Search Results (16 total)

We demonstrate that the phase response curve (PRC) can be reconstructed using a weighted spike-triggered average of an injected fluctuating input. The key idea is to choose the weight to be proportional to the magnitude of the fluctuation of the oscillatory period. Particularly, when a neuron exhibits random switching behavior between two bursting modes, two corresponding PRCs can be simultaneously reconstructed, even from the data of a single trial. This method offers an efficient alternative to the experimental investigation of oscillatory systems, without the need for detailed modeling.

The 3-GeV rapid cycling synchrotron (RCS) of the Japan Proton Accelerator Research Complex (J-PARC) was commissioned in October 2007, and successfully accomplished 3 GeV acceleration on October 31. Six run cycles through February 2008 were dedicated to commissioning the RCS, for which the initial machine parameter tuning and various underlying beam studies were completed. Then since May 2008 the RCS beam has been delivered to the downstream facilities for their beam commissioning. In this paper we describe beam tuning and study results following our beam commissioning scenario and a beam performance and operational experience obtained in the first commissioning phase through June 2008.

In the rapid cycling synchrotron (RCS) of the Japan Proton Accelerator Research Complex (J-PARC), the longitudinal painting is important to alleviate the space-charge effects. It is known that the momentum offset injection and applying the second harmonic rf voltage improves the bunching factor, which is defined as the ratio of average and peak current. Our simulation studies show that the large-amplitude second harmonic, 80% to the fundamental, is optimum, and the second harmonic phase sweep improves the bunching factor at the beginning of the injection period. We performed the beam tests of longitudinal painting in the J-PARC RCS. We proved that the longitudinal painting with the 80% second harmonic, the momentum offset of -0.2%, and the second harmonic phase sweep improved bunching factors significantly.

In view of the recent controversies on the above room-temperature ferromagnetism in pure ZnO and the transition-metal doped ZnO, the present paper aims to shed some light on the origin of ferromagnetism by investigating the detailed atomic structure of (Co, Ga)-codoped ZnO experimentally as well as theoretically. Above room-temperature ferromagnetism in nonmagnetic Co-doped ZnO, nanoparticle powders prepared by sol-gel technique were obtained by codoping with Ga. It is found that Co ions substitute Zn sites while Ga ions were located at octahedral interstitial sites together with one O vacancy. Electrons from the Ga 4p^↑ states transform to the unfilled Co 3d^↓ states. The strong hybridization between the charge carriers in the Co 3d and Ga 4p states and electronic polarization for surrounding O ions at Co ions are detected. Finally, the Ga-4p electrons merged with conduction band and polarized O ions act as medium for an indirect exchange between the Co ions, which could be the origin of ferromagnetism in the (Co, Ga)-codoped ZnO.

Temporal coherence of a continuous-wave photonic crystal nanocavity laser is investigated in detail using interference experiments at room temperature. The nanocavity laser operates at 1.3 μm with InAs quantum dot gain material and has a very high spontaneous emission coupling factor β=0.9 with a threshold absorbed pump power of ∼375 nW. The coherence around the laser threshold of such a high-β laser is not obvious because spontaneous emission efficiently couples to the lasing mode. The output power dependence of the coherence length shows linearity at and above threshold. This result indicates that the first-order coherence is not greatly reduced even at threshold, where the cumulative power of spontaneous emission of other wavelengths cannot be negligible when compared with the laser power. This can be attributed to the fact that the lasing mode of a high-β laser has a relatively large portion of the total power compared to lasers with lower β’s due to efficient coupling of the spontaneous emission to the cavity mode.

We have demonstrated for the first time the production of highly polarized short-pulse positrons with a finite energy spread in accordance with a new scheme that consists of two-quantum processes, such as inverse Compton scattering and electron-positron pair creation. Using a circularly polarized laser beam of 532 nm scattered off a high-quality, 1.28 GeV electron beam, we have obtained polarized positrons with an intensity of 2×10⁴ e⁺/bunch. The magnitude of positron polarization has been determined to be 73±15(stat)±19(syst)% by means of a newly designed positron polarimeter.

The Rapid Cycling Synchrotron (RCS) of the J-PARC complex in Tokai, Japan, is designed to accelerate a high intensity proton beam from 181 MeV, and later 400 MeV to 3 GeV in 20 ms within the 40 ms machine cycle. The beam power up to 1 MW demands a stable beam control to avoid excessive losses and activation of the accelerator chain. The fully digital control system is based on quadrature modulation and demodulation. In the amplitude control loops standard FIR filters separate the harmonics (h=2) and (h=4) after down conversion. For the phase loops at (h=2) and (h=4), intended to damp synchrotron oscillations, the delay in a FIR filter would limit the loop stability. Cascaded integrator comb filters, also called CIC filters, provide a shorter delay because they filter the longitudinal beam signal only where it is necessary. The notches are located at multiples of the revolution frequency of the proton beam. For fixed frequency accelerator applications, digital comb filters with fixed clock frequency are widely used to improve loop stability. For variable frequency accelerator applications, as in a proton synchrotron, where the frequency swing is larger than the notch width, usually the clock frequency of the comb filter is variable and chosen to be an integer multiple of the particle revolution frequency. At J-PARC RCS, the clock frequency has to be fixed. Tracking the frequency would require a variable noninteger number of filter taps. Here we present a filter, based on the weighted output of 2 CIC filters with variable length, and one tap difference. The filter function looks like a CIC with smoothly varying coefficients, where the notches follow the revolution frequency of the proton beam. The delay of this filter is approximately half of the corresponding FIR filter, so that the phase loops have a higher stability margin.

