Tohru Kawamura

Tokyo Institute of Technology, Edo, Tōkyō, Japan

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Publications (49)64.1 Total impact

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    ABSTRACT: A compact pulsed power device was proposed and constructed for plasma interaction experiments relevant to astrophysical phenomena using a configuration of counter-streaming high-speed plasma flows. A controllable, high-speed argon plasma flow with a drift velocity of ∼ 20km/s, an ion density of 1013cm-3, an electron temperature of ∼ 1eV, and a pulse length of a few microseconds was obtained using tapered z-pinch discharge. Plasma interaction experiments were carried out at high-acoustic-Mach number regime (Mi∼ 10) in which ion-ion collision was negligible. An enhancement of line emission and a decrease of probe current were observed in the interaction region of counter-streaming plasmas. These results indicate that the ion flow decelerated even without momentum transfer due to ion-ion collisions.
    IEEJ Transactions on Fundamentals and Materials 01/2015; 135(6):366-372. DOI:10.1541/ieejfms.135.366
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    ABSTRACT: We investigated the interaction of a laser ablation plasma with a longitudinal magnetic field aiming to create a directionally moving plasma for high-flux and low-emittance ion injectors. To study the plasma dynamics, time-of-flight measurements and energy analysis of the plasma ion flux were made as functions of the laser intensity and the magnetic field. Moderate magnetic field (˜0.2T) directed the fast and highly charged ions in the target normal. In addition, a slow peak appeared and increased with increasing the magnetic field. These results indicated that directional electric field is formed and recombination increases by the application of longitudinal magnetic field.
    Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 01/2014; DOI:10.1016/j.nima.2013.05.088 · 1.32 Impact Factor
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    ABSTRACT: A plasma focus system composed of a pair of counter-facing coaxial guns was proposed as a long-pulse and/or repetitive high energy density plasma source. We applied Li as the source of plasma for improvement of the conversion efficiency, the spectral purity, and the repetition capability. For operation of the system with ideal counter-facing plasma focus mode, we changed the system from simple coaxial geometry to a multi-channel configuration. We applied a laser trigger to make synchronous multi-channel discharges with low jitter. The results indicated that the configuration is promising to make a high energy density plasma with high spectral efficiency.
    The European Physical Journal Conferences 11/2013; 59:18006-. DOI:10.1051/epjconf/20135918006
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    ABSTRACT: Satellite lines with a high-lying spectator electron give unresolved spectral feature with respect to a parent resonance line. In our previous study [T. Ozawa et al., Phys. Plasmas 19, 063302 (2012)], population inversion between 3l and 2l of hydrogenlike nitrogen in a recombining phase was quantitatively estimated and the dielectronic-capture ladderlike processes, which implicitly include such doubly excited states as nln′l′ (n = 2−4,n′≫n), were found to be indispensable in the estimation of a gain. In this study, the population densities of the doubly excited states are explicitly calculated by a time-dependent collisional-radiative model with desired plasma conditions for the lasing of nitrogen Balmer-α, which is a parent resonance line of unresolved satellite lines 3ln′l′→2ln′l′. The stimulated emission of the unresolved satellite lines caused by Balmer-α is found to reduce the population inversion between 3l and 2l, leading to essential saturation of the amplification of Balmer-α laser light.
    Physics of Plasmas 08/2013; 20(8). DOI:10.1063/1.4819022 · 2.25 Impact Factor
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    ABSTRACT: Focused are the properties of population inversion with nitrogen plasma between n = 3 and 2 of a hydrogen-like state in a recombining phase. The population inversion is calculated in the framework of population kinetics, and the gain coefficient G is estimated. The maximum plasma length L is approximated by a product of the duration of population inversion {tau}{sub e} and the speed of light c. In the calculation, dielectronic-capture ladderlike (DL) processes are considered. At the plasma ion densities of 10{sup 17-18} cm{sup -3} with a rapid cooling, the DL deexcitations make a large contribution to the population inversion, resulting in GL=5-10, and the prescription of plasma cooling is found to be definitely critical.
    Physics of Plasmas 06/2012; 19(6). DOI:10.1063/1.4729172 · 2.25 Impact Factor
  • Tohru Kawamura, Kazuhiko Horioka, Fumihiro Koike
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    ABSTRACT: Kα radiation from low charge chlorine is examined for cold dense plasma diagnostics. A relativistic atomic structure calculation shows that the transition energies of Kα lines are slightly shifted to a higher-energy side according to the degree of M-shell ionization. Total spectral shift from Cl+ Kα lines up to those with a fully stripped M-shell is about 10 eV. With an assumption that collisional-radiative-equilibrium is valid, spectral calculations were carried out for a C2H3Cl-plasma heated by an ion beam, and clear deformation among spectral line-shapes is found in the range of the electron temperatures of ≤~ 30 eV. Contribution to the composite spectra of Kα lines with an excited electron in the outer-shell is also briefly discussed. Cl-Kα lines with M-shell electrons can be expected to give us distinct understandings for energy deposition by an ion beam in cold dense plasma.
