A. Ando

Tohoku University, Miyagi, Japan

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Publications (231)286.62 Total impact

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    ABSTRACT: Production of chemically active species in primary and secondary streamers is investigated using a two-dimensional axisymmetric numerical simulation model. The production processes of N2(v = 1), O(3P) and N(4S), which each have different threshold energies, are simulated using experimentally obtained pulsed voltages with peak values, V peak, of 18, 24 and 30 kV in dry air at atmospheric pressure. As V peak increases, the simulated length of the secondary streamer increases, although there is little change in the primary streamer characteristics. This means that the ratio of the secondary streamer phase to the primary streamer phase increases for increasing V peak. The simulated results show that as V peak increases, the energy efficiency of O(3P) production increases and that of N2(v = 1) production decreases. On the other hand, the energy efficiency of N(4S) production has reduced dependence on V peak. These characteristics can be explained by the spatiotemporal variations of the reduced electric field in the primary and secondary streamer.
    Journal of Physics D Applied Physics 06/2015; 48(21). DOI:10.1088/0022-3727/48/21/215203 · 2.52 Impact Factor
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    ABSTRACT: Momentum, i.e., force, exerted from a small helicon plasma thruster to a target plate is measured simultaneously with a direct thrust measurement using a thrust balance. The calibration coefficient relating a target displacement to a steady-state force is obtained by supplying a dc to a calibration coil mounted on the target, where a force acting to a small permanent magnet located near the coil is directly measured by using a load cell. As the force exerted by the plasma flow to the target plate is in good agreement with the directly measured thrust, the validity of the target technique is demonstrated under the present operating conditions, where the thruster is operated in steady-state. Furthermore, a calibration coefficient relating a swing amplitude of the target to an impulse bit is also obtained by pulsing the calibration coil current. The force exerted by the pulsed plasma, which is estimated from the measured impulse bit and the pulse width, is also in good agreement with that obtained for the steady-state operation; hence, the thrust assessment of the helicon plasma thruster by the target is validated for both the steady-state and pulsed operations.
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    ABSTRACT: Plasma and wave structures in a helicon plasma thruster, and an imparted thrust are experimentally investigated with two configurations having a mechanical aperture (MA) near the thruster exit and no MA. A standing helicon wave is excited by locating the MA, where a large amplitude and no spatial change of the phase of the axial component of the radio frequency (rf) magnetic field are detected between the rf antenna and the MA; simultaneously, a higher plasma density by a factor of 2─2.5 is obtained in the source compared with that obtained with no MA. On the other hand, the plasma density downstream of the thruster exit with the MA is lower than that obtained without the MA. The magnetic field measurement downstream of the thruster exit shows the presence of a traveling electromagnetic wave, axial and radial wavenumbers of which are in the range of the slow wave dispersion branch. The directly measured thrust with the MA is only 4.5 mN for 1.5 kW rf power due to the lower plasma density in the magnetic nozzle, while the thrust without the MA is 12 mN.
    Plasma Sources Science and Technology 11/2014; 23(6). DOI:10.1088/0963-0252/23/6/064005 · 3.06 Impact Factor
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    ABSTRACT: A sheath expansion around the Langmuir probe is known to give a significantly overestimated plasma density. Here, the sheath expansion effect suggested by Sheridan [Phys. Plasmas 7, 3084 (2000)] is successfully incorporated with no cumbersome analysis of the current–voltage (I–V) characteristics of the planar probe by measuring local plasma potential, floating potential, and ion saturation current. The probe consists of an emissive probe and two planar Langmuir probes, and is tested in low-pressure geometrically and magnetically expanding plasmas. The electron temperature estimated from the difference between the local plasma and floating potentials in the geometrically expanding plasma is in good agreement with that obtained from a classical analysis of the I–V characteristics. The plasma density computed with taking into account the sheath expansion effect shows significantly lower values than that obtained from the classical density estimation. The measurements in the magnetically expanding plasma successfully reproduce both the presence of the high-temperature population of electrons near the last field lines intersecting the radial wall at the open source exit and the presence of cold electrons outside the last field lines. The presently proposed method will lead to easy access to the two- and/or three-dimensional diagnoses of the low-pressure plasma structures.
    Japanese Journal of Applied Physics 10/2014; 54(1S):01AB01. DOI:10.7567/JJAP.54.01AB01 · 1.06 Impact Factor
  • Koichi Takahashi, Akira Ando
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    ABSTRACT: Inductively coupled plasma source connected to a diffusion vacuum chamber is operated with an inverter-type radio frequency (RF) power supply in 10-Pa argon. Plasma structure in the glass source tube is preliminarily studied by acquisition of digital images. A RF antenna is powered with a frequency of ~100 kHz, which is adjusted in order to optimize impedance matching condition. Near the RF antenna, the plasma production over the glass source tube diameter is observed, whereas the smaller diameter plasma transport and the stationary plasma striation structure are seen between the RF antenna and diffusion chamber.
