Recent progress in Large Helical Device experiments

Akio Komori,T. Shimozuma,T. Ido,T. Kobuchi,T. Seki,T. Ozaki,T. Fujita,T. Watari,T. Akiyama,T. Tokuzawa,T. Uda,T. Minami,Y. Nakamura,Y. Torii,Y. Sakamoto,Y. Takeiri,Y. Nagayama,Y. Oka,Y. Narushima,Y. Y 한국물리학회 2006 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.49 No.III

In the Large Helical Device (LHD), some reactor-oriented experiments, i.e. high beta, high ion temperature, steady state operation, have produced remarkable progress in recent experimental campaigns. By optimizing the rotational transform, an average beta value of 4.3 %, which is the highest on record for helical devices, was achieved. The ICRF sustained steady-state discharges for more than 30 minutes, these were also successfully performed with the aid of the magnetic axis swing technique for the reduction of the heat load to the plasma-facing component. In the discharge, the total input energy to the plasma reached 1.3 GJ, which also established a new record.1

Development of Learn-by-Doing Programs on Engineering Career Education Conducted with Collaboration from Private Company Engineers

Oka, T.,Tanabe, Y.,Harada, S.,Abe, K.,Ishii, N.,Maruyama, T.,Sato, T.,Imaizumi, H.,Nishimura, S.,Sengoku, M.,Ohkaw, H. Korean Society for Engineering Education 2010 공학교육연구 Vol.13 No.2

We have constructed three characteristic internship programs which were based on the different aspects of view by the grant in aid from MEXT of Japanese government. The most outstanding program is what we call "Market Internship", where the students directly approach the market in which the engineering technologies are utilized. Then they must find out the problems and advantages of engineering technologies from the view of consumers. The experience would result in the enhancement of motivation to study the engineering technologies. It would be substantially effective in engineering education curriculums that the engineers of private companies in which university students will get their own jobs in near future could take important roles to conduct them. We believe that this activity would effective in reducing gaps they would feel between before and after getting jobs in practical society, and lowering the ratio of unemployment after getting jobs.

Development of Learn-by-Doing Programs on Engineering Career Education Conducted with Collaboration from Private Company Engineers

T. Oka,Y. Tanabe,S. Harada,K. Abe,N. Ishii,T. Maruyama,T. Sato,H. Imaizumi,S. Nishimura,M. Sengoku,H. Ohkaw 한국공학교육학회 2010 공학교육연구 Vol.13 No.2

We have constructed three characteristic internship programs which were based on the different aspects of view by the grant in aid from MEXT of Japanese government. The most outstanding program is what we call “Market Internship”, where the students directly approach the market in which the engineering technologies are utilized. Then they must find out the problems and advantages of engineering technologies from the view of consumers. The experience would result in the enhancement of motivation to study the engineering technologies. It would be substantially effective in engineering education curriculums that the engineers of private companies in which university students will get their own jobs in near future could take important roles to conduct them. We believe that this activity would effective in reducing gaps they would feel between before and after getting jobs in practical society, and lowering the ratio of unemployment after getting jobs

Subchannel analysis of supercritical light water-cooled fast reactor assembly

Yoo, J.,Oka, Y.,Ishiwatari, Y.,Yang, J.,Liu, J. North-Holland Pub. Co 2007 Nuclear engineering and design Vol.237 No.10

Subchannel analyses have been carried out for supercritical water-cooled fast reactor fuel assembly. Peak cladding surface temperature difference arising from subchannel heterogeneities have been calculated by using the improved subchannel analysis code STARS and was evaluated to be about 18.5<SUP>o</SUP>C. Several suggestions have been also made for reducing the PCST difference arising from channel heterogeneity. Influences of local power peaking on deflection of cladding surface temperature are explained with pin power distribution taken from core depletion calculation in this paper. Maximum cladding surface temperature at nominal condition is evaluated to be 645.3<SUP>o</SUP>C over the cycle. Statistical thermal design uncertainty associated with PCST calculation is evaluated by Monte-Carlo sampling technique combined with subchannel analysis code. Maximum statistical design uncertainty of PCST is calculated to be 31<SUP>o</SUP>C and is in a good agreement with that from RTDP method. Influence of downward flow in seed region on system sensitivity is investigated by improved Monte-Carlo thermal design procedure. Limiting thermal condition of MCST is 681<SUP>o</SUP>C (650<SUP>o</SUP>C of nominal+31<SUP>o</SUP>C) within 95/95 limit for SWFR.

