RESUMO
In a particle accelerator with a periodic structure beam space charge force may excite resonant beam emittance growth if the particle's transverse phase advance approaches 90 degrees . A recent simulation study with the PARMILA code [D. Jeon, Phys. Rev. ST Accel. Beams 12, 054204 (2009)]10.1103/PhysRevSTAB.12.054204 has shown the feasibility of measuring the stop band of this fourth order resonance in the GSI Universal Linear Accelerator UNILAC and proposed its experimental verification, which is reported here. Measurements of transverse phase space distributions behind a periodically focusing structure reveal a fourfold symmetry characteristic of fourth order resonances as well as a resonance stop band above sigma_{0}=90 degrees per focusing cell. These experimental findings agree with results from three different beam dynamics simulation codes, i.e., DYNAMION, PARMILA, and TRACEWIN.
RESUMO
In high intensity linacs emittance exchange driven by space charge coupling may lead to the well-known "equipartitioning" phenomenon if the stop band at sigma(parallel) = sigma(perpendicular) is crossed at sufficiently slow rate. This Letter is the first experimental evidence of this phenomenon in a high intensity linear accelerator, here the UNILAC at GSI. Measurements of emittances at the entrance and exit of one drift tube linac tank comprising 15 lattice cells are taken for a set of transverse and longitudinal tunes. The onset of exchange on the stop band of previously derived "stability charts" confirms theoretical predictions. The measured transverse emittance growth also compares well with results from the beam dynamics simulation codes DYNAMION and TRACEWIN.
