Search Results (6 total)

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%.

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.

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.

A one-dimensional dissipative magnetohydrodynamics code is used to investigate the discharge dynamics of a waveguide for high-intensity laser pulses: the gas-filled capillary discharge waveguide. Simulations are performed for the conditions of a recent experimental measurement of the electron density profile in hydrogen-filled capillaries [D. J. Spence et al., Phys. Rev. E 63, 015401 (R) (2001)], and are found to be in good agreement with those results. The evolution of the discharge in this device is found to be substantially different to that found in Z-pinch capillary discharges, owing to the fact that the plasma pressure is always much higher than the magnetic pressure. Three stages of the capillary discharge are identified. During the last of these the distribution of plasma inside the capillary is determined by the balance between ohmic heating, and cooling due to electron heat conduction. A simple analytical model of the discharge during the final stage is presented, and shown to be in good agreement with the magnetohydrodynamic simulations.

A theoretical study of the interchangelike (Rayleigh-Taylor) instability of a thin slab of weakly ionized plasma is presented. We have found an analytical solution for the stationary motion of a plasma slab under the effect of the magnetic field pressure. This solution describes a shock-wave-like structure of the magnetic field and is unstable against the interchange mode. Using an approach developed earlier [E. Ott, Phys. Rev. Lett. 29, 1429 (1972); S.V. Bulanov, and P.V. Sasorov, Sov. J. Plasma Phys. 4, 418 (1978)] we have obtained exact solutions, in terms of analytical functions of a complex variable, of the Cauchy problem for the evolution of nonlinear perturbations. We have investigated the formation of the typical singularities that correspond to different wave breaking regimes in an unstable medium. We discuss Ott’s problem of the Rayleigh-Taylor instability of initially nonplanar shells.