- Email: [email protected]
Microparticle acceleration by a Van de Graaff accelerator and application to space and material sciences
Radiation Physics and Chemistry 60 (2001) 277–282
Microparticle acceleration by a Van de Graaff accelerator and application to space and material sciences Hiromi Shibataa,*, Koichi Kobayashia, Takeo Iwaia, Yoshimi Hamabeb, Sho Sasakib, Sunao Hasegawac, Hajime Yanoc, Akira Fujiwarac, Hideo Ohashid, Toru Kawamurae, Ken-ichi Nogamie a
Research Center for Nuclear Science and Technology, The University of Tokyo, Tokyo 113-0032, Japan b Department of Earth and Planetary Science, The University of Tokyo, Tokyo 113-0033, Japan c Research Division for Planetary Science, Institute of Space and Astronautical Science, Kanagawa 229-0022, Japan d Department of Ocean Sciences, Tokyo University of Fisheries, Tokyo 108-8477, Japan e Dokkyo University School of Medicine, Tochigi 321-0293, Japan
Abstract A microparticle (dust) ion source has been installed in the 3.75 MV Van de Graaff electrostatic accelerator and a new beam line for microparticle experiments has been built at the HIT facility of Research Center for Nuclear Science and Technology, the University of Tokyo. Microparticle acceleration has been successful in obtaining expected velocities of 1–20 km/s or more for micron- or submicron-sized particles. Development of in situ dust detectors on board satellites and spacecraft in the expected mass and velocity range of micrometeoroids and investigation of hypervelocity impact phenomena by using time-of-flight mass spectrometry, impact flash measurement and scanning electron microscope observation for metals, polymers and semiconductors bombarded by micron-sized particles have been started. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Microparticle; Dust; Van de Graaff accelerator; Impact rates
1. Introduction Recently, hypervelocity impact phenomena in space have been paid much attention, because extreme transient pressure with relatively low temperature (but enough temperature to ionize or vaporize materials) and production of shock wave in materials are not caused by a normal situation of collision phenomena (McDonnell, 1978; Kissel and Krueger, 1987). These phenomena have much attraction to material science as well as space science and can be achieved by
*Corresponding author. Correspondence address: Research Center for Nuclear Science and Technology, The University of Tokyo, 2-22 Shirakata-Shirane, Tokai-mura, Ibaraki 319-1188, Japan. Tel.: +81-29-287-8476; fax: +81-29-287-8490. E-mail address: [email protected] (H. Shibata).
acceleration of micron sized particles to velocities of several km/s. The usual method of accelerating a microparticle (or dust, hereafter we use ‘dust’ mainly instead of ‘microparticle’) to velocities in excess of 1 km/s is to generate charges on the particle and then accelerate it across a large potential difference. The electrostatic dust acceleration method was devised by Shelton et al. (1960). A dust ion source was connected to the top terminal of a Van de Graaff accelerator. Their techniques were used and modified for the Van de Graaff accelerator at Max–Planck-Institut fu¨r Kernphysik in Heidelberg (Fechtig et al., 1978), Unit for Space Sciences and Astrophysics, University of Kent at Canterbury (Green et al., 1988; Burchell et al., 1999), Los Alamos National Laboratory (Keaton et al., 1990) and the Faculty of Engineering, Kyoto University (Fukuzawa, 1991).
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Fig. 1. Schematic view of the accelerator system.
We developed a 25 kV dust accelerator several years ago and fundamental research for the advanced facility has been performed. The velocity of dust particle using our 25 kV dust accelerator was not sufficient to calibrate in situ dust detectors and collectors, which were generally used for the detection of meteoroids and space debris on board satellites and spacecraft. All methods of the dust detection in space involve hypervelocity impact phenomena and in situ dust detectors should be calibrated by the dust accelerator in the expected mass and velocity range of micrometeoroids. We have developed two higher voltage accelerators for the above reason. One is a modified 3.75 MV Van de Graaff accelerator, which is not a dedicated machine for the dust acceleration (Hasegawa et al., 1999), and the other is a new 100 kV accelerator dedicated for dust experiments. In following chapters, we will report our dust acceleration system, results of the dust acceleration and some results of hypervelocity impact phenomena.
