arXiv subjects

M. Hasegawa

Publications and source records attributed to M. Hasegawa.

89 records · Page 5Linked to original sources

arXiv2004

Ion acceleration during internal magnetic reconnection events in TST-2

Characteristics of ion acceleration in the internal magnetic reconnection events (IRE) have been studied by means of a neutral particle energy analyzer (NPA) in Tokyo Spherical Tokamak (TST-2). The major and minor radii are 0.38 m and 0.25m, respectively. The magnetic field strength is 0.3T and the maximum plasma current is up to 140 kA. The electron and ion temperatures are 0.4-0.5 keV and 0.1 keV, respectively and the electron density is ~1x1019 m-3. The NPA can be scanned toroidally from q = 74° (cw) to q = 114° (ccw), where q = 90° corresponds to the perpendicular sightline. The direction of the plasma current is cw. The NPA signals are digitized at every 50 ms. The NPA is calibrated in the energy range of 0.1 keV < E < 8.4 keV. When the IRE occurs, it is observed that the plasma current increases by ~ 20% and the loop voltage drops from 0.6 V to-5 V for ~ 0.1 ms. The enhanced charge exchange flux is observed by more than one order of magnitude at ~ 1 keV for this reconnection phase. The ion temperature increases by 80 eV at IREs. The angle q dependence of increment of Ti shows that DTi (q = 74°) is higher than that for q = 114°. This observation suggests that an ion is accelerated initially in the direction of magnetic field lines. The time evolution of the ion distribution function is simulated with a Fokker-Planck code taking into account the electric field effects.

physics.plasm-ph↗

arXiv2004

Estimation of Power Balance in Steady State LHCD Discharges on TRIAM-1M

On TRIAM-1M, a long duration discharge for more than 5 h was achieved successfully by fully non-inductive lower hybrid current drive. Heat load distribution to the plasma facing components (PFCs) in the 5 h discharge was investigated by using calorimetric measurements. The injected RF power was coincident with the total amount of heat load to PFCs estimated by calorimetric measurement. The power balance including the portion of direct loss power of fast electron, heat flux due to the charge exchange process was able to be estimated in this long duration discharge.

physics.plasm-ph↗

arXiv2004

Transport barrier formation by LHCD on TRIAM-1M

Internal transport barrier (ITB) has been obtained in full lower hybrid current driven (LHCD) plasmas on a superconducting tokamak, TRIMA-1M (R=0.84m, a x b=0.12mx0.18m, BT<8T). The formation of ITB depends on the current density profile, j(r), varied by the power deposition of the lower hybrid (LH). The plasma with ITB can be maintained by the LH power deposited around the foot point of ITB up to 25 sec, which corresponds to more than 100 times of current diffusion time, $τ$L/R. ITB is terminated by the reduction of current drive efficiency caused by metal impurities accumulation. In some condition, self-organized slow sawtooth oscillations (SSSO) of plasma current, density, temperature, and so on with the period comparable to the current diffusion time have been also observed during ITB discharge. The oscillation has the capability of particle exhaust, as the result, it may play an role in the avoidance of the impurity accumulation and the dilution in the future steady state fusion plasma with ITB, as the edge-localized mode in H-mode.

physics.plasm-ph↗

arXiv2004

Shape transition and oblate-prolate coexistence in N=Z fpg-shell nuclei

Nuclear shape transition and oblate-prolate coexistence in $N=Z$ nuclei are investigated within the configuration space ($2p_{3/2}$, $1f_{5/2}$, $2p_{1/2}$, and $1g_{9/2}$). We perform shell model calculations for $^{60}$Zn, $^{64}$Ge, and $^{68}$Se and constrained Hartree-Fock (CHF) calculations for $^{60}$Zn, $^{64}$Ge, $^{68}$Se, and $^{72}$Kr, employing an effective pairing plus quadrupole residual interaction with monopole interactions. The shell model calculations reproduce well the experimental energy levels of these nuclei. From the analysis of potential energy surface in the CHF calculations, we found shape transition from prolate to oblate deformation in these $N=Z$ nuclei and oblate-prolate coexistence at $^{68}$Se. The ground state of $^{68}$Se has oblate shape, while the shape of $^{60}$Zn and $^{64}$Ge are prolate. It is shown that the isovector matrix elements between $f_{5/2}$ and $p_{1/2}$ orbits cause the oblate deformation for $^{68}$Se, and four-particle four-hole ($4p-4h$) excitations are important for the oblate configuration.

