89614487
High energy electron acceleration by stochastic acceleration mechanism
A stochastic acceleration mechanism which explains the high energy electron acceleration by intense laser pulses is analyzed by Fokker‐Planck approach. The Fokker‐Planck equation describing is derived from the equation of motion of electrons interacting with filamented laser field. The time evolution of electron distribution function, energy distribution, effective temperature are obtained for initially rest plasma. The acceleration of 10 MeV electron beam is examined and shown to be effective.
138845175
Surface Study on Niobium Stain after Electro-polishing for Super-conducting RF Cavity
In development of superconducting radio-frequency niobium cavities, there are problems in low performances of electro-polished (EP) cavities with a fresh EP solution due to stains on the surfaces with discoloration. Although the stain problems have been known from the past researches, the detailed study with surface analysis has not been carried out. In this study, the stains on the niobium surfaces were observed with x-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy and scanning electron microscope. According to results of XPS, there are some differences in atomic components at the stained and non-stained surfaces, ex, a little amount of fluorine and no metal oxide were found only at the stained surface. In this article, we will describe the detail of the XPS results.
8862999
Recent progress of KEKB
We summarize the machine operation of KEKB during past one year focusing on progress for this period.
135949224
Residual gas analysis and electron cloud measurement of DLC and TiN coated chambers at KEKB LER
For future high-intensity positron or proton accelerators, beam instability caused by electron cloud is one of the most important problems. Some coatings on inner surface of beam chambers with materials having low secondary emission yields such as titanium nitride (TiN), nonevaporable getter and so on have represented good effects against the electron cloud instability. In this study, diamond like carbon (DLC) and TiN coated chambers, and a copper chamber without coating were installed to an arc section of KEKB LER to make comparisons of total pressure, residual gas components and electron cloud activity during the beam operation under the same condition. Residual gas observation for the DLC coating revealed much higher hydrogen gas desorption because a process gas including hydrogen was used for the film growth. No remarkable hydrocarbon gas desorption was found. On the other hand, a mass peak of amu = 14, that is N was prominent in the TiN coating. The electron cloud activity in the DLC coating was lower than the TiN coating and the copper chamber.
120600649
Field emission from carbon nanotubes in DC and pulsed mode
Abstract Multi-wall Carbon Nanotube (CNT) emitters were tested in a combined diode-RF electron gun. Field emission of the nanotubes was observed at 5–30 MV/m, using a 250 ns FWHM long pulse with a peak voltage of 80–470 kV. The field emission threshold is compatible with that found from previous DC testing. We have extracted from a continuous field emitter up to a nanoCoulomb of charge and measured an emittance of 4 mm mrad with a 2 pC electron beam. The total charge emission during RF operation, using the 1.5 GHz, 2 cell RF structure, was found dependent on its period. RF operation showed that back bombarding electrons with up to 5 MeV did not impair the emission stability of the CNTs.
96731395
Graphitization of inner surface of copper beam duct of KEKB positron ring
In order to evaluate the best technique to reduce electron cloud instability, we measured the electron cloud density in copper ducts coated with titanium nitride and non-evaporable getter used in the KEK B-factory (KEKB) positron ring. In this study, we developed a new system for the graphitization of copper beam ducts. The graphitization of a copper surface by intense electron bombardment resulted in the reduction in the secondary electron yield (SEY) to 1.0 in a laboratory experiment and to 1.1 in a real beam duct. The electron cloud density in the graphitized duct is 30% lower than that in an uncoated copper duct. Thus, it has been confirmed that the graphitization of a beam duct leads to a reduction in the electron cloud density in the duct.
120413457
Coherent bremsstrahlung from the internal target of the 1.3 GeV electron synchrotron
Abstract A Si single crystal has been mounted on the conventional internal target holder of the 1.3 GeV electron synchrotron at the Institute for Nuclear Study, Tokyo. The energy spectrum measured with the pair spectrometer has a clear enhancement due to the coherent bremsstrahlung process. The linear polarization of the photon beam thus obtained is estimated to be 27% and 10% at the photon energy of 200 and 540 MeV, respectively.