2. Accelerator system for microparticle The 3.75 MV single-ended Van de Graaff accelerator installed with an RF or PIG ion source and four beam lines is operated by HIT (High Fluence Irradiation Facility, the University of Tokyo) group of Research Center for Nuclear Science and Technology, the University of Tokyo. This accelerator and the 1.0 MV Tandetron accelerator are used for studies of the ionbeam simulation of neutron irradiation on nuclear fusion materials (dual beam irradiation), ion-beam irradiation effects of polymers, ceramics and metals, and advanced techniques of microbeam and positronbeam analyses. The modification from an ion accelerator to a dust one is usually achieved by replacing an ion source to a dust ion source at the high-voltage terminal of a Van de Graaff accelerator. The electrostatic dust
Fig. 2. The top view of microparticle (dust) source/reservoir.
accelerator system consists of four sections: a dust source section, an accelerator section, a charge and velocity measurement section, and an experimental section including a target chamber. Fig. 1 shows a schematic view of these sections. The dust source, which produces positively charged dust particles, is located inside the high-voltage terminal of the accelerator. Fig. 2 shows the top view of the dust source and the dust sample powder is reserved below the tongue electrode. Two kinds of high voltages are supplied to the two electrodes. One is continuous dc high voltage around +15 kV, which is added to the surrounding electrode. The other is pulsed dc high voltage from 0 to +15 kV (the maximum voltage equals the continuous dc high voltage) applied to the tongue electrode to excite dust particles vibrating between the walls inside the dust reservoir. The pulse width is 10 ms and the duration is 50 ms. The dust particle reaching the needle tip accidentally gets charges from the tip whose shape is spherical. The maximum charge received from the tip is limited by the effect of the field emission from the dust particle (Keaton et al., 1990).
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Charged-up particles are accelerated toward ground potential. When each particle arrives at ground potential, its kinetic energy E ¼ mv2 =2, where m is the particle mass and v is its velocity. The final velocity v of the particle accelerated through the voltage difference of U is given by v ¼ ð2QU=mÞ1=2 ;
ð1Þ
where Q is the particle charge. The accelerated dust particle is adjusted to the center of the beam line by the two sets of steerers in Fig. 1. A new beam line for dust experiments is installed at the zero-degree port of the accelerator. On this beam line three cylindrical detectors (‘‘Detector 1–3’’ in Fig. 1) are set to measure projectile dust charges induced as a dust particle passes through the detector. The cylindrical detector is connected to a charge-sensitive amplifier (CSA). The output voltage of the charge-sensitive amplifier is proportional to the particle charge. The particle velocity is calculated by the time of flight (TOF) between two cylindrical detectors. Thus, one can calculate the particle mass, momentum and energy by substituting the known value for the charge Q, the velocity v and the acceleration voltage U into Eq. (1). If the density of the particle is known, one can calculate the particle diameter by assuming that it is spherical. The particle selector is indispensable to select a specific particle condition, because a beam of accelerated particles has a broad distribution of mass and velocity range. It is composed of a deflector, a high-voltage switching circuit and a processing unit. When a particle passes parallel plates (‘‘Deflector’’ in Fig. 1) in the beam line, only a desirable dust particle can pass that deflector by switching deflection voltage between them. A target chamber consists of a vacuum vessel with a turbomolecular pump, a movable target stage and several view ports for impact flash (UV, visible, IR, and X-ray) measurements.
