Halo Formation in Spheroidal Bunches with Self-Consistent Stationary Distributions

A new class of self-consistent 6-D phase space stationary distributions is constructed both analytically and numerically. The beam is then mismatched longitudinally and/or transversely, and we explore the beam stability and halo formation for the case of 3-D axisymmetric beam bunches using particle-in-cell simulations. We concentrate on beams with bunch length-to-width ratios varying from 1 to 5, which covers the typical range of the APT linac parameters. We find that the longitudinal halo forms first for comparable longitudinal and transverse mismatches. An interesting coupling phenomenon - a longitudinal or transverse halo is observed even for very small mismatches if the mismatch in the other plane is large - is discovered.Comment: 3 pages, 3 figures; presented at European Particle Accelerator Conference, Stockholm, Sweden (June 22-26, 1998

Analysis of Shielding Charged Particle Beams by thin Conductors

We present an analysis of shielding of electromagnetic fields excited by beams of charged particles surrounded by thin conducting layers or metal stripes inside an external structure of finite length. The ability of shielding by a layer thinner than the skin depth is explained and expressions for the impedance are derived. A previous result[1] showing preferential penetration through the shielding layer at the resonant frequencies of the surrounding structure is verified, and extended to include finite resistivity of the outer structure. Integration over the spectrum of the beam bunch shows that penetration is (nearly) independent of the quality factors of the resonances. The transition of these results to those for a geometry of infinite length requires numerical evaluation

Energy Loss to Coaxial Vacuum Chambers in LEP and LHC

In many hig-energy storage rings the beam chamber is connected to a separate pump chamber by a metallic wall with many holes or slots whic permits passage of the rest gas. In LEP, the pump chamber contains a metallic 'negstrip' pump, and thereby becomes a coazial transmission line. Also in LHC, a coaxial line is formed by the 'liner' and the surrounding cold vacuum chamber which it shields from heating by sznchrotron radiation. Since the phase velocity of electro-magnetic fields in a coax line is close to light velocity, the fields will be almost in sznchronism with the particle beam and the pump chamber, which may result in a large resistive impedance and could lead to isntability, loss of beam energy, and excessive heating of the chamber walls. Here we estimate the rate of field buildup analytically, and in a subsequent report we will compare these results with numerical computations using 3-D computer codes. The results are tested for diagnostic purposes on a 'slot coupler' with short and wide holes designed to extract energy efficiency