate and intensity of the magnetic field. Investigations of the ion source parameters were carried out by experiments to study the influence of the operating parameters on the end product of the ion beam current. The experiments were executed on an ion source testing device.
Through the investigations, it would be to get data
for determining further research
and development of ion source design for DECY-13 cyclotron, which the goal will be obtaining negative hydrogen ion beam current by several hundreds of µA.
EXPERIMENTAL METHOD
The Penning Source and Its Testing Device The Penning ion source for a cyclotron and scheme of testing device is shown in Figure 1.
Figure 1. Scheme of the ion source and the testing device
Plasma is produced in the anode cavity caused by a hydrogen gas ionization discharge between cathode and anode those are connected to 0-3 kV DC source.
Intensity of ionization is amplified by the magnetic field existence of 0-1000 gauss. Negative hydrogen ions are pulled by puller that is applied 0-10 DC voltage compared to the anode. The ion beams are measured by a collector and a current meter.
Existing geometry of the ion source, especially surface of the anode and the puller follow the shape of common H- ion source [1,3]. The distance between the anode aperture (slit) and the puller aperture is 0.5 cm and the distance between the puller aperture and collector is 3 cm. The material of the anode is W-Cu alloy, the cathode is tantalum and the puller is graphite. The magnet was generated by electric current winding of 0-15 A.
Experimental Procedures
The main operating parameters of ion source are:
- Cathode electric current, referred to as electron current that ionizing gas on the anode cavity.
- Magnetic field strength of electro-magnet that affect the intensity of ionization on the anode cavity.
- Hydrogen gas flow that affects on the plasma pressure.
- Puller voltage that affects on extracted ion beam current out of the anode cavity.
Experiments were done by making the highest vacuum conditions be reached. The next steps are observation an ionization on the anode cavity by following steps:
1. Hydrogen gas was piped into the head until ap- proximately some cc/minute.
2. Cathode voltage was raised to over 1800 V; this is the voltage that allows electrons from the tan- talum cathode surface escape toward anode cav- ity.
3. Electrical current generating magnetic field was raised to observe a glow on the anode slit, as a sign that glow discharge ionization has occurred.
After ionization is observed, the next step is observing the influence of ion beam current by ion source operating parameters by following steps:
1. Observing of influence of puller voltage change against ion beam current.
2. Observing of influence of magnetic field against ion beam current, and the calibration to
determine the relationship between electrical current and magnetic field was done before.
3. Observing of influence of gas flow rate change against ion beam current.
4. Observing of influence of cathode current change against ion beam current.
RESULTS AND DISCUSSION
After the flow of hydrogen gas was raised to 2 cc/min, the vacuum came down from 5 × 10-6 Torr to 5 × 10-5 Torr. After cathode voltage was raised to slightly over 1800 V and electro-magnet current was raised to 10 A (based on the calibration this value is equivalent to 465 gauss magnetic field), a visible glow can be observed on the anode slit. It is an indication that a plasma has been occurred. The occurrence of ionization also was ensured by the cathode current of 5 mA. After the occurrence of ionization, the observed results of the influence of
the parameters on the ion beam current are as follows.
Measurement data results of the influence the puller voltage on the beam current is shown in Figure 2.
Figure 2. The influence of the puller voltage on the ion beam current; :measurement data, :
calculation data using the least square fit method by I ~ const. V3/2
The data were acquired for puller voltage up to 8 kV, above this value the excessive spark was occurring in the space between the anode and puller, so the ion beam current was difficult to be measured. The excessive spark due to the vacuum being less high and that is because a less powerful of vacuum pump. At the 8 kV puller voltage an ion beam current of 35 μA is obtained.
In the theoretical approach, the influence of puller voltage is expected to comply with the Child- Langmuir equation, i.e. the ion beam current is proportional to power 3/2 of the extraction (puller) voltage, which is I const ×V
3
2 [4]. In Figure 3 are shown curves of measured and theoretically calculated ion beam current, and two curves are seemed to coincide with each other. From these two curves there are two things that can be concluded
that firstly optics of an anode-puller functions properly for extracting ion beam and secondly the space charge effect of ion beam does not affect on ion beam current.
The observation was done on the gas flow rate of the same as the earlier observation, but the puller voltage was reduced to 7 kV (so that the excessive spark effect can be reduced).When the electric current of electro-magnet was varied, the effect of variation of magnetic field on the ion beam current is shown in the Figure 3.
Figure 3. Effect of magnetic field on the ion beam current.
From above results seem to be a constancy of the ion beam current even if the magnetic field is in- creased. So up to 700 gauss magnetic field, it can be concluded that the ion beam current cannot be in- creased by using increasing the magnetic field. But this phenomena is actually still to be examined fur- ther because some references mentioned that testing of this type of ion source generally is done at several kilogauss magnetic field [1,2].