3. Results of microparticle acceleration Results of the dust acceleration using the Van de Graaff accelerator are shown in Fig. 3. Fig. 3(a) shows the accelerated particle velocity as a function of the particle diameter under 2.0 and 3.0 MV conditions with those obtained at Max–Planck-Institut and Kent University. Accelerated particles were sphere-shaped silver and carbon microparticles whose size distributed over 0.2–2 mm in diameter (1017–1013 kg) in our case, and iron microparticles distributed over 0.01–2 mm in diameter (1019–1013 kg) were accelerated at Max–PlanckInstitut and Kent University. The particle velocity distribution achieved by using the HIT Van de Graaff accelerator is almost equal to that reported in other acceleration facilities under the same particle mass
Fig. 3. Experimental results of carbon, silver and iron particle acceleration using the 3.75 MV Van de Graaff accelerator of HIT and 2 MV Van de Graaff accelerators of Max–PlankInsitut and Kent University. (a) The particle velocity as a function of the particle diameter. (b) The specific charge versus the particle diameter.
conditions (Fechtig et al., 1978; Green et al., 1988). Fig. 3(b) shows the specific charge Q=m versus the particle diameter. The specific charge Q=m depends mainly on charging methods. In our case, the accepted charge is controlled by the effect of the field emission. The typical charge of 1 mm-sized particle is the order of 1014 C (the charge number is about 105), and the particle mass is the order of 1014 kg, which corresponds to the particle number of about 1011.
4. Time-of-flight mass measurement In order to reveal the origin of the dust particles, it is necessary to analyze chemical elements directly on board satellites as well as fluxes, velocities, directions and masses of micrometeoroids for hypervelocity impacts.
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H. Shibata et al. / Radiation Physics and Chemistry 60 (2001) 277–282
For this purpose, we are developing a new type of impact-ionization dust analyzer with a TOF mass spectrometer, which has to be compact and lightweight, because this type of instrument will be loaded on a satellite in future. As the first step, TOF mass measurements were made for a pulsed IR laser irradiation, ion bombardments and microparticle impacts on metals, such as aluminum, copper, gold and molybdenum targets by using a linear-type TOF mass spectrometer. In the second step, a reflectron-type mass spectrometer with a new idea is being developed for in situ dust analyzer on board spacecraft. To simulate hypervelocity impact phenomena of micrometeoroids in space, simulation experiments are made by using the accelerated dust with a mass and velocity range comparable to that of micrometeoroids. In these experiments, silver particles of around 1 mm in diameter were used as projectiles, and they were accelerated to several km/s by the Van de Graaff accelerator at HIT. The IR laser irradiation has not produced so much secondary ions from the Mo target in our experiment. On the other hand, 1.0 MeV H+ ion beam bombardments produced a rather complicated spectrum of secondary ions. By these laser and ion beam experiments, we could confirm the performance of our linear TOF mass spectrometer. The typical mass spectra of the linear TOF mass spectrometer by the silver-dust impact experiments are shown in Figs. 4(a) and (b). In Fig. 4 the ions from (a) molybdenum and (b) gold targets were registered together with the target charge signals induced by the impacts of the single silver dust projectile. Though several peaks were found in the spectra, we could not assign all of them. The spectra are divided into two types. One case is that only one particle impacts on the target with charges. The other is that several particles impact on the target at the same time or fragments produced by the collision of the projectile with the front mesh of the ion source part of the mass spectrometer impact on the target. Since the initial energies of ions from both the target and the projectile are less than that by laser irradiation, the mass resolution becomes higher for the dust impacts than for the laser irradiation. We will verify the performance of the new reflectrontype TOF mass spectrometer with hypervelocity particle impacts, and compare the results with pulsed IR laser irradiation.
5. Scanning electron microscope observation of impact craters It is very important to know the mechanism of impact crater creation and the distribution of both projectile and target materials in the crater under hypervelocity impacts both in space and material sciences.
Fig. 4. Typical time-of-flight ion mass spectra from (a) molybdenum and (b) gold targets together with the target charge signals bombarded by hypervelocity silver particles. These mass spectra and charge signals were registered by the one-particle impact.
