Search Results (15 total)

Transition radiation (TR) and diffraction radiation (DR) has widely been used for both electron beam diagnostics and generation of intense radiation beams in the millimeter and the submillimeter wavelength range. Recently, it was theoretically predicted that TR and DR properties change either at extremely high energies of electrons or at long radiation wavelengths. This phenomenon was called a prewave zone effect. We have performed the first observation and detailed investigation of the prewave zone effect in optical diffraction radiation at 1.28 GeV electron beam at the KEK-Accelerator Test Facility (KEK-ATF). The beam energy at KEK-ATF is definitely not the highest one achieved in the world. Since we could easily observe the effect, at higher energies it might cause serious problems. We developed and applied a method for prewave zone suppression valid for optical wavelengths. Furthermore, a method for prewave zone suppression applicable for longer radiation wavelengths is discussed.

We report on pressure-induced unconventional superconductivity (SC) in the heavy-fermion (HF) antiferromagnet CeIn₃ by means of nuclear-quadrupole-resonance (NQR) studies conducted under a high pressure. The temperature (T) and pressure (P) dependences of the In-NQR spectra have revealed a first-order quantum-phase transition (QPT) from antiferromagnetism (AFM) to paramagnetism (PM) at a critical pressure P_c=2.46  GPa at which AFM disappears with a minimum value of T_N(P_c)=1.2  K. High-energy x-ray scattering measurements under P show a progressive decrease in the lattice density without any change in the crystal structure, whereas an increase in the NQR frequency (ν_Q) indicates an increase in the hybridization between 4f electrons and conduction electrons, which stabilizes the HF-PM state. This competition between the AFM phase where T_N is reduced and the formation of the HF-PM phase triggers the first-order QPT at P_c=2.46  GPa. Despite the lack of an AFM quantum critical point in the P-T phase diagram, we highlight the fact that unconventional SC occurs in both phases of AFM and PM. The measurements of the nuclear spin-lattice relaxation rate 1∕T₁ in the AFM phase have provided evidence for the uniformly coexisting AFM+SC phase. Remarkably, the significant increase in 1∕T₁ upon cooling in the AFM phase has revealed the development of low-lying magnetic excitations down to T_c in the AFM phase; it is indeed relevant to the onset of the uniformly coexisting AFM+SC phase. In the HF-PM phase where AFM fluctuations are not developed, 1∕T₁ decreases without the coherence peak just below T_c, followed by a power-law-like T dependence that indicates an unconventional SC with a line-node gap. Remarkably, T_c has a peak around P_c in the HF-PM phase as well as in the AFM phase. In other words, an SC dome exists with a maximum value of T_c=230  mK around P_c, indicating that the origin of the pressure-induced HF SC in CeIn₃ is not relevant to AFM spin fluctuations but to the emergence of the first-order QPT in CeIn₃. These phenomena observed in CeIn₃ should be understood in terms of the first-order QPT because these new phases of matter are induced by applying P. When the AFM critical temperature is suppressed at the termination point of the first-order QPT, P_c=2.46  GPa, the diverging AFM spin-density fluctuations emerge at the critical point from AFM to PM. The results with CeIn₃ leading to a new type of quantum criticality deserve further theoretical investigations.

An experiment on the investigation of optical diffraction radiation (ODR) from a slit target as a possible tool for noninvasive electron beam-size diagnostics has been performed at the KEK accelerator test facility. The experimental setup has been installed at the diagnostics section of the extraction line. We have performed the first incoherent ODR observation from a slit target. The measured angular distributions are in reasonable agreement with the theoretical expectation. The beam-size effect onto the ODR angular pattern has been observed. Moreover, the sensitivity to the beam size as small as 14  μm has been achieved.

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.

Systematic neutron-scattering experiments have been carried out in order to reveal the crystal and magnetic structures as well as the magnetic excitation in the Ce-based intermetallic compound CeAgSb₂. It was clarified that the Ag atoms in CeAgSb₂ (space group: P4/nmm) occupy the 2b site, while Sb atoms occupy 2a and 2c sites, which have, to date, been controversial in literature. From the analysis of neutron powder-diffraction profile, we found the existence of the ferromagnetic component of about 0.41μ_B/Ce oriented along the c axis, consistent with the result of previous studies. On the other hand, however, no trace of the antiferromagnetic or spin-density wave peak was detected within our range of experimental accuracy. We observed a considerable spin-wave excitation below 3 meV. The dispersion relation was explained by the anisotropic Heisenberg model. This means that CeAgSb₂ has a simple ferromagnetic structure. Furthermore, crystalline electric-field (CEF) excitations were observed at 5.2 and 12.5 meV in the paramagnetic state, which is indicative of the localized nature of the Ce-4f electron. Our analysis revealed that |J_z=±1/2〉 is the ground state, while the first and second excited levels are mainly due to |±3/2〉 and |±5/2〉, respectively. The observed ferromagnetic moment is in good agreement with the ground-state saturation moment, g_Jμ_BJ_z∼0.43μ_B. The susceptibility, magnetization curve, and magnetostriction can be explained in terms of the CEF level scheme with anisotropic exchange interaction.