    IEEE Transactions on Plasma Science 10/2014; 42(10):2784-2785. DOI:10.1109/TPS.2014.2308893 · 0.95 Impact Factor
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    ABSTRACT: A high density argon plasma produced in a compact helicon source is transported by a convergent magnetic field to the central region of a substrate located downstream of the source. The magnetic field converging near the source exit is applied by a solenoid and further converged by installing a permanent magnet (PM) behind the substrate, which is located downstream of the source exit. Then a higher plasma density above 5 × 1012 cm−3 can be obtained in 0.2 Pa argon near the substrate, compared with the case without the PM. As no noticeable changes in the radially integrated density near the substrate and the power transfer efficiency are detected when testing the source with and without the PM, it can be deduced that the convergent field provided by the PM plays a role in constricting the plasma rather than in improving the plasma production. Furthermore it is applied to physical ion etching of silicon and aluminum substrates; then high etching rates of 6.5 µm min−1 and 8 µm min−1 are obtained, respectively.
    Journal of Physics D Applied Physics 09/2014; 47(42):425201. DOI:10.1088/0022-3727/47/42/425201 · 2.52 Impact Factor
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    ABSTRACT: Plasma cross-field diffusion in a magnetic nozzle is inhibited by increasing the magnetic field strength in a helicon plasma thruster attached to a pendulum thrust balance, while maintaining constant plasma density and electron temperature in the source tube, i.e. a constant plasma injection into the magnetic nozzle, where the field strength near the radio frequency (rf) antenna is less than 210 G and the operating argon pressure in the vacuum chamber is 0.8 mTorr. Inhibition of the cross-field diffusion yields a higher electron pressure in the magnetic nozzle and a resultant larger thrust. The thrust component arising from the magnetic nozzle approaches the theoretical limit derived from an ideal magnetic nozzle approximation where no plasma is lost from the nozzle and there is an azimuthal plasma current originating from the electron diamagnetic drift. It is also shown that the momentum of the plasma lost from the magnetic nozzle is captured by a physical nozzle attached at the source exit resulting in a larger thrust. Two physical nozzles of different sizes (nozzle 1 : 10.5 cm in length with a maximum diameter of 20 cm, nozzle 2 : 26 cm in length with a maximum diameter of 36 cm) are tested. The maximum thrust of 20 ± 1 mN is obtained for 25 sccm argon propellant and 2 kW rf power with a reflection power less than 5 W, which gives a specific impulse of 2750 ± 165 s and a thrust efficiency of 13.5 ± 1.5%.
    Plasma Sources Science and Technology 07/2014; 23(4):044004. DOI:10.1088/0963-0252/23/4/044004 · 3.06 Impact Factor
  • Proceedings of the 12th Asia Pacific Physics Conference (APPC12); 03/2014
  • Proceedings of the 12th Asia Pacific Physics Conference (APPC12); 03/2014
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    ABSTRACT: Uniform axial magnetic field of about 70 G is applied to a radiofrequency (rf) hydrogen ion source by arrays of permanent magnets. The plasma density and electron temperature downstream of the source and near the magnetic filter are compared with those in the previously described ion source, where the axial field has been applied by two solenoids. The source is operated at ∼350 kHz and above 10 kW rf power with a field-effect-transistor-based invertor power supply in 1.5 Pa hydrogen. The results show that the plasma density of ∼10(19) m(-3) near the source exit and ∼10(18) m(-3) near the magnetic filter can be obtained, which are higher than those with the solenoids.
    The Review of scientific instruments 02/2014; 85(2):02B124. DOI:10.1063/1.4849696 · 1.58 Impact Factor
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    ABSTRACT: Diameter of a permanent-magnets-expanded, radiofrequency (rf) plasma source is enlarged up to ∼13 cm for an application to a space propulsion device and tested with being attached to a diffusion chamber. The source is operated at 13.56 MHz 300 W rf power in low-pressure (40 mPa) argon. Measurement of ion energy distribution functions downstream of the source exit shows generation of a supersonic ion beam of about 20 eV. The detailed radial measurements demonstrate that the diameter and energy of the ion beam corresponds to the source tube diameter and the potential difference between the source and downstream plasmas, and that the radial profile of the beam flux is similar to the plasma density profile in the source cavity.
    The Review of scientific instruments 02/2014; 85(2):02C101. DOI:10.1063/1.4826542 · 1.58 Impact Factor
  • Transactions of the Japan Society for Aeronautical and Space Sciences, Aerospace Technology Japan 01/2014; 12(ists29):Pb_1-Pb_6. DOI:10.2322/tastj.12.Pb_1
  • Transactions of the Japan Society for Aeronautical and Space Sciences, Aerospace Technology Japan 01/2014; 12(ists29):Tb_5-Tb_9. DOI:10.2322/tastj.12.Tb_5
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    ABSTRACT: The performance of a permanent magnet helicon plasma thruster (PM-HPT) is improved by modifying the magnetic field configuration and increasing the magnetic field strength for operating source conditions of 13.56 MHz radiofrequency power up to 2 kW and 24 sccm of argon (pressure of ~0.8 mTorr). A convergent–divergent magnetic nozzle giving a maximum field strength of ~300 G is provided by arrays of permanent magnets, giving a higher plasma density downstream of the thruster exit (hence a larger Lorentz force within the magnetic nozzle) compared with that measured in the previous PM-HPT experiments (Takahashi et al 2011 Appl.Phys. Lett. 98 141503; Takahashi et al 2011 Phys. Rev. Lett. 107 235001). The directly measured thrust and specific impulse are about 15 mN and 2000 s, respectively, for a thrust efficiency of 7.5%.