Observations of the impact craters as well as the mass analysis of the ions produced by hypervelocity impacts have been carried out. Aluminum, molybdenum and gold were used as target materials. Typical scanning electron microscope (SEM) and emitted characteristic X-ray photographs of craters formed on (a) molybdenum and (b) gold targets by hypervelocity silver dust impact are shown in Fig. 5. The distribution of the projectile silver was mapped by the characteristic X-ray measurement. We could observe silver particles distributed in both molybdenum and gold targets. The incident velocities and masses of projectiles were not measured, but these velocities were estimated to be about 2–8 km/s from the collected data for the experimental condition in this photograph. The differences of target materials and impact velocities cause the different types of impact craters in shape and size. In the case of molybdenum as the target material, for example, two types of craters were observed. One was a ripple-shaped crater created by vaporizing silver projectile. The other comprised, on the contrary, silver residuals which were observed in the crater and on the rim. The number ratio of the rippleshaped craters and the craters with silver residuals was about 13 and 75%, respectively. Taking the distribution of the impact velocity into consideration,
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Fig. 5. Typical photographs of scanning electron microscope and emitted characteristic X-ray observation of craters formed on (a) molybdenum and (b) gold targets.
the ripple-shaped craters are produced by higher velocity impacts of the projectiles than the craters with silver residuals were made. In the case of the gold target, the mean diameter of the craters was about 3.1 mm, which was larger than that of the molybdenum target. Moreover, in the gold target the silver residuals distributed over broader range than in the molybdenum target. These phenomena may be attributed to the melting point and the high ductility of gold.
6. Conclusion The acceleration of microparticles has been achieved to a hypervelocity of 10 km/s for about 1 mm in diameter and more for sub-micron particles by using the 3.75 MV single-ended Van de Graaff accelerator. The secondary ion mass spectrometry by the laser irradiation, ion beam bombardments and dust hypervelocity impacts on metal targets has been done using the linear TOF mass spectrometer for the development of in situ dust detectors on board spacecraft and the investigation of hypervelocity impact phenomena. The SEM observation for molybdenum and gold materials has shown two kinds of impact craters. In the near future, we will offer statistical data of impact craters in shape and size, and element mapping by using STM and SEM/EPMA.
Acknowledgements We are grateful to Mr. G. Schafer, Dr. R. Srama and Prof. E. Gru¨n for discussions and for their advice about detail of the dust accelerators, and for providing us their dust source. We are also grateful to Mr. M. Cole, Dr. M.J. Burchell and Prof. J.A.M. McDonnell for giving us useful advice on accelerator design. We also thank Mr. M. Narui and Mr. T. Omata for their help with dust experiments. This study was carried out as a part of ‘‘Ground Research Announcement for Space Utilization’’ promoted by Japan Space Forum.
References Burchell, M.J., Cole, M.J., McDonnell, J.A.M., Zarnecki, J.C., 1999. Hypervelocity impact studies using 2 MV Van de Graaff accelerator and two-stage light gas gun of the University of Kent at Canterbury. Meas. Sci. Technol. 10, 41. Fechtig, H., Gru¨n, E., Kissel, J., 1978. Laboratory simulation. In: McDonnell, J.A.M. (Ed.), Cosmic Dust. Wiley-Interscience Publications, New York, pp. 607. Fukuzawa, F., 1991. Microparticle ion beam (Macron beam). Oyo Buturi 60, 21 (in Japanese). Green, S.F., Clarke, C.D., Stevenson, T.J., 1988. A 2 MV Van de Graaff accelerator for cosmic dust impact simulation. J. Br Interplanet. Soc. 31, 393.
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Hasegawa, S., Fujiwara, A., Yano, H., Nishimura, T., Sasaki, S., Ohashi, H., Iwai, T., Kobayashi, K., Shibata, H., 1999. Design of electrostatic accelerators for microparticle detection in Japan. Adv. Space Res. 23, 119. Keaton, P.W., Idzorek, G.C., Rowton Sr., L.J., Seagrave, J.D., Stradling, G.L., Bergeson, S.D., Collopy, M.T., Curling Jr., H.L., McColl, D.B., Smith, J.D., 1990. A hypervelocitymicroparticle-impacts laboratory with 100-km/s projectiles. Int. J. Impact Engng. 10, 295.