For high luminosity in electron-positron linear colliders, it is essential to generate low vertical emittance beams. We report on the smallest vertical emittance achieved in single-bunch-mode operation of the Accelerator Test Facility, which satisfies the requirement of the x-band linear collider. The emittances were measured with a laser-wire beam-profile monitor installed in the damping ring. The bunch length and the momentum spread of the beam were also recorded under the same conditions. The smallest vertical rms emittance measured at low intensity is 4 pm at a beam energy of 1.3 GeV, which corresponds to the normalized emittance of 1.0×1.0^-8   m. It increases by a factor of 1.5 for a bunch intensity of 10¹⁰ electrons. The measured data agreed to the calculation of intrabeam scattering within much better than a factor of 2.

We present the measurement results of electron beam emittance in the Accelerator Test Facility damping ring operated in multibunch modes. The measurements were carried out with an upgraded laser wire beam profile monitor. The monitor has now a vertical wire as well as a horizontal one and is able to make much faster measurements thanks to an increased effective laser power inside the cavity. The measured emittance shows no large bunch-to-bunch dependence in either the horizontal or vertical directions. The values of the vertical emittance are similar to those obtained in the single-bunch operation. The present results are an important step toward the realization of a high-energy linear collider.

Little is known theoretically about the associative memory capabilities of neural networks in which information is encoded not only in the mean firing rate but also in the timing of firings. Particularly, in the case of sparsely coded patterns, it is biologically important to consider the timings of firings and to study how such consideration influences storage capacities and quality of recalled patterns. For this purpose, we propose a simple extended model of oscillator neural networks to allow for expression of a nonfiring state. Analyzing both equilibrium states and dynamical properties in recalling processes, we find that the system possesses good associative memory.

The homoepitaxial island growth on hexagonally reconstructed Au(100) is studied using molecular dynamics based on a well-tested many-atom interatomic potential. Our study reveals that the stable islands of rectangular shape are hexagonally reconstructed in conformity with the patterns of the reconstructed Au(100) surface and suggests the “magic” stable width for the reconstructed islands in agreement with experimental observations. Furthermore, our results on the adatom diffusion indicate that the experimentally observed strong anisotropic effect is attributed to the long-range exchange diffusion.

A general formula for the optical-luminescence yield spectra arising from the core excitation is derived, making use of a simple phenomenological model. In this model, it is assumed that various excitations from core electron levels give rise to an identical luminescence in the individual yields. A smaller yield of the inner-core electron excitation than that of the outer-core electron excitation results in a variety of anomalous yield spectra such as those with a negative jump, a positive jump with an inverted oscillation part, and an overtone oscillation. Effective thickness is introduced to explain the optical-luminescence yield spectrum of an opaque powdered specimen. It is pointed out, within the framework of the present model, that for a certain physical condition of the specimens, the yield spectra are deformed and its oscillatory part lacks the precise structural information. The theory is examined by applying it to the yield spectra observed in CaF₂, where the original spectra taken from the differently prepared forms—the powdered layers in different thicknesses, a pressed pellet, and a single crystal—at 30 and 90 K are presented. A switching of the edge jump is observed in the single crystal and the pressed pellet with increasing temperature. Experimental results are consistently explained by the proposed model and the observed yield spectra are well reproduced from the absorption spectrum.

The absolute electrofission cross section of ²⁰⁸Pb was measured in the energy range 35–250 MeV, and the photofission cross section was deduced by means of the virtual-photon technique. A structure observed in the (γ,f) curve, around 200 MeV, was interpreted on the basis of recent calculations for the thermalization process which were performed in the framework of the intranuclear cascade model.

Recently, Karwowski, Duch, and Valdemoro have developed a method for constructing a matrix representation of a spin-adapted reduced Hamiltonian based on evaluation of traces of occupation number operators over finite spaces of many electrons being coupled to a definite spin S. We show that the spectral-distribution method, used in nuclear physics, leads to a unified and general reduction formula for the fixed-S traces.

The D(e, e^′p) coincidence cross section was measured at angles out of the plane, at an excitation energy of approximately 18 MeV and a momentum transfer of 0.33 fm^-1. This is the first reported measurement of the longitudinal-transverse and transverse-transverse interference terms of the electrodisintegration cross section of the deuteron. The longitudinal plus transverse cross section is also reported. The results are in good agreement with a theoretical calculation carried out with use of the Paris potential.

Differential cross sections and analyzing powers of the ¹³C(p,t)¹¹C and ¹³C(p,³He)¹¹B reactions to the ground and the first excited states were measured at E_p=65 MeV. They were analyzed by means of distorted-wave Born approximation calculations including both one- and two-step processes of p-d-t or -³He. Attention was paid to an isospin symmetry of the reaction process consisting of both isospin 0 and 1 channels, which were important in a comparable order. As intermediate states of ¹²C in the sequential process, five states of either isospin 0 or 1, which were strongly excited by the ¹³C(p,d)¹²C reaction, were taken into account. A good fit between the data and the theoretical values was obtained, especially for the (p,t) reactions, concerning both differential cross sections and analyzing powers. The calculations including one-step and sequential processes induced, in some cases, remarkable differences from the one-step calculation for backward angles.

NUCLEAR REACTIONS ¹³C(p,t)¹¹C, ¹³C(p,³He)¹¹B, E=65 MeV; measured σ(θ) and analyzing powers for 4 transitions. One-step and sequential processes DWBA analysis.