As the puller voltage of 7 kV and electro-magnet electric current of 10 A, the influence of the gas flow rate on the ion beam current is shown in Figure 4.
Figure 4. Influence of gas flow rate on ion beam current at puller voltage of 7 kV and electro-magnet
electric current of 10 A
There is a minimum flow rate of value of 2 cc/min and no optimum value until 10 cc/min. The ion beam increase is not too sharp compared with the influence of the puller voltage. A result of simulation study using Fluent 6 showed that the gas flow rate will affect linearly to gas pressure in the anode cavity [5], and the gas pressure affects linearly to the intensity of the ionization [6]. The linearity in that experiment didn’t happen, it can be caused by no linear effect of gas flow rate on the gas pressure because not all of the molecules will be ionized after entry into the anode cavity but part of them out of the cavity as neutral molecules. It was obtained a moderate range of flow rate on the value of 5-10 cc/min, which producing a high beam current but does not affect on the decrease of vacuum. This, according to Penning-type ion source operation on cyclotron in general [7,8]. In the experiment, the cathode current rising from originally 6 mA to 10 mA at gas flow rate of 2 cc/min and 10 cc/min respectively.
The influence of cathode current on the ion beam current at the gas flow rate of 2 cc/min, the puller voltage of 6 kV and electric current of electro- magnet of 10.5 A (concurrent with 465 gauss magnetic field) is shown in Figure 5.
Figure 5. The influence of the cathode current on the ion beam current
The influence of cathode current on the ion beam current at the gas flow rate of 2 cc/min, the puller voltage of 6 kV and electric current of electro- magnet of 10.5 A (concurrent with 465 gauss magnetic field) is shown in Figure 6. To increase the ion beam current into the larger amount it requires a change in the ionization from the glow discharge state to the arc discharge state, which can be done by making the process of the release of electrons from the cathode surface in thermionic mode.
Technically, this can be done by enlarging the cathode current up to several hundreds of mA.
CONCLUSION
The potential of the puller or the puller voltage is the most substantial role in giving effect to changes in ion beam current, and this parameter is still likely
to be raised if the current beam will enlarge. But the excessive spark is a constraint to raise the puller voltage, and this can be overcome when the vacuum is to be higher than before. Change in the magnetic field showed saturation the ion beam current after the magnitude of magnetic field is approximately 450 gauss, this uncommon phenomenon is necessary for the next research object. It was obtained a moderate range of gas flow rate on the value of 5-10 cc/min, which producing a high beam current without effect on the significant decrease of vacuum. The increase of the cathode current gives significant effect until 1 mA and then does not affect on the increase of ion beam current. So it should be tried in further experiments on several tens or hundreds mA cathode current. As a final conclusion is that in order to increase the ion beam current, it is necessary to do two things, first is an increase in the vacuum of experiment chamber and the second is an increase in cathode current source operation up to several hundred miliamper.
ACKNOWLEDGMENT
This experiment was funded by an annual R&D program of Center for Accelerator Science and Technol- ogy, the National Nuclear Energy Agency of Indonesia, so the authors would like to thank the head of the center.
The authors also would like to thank Mr. Sunarto for his support throughout the experiments and measurements.
REFERENCES
1. J. Long J, Z. Yang, P. Dong , et al, Study on a cold-cathode PIG-type ion source, Nuclear Science and Techniques 24 (2013) 040201.
2. Y.H. Yeon, H.S. Kim, K.M. Gad, et al,
“Development Study of Penning Ion Source for Compact 9 MeV Cyclotron”, Proceedings of Cyclotrons(2013), Vancouver, BC, Canada
3. D.P. Moehs, J. Peters and J. Sherman, Negative Hydrogen Ion Sources for Accelerators, IEEE Transactions on Plasma Science, Vol. 33, No. 6, December 2005.
4. M.E.M. Eisa, J.L. Conradie and P.J.Cellier, World Journal of Nuclear Science and Technology, 3 (2013) 83-86.
5. Silakhuddin, “Influence of a Geometry of Circulated Chamber on Distribution of Gas Pressure; a Case in the Cyclotron Ion Source”
Journal of “Saintek”, 17 (1) (2012), LPPM- Yogyakarta State University.
6. D.M. Goebe, R.E. Wirz and I. Katz, Journal Propulsion and Power, 23 (5) (2007) 5
7. B.C. Lee, H.J. Lee and J.H. Park, Journal of the Korean Physical Society, 57 (6), (December 2010) 1376-1380
8. Anonym, Technical Specifications of Sumitomo HM-12S of PET Radio-Tracer Production System for RS KankerDharmais, (21 February 2008), Sumitomo Heavy Industries Ltd.
