Polarisation potentials for positron-molecule collisions: positron-H, scattering
Model polarisation potentials for use in calculating positron-molecule cross sections are often based on approximations in which terms that depend on the sign of the charge of the projectile are neglected. Thus, these potentials do not fully distinguish between electron and positron scattering. The validity of using such potentials for positron- H2 collisions is investigated by performing scattering calculations using electron- and positron-H, model polarisation potentials. Results show that, because of differences in the way electrons and positrons distort the target charge cloud, sign-dependent terms in the adiabatic potential can significantly influence the total cross section. Low-energy cross sections are also shown to be rather sensitive to how non-adiabatic polarisation effects are taken into account. Using a cut-off fully adiabatic positron-H, polarisation potential, theoretical total cross sections in reasonably good agreement with recent measurements are obtained.
The AB Initio Inclusion of Polarisation Effects in Low-Energy Positron-Molecule Collisions Using the R-Matrix Method
Positron-molecule collisions at low energy have become a subject of great experimental interest (Charlton, 1985a, 1985b, and other articles in this volume), providing a concomitant theoretical challenge. As pointed out by Massey (1976) and others, collisions involving low-energy positrons provide one of the most stringent tests of our theoretical understanding of scattering. In electron collisions, the static and polarisation contributions to the overall potential are both attractive. In positron scattering, the static potential is generally repulsive leading to a cancellation between interactions which is difficult to represent.
DESIGN CONSIDERATION TO PPM KLYSTRONS FOR INDUSTRIAL LINAC
Toshiba has been developing the C-band and X-band Periodic Permanent Magnet (PPM) focused klystron in collaboration with KEK and manufactured prototypes for the JLC project. [1] The PPM klystron has an advantage to enhance the total efficiency as a result of eliminating the electric power for the focusing magnet. From experience on the development the PPM focused klystrons for the JLC project, we plan to apply the PPM focused klystron to the medical and industrial linac. The periodic magnet focusing scheme has some technical difficulties such as stop band voltage. To design the PPM klystron for the medical and industrial linac, it is necessary to give heed to electron beam instability; so that the tube parameters of the PPM focused klystrons have some constraints. The size of the accelerator system is substantially reduced if a PPM focused klystron is used. The majority of medical and industrial linacs use frequencies of S-band range. We discuss the availability of the S-band PPM focused klystron in this paper.