A high intensity 200 mA proton source for the FRANZ-Project (Frankfurt-NeutronSource at the Stern-Gerlach-Center)a) W. Schweizer, U. Ratzinger, B. Klump, and K. Volk Citation: Review of Scientific Instruments 85, 02A743 (2014); doi: 10.1063/1.4842335 View online: http://dx.doi.org/10.1063/1.4842335 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/85/2?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Improved design of proton source and low energy beam transport line for European Spallation Sourcea) Rev. Sci. Instrum. 85, 02A723 (2014); 10.1063/1.4832135 Design and construction of a compact microwave proton source for a proton linaca) Rev. Sci. Instrum. 81, 02A314 (2010); 10.1063/1.3271170 Radio frequency-driven proton source with a back-streaming electron dumpa) Rev. Sci. Instrum. 81, 02B312 (2010); 10.1063/1.3267832 Development of a high efficiency proton source for the Frankfurter-Neutronen-Quelle am Stern-GerlachZentruma) Rev. Sci. Instrum. 79, 02B316 (2008); 10.1063/1.2838245 Study on proton fraction of beams extracted from electron cyclotron resonance ion sourcea) Rev. Sci. Instrum. 79, 02B713 (2008); 10.1063/1.2805644
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REVIEW OF SCIENTIFIC INSTRUMENTS 85, 02A743 (2014)
A high intensity 200 mA proton source for the FRANZ-Project (Frankfurt-Neutron-Source at the Stern-Gerlach-Center)a) W. Schweizer,b) U. Ratzinger, B. Klump, and K. Volk Institute of Applied Physics, Goethe-Universität Frankfurt, 60438 Frankfurt am Main, Germany
(Presented 12 September 2013; received 5 September 2013; accepted 24 October 2013; published online 28 January 2014) At the University of Frankfurt a high current proton source has been developed and tested for the FRANZ-Project [U. Ratzinger, L. P. Chau, O. Meusel, A. Schempp, K. Volk, M. Heil, F. Käppeler, and R. Stieglitz, “Intense pulsed neutron source FRANZ in the 1–500 keV range,” ICANS-XVIII Proceedings, Dongguan, April 2007, p. 210]. The ion source is a filament driven arc discharge ion source. The new design consists of a plasma generator, equipped with a filter magnet to produce nearly pure proton beams (92 %), and a compact triode extraction system. The beam current density has been enhanced up to 521 mA/cm2 . Using an emission opening radius of 4 mm, a proton beam current of 240 mA at 50 keV beam energy in continuous wave mode (cw) has been extracted. This paper will present the current status of the proton source including experimental results of detailed investigations of the beam composition in dependence of different plasma parameters. Both, cw and pulsed mode were studied. Furthermore, the performance of the ion source was studied with deuterium as working gas. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4842335] I. INTRODUCTION
FRANZ is a research project of the Goethe University Frankfurt.1 It allows to intensify well-established accelerator research activities by providing very intense 200 mA proton beams with energies of 120 keV in cw and pulsed mode in the low energy beam transport section. In pulsed and cw mode the beam pulses will be provided by a chopper system which delivers a pulse duration of 250 kHz. Time dependent space charge compensation effects and beam wall interaction phenomena can also be investigated and optimized.2 At the envisaged beam currents a 2 MeV RFQ had been shown to be rather long and to consume a lot of rf power. That is why a coupled rf cavity was chosen finally, consisting of a 700 keV 4-Rod-RFQ and a 2 MeV IH-DTL. The total rf power losses will stay below 200 kW.2 II. EXTRACTION SYSTEM DESIGN AND BEAM FORMATION SIMULATIONS
The main object was to produce a cw 200 mA proton beam at 120 keV beam energy. During the developing phase of the ion source, it was operated at a separated test facility with a maximum available voltage of 65 kV. Therefore, in a first step a 65 kV triode extraction system was designed with the computer code IGUN.3 Usually, a source operation with hydrogen leads to ion beams consisting of a mixture of three species: H1 + , H2 + , and H3 + . Hence, during the layout of the extraction system, the additional space charge of the undesirable beam species H2 + and H3 + has to be taken into consideration according to the Child-Langmuir-Law.