    Journal of Physics D Applied Physics 08/2013; 46(35):352001. DOI:10.1088/0022-3727/46/35/352001 · 2.52 Impact Factor
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    ABSTRACT: A compact magnetically expanding plasma source having a strong magnetic field is developed, where a pulsed solenoid current is provided by an insulated gate bipolar transistor switching circuit. The field strength is successfully increased up to ~6 kG. The 13.56 MHz radio-frequency power with a pulse width of 28 ms and the operating argon pressure are maintained at 600 W and 3 mTorr, respectively. When the magnetic field strength is increased, the plasma density downstream of the source cavity increases due to the inhibition of cross-field diffusion and appears to saturate above ~3.5 kG, while maintaining a plasma density of 5 × 1012 cm−3 within the source cavity.
    Plasma Sources Science and Technology 08/2013; 22(5):055002. DOI:10.1088/0963-0252/22/5/055002 · 3.06 Impact Factor
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    ABSTRACT: This paper briefly summarizes a workshop held in Jyvaskyla the day after NIBS'12. The half-day workshop aimed at globally capturing the issue of performance variations in H- sources. There was a focus on production facilities and facilities that work under production-like conditions, because there are often high expectations to be met.
    02/2013; DOI:10.1063/1.4792832
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    ABSTRACT: Measurement of negative hydrogen ions (H-) is one of the important diagnostics for development of negative ion sources. Cavity-Ring-Down (CRD) measurement system was successfully implemented for the H- density measurements in the Field-Effect-Transistor based Radio Frequency ion source. Reliability of the CRD system was experimentally confirmed, and basic characteristics of the H- ions and the ion source were studied. The measured H- density varied from 1015 to 1016 m-3 depending on the source parameters, such as input RF power, axial magnetic field in the driver region, and source pressure. The H- density increased linearly with increase of the RF power and the axial magnetic field increased, due to improvement of power transmission and plasma production efficiency. Source gas pressure dependence of the Hdensity showed an increase of H- density with the decrease of pressure. Higher electron temperature and lower neutral gas resulted in increased production of vibrationally excited molecules and decreased destructions of H- ions, which was feasible for in high H- density at low source pressure.
    02/2013; DOI:10.1063/1.4792793
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    ABSTRACT: To evaluate retrospectively the long-term effects of initial trabeculotomy combined with sinusotomy performed inferiorly. Enrolled were 128 eyes of 100 patients who received initial glaucoma surgery. In 36 eyes, the removal of Schlemm's canal endothelium was also performed (removed group). The results were compared with the intact group In the primary open angle glaucoma (POAG), mean intraocular pressure (IOP) at 3 years after surgery was 14.6 (intact) and 15.4 mmHg (removed). Kaplan-Meier life-table analysis showed that qualified success rates for the intact group at 8 years were 62.2% and for the removed group at 5 years 45.2% defined by 20 mmHg or lower. The results in developmental glaucoma (DG) were similar to those in POAG. No statistical differences in postoperative IOP between the intact and removed groups were seen in either POAG or DG. In exfoliation glaucoma (XFG), mean IOPs for the intact group at 3 years were 17.3 mmHg and for the removed group at 2 years 15.4 mmHg. The success rates for the intact group at 3.5 years were 25.2% and for the removed group at 4.5 years 64.3%. The results in the intact group were worse than in the POAG patients. Although visual disturbance was seen in 13% of the patients, the major cause was the progression of the cataracts. The long-term results were the same as those of previous reports on surgery performed superiorly, including the frequency of visual disturbance. However the removal of Schlemm's canal endothelium is necessary in XFG for better IOP control.
    Nippon Ganka Gakkai zasshi 08/2012; 116(8):740-50.
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    ABSTRACT: Hydrogen negative ion density measurements are required to clarify the characteristics of negative ion production and ion source performance. Both of laser photodetachment and cavity ring down (CRD) measurements have been implemented to a field-effect-transistor based radio-frequency ion source. The density ratio of negative hydrogen ions to electrons was successfully measured by laser photodetachment and effect of magnetic filter field on negative ion density was confirmed. The calculated CRD signal showed that CRD mirrors with >99.990% reflectivity are required and loss of reflectivity due to cesium contamination should be minimized.
    The Review of scientific instruments 02/2012; 83(2):02A731. DOI:10.1063/1.3680549 · 1.58 Impact Factor
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    ABSTRACT: H(-) beam was successfully extracted from a cesium seeded ion source operated using a field effect transistor inverter power supply as a radio frequency (RF) wave source. High density hydrogen plasma more than 10(19) m(-3) was obtained using an external type antenna with RF frequency of lower than 0.5 MHz. The source was isolated by an isolation transformer and H(-) ion beam was extracted from a single aperture. Acceleration current and extraction current increased with the increase of extraction voltage. Addition of a small amount of cesium vapor into the source enhanced the currents.
    The Review of scientific instruments 02/2012; 83(2):02B122. DOI:10.1063/1.3684720 · 1.58 Impact Factor