nucl-th↗

arXiv2004

Particle alignments and shape change in $^{66}$Ge and $^{68}$Ge

The structure of the $N \approx Z$ nuclei $^{66}$Ge and $^{68}$Ge is studied by the shell model on a spherical basis. The calculations with an extended $P+QQ$ Hamiltonian in the configuration space ($2p_{3/2}$, $1f_{5/2}$, $2p_{1/2}$, $1g_{9/2}$) succeed in reproducing experimental energy levels, moments of inertia and $Q$ moments in Ge isotopes. Using the reliable wave functions, this paper investigates particle alignments and nuclear shapes in $^{66}$Ge and $^{68}$Ge. It is shown that structural changes in the four sequences of the positive- and negative-parity yrast states with even $J$ and odd $J$ are caused by various types of particle alignments in the $g_{9/2}$ orbit. The nuclear shape is investigated by calculating spectroscopic $Q$ moments of the first and second $2^+$ states, and moreover the triaxiality is examined by the constrained Hatree-Fock method. The changes of the first band crossing and the nuclear deformation depending on the neutron number are discussed.

nucl-th↗

arXiv2004

Proton-neutron alignment in the yrast states of $^{66}$Ge and $^{68}$Ge

The $^{66}$Ge and $^{68}$Ge nuclei are studied by means of the shell model with the extended $P+QQ$ Hamiltonian, which succeeds in reproducing experimentally observed energy levels, moments of inertia and other properties. The investigation using the reliable wave-functions predicts T=0, J=9 one-proton-one-neutron ($1p1n$) alignment in the $g_{9/2}$ orbit, at high spins ($14_1^+$, $16_1^+$ and $18_1^+$) in these $N \approx Z$ even-even nuclei. It is shown that a series of the even-$J$ positive-parity yrast states (observed up to $26_1^+$ for $^{68}$Ge) consists of the ground-state band and successive three bands with different types of particle alignments (two-neutron, $1p1n$, two-proton-two-neutron) in the $g_{9/2}$ orbit.

nucl-th↗

arXiv2004

Search for Dark Matter WIMPs using Upward Through-going Muons in Super-Kamiokande

We present the results of indirect searches for Weakly Interacting Massive Particles (WIMPs) with 1679.6 live days of data from the Super-Kamiokande detector using neutrino-induced upward through-going muons. The search is performed by looking for an excess of high energy muon neutrinos from WIMP annihilations in the Sun, the core of the Earth, and the Galactic Center, as compared to the number expected from the atmospheric neutrino background. No statistically significant excess was seen. We calculate flux limits in various angular cones around each of the above celestial objects. We obtain conservative model-independent upper limits on WIMP-nucleon cross-section as a function of WIMP mass and compare these results with the corresponding results from direct dark matter detection experiments.

hep-ex↗

arXiv2004

Pairing transition of nuclei at finite temperature

Pairing transition at finite temperature was investigated by the shell model and BCS calculations. The definitive signature of pairing transition is identified by a "transition temperature" $T_t$ estimated from a "thermal" odd-even mass difference, while there is no sharp phase transition because of the finiteness of nucleus. It is found that $T_t$ is in good agreement with predictions of critical temperature $T_c$ in the BCS approximation, and the pairing correlations almost vanish at two points of the transition temperature $T\approx 2T_{t}$. The BCS calculations show that the critical temperature $T_c$ increases with increasing deformation.

