Experimental Evidence of the 90° Stop Band in the GSI UNILAC

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°. A recent simulation study with the PARMILA code [ D. Jeon et al. Phys. Rev. ST Accel. Beams 12 054204 (2009) 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 σ₀=90° per focusing cell. These experimental findings agree with results from three different beam dynamics simulation codes, i.e., DYNAMION, PARMILA, and TRACEWIN.