Kissel, J., Krueger, F.R., 1987. Ion formation by impact of fast dust particles and comparison with related technique. Appl. Phys. A 42, 69. McDonnell, J.A.M., 1978. Microparticle studies by space instrumentation. In: McDonnell, J.A.M. (Ed.), Cosmic Dust. Wiley-Interscience Publications, New York, pp. 337. Shelton, H., Hendricks Jr., C.D., Wuerker, R.F., 1960. Electrostatic acceleration of microparticles to hypervelocity. J. Appl. Phys. 31, 1243.
Microparticle acceleration by a Van de Graaff accelerator and application to space and material sciences Hiromi Shibataa,*, Koichi Kobayashia, Takeo Iwaia, Yoshimi Hamabeb, Sho Sasakib, Sunao Hasegawac, Hajime Yanoc, Akira Fujiwarac, Hideo Ohashid, Toru Kawamurae, Ken-ichi Nogamie a
Research Center for Nuclear Science and Technology, The University of Tokyo, Tokyo 113-0032, Japan b Department of Earth and Planetary Science, The University of Tokyo, Tokyo 113-0033, Japan c Research Division for Planetary Science, Institute of Space and Astronautical Science, Kanagawa 229-0022, Japan d Department of Ocean Sciences, Tokyo University of Fisheries, Tokyo 108-8477, Japan e Dokkyo University School of Medicine, Tochigi 321-0293, Japan
Abstract A microparticle (dust) ion source has been installed in the 3.75 MV Van de Graaff electrostatic accelerator and a new beam line for microparticle experiments has been built at the HIT facility of Research Center for Nuclear Science and Technology, the University of Tokyo. Microparticle acceleration has been successful in obtaining expected velocities of 1–20 km/s or more for micron- or submicron-sized particles. Development of in situ dust detectors on board satellites and spacecraft in the expected mass and velocity range of micrometeoroids and investigation of hypervelocity impact phenomena by using time-of-flight mass spectrometry, impact flash measurement and scanning electron microscope observation for metals, polymers and semiconductors bombarded by micron-sized particles have been started. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Microparticle; Dust; Van de Graaff accelerator; Impact rates
1. Introduction Recently, hypervelocity impact phenomena in space have been paid much attention, because extreme transient pressure with relatively low temperature (but enough temperature to ionize or vaporize materials) and production of shock wave in materials are not caused by a normal situation of collision phenomena (McDonnell, 1978; Kissel and Krueger, 1987). These phenomena have much attraction to material science as well as space science and can be achieved by
*Corresponding author. Correspondence address: Research Center for Nuclear Science and Technology, The University of Tokyo, 2-22 Shirakata-Shirane, Tokai-mura, Ibaraki 319-1188, Japan. Tel.: +81-29-287-8476; fax: +81-29-287-8490. E-mail address: [email protected] (H. Shibata).
acceleration of micron sized particles to velocities of several km/s. The usual method of accelerating a microparticle (or dust, hereafter we use ‘dust’ mainly instead of ‘microparticle’) to velocities in excess of 1 km/s is to generate charges on the particle and then accelerate it across a large potential difference. The electrostatic dust acceleration method was devised by Shelton et al. (1960). A dust ion source was connected to the top terminal of a Van de Graaff accelerator. Their techniques were used and modified for the Van de Graaff accelerator at Max–Planck-Institut fu¨r Kernphysik in Heidelberg (Fechtig et al., 1978), Unit for Space Sciences and Astrophysics, University of Kent at Canterbury (Green et al., 1988; Burchell et al., 1999), Los Alamos National Laboratory (Keaton et al., 1990) and the Faculty of Engineering, Kyoto University (Fukuzawa, 1991).
0969-806X/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 9 - 8 0 6 X ( 0 0 ) 0 0 3 6 2 - 5
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H. Shibata et al. / Radiation Physics and Chemistry 60 (2001) 277–282
Fig. 1. Schematic view of the accelerator system.