nucl-th↗

arXiv2003

Different particle alignments in N \approx Z Ru isotopes studied by the shell model

Experimentally observed heaviest $N \approx Z$ nuclei, Ru isotopes, are investigated by the shell model on a spherical basis with the extended $P+QQ$ Hamiltonian. The energy levels of all the Ru isotopes can be explained by the shell model with a single set of force parameters. The calculations indicate enhancement of quadrupole correlations in the $N=Z$ nucleus $^{88}$Ru as compared with the other Ru isotopes, but the observed moments of inertia seem to require much more enhancement of quadrupole correlations in $^{88}$Ru. It is discussed that the particle alignment takes place at $8^+$ in $^{90}$Ru but is delayed in $^{88}$Ru till $16^+$ where the simultaneous alignments of proton and neutron pairs take place. The calculations present interesting predictions for $^{89}$Ru that the ground state is the $1/2^-$ state and there are three $ΔJ=2$ bands with different particle alignments including the T=0 $p-n$ pair alignment.

nucl-th↗

arXiv2003

Antiferromagnetic Vortex Core of Tl_2Ba_2CuO_{6+x} Studied by Nuclear Magnetic Resonance

Spatially-resolved NMR is used to probe the magnetism in and around vortex cores of nearly optimally-doped Tl_2Ba_2CuO_{6+x} (Tc=85 K). The NMR relaxation rate 1/T_1 at Tl site provides a direct evidence that the AF spin correlation is significantly enhanced in the vortex core region. In the core region Cu spins show a local AF ordering with moments parallel to the layers at T_N=20K. Above T_N the core region is in the paramagnetic state which is a reminiscence of the state above the pseudogap temperature (T*~120 K), indicating that the pseudogap disappears within cores.

cond-mat.supr-con↗

arXiv2002

Antiferromagnetic ordering and disappearance of pseudogap within the vortex core of Tl_2Ba_2CuO_{6+δ}

Spatially-resolved NMR is used to probe the magnetism in and around the vortex core of nearly optimally-doped Tl_2Ba_2CuO_{6+δ} (T_c=85K). The NMR relaxation rate T_1^{-1} at ^{205}Tl site, at which antiferromagnetic (AF) fluctuation can be monitored sensitively, provides a direct evidence that the AF spin correlation is significantly enhanced in the vortex core region. In the core region Cu spins show a local AF ordering with moment ~ 0.1μ_B parallel to the layers at T_N=20K. Above T_N the core region is in the paramagnetic state which is a reminiscence of the state above the pseudogap temperature (T*=120K), indicating that the pseudogap disappears within the core.

cond-mat.supr-con↗

arXiv2002

Free flux flow resistivity in strongly overdoped high-T_c cuprate; purely viscous motion of the vortices in semiclassical d-wave superconductor

We report the free flux flow (FFF) resistivity associated with a purely viscous motion of the vortices in moderately clean d-wave superconductor Bi:2201 in the strongly overdoped regime (T_c=16K) for a wide range of the magnetic field in the vortex state. The FFF resistivity is obtained by measuring the microwave surface impedance at different microwave frequencies. It is found that the FFF resistivity is remarkably different from that of conventional s-wave superconductors. At low fields (H<0.2H_c2) the FFF resistivity increases linearly with H with a coefficient which is far larger than that found in conventional s-wave superconductors. At higher fields, the FFF resistivity increases in proportion to \sqrt H up to H_c2. Based on these results, the energy dissipation mechanism associated with the viscous vortex motion in "semiclassical" d-wave superconductors with gap nodes is discussed. Two possible scenarios are put forth for these field dependence; the enhancement of the quasiparticle relaxation rate and the reduction of the number of the quasiparticles participating the energy dissipation in d-wave vortex state.

