Search Results (29 total)

A system for inspecting the inner surface of superconducting rf cavities is developed in order to study the relation between the achievable field gradient and the defects in the inner surface. The inspection system consists of a high resolution complementary metal-oxide-semiconductor camera and a special illumination system built in a cylinder that has a diameter of 50 mm. The camera cylinder can be inserted into the L-band 9 cell superconducting cavity. The system provides a resolution of about 7.5  μm/pixel. Thus far, there have been good correlations between locations identified by thermometry measurements and positions of defects found by this system. The heights or depths of the defects can also be estimated by measuring wall gradients using the reflection angle relation between the camera position and the strip illumination position. This paper presents a detailed description of the system and the data obtained from it.

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.

We describe the improvements of a Fresnel zone plate (FZP) beam-profile monitor, which is being developed at the KEK-ATF damping ring to measure ultralow emittance electron-beam profiles. This monitor, which is designed to have a submicrometer spatial resolution, is based on x-ray imaging optics composed of two FZPs. Various improvements were performed to the old setup. First, a new crystal monochromator was introduced to suppress the beam image drift. Second, the two FZP folders were improved in order to realize a precise beam-based alignment during x-ray imaging. Third, a fast mechanical shutter was installed to achieve a shorter time resolution, and an x-ray mask system was also installed to obstruct direct synchrotron radiation through the FZPs. These improvements could make beam-profile measurements more precise and reliable. The beam profiles with less than 50   μm horizontal beam size and less than 6   μm vertical beam size could be measured within a 1 ms time duration. Furthermore, measurements of the damping time and the coupling dependence of the ATF damping ring were successfully carried out with this upgraded FZP monitor.

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.

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.

The use of TE₁₂ in circular waveguide with smooth walls was suggested for low-loss transport of rf signals in multimoded systems [S. G. Tantawi et al., in Advanced Accelerator Concepts: Eighth Workshop, edited by Wes Lawson, AIP Conf. Proc. No. 472 (AIP, New York, 1999), pp. 967–974]. Such systems use the same waveguide to transport different signals over different modes. In this report we detail a series of experiments designed to measure the characteristics of this mode. We also describe the different techniques used to generate it and receive it. The experiments were done at X band around a frequency of 11.424 GHz, the frequency of choice for future linear colliders at X band [The NLC Design Group, Report No. LBNL-PUB-5424, SLAC Report No. 474, Report No. UCRL-ID 124161, 1996; The JLC Design Group, KEK-REPORT-97-1, 1997]. The transportation medium is 55 m of highly overmoded circular waveguide. The design of the joining flanges is also presented.

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.

First experimental results from the final focus test beam (FFTB) are reported. The vertical dimension of a 47-GeV electron beam from the SLAC linac has been reduced at the focal point of the FFTB by a demagnification of 320 to a beam height of approximately 70 nm.

The deep-inelastic scattering reaction ν_μN→μ^-X has been studied using the deuterium-filled 15-foot bubble chamber at Fermilab. The data have been analyzed under the assumption of isospin invariance to extract x(u_V-d_V) for the proton, where xu_V(x) and xd_V(x) are the valence up- and down-quark momentum distributions, respectively. The results are compared with other data and with different theoretical fits. The ratio νn/νp as a function of x is also presented.

High-statistics data from two exposures of the BNL 7-foot deuterium bubble chamber to a wide-band-neutrino beam with an average energy of 1.6 GeV were analyzed for exclusive pion production in charged-current neutrino-deuterium interactions. For the single-pion production reactions, the properties of Δ⁺⁺ production and decay as well as the final results of an isospin analysis are presented. New results for single-pion and multipion production are also presented.

We compare cross sections derived from deep-inelastic neutrino scattering on neon and deuterium targets. Systematic effects are minimized by imposing identical analysis criteria on two experiments in the same detector with similar incident neutrino spectra, and by using variables independent of the beam energy estimate. Our results favor a previously observed dependence of nucleon structure functions on the target nucleus (the European Muon Collaboration effect), but do not require the existence of this phenomenon.

