Search Results (22 total)

We have developed a high-resolution cavity-beam position monitor (BPM) to be used at the focal point of the ATF2, which is a test beam line that is now being built to demonstrate stable orbit control at ∼nanometer resolution. The design of the cavity structure was optimized for the Accelerator Test Facility (ATF) beam in various ways. For example, the cavity has a rectangular shape in order to isolate two dipole modes in orthogonal directions, and a relatively thin gap that is less sensitive to trajectory inclination. A two stage homodyne mixer with highly sensitive electronics and phase-sensitive detection was also developed. Two BPM blocks, each containing two cavity BPMs, were installed in the existing ATF beam line using a rigid support frame. After testing the basic characteristics, we measured the resolution using three BPMs. The system demonstrated 8.7 nm position resolution over a dynamic range of 5  μm.

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.

Electron storage rings of GeV energy with laser pulse stacking cavities are promising intense sources of polarized hard photons which, via pair production, can be used to generate polarized positron beams. In this paper, the dynamics of electron bunches circulating in a storage ring and interacting with high-power laser pulses is studied both analytically and by simulation. Both the common features and the differences in the behavior of bunches interacting with an extremely high power laser pulse and with a moderate pulse are discussed. Also considerations on particular lattice designs for Compton gamma rings are presented.

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.

A free relativistic electron in an electromagnetic field is a pure case of a light-matter interaction. In the laboratory environment, this interaction can be realized by colliding laser pulses with electron beams produced from particle accelerators. The process of single photon absorption and reemission by the electron, so-called linear Thomson scattering, results in radiation that is Doppler shifted into the x-ray and γ-ray regions. At elevated laser intensity, nonlinear effects should come into play when the transverse motion of the electrons induced by the laser beam is relativistic. In the present experiment, we achieved this condition and characterized the second harmonic of Thomson x-ray scattering using the counterpropagation of a 60 MeV electron beam and a subterawatt CO₂ laser beam.

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 have developed a polarimetry of ultrashort pulse γ rays based on the fact that γ rays penetrating in the forward direction through a magnetized iron carry information on the helicity of the original γ rays. Polarized, short-pulse γ rays of (1.1±0.2)×10⁶/bunch with a time duration of 31 ps and a maximum energy of 55.9 MeV were produced via Compton scattering of a circularly polarized laser beam of 532 nm off an electron beam of 1.28 GeV. The first demonstration of asymmetry measurements of short-pulse γ rays was conducted using longitudinally magnetized iron of 15 cm length. It is found that the γ-ray intensity is in good agreement with the simulated value of 1.0×10⁶. Varying the degree of laser polarization, the asymmetry for 100% laser polarization was derived to be (1.29±0.12)%, which is also consistent with the expected value of 1.3%.

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.

Based on the requirements from a conceptual design of a polarized positron beam for future linear colliders, we constructed a special collision system with a short focal length of 150 mm of the laser beams so as to produce γ rays through inverse Compton scattering. In order to achieve efficient laser-electron collisions, we created a special optics to produce very small e^--beam sizes of σ_ex0=7.6   μm and σ_ey0=5.4   μm in the horizontal and vertical directions at the collision point. Using laser light with a wavelength of 532 nm and an e^- beam of 1.28 GeV, provided from the ATF-damping ring at KEK, we generated 2×10⁵ γ rays with a time duration of 26 ps in rms, leading to a peak brightness of 1.7×10¹⁸/(mrad² mm² 0.1%bandwidth s) near to the maximum energy of 56 MeV.

We describe our studies of the generation of plasma wake fields by a relativistic electron bunch and of phasing between the longitudinal and transverse fields in the wake. The leading edge of the electron bunch excites a high-amplitude plasma wake inside the overdense plasma column, and the acceleration and focusing wake fields are probed by the bunch tail. By monitoring the dependence of the acceleration upon the plasma’s density, we approached the beam-matching condition and achieved an energy gain of 0.6 MeV over the 17 mm plasma length, corresponding to an average acceleration gradient of 35  MeV/m. Wake-induced modulation in energy and angular divergence of the electron bunch are mapped within a wide range of plasma density. We confirm a theoretical prediction about the phase offset between the accelerating and focusing components of plasma wake.

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.

We derive a simple relation for estimating the relative emittance growth in x and y due to intrabeam scattering (IBS) in electron storage rings. We show that IBS calculations for the Accelerator Test Facility (ATF) damping ring, when using the formalism of Bjorken-Mtingwa, a modified formalism of Piwinski (where η²/β has been replaced by H), or a simple high-energy approximate formula all give results that agree well. Comparing theory, including the effect of potential well bunch lengthening, with a complete set of ATF steady-state beam size versus current measurements we find reasonably good agreement for energy spread and horizontal emittance. The measured vertical emittance, however, is larger than theory in both offset (zero current emittance) and slope (emittance change with current). Almost all the offset error can be accounted for by considering the expected projected vertical emittance due to machine errors rather than the real emittance. This result is consistent with the assumed Coulomb log factor being close to the correct one. The slope error indicates measurement error and/or additional current-dependent physics at the ATF.