Progress of positive-ion-based NBI system in LHD

O. Kaneko,Y. Takeiri,M. Osakabe,K. Nagaoka,K. Tsumori,K. Ikeda,Y. Oka,E. Asano,S. Komada,T. Kondo,M. Sato,M. Shibuya 한국진공학회 2007 한국진공학회 학술발표회초록집 Vol.16 No.2

Illumination Condition and Mask Bias for 0.18 mm Pattern with KrF Lithography

H. Tabuchi,Y. Shichijo,I. Okabe,N. Oka,M. Inoue,K. Iguchi 대한전자공학회 1998 International Conference on Electronics, Informati Vol.- No.-

Electron-Spin Relaxation Induced by Mn Spins and Eects of LO-Phonon Scattering in CdMnTe Quantum Wells

K. Saito,I. Souma,Y. Oka,A. Murayama 한국물리학회 2008 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.53 No.1

We study electron-spin relaxation induced by Mn spins and eects of LO-phonon scattering in CdMnTe quantum wells, where Mn contents of 3.6 and 5.0 at% are examined. By using pump-probe absorption a under magnetic eld, the time constant of the electron-spin relaxation e is deduced as a function of the magnetic eld. Values of e under magnetic elds are ve times dierent between both the Mn contents, while those at 0 T re ect the Mn content. The latter fact for the spin-relaxation times at 0 T shows that the electron-spin relaxation is induced by Mn spins via the s-d exchange interaction. However, the former signicant dierence in e suggests that the number of eective Mn spins involved in the electron-energy relaxation is greatly dierent in the spin-relaxation between the spin-splitting electron levels under dierent magnetic elds. The eects of LO-phonon scattering on the electron-spin relaxation are observed when the spin-splitting is close to the one-LO-phonon energy, indicating an eective path of the electron-spin relaxation involving both the LO phonon and the Mn spins. We study electron-spin relaxation induced by Mn spins and eects of LO-phonon scattering in CdMnTe quantum wells, where Mn contents of 3.6 and 5.0 at% are examined. By using pump-probe absorption a under magnetic eld, the time constant of the electron-spin relaxation e is deduced as a function of the magnetic eld. Values of e under magnetic elds are ve times dierent between both the Mn contents, while those at 0 T re ect the Mn content. The latter fact for the spin-relaxation times at 0 T shows that the electron-spin relaxation is induced by Mn spins via the s-d exchange interaction. However, the former signicant dierence in e suggests that the number of eective Mn spins involved in the electron-energy relaxation is greatly dierent in the spin-relaxation between the spin-splitting electron levels under dierent magnetic elds. The eects of LO-phonon scattering on the electron-spin relaxation are observed when the spin-splitting is close to the one-LO-phonon energy, indicating an eective path of the electron-spin relaxation involving both the LO phonon and the Mn spins.

Transfer Dynamics of Spin-Polarized Excitons in ZnCdMnSe/ZnCdSe Double Quantum Wells

J. H. Park,I. Souma,Y. Oka,A. Murayama,D. Dagnelund,I. A. Buyanova,W. M. Chen 한국물리학회 2008 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.53 No.1

We study the transfer dynamics of spin-polarized excitons in diluted magnetic semiconductor (DMS) double quantum wells (QWs) of ZnCdMnSe/ZnCdSe, where the spin-polarized excitons tunnel from the non-magnetic ZnCdSe to DMS-ZnCdMnSe wells and subsequently relax to the ground state in the DMS well with phonon emissions. The exciton-transfer time decreases when the exciton-energy dierence between both the wells is equal to the one-LO-phonon energy, indicating the LO-phonon-assisted acceleration of the spin transfer. Moreover, the exciton transfer alters into the separate transfer of the electron and the heavy hole when the electron potential dierence becomes larger than the LO-phonon energy. This observation demonstrates the importance of energy-relaxation dynamics in the spatial transport of spins in semiconductor quantum structures. We study the transfer dynamics of spin-polarized excitons in diluted magnetic semiconductor (DMS) double quantum wells (QWs) of ZnCdMnSe/ZnCdSe, where the spin-polarized excitons tunnel from the non-magnetic ZnCdSe to DMS-ZnCdMnSe wells and subsequently relax to the ground state in the DMS well with phonon emissions. The exciton-transfer time decreases when the exciton-energy dierence between both the wells is equal to the one-LO-phonon energy, indicating the LO-phonon-assisted acceleration of the spin transfer. Moreover, the exciton transfer alters into the separate transfer of the electron and the heavy hole when the electron potential dierence becomes larger than the LO-phonon energy. This observation demonstrates the importance of energy-relaxation dynamics in the spatial transport of spins in semiconductor quantum structures.