We have measured the cross-section ratio R=σ(νpπ^-) / σ(μ^-pπ⁺)=0.11±0.023, 0.13±0.023, 0.15±0.025 for M_pπ<1.4, 1.6, 2.0 GeV, respectively, in the energy range 0.4<E_ν<3.0 GeV. The data are consistent with various weak one-pion production models incorporating the Weinberg-Salam structure of neutral currents.

Three charmed-baryon candidates produced in the quasielastic reaction νn→μ^-Λ_c⁺→μ^-Λπ⁺ were found in an exposure of the Fermilab deuterium-filled 15-ft bubble chamber to a wide-band neutrino beam. We also found that the cross section of the associated-production reaction νn→μ^-ΛK⁺ is (5.5±1.3±1.1) × 10^-40 cm² for E_ν>10 GeV, and is approximately independent of energy. In addition we studied the cross sections of some quasi-two-body associated strange-particle production reactions other than μ^-ΛK⁺.

We have studied the quasielastic reaction ν_μn→μ^-p in an exposure of the Fermilab deuterium-filled 15-foot bubble chamber to a high-energy wide-band neutrino beam. From an analysis of the Q² distribution based on the standard V-A theory, the axial-vector mass in a dipole parametrization of the axial-vector form factor is determined to be M_A=1.05_-0.16^+0.12 GeV, consistent with the values previously reported from low-energy experiments.

We apply a new method to determine the probability of the uu diquark from ν p interactions fragmenting as independent quarks. In particular, by assuming that all Λ's which do not come from Y^*+, N^*+, or charmed baryons must come from independent fragmentation, we show this probability to be about 50%. We also provide information from ν n interactions on the fragmentation of ud diquarks into Λ's, and from this determine the probability of capturing a strange quark from the sea in the target region to be τ_s=0.051±0.013, about half the value found in the current region, γ_s=0.12±0.02. Overall Λ production rates in ν n and ν p reactions are (7.0 ± 0.8)% and (7.0 ± 1.2)%, respectively, when cuts of W>4 GeV and x_B>0.2 are made on the data.

Charged-hadron multiplicity distributions in νn and νp interactions are measured in an exposure of the Fermilab deuterium-filled 15-foot bubble chamber to a wide-band neutrino beam. Multiplicity moments are studied as functions of the invariant mass of the hadronic system W in the range 1<W<15 GeV. The mean multiplicities for both νn and νp are found to increase logarithmically with W² with identical slopes but slightly different intercepts. A linear rise of the dispersion D with the mean multiplicity 〈n_ch〉 is observed with an approximate slope of 0.35. This value is similar to that found in e⁺e^- and p̅ p annihilation and is much less than the asymptotic prediction of QCD. A close similarity between D_- vs 〈n_-〉 in νn and νp interactions is seen. Negative correlations between the multiplicity distributions in the forward and backward hadronic center-of-mass hemispheres are found and discussed.

Single-Λ production with no other strange particles in the final state is observed in charged-current neutrino interactions in the deuterium filled 15-ft bubble chamber at Fermilab. This phenomenon is interpreted as a signal for inclusive production of charmed baryons followed by Λ_c⁺→Λ+X decays. The product of total Λ_c⁺ cross section by branching ratio for this process is found to be (2.0±0.74±0.65)% of the total charged-current neutrino cross section.

From 12000 charged-current ν_μ D events obtained in an exposure of the Fermilab 15-ft bubble chamber to a high-energy wide-band neutrino beam, the absolute neutrino flux is determined using the reaction ν_μn→μ^-p. For the total charged-current cross section, σ_t=kE_ν, k=(0.68±0.04±0.10)×10^-38 cm²/GeV is obtained for E_ν between 10 and 200 GeV. No clear energy dependence of the slope parameter k is observed.