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 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.

Both temperature and field dependences of specific heat C and magnetocaloric effect (MCE) were measured on a HoGa₂ single crystal in fields up to 8 T at low temperatures down to 0.1 K. The magnetic fields were applied in the [100] direction, for which intermediate magnetic phases appear. Significant Ho nuclear contribution to C is found to dominate below 1 K and its field-independent character reveals that the value of Ho magnetic moments does not change visibly in the measured field range irrespective of the magnetic phases. From anomalies observed in the C and MCE data, the reported field-vs-temperature phase diagram is corroborated thermodynamically. One of the phase transitions occurring at T_t=6.6 K in zero field is found to be of first-order transition. Magnetic entropy determined by the MCE measurements shows an enhancement in the intermediate phases and small humps at the phase boundaries. Based on a crystalline-electric-field model, the nuclear specific heat, magnetic entropy of the 4f electrons, and anisotropy in the magnetization can be explained consistently. Possible origins of the reported giant magnetoresistance caused by the appearance of the intermediate phases will be discussed.

7.6×10⁶ x-ray photons per 3.5 ps pulse are detected within a 1.8–2.3 Å spectral window during a proof-of-principle laser synchrotron source experiment. A 600 MW CO₂ laser interacted in a head-on collision with a 60 MeV, 140 A, 3.5 ps electron beam. Both beams were focused to a σ  =  32 μm spot. Our next plan is to demonstrate 10¹⁰ x-ray photons per pulse using a CO₂ laser of ∼1 TW peak power.

A two-photon correlator has been constructed and successfully operated in measuring the true two-photon correlation of synchrotron radiation in the soft-x-ray region. The strong influence of the nonstationary features of the optical field, which is the so-called accidental correlation induced by the systematic bunched structure of the stored current in a storage ring, has been removed by a coherence time modulation technique. The explicit bunching effect of the measured two-photon correlation shows that synchrotron radiation is a chaotic rather than coherent radiation, indicating a chaotic nature of the stored current. This experimental method provides a way to measure the instantaneous emittance with the time scale of the coherence time τ_c, and to characterize the coherence property of incomplete free-electron lasers, such as self-amplified spontaneous emission.

The KEK Accelerator Test Facility (KEK-ATF) was constructed to develop technologies for producing a low-emittance beam which will be required by future linear colliders. The KEK-ATF consists of an injector linac, a damping ring, and a beam extraction line. The basic optical structure of the damping ring is a FOBO lattice, which reduces the horizontal dispersion at the center of the bending magnets and, as a consequence, can produce an extremely small emittance beam. To verify the performance of such a unique, low-emittance lattice, it is crucial to measure the horizontal emittance. The horizontal emittance was measured using wire scanners in the beam extraction line. Since the horizontal beam position was not stable, we established a method to correct the measured beam size for position fluctuation (“jitter”) and we succeeded in the observation of the so far smallest horizontal emittance in any accelerator. The measured horizontal emittance was 1.37±0.03nm at a beam energy of 1.285 GeV and a bunch population of \(3–5\)×10⁹, in agreement with the design value of 1.27–1.34 nm at the beam energy and the bunch population.

We have investigated the Hall effect in Y_1-xU_xPd₃ and CeNi₂Ge₂, both of which are reported to exhibit so-called non-Fermi-liquid behavior. The Hall coefficient R_H of Y_1-xU_xPd₃ first increases with decreasing temperature and tends to saturate at lower temperatures for small x, as expected from the ordinary skew scattering contribution. With increasing x, an increase of R_H at low temperatures becomes evident, and finally for x=0.3 it shows a faint peak near 2 K reflecting a spin-glass transition. At a critical concentration x∼0.2, it varies linearly as -log(T) below ∼40 K. The anomalous R_H in CeNi₂Ge₂ is found to be negative and anisotropic, in contrast with that in CeRu₂Si₂. At low temperatures, we found a breakdown of the scaling relation between R_H and the resistivity (ρ), i.e., R_H∼ρ², reported to be fulfilled in several heavy fermion compounds including CeRu₂Si₂. We found that R_H∝ln(ρ) is fulfilled over the wider temperature range of 0.5–15 K.

An experiment to search for the production of neutral penetrating particles decaying into electron-positron pairs was performed with a 2.5-GeV electron beam. A total of 0.027 C was injected into a tungsten target. No such particle is found. Constraints on coupling constants α_e and α_γ are given.