We developed a 25 kV dust accelerator several years ago and fundamental research for the advanced facility has been performed. The velocity of dust particle using our 25 kV dust accelerator was not sufficient to calibrate in situ dust detectors and collectors, which were generally used for the detection of meteoroids and space debris on board satellites and spacecraft. All methods of the dust detection in space involve hypervelocity impact phenomena and in situ dust detectors should be calibrated by the dust accelerator in the expected mass and velocity range of micrometeoroids. We have developed two higher voltage accelerators for the above reason. One is a modified 3.75 MV Van de Graaff accelerator, which is not a dedicated machine for the dust acceleration (Hasegawa et al., 1999), and the other is a new 100 kV accelerator dedicated for dust experiments. In following chapters, we will report our dust acceleration system, results of the dust acceleration and some results of hypervelocity impact phenomena.
2. Accelerator system for microparticle The 3.75 MV single-ended Van de Graaff accelerator installed with an RF or PIG ion source and four beam lines is operated by HIT (High Fluence Irradiation Facility, the University of Tokyo) group of Research Center for Nuclear Science and Technology, the University of Tokyo. This accelerator and the 1.0 MV Tandetron accelerator are used for studies of the ionbeam simulation of neutron irradiation on nuclear fusion materials (dual beam irradiation), ion-beam irradiation effects of polymers, ceramics and metals, and advanced techniques of microbeam and positronbeam analyses. The modification from an ion accelerator to a dust one is usually achieved by replacing an ion source to a dust ion source at the high-voltage terminal of a Van de Graaff accelerator. The electrostatic dust
Fig. 2. The top view of microparticle (dust) source/reservoir.
accelerator system consists of four sections: a dust source section, an accelerator section, a charge and velocity measurement section, and an experimental section including a target chamber. Fig. 1 shows a schematic view of these sections. The dust source, which produces positively charged dust particles, is located inside the high-voltage terminal of the accelerator. Fig. 2 shows the top view of the dust source and the dust sample powder is reserved below the tongue electrode. Two kinds of high voltages are supplied to the two electrodes. One is continuous dc high voltage around +15 kV, which is added to the surrounding electrode. The other is pulsed dc high voltage from 0 to +15 kV (the maximum voltage equals the continuous dc high voltage) applied to the tongue electrode to excite dust particles vibrating between the walls inside the dust reservoir. The pulse width is 10 ms and the duration is 50 ms. The dust particle reaching the needle tip accidentally gets charges from the tip whose shape is spherical. The maximum charge received from the tip is limited by the effect of the field emission from the dust particle (Keaton et al., 1990).
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Charged-up particles are accelerated toward ground potential. When each particle arrives at ground potential, its kinetic energy E ¼ mv2 =2, where m is the particle mass and v is its velocity. The final velocity v of the particle accelerated through the voltage difference of U is given by v ¼ ð2QU=mÞ1=2 ;
ð1Þ
where Q is the particle charge. The accelerated dust particle is adjusted to the center of the beam line by the two sets of steerers in Fig. 1. A new beam line for dust experiments is installed at the zero-degree port of the accelerator. On this beam line three cylindrical detectors (‘‘Detector 1–3’’ in Fig. 1) are set to measure projectile dust charges induced as a dust particle passes through the detector. The cylindrical detector is connected to a charge-sensitive amplifier (CSA). The output voltage of the charge-sensitive amplifier is proportional to the particle charge. The particle velocity is calculated by the time of flight (TOF) between two cylindrical detectors. Thus, one can calculate the particle mass, momentum and energy by substituting the known value for the charge Q, the velocity v and the acceleration voltage U into Eq. (1). If the density of the particle is known, one can calculate the particle diameter by assuming that it is spherical. The particle selector is indispensable to select a specific particle condition, because a beam of accelerated particles has a broad distribution of mass and velocity range. It is composed of a deflector, a high-voltage switching circuit and a processing unit. When a particle passes parallel plates (‘‘Deflector’’ in Fig. 1) in the beam line, only a desirable dust particle can pass that deflector by switching deflection voltage between them. A target chamber consists of a vacuum vessel with a turbomolecular pump, a movable target stage and several view ports for impact flash (UV, visible, IR, and X-ray) measurements.