Formula (1) presents the proton equivalent total current IPETC , which is the sum of the partial currents of the three ion species normalized to the mass 1: AH+1 AH+2 AH+1 + IH+2 + + IH+1 . (1) IPETC = IH+1 1 1 1 Figure 1 shows IPETC as a function of the proton fraction for a fixed IH+2 /IH+3 ratio of 1 and for a fixed IH+1 = 200 mA. With decreasing proton fraction the IPETC is increasing. This fact has to be considered for the layout of the extractor. It requires an enhancement of the emission opening area, which leads to a higher gas flow through the extraction system. With increasing gas pressure in the extractor, the maximum applicable electrical field strength in the acceleration gap decreases, which reduces the extractable beam current. Therefore, it was an essential development goal to achieve an acceptable proton fraction in the ion beam. For the IGUN simulations a proton fraction of 90 % was assumed, leading to an IPETC of 235 mA. In order to keep the emission opening radius at an acceptable value, the used
a) Contributed paper, published as part of the Proceedings of the 15th
International Conference on Ion Sources, Chiba, Japan, September 2013. b) Electronic mail: [email protected].
0034-6748/2014/85(2)/02A743/3/$30.00
FIG. 1. Proton equivalent total current as a function of the proton fraction. 85, 02A743-1
© 2014 AIP Publishing LLC
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02A743-2
Schweizer et al.
Rev. Sci. Instrum. 85, 02A743 (2014)
FIG. 2. IGUN simulation of a 50 keV/200 mA H1 + beam. Ti = 0 eV.
electrical field strength has to be relatively high (9.4 kV/mm). Taking these values into account, ion beam formation simulations were performed with IGUN (Fig. 2). With an emission opening radius of 4 mm, an extraction voltage of 50 kV, and an aspect ratio of 0.62 the beam course in the extraction system is matched. At the end of the extractor the beam radius is about 1.5 mm at a divergence angle below 20 mrad. III. EXPERIMENTAL SETUP
In Fig. 3 a schematic cross-sectional view of the high current proton ion source is shown. The source consists of a plasma generator (input power up to 12 kW) and a 65 kV single hole triode extraction system. The new development of this proton ion source is based on former investigations of high current ion sources at the Institute of Applied Physics.4 The plasma generator is equipped with a cathode system (which includes a 2.4 mm thick tungsten filament), a solenoid for plasma confinement and a transversal magnetic filter. The front end of the plasma generator consists of the plasma electrode, which is insulated from the anode and can
FIG. 3. Schematic drawing and electric circuit of the proton ion source.
FIG. 4. Ion fraction vs. total arc power.
be biased negatively with respect to it. Hence, the total arc power is the sum of the arc voltage times the arc current and the bias voltage of the plasma electrode times the plasma electrode current. The source can be operated in cw as well as in pulsed mode. In pulsed mode the arc discharge is pulsed and the arc power is provided by an array of capacitors and gated by a high current switch. Typically, the pulse length can be adjusted from 0.15 to 1.2 ms while achieving repetition rates from 1 to 80 Hz. Furthermore, a buffer capacitor (CHV = 36 nF) is added to the system in order to achieve beam currents beyond the limit of the extraction power supply (65 kV/300 mA) during the pulse mode operation. IV. EXPERIMENTAL RESULTS
Figure 4 shows the ion beam fractions as a function of the total arc power. For a constant arc voltage of 85 V, the H1 + fraction increases with increasing total arc power. In this operation mode with a relative high gas pressure and a high solenoid and filter field strength, it is possible to raise the H1 + fraction above 90 %. At the same time, the H3 + fraction decreases while the H2 + fraction does not change. Figure 5 shows a spectrum taken for a total arc power of 9.7 kW. The spectrum shows ion fractions in the ion beam of 92 % H1 + , 7 % H2 + , and 1 % H3 + , respectively.
FIG. 5. Hydrogen spectrum.
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02A743-3
Schweizer et al.
FIG. 6. Proton emission current density as a function of total arc power.
FIG. 7. Time evolution of the extracted total current.
FIG. 8. Photo of the ion beam just behind the extractor.