An experiment to investigate the diffraction radiation from a single edge target has been performed at the accelerator test facility of KEK with the aim of developing noninvasive beam diagnostics. The yield and the angular distribution of diffraction radiation as a function of the impact parameter was measured in the visible light region. The distributions were qualitatively consistent with the theoretical expectation. This work exhibits the first observation of the incoherent diffraction radiation in the visible light region.

We describe in this paper a measurement of vertical emittance in the Accelerator Test Facility (ATF) damping ring at KEK with a laser wire beam profile monitor. This monitor is based on the Compton scattering process of electrons with a laser light target which is produced by injecting a cw laser beam into a Fabry-Perot optical cavity. We installed the monitor at a straight section of the damping ring and measured the vertical emittance with three different ring conditions. In all cases, the ATF ring was operated at 1.28 GeV in a single bunch mode. When the ring was tuned for ultralow emittance, the vertical emittance of ε_y=(1.18±0.08)×10^-11   mrad was achieved. This shows that the ATF damping ring has realized its target value also vertically.

Evolution of the pressure-induced heavy fermion state and superconductivity in CeRhIn₅ have been established by high-pressure Fermi surface studies using dHvA technique. The effective mass of the conduction electrons named branch β₂ increases from 5.5 m₀ at ambient pressure to 20 m₀ at 1.6 GPa where superconductivity sets in. This heavy electron mass continues to increase steeply up to 2.1 GPa, 45 m₀ at 2.1 GPa, with a concomitant increase of the superconducting transition temperature. However, the underlying Fermi surface remains approximately same even at 2.1 GPa, strongly suggesting that antiferromagnetic fluctuations of Ce moments do not contribute to the volume of the Fermi surface, but enhance the cyclotron mass of the conduction electrons.

We report evidence for unconventional superconductivity in CeIn₃ at a pressure P=2.65 GPa above the critical pressure (P_c∼2.5 GPa) revealed by measurements of the nuclear-spin-lattice-relaxation time (T₁) and ac susceptibility (ac-χ). Both measurements of T₁ and ac-χ have pointed to a superconducting transition at T_c=95 mK, which is much lower than the onset temperature T_onset=0.15 K at zero resistance. The temperature dependence of 1/T₁ shows no coherence peak just below T_c, indicative of an unconventional nature for the superconductivity induced in CeIn₃.

Electron beams with the lowest, normalized transverse emittance recorded so far were produced and confirmed in single-bunch-mode operation of the Accelerator Test Facility at KEK. We established a tuning method of the damping ring which achieves a small vertical dispersion and small x-y orbit coupling. The vertical emittance was less than 1% of the horizontal emittance. At the zero-intensity limit, the vertical normalized emittance was less than 2.8×10^-8 rad m at beam energy 1.3 GeV. At high intensity, strong effects of intrabeam scattering were observed, which had been expected in view of the extremely high particle density due to the small transverse emittance.

We report the pressure (P)-temperature (T) phase diagram of antiferromagnetism and superconductivity in CeRhIn₅ and CeIn₃ revealed by the ¹¹⁵In nuclear-spin-lattice-relaxation (T₁) measurement. In the itinerant magnet CeRhIn₅, we found that the Néel temperature T_N is reduced at P>~1.23 GPa with an emergent pseudogap behavior. In CeIn₃, the localized magnetic character is robust against the application of pressure up to P∼1.9 GPa, beyond which the system evolves into an itinerant regime in which the resistive superconducting phase emerges. We discuss the relationship between the phase diagram and the magnetic fluctuations.

That the 4f electrons in the rare-earth antiferromagnetic compound CeRh₂Si₂ should be considered as localized electrons at cerium sites has been confirmed by comparing a de Haas–van Alphen experiment to the result of energy band calculations for LaRh₂Si₂. When pressure p is applied to the compound, the Néel temperature T_N1=36 K decreases and finally becomes zero at p_c≃1.0–1.1 GPa. The topology of the Fermi surface is found to be almost unchanged up to 1.0 GPa, but the cyclotron effective mass increases with increasing pressure. Above 1.1 GPa, the topology of the Fermi surface changes abruptly. A new Fermi surface is explained by the 4f-itinerant-band model. The corresponding cyclotron mass is large, being about 22m₀.

We describe the first measurement of an electron beam size in the accelerator test facility damping ring at KEK with a laser wire beam profile monitor. This monitor is based upon the Compton scattering process of electrons with a laser light target, which is produced by injecting a cw laser beam into a Fabry-Pérot optical cavity. We have observed clear signals of the Compton scattered photons and confirmed that the observed energy spectrum as well as the count rate agree with the expected ones. From the measurement, we have deduced the vertical beam size σ_b to be 9.8±1.1±0.4 μm, where the first (second) error represents statistical (systematic) uncertainty. Various improvements are in progress to enhance the signal-to-noise ratio, which is essential for the detailed study of the beam dynamics.

We have measured the ac magnetic susceptibility in various fields and thermal expansion of single crystals of the heavy fermion compound CeCu₆ at temperatures down to 250 μK. The susceptibility of CeCu₆ shows a peak at about 2 mK and has a large anisotropy. We also detected an anomaly of thermal expansion at the same temperature. The observed behaviors of the susceptibility and the thermal expansion in CeCu₆ indicate the occurrence of an antiferromagnetic order.