    Laser and Particle Beams 02/2011; 29(01):135 - 140. DOI:10.1017/S0263034611000036 · 1.70 Impact Factor
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    ABSTRACT: Formation Mechanism of Dense High-Speed Plasma Flow in Tapered Capillary Discharge
    Plasma and Fusion Research 01/2011; 6. DOI:10.1585/pfr.6.1201019
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    ABSTRACT: Conductivities of dense aluminum, copper, and tungsten are evaluated using exploding wire discharges in water. Evolutions of the radius and the electrical resistance of exploding wire are measured together with direct pyrometric estimation of the temperature. The conductivities are evaluated based on the measurements and their density dependence is compared with theoretical predictions at a fixed temperature. The results indicate that regardless of materials, the conductivity has a minimum around 3% of solid density at temperature of 5000 K.
    Physics of Plasmas 08/2010; 17(8):084501-084501-4. DOI:10.1063/1.3475430 · 2.25 Impact Factor
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    ABSTRACT: Strong shock waves play a crucial role in many astrophysical phenomena. Ion-electron relaxation process and radiation affect the structure of strong shock waves. Since the non-linear nature of the relaxation process makes the plasma behavior extremely complicated, it requires well-defined shock wave formation to estimate the structure. We investigate electro-magnetically driven shock in laboratory experiments. The pulse power device with tapered electrodes can generate a quasi steady and 1-D shock [1], which allows for analysis of ion-electron relaxation and radiation processes. We will show results of electron temperature measurement by a line pair method and radiative cooling, which restricts the increase of electron temperature. [4pt] [1] K. Kondo, M. Nakajima, T. Kawamura and K. Horioka, Rev. Sci. Instr. 77, 036104 (2006).
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    ABSTRACT: A quasi-statically tamped target for warm dense matter (WDM) experiments using all-ion accelerator (AIA) is presented. We compare the target state and homogeneity based on the KEK AIA with and without injector. For making WDM based on KEK AIA (without injector), we evaluated achievable state of aluminum target as a function of beam radius and target density using a two-dimensional cylindrical hydrodynamics code including heavy ion beam deposition profile. Results show that the beam radius should be less than 0.2mm to make a WDM state. To evaluate uniformity of KEK AIA (with injector) driven targets, we compared the behaviors of a mono-layer and a tamped cylinder target. Results show that both targets can reach a warm dense state. In particular, the tamped target can achieve a well-defined warm dense state with long time scale. The comparison of the target behaviors of the KEK AIA with and without injector indicated that the estimated homogeneity of the target depends on the formulation of sound velocity of target material at beam irradiation time.
    Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 07/2009; 606(1):161-164. DOI:10.1016/j.nima.2009.03.091 · 1.32 Impact Factor
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    ABSTRACT: The Breit-Pauli R-matrix method is used to calculate integral cross sections between magnetic sublevels for He-like Cu ions by electron impact for x-ray polarization spectroscopy of ultrahigh-intensity laser-produced plasma. The integral cross sections for inelastic scattering that produce aligned states are calculated. And, the integral cross sections for elastic scattering that destroy the alignment of excited states are also calculated for transitions between magnetic sublevels. The data is useful to analyze the anisotropic plasmas using the anisotropic collisional-radiative atomic-kinetics model.
    Journal of Physics Conference Series 06/2008; 112(2):022104. DOI:10.1088/1742-6596/112/2/022104
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    ABSTRACT: Polarized Heα of chlorine from ultra-intense laser produced plasmas was calculated. The experiment with 800 nm laser pulses of 100 — 130 mJ in 130 fs was also carried out, and the resultant polarization of time- and space-integrated Heα is so high (≥ 10%). A new time-dependent atomic kinetics code was developed to gain insight into the generation of polarized Heα by fast electron transport relevant to fast ignition. The polarized feature is due to anisotropic electron impacts by fast electrons, and the aspect ratio of fast electron temperatures associated with longitudinal and transverse directions is found to be essential with the numerical calculation. In the calculation, so small polarization (≤ 2 ~ 3%) is predicted in dense plasma region (≥ 100 times the critical density), which is due to frequent elastic atomic transitions between magnetic atomic sublevels, while high polarization (≥ 10%) is observable in low density region (≤ 10 times the critical density). The collimated fast electrons are generated by electromagnetic instability, resulting in large anisotropy along the propagation direction in low density region.
    Journal of Physics Conference Series 06/2008; 112(2):022097. DOI:10.1088/1742-6596/112/2/022097
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    ABSTRACT: A new approach for warm dense matter (WDM) experiments using intense heavy ion beams is presented. To improve uniformity of the ion beam driven targets, we compared behaviors of a mono-layer and a tamped cylinder target. For the estimation of target behaviors, a two-dimensional cylindrical hydrodynamics code including ion beam deposition profile was developed. Results show that both targets can reach a warm dense state. In particular, the tamped target can achieve a well defined warm dense state with long time scale.