119407416
Proton compton scattering at backward angles in the energy range from 400 MeV to 1050 MeV
Abstract Differential cross sections of proton Compton scattering have been measured in the energy range between 400 MeV and 1050 MeV at C.M.S. angles of 150° and 160°. The recoil proton was detected with a magnetic spectrometer using multi-wire proportional chambers and multi-wire spark chambers. The scattered photon was detected with a lead glass Cerenkov counter of the total absorptive type with two layers of multi-wire proportional chambers which measured the horizontal and vertical positions of the scattered photon. The proton Compton scattering was differentiated from the background of the π 0 photoproduction events with the use of angular correlations between the recoil proton and the scattered photon. The cross-section data show the clear second resonance peak at E γ = 750 MeV and a bump around E γ = 925 MeV at both angles. Also the angular distributions show the backward peak in the energy range between 700 MeV and 950 MeV. The data are compared with theoretical calculations based on an isobar model with some phases for the resonances. In this calculation, the proton Compton data were fitted varying the photocouplings of the nucleon resonances around the bump and the phases below the third resonances as free parameters. The calculation reproduced the angular distribution of differential cross sections well. However, it was inevitable that we use large photocouplings of P 33 (1600) in order to fit the bump.
523124
Development of a 56 mm aperture superconducting dipole model magnet for LHC
A 56 mm single aperture superconducting dipole model magnet, which is one of the frameworks of the LHC collaboration between KEK and CERN, has been developed. A central field of 8.65 T is provided at a load line ratio of 90% with a 5 block coil wound with NbTi superconducting cables. The magnet has a straight section of 0.8 m and a total length of 1.2 m. The distinctive features of the magnet are a magnetically symmetrical configuration being applicable to the twin aperture magnet and mechanical consideration sharing the magnetic forces with a high manganese steel collar and a horizontally split iron yoke. The magnet showed almost no training at 4.4 K. The first quench current at 1.8 K was 91% of the short sample I/sub c/ and the magnet reached a central magnetic field of 8.8 T. Finally it could be excited with the current of more than 99% of the short sample limit which corresponds to a central field of 9.63 T.
121185850
New lead glasses for Cherenkov counters
Abstract Attempts have been made to improve the characteristics of glasses suitable for Cherenkov counters of the total-absorption type. Many kinds of new lead glasses were prepared and examined in the transmittance, radiation lengths, and methods of manufacturing. Several kinds of glass blocks having the dimension of 65 × 65 × 235 mm 3 were manufactured successfully. The transmittance of one of the new glasses is higher than the usual lead glass of F-2 ∗ . It was also found that the discoloration of the new glasses is smaller than the usual lead glasses upon irradiation of 60 Co γ rays of 10 4 to 10 7 Roentgens. The Cherenkov counters of the total-absorption type with the new glasses, together with the ones with usual lead glasses of F-2, SF-2, SF-5, and SF-6, were tested by using electron beams with energies from 300–1000 MeV. It was found that the energy resolution of the first ones is better by a few percents than that of the latter in the whole energy region.
118388927
Excitation of Trapped g-Mode Oscillations in Warped Disks around Black Holes
In order to study the origin of high-frequency quasi-periodic oscillations observed in X-ray binaries, Kato (2004) suggested a resonant excitation mechanism of disk oscillations in deformed disks. In this paper, we study numerically, following his formulation, whether trapped g-mode oscillations in a warped disk, where the warp amplitude varies with radius, can be excited by this mechanism. For simplicity, we adopt Newtonian hydrodynamic equations with relativistic expressions for the characteristic frequencies of disks. We also assume that the accretion disk is isothermal. We find that the fundamental modes of trapped g-mode oscillations with eigenfrequencies close to the maximum of epycyclic frequency are excited. The intermediate oscillations found are isolated in a narrow region around the resonance radius. After varying some parameters, we find that the growth rate increases as the warp amplitude or the black hole spin parameter increases, while it decreases as the sound speed increases.