Rev. Sci. Instrum. 85, 02A743 (2014)
After having achieved reproducible conditions for generating beams with proton fraction beyond 90 %, the source was optimized to produce high currents in cw and pulsed mode. Figure 6 shows the extracted H1 + emission current density as a function of the total arc power. In cw mode, at the total arc power of 9.7 kW, a H1 + beam current density of 480 mA/cm2 has been achieved. Using an emission opening radius of 4 mm, a H1 + ion beam current of 240 mA at 50 keV beam energy has been extracted. In pulsed mode with a total arc power of 14.5 kW a H1 + beam current density of 696 mA/cm2 has been achieved with the same extraction system. A 60 keV/350 mA H1 + ion beam has been generated. The time evolution of the extracted total current is presented in Fig. 7. The duty cycle was 5 % and the pulse length was 1 ms. After a rise time of 60 μs, the total current reaches its maximum of 397 mA and falls down within 15 μs. Figure 8 shows a photo of a 40 keV/110 mA H1 + ion beam. Obviously, the beam has an excellent quality, with a very small divergence angle. While using a pepperpot device it was possible to make a first estimation of the emittance for a 56 keV/75 mA H1 + ion beam. Due to the high beam power density at the pepperpot aperture the emittance measurement was carried out in pulsed mode. Employing this method the (100%) normalized rms beam emittance after the ion source was determined to have a value of 0.15 π mm mrad. The spatial uncertainty is 2 mm and the angle resolution is in the range of 1.8 mrad (Fig. 9). In the cw operation mode, while using the same filament, the ion source lifetime was tested to be at least 320 h. The test was performed with a total arc power of 5 kW while extracting a 40 keV/112 mA H1 + ion beam. The average number of high voltage breakdowns was three times per hour. The experiments were initially carried out with hydrogen and then continued with deuterium. Due to similar atomic structure of H2 and D2 it was interesting to operate the source with D2 as a working gas. With the same extraction system, which is described above, at the D1 + fraction of 88% a 52 keV/205 mA D1 + ion beam has been generated. Due to the d-d reaction in the faraday cup from massive copper, a neutron source activity of 0.2 × 109 n/s was measured.
1 U.
FIG. 9. x − x phase space diagram.
Ratzinger, L. P. Chau, O. Meusel, A. Schempp, K. Volk, M. Heil, F. Käppeler, and R. Stieglitz, “Intense pulsed neutron source FRANZ in the 1–500 keV range,” ICANS-XVIII Proceedings, Dongguan, April 2007, p. 210. 2 U. Ratzinger, M. Basten, L. P. Chau, H. Dinter, M. Droba, M. Heilmann, M. Lotz, D. Mäder, O. Meusel, I. Müller, Y. Nie, D. Noll, H. Podlech, A. Schempp, W. Schweizer, K. Volk, C. Wiesner, and C. Zhang, “The driver Linac of the neutron source FRANZ,” IPAC2011 Proceedings, San Sebastián, September 2011, p. 2577. 3 R. Becker and W. Herrmannsfeldt, Rev. Sci. Instrum. 63, 2756 (1992). 4 R. Hollinger, P. Beller, K. Volk, M. Weber, and H. Klein, Rev. Sci. Instrum. 71, 836 (2000).
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 141.218.1.105 On: Mon, 18 Aug 2014 06:54:30
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 141.218.1.105 On: Mon, 18 Aug 2014 06:54:30
REVIEW OF SCIENTIFIC INSTRUMENTS 85, 02A743 (2014)
A high intensity 200 mA proton source for the FRANZ-Project (Frankfurt-Neutron-Source at the Stern-Gerlach-Center)a) W. Schweizer,b) U. Ratzinger, B. Klump, and K. Volk Institute of Applied Physics, Goethe-Universität Frankfurt, 60438 Frankfurt am Main, Germany
(Presented 12 September 2013; received 5 September 2013; accepted 24 October 2013; published online 28 January 2014) At the University of Frankfurt a high current proton source has been developed and tested for the FRANZ-Project [U. Ratzinger, L. P. Chau, O. Meusel, A. Schempp, K. Volk, M. Heil, F. Käppeler, and R. Stieglitz, “Intense pulsed neutron source FRANZ in the 1–500 keV range,” ICANS-XVIII Proceedings, Dongguan, April 2007, p. 210]. The ion source is a filament driven arc discharge ion source. The new design consists of a plasma generator, equipped with a filter magnet to produce nearly pure proton beams (92 %), and a compact triode extraction system. The beam current density has been enhanced up to 521 mA/cm2 . Using an emission opening radius of 4 mm, a proton beam current of 240 mA at 50 keV beam energy in continuous wave mode (cw) has been extracted. This paper will present the current status of the proton source including experimental results of detailed investigations of the beam composition in dependence of different plasma parameters. Both, cw and pulsed mode were studied. Furthermore, the performance of the ion source was studied with deuterium as working gas. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4842335] I. INTRODUCTION