3. Results of microparticle acceleration Results of the dust acceleration using the Van de Graaff accelerator are shown in Fig. 3. Fig. 3(a) shows the accelerated particle velocity as a function of the particle diameter under 2.0 and 3.0 MV conditions with those obtained at Max–Planck-Institut and Kent University. Accelerated particles were sphere-shaped silver and carbon microparticles whose size distributed over 0.2–2 mm in diameter (1017–1013 kg) in our case, and iron microparticles distributed over 0.01–2 mm in diameter (1019–1013 kg) were accelerated at Max–PlanckInstitut and Kent University. The particle velocity distribution achieved by using the HIT Van de Graaff accelerator is almost equal to that reported in other acceleration facilities under the same particle mass
Fig. 3. Experimental results of carbon, silver and iron particle acceleration using the 3.75 MV Van de Graaff accelerator of HIT and 2 MV Van de Graaff accelerators of Max–PlankInsitut and Kent University. (a) The particle velocity as a function of the particle diameter. (b) The specific charge versus the particle diameter.
conditions (Fechtig et al., 1978; Green et al., 1988). Fig. 3(b) shows the specific charge Q=m versus the particle diameter. The specific charge Q=m depends mainly on charging methods. In our case, the accepted charge is controlled by the effect of the field emission. The typical charge of 1 mm-sized particle is the order of 1014 C (the charge number is about 105), and the particle mass is the order of 1014 kg, which corresponds to the particle number of about 1011.
4. Time-of-flight mass measurement In order to reveal the origin of the dust particles, it is necessary to analyze chemical elements directly on board satellites as well as fluxes, velocities, directions and masses of micrometeoroids for hypervelocity impacts.
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For this purpose, we are developing a new type of impact-ionization dust analyzer with a TOF mass spectrometer, which has to be compact and lightweight, because this type of instrument will be loaded on a satellite in future. As the first step, TOF mass measurements were made for a pulsed IR laser irradiation, ion bombardments and microparticle impacts on metals, such as aluminum, copper, gold and molybdenum targets by using a linear-type TOF mass spectrometer. In the second step, a reflectron-type mass spectrometer with a new idea is being developed for in situ dust analyzer on board spacecraft. To simulate hypervelocity impact phenomena of micrometeoroids in space, simulation experiments are made by using the accelerated dust with a mass and velocity range comparable to that of micrometeoroids. In these experiments, silver particles of around 1 mm in diameter were used as projectiles, and they were accelerated to several km/s by the Van de Graaff accelerator at HIT. The IR laser irradiation has not produced so much secondary ions from the Mo target in our experiment. On the other hand, 1.0 MeV H+ ion beam bombardments produced a rather complicated spectrum of secondary ions. By these laser and ion beam experiments, we could confirm the performance of our linear TOF mass spectrometer. The typical mass spectra of the linear TOF mass spectrometer by the silver-dust impact experiments are shown in Figs. 4(a) and (b). In Fig. 4 the ions from (a) molybdenum and (b) gold targets were registered together with the target charge signals induced by the impacts of the single silver dust projectile. Though several peaks were found in the spectra, we could not assign all of them. The spectra are divided into two types. One case is that only one particle impacts on the target with charges. The other is that several particles impact on the target at the same time or fragments produced by the collision of the projectile with the front mesh of the ion source part of the mass spectrometer impact on the target. Since the initial energies of ions from both the target and the projectile are less than that by laser irradiation, the mass resolution becomes higher for the dust impacts than for the laser irradiation. We will verify the performance of the new reflectrontype TOF mass spectrometer with hypervelocity particle impacts, and compare the results with pulsed IR laser irradiation.
