Non-gaussian beam tails at the Advanced Light Source
The coordinate and energy distributions of the electron beam density at the Advanced Light Source have been obtained from measurements of the beam lifetime at various storage-ring apertures in the horizontal and vertical planes. It is shown that these distributions have a gaussian core extended approximately up to five rms beam sizes. Beyond this point the electron density is defined by the electron scattering on the residue gas and electron intra-beam scattering and decreases as the cubic of the distance from the beam center. This behavior continues approximately up to ~;30 rms beam sizes in the horizontal plane and ~;60 rms beam sizes in the vertical plane. The horizontal and vertical emittances obtained from the measurements near the beam core suggest a presence of ~; 8.5 percent coupling in the ring at the time of measurements.