FRANZ is a research project of the Goethe University Frankfurt.1 It allows to intensify well-established accelerator research activities by providing very intense 200 mA proton beams with energies of 120 keV in cw and pulsed mode in the low energy beam transport section. In pulsed and cw mode the beam pulses will be provided by a chopper system which delivers a pulse duration of 250 kHz. Time dependent space charge compensation effects and beam wall interaction phenomena can also be investigated and optimized.2 At the envisaged beam currents a 2 MeV RFQ had been shown to be rather long and to consume a lot of rf power. That is why a coupled rf cavity was chosen finally, consisting of a 700 keV 4-Rod-RFQ and a 2 MeV IH-DTL. The total rf power losses will stay below 200 kW.2 II. EXTRACTION SYSTEM DESIGN AND BEAM FORMATION SIMULATIONS
The main object was to produce a cw 200 mA proton beam at 120 keV beam energy. During the developing phase of the ion source, it was operated at a separated test facility with a maximum available voltage of 65 kV. Therefore, in a first step a 65 kV triode extraction system was designed with the computer code IGUN.3 Usually, a source operation with hydrogen leads to ion beams consisting of a mixture of three species: H1 + , H2 + , and H3 + . Hence, during the layout of the extraction system, the additional space charge of the undesirable beam species H2 + and H3 + has to be taken into consideration according to the Child-Langmuir-Law.
Formula (1) presents the proton equivalent total current IPETC , which is the sum of the partial currents of the three ion species normalized to the mass 1: AH+1 AH+2 AH+1 + IH+2 + + IH+1 . (1) IPETC = IH+1 1 1 1 Figure 1 shows IPETC as a function of the proton fraction for a fixed IH+2 /IH+3 ratio of 1 and for a fixed IH+1 = 200 mA. With decreasing proton fraction the IPETC is increasing. This fact has to be considered for the layout of the extractor. It requires an enhancement of the emission opening area, which leads to a higher gas flow through the extraction system. With increasing gas pressure in the extractor, the maximum applicable electrical field strength in the acceleration gap decreases, which reduces the extractable beam current. Therefore, it was an essential development goal to achieve an acceptable proton fraction in the ion beam. For the IGUN simulations a proton fraction of 90 % was assumed, leading to an IPETC of 235 mA. In order to keep the emission opening radius at an acceptable value, the used
a) Contributed paper, published as part of the Proceedings of the 15th
International Conference on Ion Sources, Chiba, Japan, September 2013. b) Electronic mail: [email protected].
0034-6748/2014/85(2)/02A743/3/$30.00
FIG. 1. Proton equivalent total current as a function of the proton fraction. 85, 02A743-1
© 2014 AIP Publishing LLC
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02A743-2
Schweizer et al.
Rev. Sci. Instrum. 85, 02A743 (2014)
FIG. 2. IGUN simulation of a 50 keV/200 mA H1 + beam. Ti = 0 eV.
electrical field strength has to be relatively high (9.4 kV/mm). Taking these values into account, ion beam formation simulations were performed with IGUN (Fig. 2). With an emission opening radius of 4 mm, an extraction voltage of 50 kV, and an aspect ratio of 0.62 the beam course in the extraction system is matched. At the end of the extractor the beam radius is about 1.5 mm at a divergence angle below 20 mrad. III. EXPERIMENTAL SETUP
In Fig. 3 a schematic cross-sectional view of the high current proton ion source is shown. The source consists of a plasma generator (input power up to 12 kW) and a 65 kV single hole triode extraction system. The new development of this proton ion source is based on former investigations of high current ion sources at the Institute of Applied Physics.4 The plasma generator is equipped with a cathode system (which includes a 2.4 mm thick tungsten filament), a solenoid for plasma confinement and a transversal magnetic filter. The front end of the plasma generator consists of the plasma electrode, which is insulated from the anode and can
FIG. 3. Schematic drawing and electric circuit of the proton ion source.