5. Scanning electron microscope observation of impact craters It is very important to know the mechanism of impact crater creation and the distribution of both projectile and target materials in the crater under hypervelocity impacts both in space and material sciences.
Fig. 4. Typical time-of-flight ion mass spectra from (a) molybdenum and (b) gold targets together with the target charge signals bombarded by hypervelocity silver particles. These mass spectra and charge signals were registered by the one-particle impact.
Observations of the impact craters as well as the mass analysis of the ions produced by hypervelocity impacts have been carried out. Aluminum, molybdenum and gold were used as target materials. Typical scanning electron microscope (SEM) and emitted characteristic X-ray photographs of craters formed on (a) molybdenum and (b) gold targets by hypervelocity silver dust impact are shown in Fig. 5. The distribution of the projectile silver was mapped by the characteristic X-ray measurement. We could observe silver particles distributed in both molybdenum and gold targets. The incident velocities and masses of projectiles were not measured, but these velocities were estimated to be about 2–8 km/s from the collected data for the experimental condition in this photograph. The differences of target materials and impact velocities cause the different types of impact craters in shape and size. In the case of molybdenum as the target material, for example, two types of craters were observed. One was a ripple-shaped crater created by vaporizing silver projectile. The other comprised, on the contrary, silver residuals which were observed in the crater and on the rim. The number ratio of the rippleshaped craters and the craters with silver residuals was about 13 and 75%, respectively. Taking the distribution of the impact velocity into consideration,
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Fig. 5. Typical photographs of scanning electron microscope and emitted characteristic X-ray observation of craters formed on (a) molybdenum and (b) gold targets.
the ripple-shaped craters are produced by higher velocity impacts of the projectiles than the craters with silver residuals were made. In the case of the gold target, the mean diameter of the craters was about 3.1 mm, which was larger than that of the molybdenum target. Moreover, in the gold target the silver residuals distributed over broader range than in the molybdenum target. These phenomena may be attributed to the melting point and the high ductility of gold.
6. Conclusion The acceleration of microparticles has been achieved to a hypervelocity of 10 km/s for about 1 mm in diameter and more for sub-micron particles by using the 3.75 MV single-ended Van de Graaff accelerator. The secondary ion mass spectrometry by the laser irradiation, ion beam bombardments and dust hypervelocity impacts on metal targets has been done using the linear TOF mass spectrometer for the development of in situ dust detectors on board spacecraft and the investigation of hypervelocity impact phenomena. The SEM observation for molybdenum and gold materials has shown two kinds of impact craters. In the near future, we will offer statistical data of impact craters in shape and size, and element mapping by using STM and SEM/EPMA.
Acknowledgements We are grateful to Mr. G. Schafer, Dr. R. Srama and Prof. E. Gru¨n for discussions and for their advice about detail of the dust accelerators, and for providing us their dust source. We are also grateful to Mr. M. Cole, Dr. M.J. Burchell and Prof. J.A.M. McDonnell for giving us useful advice on accelerator design. We also thank Mr. M. Narui and Mr. T. Omata for their help with dust experiments. This study was carried out as a part of ‘‘Ground Research Announcement for Space Utilization’’ promoted by Japan Space Forum.
References Burchell, M.J., Cole, M.J., McDonnell, J.A.M., Zarnecki, J.C., 1999. Hypervelocity impact studies using 2 MV Van de Graaff accelerator and two-stage light gas gun of the University of Kent at Canterbury. Meas. Sci. Technol. 10, 41. Fechtig, H., Gru¨n, E., Kissel, J., 1978. Laboratory simulation. In: McDonnell, J.A.M. (Ed.), Cosmic Dust. Wiley-Interscience Publications, New York, pp. 607. Fukuzawa, F., 1991. Microparticle ion beam (Macron beam). Oyo Buturi 60, 21 (in Japanese). Green, S.F., Clarke, C.D., Stevenson, T.J., 1988. A 2 MV Van de Graaff accelerator for cosmic dust impact simulation. J. Br Interplanet. Soc. 31, 393.
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