Search Results (6 total)

The effects of electron clouds on positively charged beams have been an active area of research in recent years at particle accelerators around the world. Transverse beam-size blowup due to electron clouds has been observed in some machines and is considered to be a major limiting factor in the development of higher-current, higher-luminosity electron-positron colliders. The leading proposed mechanism for beam blowup is the excitation of a fast head-tail instability due to short-range wakes within the electron cloud. We present here observations of betatron oscillation sidebands in bunch-by-bunch spectra that may provide direct evidence of such head-tail motion in a positron beam.

A vertical coupled-bunch instability was observed for a positron beam at the Beijing Electron Positron Collider (BEPC). The experimental results show that the instability has similar characteristics as that observed in the Photon Factory of KEK several years ago. The instability at BEPC can be explained by the effect of an electron cloud which is produced in the beam chamber by synchrotron light hitting the wall.

The momentum dependence of the analyzing power A_y in proton-proton elastic scattering has been measured in small steps using an internal target during polarized beam acceleration from 1 to 3 GeV/c. The momentum bin size ranges from 5 to 18 MeV/c. The relative uncertainty of A_y is typically less than 0.01 for each momentum bin. Narrow structures have been found in the two-proton invariant mass distribution of A_y.

Ion-channel guiding of an intense relativistic electron beam in a microwave (cm–mm) steady-state free-electron laser (FEL) is proposed. By averaging equations of motion over the high-frequency variation of the radio-frequency field, the expulsion mechanism of plasma electrons is examined and a criterion to allow development of an ion channel in a steady-state FEL is derived. Key issues on FEL operation induced by ion-channel guiding are discussed.

The distorted spectral function of the protons in the reaction ⁴⁰Ca(e, e^′p) is measured. For the 2s-1d region the distorted-wave impulse-approximation calculation using the Elton-Swift wave functions explains the data within 20%. The 1p occupation number is 1.7 times that of the shell-model prediction. After correcting for the radiative effects and for the multiple-collision background the 1s state is clearly observed at E_S≃58 MeV with a reasonable occupation number.

The differential asymmetry ratio for the process γ+n→p+π^- was measured at 90° in the center-of-mass system and for incident photon energies from 352 to 550 MeV. The observed asymmetries are larger than the values predicted from the theory by Berends, Donnachie, and Weaver. A smaller M₁- amplitude gives better agreement between the experiment and the theory.