    Journal of Physics Conference Series 06/2008; 112(4):042027. DOI:10.1088/1742-6596/112/4/042027
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    ABSTRACT: A high rep-rate, compact and low-debris xenon Z-pinch discharge system has been designed and fabricated as an EUV light source, in which a newly developed gas jet-type Z-pinch source is used. The discharge head has a coaxial double nozzle and diffuser. Xenon Z-pinch plasma that emits EUV light is produced in between the inner nozzle and the corresponding diffuser. An annular shell of a helium gas curtain produced by the outer nozzle is specially designed for shielding the debris and suppressing the inner gas expansion. In this work, in order to get higher EUV output power, a new pulse power supply system has been developed. This power supply delivers a current with amplitude of 22 kA, rise time of 110 ns and pulse duration of 260 ns to a low inductance load. In addition, a laser triggered discharge produced tin plasma light source has been developed. Experimental parameters such as electrode separation and laser irradiation power are varied to optimize EUV emission power. It is found that EUV emissions cannot be obtained when the laser irradiation power is too high.
    Journal of Physics Conference Series 06/2008; 112(4):042063. DOI:10.1088/1742-6596/112/4/042063
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    ABSTRACT: A method of measuring equation of state and electrical conductivity is proposed relevant to conditions in warm dense matter (WDM). The WDM was produced by exploding wire discharges in water. The wire/plasma parameters were measured spectroscopically and also by a shadow graph method. The density and the temperature were estimated to be typically ~ 0.01 ρsolid and ~ 15 kK, respectively, at 2 μsec from beginning of the discharges. We have compared the behaviors of the plasma boundary and an accompanied shock wave in water to a numerical estimation based on a model of equation of state (EOS). We indicate that the comparative study of the experimental and the numerical results, is a possible approach for a 'semi-empirical' scaling of EOS.
    Journal of Physics Conference Series 06/2008; 112(4):042026. DOI:10.1088/1742-6596/112/4/042026
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    ABSTRACT: Study on x-ray polarization spectroscopy was performed for intense-laser-produced plasma under laser-oblique incidence. The laser pulse was focused onto a polyvinylchloride target at angles of 67 degrees and 7 degrees to the target normal at average intensities of 1017-18 W/cm2. There were differences in the spectra for shifted Kα lines of C1 atoms between the oblique and nearly normal incidence, indicating the low laser-energy absorption in the oblique incidence. A difference in polarization degrees of C1 Heα lines was also observed, and this means the polarization of incident laser pulse affect velocity distribution functions of fast electrons in intense-laser-produced plasma.
    Journal of Physics Conference Series 06/2008; 112(2):022101. DOI:10.1088/1742-6596/112/2/022101
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    ABSTRACT: A study on electro-magnetically driven shock in laboratory experiments is reported. The shock waves are driven by a compact pulse device at 40 km/s into a high-Z gas with 1 mug/cm3. The pulse power device with tapered electrodes can generate a quasi steady and one-dimensional shock. Ion-electron and ionization relaxation processes, based on a steady and 1-Dimensional shock condition, are calculated. Comparison of the experimental result to calculation one indicates that the shocks produced by the pulse power device have potential to observe ion-electron and ionization relaxation layer with an appropriate spatial scale.
    Journal of Physics Conference Series 05/2008; 112(4). DOI:10.1088/1742-6596/112/4/042028
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    ABSTRACT: The timing, intensity, and pulse width of lasing signals in fast capillary discharge plasma at a strong 2p53p-2p53s transition of Ne-like Ar are experimentally investigated. To clarify the lasing mechanism, the results are compared with those obtained by numerical calculation using a one-dimensional magneto-hydrodynamics (MHD) simulation code. The comparison indicates that the lasing occurs when a cylindrically imploding shock wave arrives at the axis of the capillary and it depends not only on the electron density and temperature but also strongly on the effects of time-varying refraction and the opacity of the pinching plasma.
    Japanese Journal of Applied Physics 04/2008; 47:2250-2258. DOI:10.1143/JJAP.47.2250 · 1.06 Impact Factor
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    ABSTRACT: Evolutions of copper conductivity in a warm dense state are evaluated using pulsed-power discharges in water. In this system, the wire/plasma evolves with coaxial symmetry from the beginning of discharge until observing time of a few micro seconds. Results show that the wire/plasma behaviors could be controlled by choosing initial wire diameter and/or discharge voltage in a broad density–temperature regime. The experimental results are compared to those obtained using conventional theoretical models. Difference between the experimental observations and the theoretical values increases in a low-temperature region (≤2 eV).
    Progress in Nuclear Energy 03/2008; DOI:10.1016/j.pnucene.2007.11.048 · 0.88 Impact Factor
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    ABSTRACT: An atomic kinetics code is developed to gain insight into the generation of polarized Healpha by fast electron transport relevant to fast ignition. The calculation predicts a very small polarization in the dense region (>or=100 times the critical density) due to frequent elastic transitions between magnetic sublevels, while high polarization is observable in the low density region (<or=10 times the critical density). It is inferred that fast electrons are collimated due to electromagnetic instability, resulting in the generation of anisotropic fast electrons along the propagation axis in the low density region.
    Physical Review Letters 09/2007; 99(11):115003. · 7.73 Impact Factor