Publication Stats

2k Citations
286.62 Total Impact Points

Institutions

  • 1995–2015
    • Tohoku University
      • • Department of Electrical Engineering
      • • Department of Electronic Engineering
      • • Graduate School of Engineering
      Miyagi, Japan
  • 2011
    • Niigata University
      • Graduate School of Science and Technology
      Niahi-niigata, Niigata, Japan
  • 1997–2011
    • Kansai Medical University
      • Department of Ophthalmology
      Moriguchi, Ōsaka, Japan
  • 1989–2011
    • National Institute for Fusion Science
      • Department of Helical Plasma Research
      Tokitsu-chō, Gifu, Japan
  • 1983–2011
    • Kyoto University
      • Department of Physics II
      Kyoto, Kyoto-fu, Japan
  • 2000–2002
    • Johns Hopkins University
      • Wilmer Eye Institute
      Baltimore, Maryland, United States
  • 1998–2001
    • Himeji Institute of Technology
      Himezi, Hyōgo, Japan
  • 1996
    • Budker Institute of Nuclear Physics
      Novo-Nikolaevsk, Novosibirsk, Russia
  • 1990–1993
    • Osaka University
      • Research Center for Nuclear Physics
      Suika, Ōsaka, Japan
    • Nagoya University
      • Department of Energy Engineering and Science
      Nagoya-shi, Aichi-ken, Japan
  • 1986
    • University of Tsukuba
      Tsukuba, Ibaraki, Japan