Spin-Injection Dynamics and Eects of Spin Relaxation in Self-Assembled Quantum Dots of CdSe

T. Furuta,K. Hyomi,I. Souma,Y. Oka,A. Murayama,D. Dagnelund,I. A. Buyanova,W. M. Chen 한국물리학회 2008 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.53 No.1

We study the spin injection dynamics and the eects of spin relaxation in self-assembled quantum dots (QDs) of CdSe coupled with a diluted magnetic semiconductor (DMS) layer of ZnMnSe, where spin-polarized excitons can be injected from the DMS into the QDs because of the potential dierence. The degree of circular polarization, P, of excitonic photoluminescence (PL) at 5 T in the coupled QDs shows a rapid increase with increasing delay time after a linearly polarized pulse excitation, indicating the spin-injection dynamics. The P value tends to decay gradually because of the exciton-spin relaxation in the QDs after the spin injection. The spin-polarized excitons in the QD ensemble migrate simultaneously from QDs with higher exciton energies to those with lower exciton energies. This inter-dot transfer of excitons also aects the P value in the lower energy region of the QD-emission band because the excitons lose their spin polarizations due to the spin relaxation in the dots during the migration. The detailed analysis for the exciton-spin transfer is presented in a coupled QD system after spin injection. We study the spin injection dynamics and the eects of spin relaxation in self-assembled quantum dots (QDs) of CdSe coupled with a diluted magnetic semiconductor (DMS) layer of ZnMnSe, where spin-polarized excitons can be injected from the DMS into the QDs because of the potential dierence. The degree of circular polarization, P, of excitonic photoluminescence (PL) at 5 T in the coupled QDs shows a rapid increase with increasing delay time after a linearly polarized pulse excitation, indicating the spin-injection dynamics. The P value tends to decay gradually because of the exciton-spin relaxation in the QDs after the spin injection. The spin-polarized excitons in the QD ensemble migrate simultaneously from QDs with higher exciton energies to those with lower exciton energies. This inter-dot transfer of excitons also aects the P value in the lower energy region of the QD-emission band because the excitons lose their spin polarizations due to the spin relaxation in the dots during the migration. The detailed analysis for the exciton-spin transfer is presented in a coupled QD system after spin injection.

Characteristics of Beam Attenuation in the High Temperature Plasma with High-Z Discharge in LHD

katsunori Ikeda,K. Tsumori,K. Nagaoka,M. Osakabe,O. Kaneko,S. Morita,Y. Oka,Y. Takeiri 한국물리학회 2006 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.49 No.III

Behavior in attenuation of a high-energy neutral beam has been investigated by visible spectroscopy in the Large Helical Device (LHD). The measurement system consists of optical fibers, a spectrometer and an ICCD detector. A spectrum of beam emission is clearly observed by the Doppler effect. In order to increase the ion-heating power, we have adopted a high-Z discharge with argon gas. An ion temperature of 13.5 keV ± 2 keV is achieved by the injection of a neutral beam power of 10.5 MW. We have observed the behavior of the neutral beam emission in the high-temperature plasma. The intensity of the beam emission in the argon discharge is lower than that in the hydrogen discharge when the ion temperature increases. After the maximum ion temperature, the intensity of the beam emission increases as the electron density increases. This behavior is similar to the hydrogen discharge. It is indicated that the particle configuration of the plasma has changed. This result will contribute to the monitoring of the neutral beam absorption in fusion plasma.