cond-mat.supr-con↗

arXiv2000

Improvement of the extended P+QQ interaction by modifying the monopole field

The extended pairing plus $QQ$ interaction with the $J$-independent isoscalar proton-neutron force as an average monopole field, which has succeeded in describing collective yrast states of $N \approx Z$ even-$A$ nuclei, is improved. The improvement is accomplished by adding small monopole terms (relevant to spectroscopy) to the average monopole field (indispensable to the binding energy). This modification extends the applicability of the interaction to nuclei with $N \approx 28$ such as $^{48}$Ca, and moreover improves energy levels of noncollective states. The modified interaction successfully describes not only even-$A$ but also odd-$A$ nuclei in the $f_{7/2}$ shell region. Results of exact shell model calculations in the model space ($f_{7/2}$, $p_{3/2}$, $p_{1/2}$) (its usefulness has been demonstrated previously) are shown for $A$=47, 48, 49, 50 and 51 nuclei.

nucl-th↗

arXiv1999

Application of the extended P+QQ force model to $N \approx Z$ fp shell nuclei

To study collective motion, the extended pairing plus $QQ$ force model proposed recently is applied to $A$=46, 48 and 50 nuclei in the $fp$ shell region. Exact shell model calculations in the truncated model space $(f_{7/2},p_{3/2},p_{1/2})$ prove the usefulness of the interaction. The simple model with the pairing plus quadrupole pairing plus $QQ$ force and $J$-independent isoscalar proton-neutron force reproduces unexpectedly well observed binding energies, energy levels of collective (yrast) states and reduced $E2$ transition probabilities in $^{46}$Ti, $^{46}$V, $^{48}$V, $^{48}$Cr, $^{50}$Cr and $^{50}$Mn. The correspondence between theory and experiment is almost comparable to that attained by the full $fp$ shell model calculations with realistic effective interactions. Some predictions are made for energy levels and variations of $B(E2)$ in the yrast bands, in these nuclei. Characteristics of the interaction are investigated by comparing with the realistic effective interactions.

nucl-th↗

arXiv1999

Global features of proton-neutron interactions and symmetry energy

We study global features of proton-neutron (p-n) interactions and symmetry energy over a wide range of nuclei, using a schematic model interaction with four forces proposed recently. Calculations are performed by the BCS approximation in N,Z=20-50 and N,Z=50-82 regions. The experimental double differences of binding energies and symmetry energy are reproduced quite well. It is shown that the isoscalar p-n interactions with all J are indispensable for explaining the binding energies of not only $N\approx Z$ but also N>Z nuclei in the A=40-160 region.

nucl-th↗

arXiv1999

Roles of proton-neutron interactions in alpha-like four-nucleon correlations

An extended pairing plus QQ force model, which has been shown to successfully explain the nuclear binding energy and related quantities such as the symmetry energy, is applied to study the alpha-like four-nucleon correlations in 1f_{7/2} shell nuclei. The double difference of binding energies, which displays a characteristic behavior at $N \approx Z$, is interpreted in terms of the alpha-like correlations. Important roles of proton-neutron interactions forming the alpha-like correlated structure are discussed.

nucl-th↗

arXiv1999

Proton-neutron interactions in N \approx Z nuclei

Proton-neutron (p-n) interactions and its various aspects in $N\approx Z$ nuclei of $g_{9/2}$- and $f_{7/2}$ subshell are studied using a schematic model interaction with four force parameters proposed recently. It is shown that the model interaction well reproduces observed physical quantities: the double differences of binding energies, symmetry energy, Wigner energy, odd-even mass difference and separation energy, which testifies the reliability of the model interaction and its p-n interactions. First of all, the double differences of binding energies are used for probing the p-n interactions. The analysis reveals different contributions of the isoscalar and isovector p-n pairing interactions to two types of double difference of binding energies, and also indicates the importance of a unique form of isoscalar p-n pairing force with all $J$ components. Next, it is shown that this p-n force is closely related to the symmetry energy and the Wigner energy. Other calculations demonstrate significant roles of p-n interactions in the odd-even mass difference and in the separation energy at $N=Z$.

nucl-th↗