FIG. 4. Ion fraction vs. total arc power.
be biased negatively with respect to it. Hence, the total arc power is the sum of the arc voltage times the arc current and the bias voltage of the plasma electrode times the plasma electrode current. The source can be operated in cw as well as in pulsed mode. In pulsed mode the arc discharge is pulsed and the arc power is provided by an array of capacitors and gated by a high current switch. Typically, the pulse length can be adjusted from 0.15 to 1.2 ms while achieving repetition rates from 1 to 80 Hz. Furthermore, a buffer capacitor (CHV = 36 nF) is added to the system in order to achieve beam currents beyond the limit of the extraction power supply (65 kV/300 mA) during the pulse mode operation. IV. EXPERIMENTAL RESULTS
Figure 4 shows the ion beam fractions as a function of the total arc power. For a constant arc voltage of 85 V, the H1 + fraction increases with increasing total arc power. In this operation mode with a relative high gas pressure and a high solenoid and filter field strength, it is possible to raise the H1 + fraction above 90 %. At the same time, the H3 + fraction decreases while the H2 + fraction does not change. Figure 5 shows a spectrum taken for a total arc power of 9.7 kW. The spectrum shows ion fractions in the ion beam of 92 % H1 + , 7 % H2 + , and 1 % H3 + , respectively.
FIG. 5. Hydrogen spectrum.
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 141.218.1.105 On: Mon, 18 Aug 2014 06:54:30
02A743-3
Schweizer et al.
FIG. 6. Proton emission current density as a function of total arc power.
FIG. 7. Time evolution of the extracted total current.
FIG. 8. Photo of the ion beam just behind the extractor.
Rev. Sci. Instrum. 85, 02A743 (2014)
After having achieved reproducible conditions for generating beams with proton fraction beyond 90 %, the source was optimized to produce high currents in cw and pulsed mode. Figure 6 shows the extracted H1 + emission current density as a function of the total arc power. In cw mode, at the total arc power of 9.7 kW, a H1 + beam current density of 480 mA/cm2 has been achieved. Using an emission opening radius of 4 mm, a H1 + ion beam current of 240 mA at 50 keV beam energy has been extracted. In pulsed mode with a total arc power of 14.5 kW a H1 + beam current density of 696 mA/cm2 has been achieved with the same extraction system. A 60 keV/350 mA H1 + ion beam has been generated. The time evolution of the extracted total current is presented in Fig. 7. The duty cycle was 5 % and the pulse length was 1 ms. After a rise time of 60 μs, the total current reaches its maximum of 397 mA and falls down within 15 μs. Figure 8 shows a photo of a 40 keV/110 mA H1 + ion beam. Obviously, the beam has an excellent quality, with a very small divergence angle. While using a pepperpot device it was possible to make a first estimation of the emittance for a 56 keV/75 mA H1 + ion beam. Due to the high beam power density at the pepperpot aperture the emittance measurement was carried out in pulsed mode. Employing this method the (100%) normalized rms beam emittance after the ion source was determined to have a value of 0.15 π mm mrad. The spatial uncertainty is 2 mm and the angle resolution is in the range of 1.8 mrad (Fig. 9). In the cw operation mode, while using the same filament, the ion source lifetime was tested to be at least 320 h. The test was performed with a total arc power of 5 kW while extracting a 40 keV/112 mA H1 + ion beam. The average number of high voltage breakdowns was three times per hour. The experiments were initially carried out with hydrogen and then continued with deuterium. Due to similar atomic structure of H2 and D2 it was interesting to operate the source with D2 as a working gas. With the same extraction system, which is described above, at the D1 + fraction of 88% a 52 keV/205 mA D1 + ion beam has been generated. Due to the d-d reaction in the faraday cup from massive copper, a neutron source activity of 0.2 × 109 n/s was measured.
1 U.
FIG. 9. x − x phase space diagram.
Ratzinger, L. P. Chau, O. Meusel, A. Schempp, K. Volk, M. Heil, F. Käppeler, and R. Stieglitz, “Intense pulsed neutron source FRANZ in the 1–500 keV range,” ICANS-XVIII Proceedings, Dongguan, April 2007, p. 210. 2 U. Ratzinger, M. Basten, L. P. Chau, H. Dinter, M. Droba, M. Heilmann, M. Lotz, D. Mäder, O. Meusel, I. Müller, Y. Nie, D. Noll, H. Podlech, A. Schempp, W. Schweizer, K. Volk, C. Wiesner, and C. Zhang, “The driver Linac of the neutron source FRANZ,” IPAC2011 Proceedings, San Sebastián, September 2011, p. 2577. 3 R. Becker and W. Herrmannsfeldt, Rev. Sci. Instrum. 63, 2756 (1992). 4 R. Hollinger, P. Beller, K. Volk, M. Weber, and H. Klein, Rev. Sci. Instrum. 71, 836 (2000).
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