Experimental Study of the Main Ring Transition Crossing

The longitudinal emittance increase and the particle loss through transition as a function of the initial emittance in the Main Ring was studied. A phase mismatch at injection was used to change the initial longitudinal emittance and measurements were taken for different intensities and RF voltages around tran-sition. The results of our measurements are in good agreement with simulation results from ESME and indicate that nonlinear effects dominate the transition crossing in the Main Ring under the present operating conditions (c, 2 0.14 eV-set). Introduct ion A number of different experiments were proposed as part of the Fermilab III Instabilities Workshop in order to study the transition crossing in the Main Ring. Due to time limitations and operational restrictions only one of the experiments was actually performed. The results and analysis presented here are preliminary. We used an injection mismatch to deliberately blow up the lon-gitudinal emittance in the MR 29 cycles, and measured the increase in bunch emittance and the particle loss through tran-sition as functions of the initial bunch emittance. The experi-ment was repeated for different intensities (2, 4 booster turns) and for different rf voltages around transition. The purpose of the experiment was to distinguish the mechanism that is responsible for emittance growth and particle loss across tran-sition. Two mechanisms can lead to the growth of bunch emittance and particle loss across transition. The first one is nonlinearity, which is due to the nonlinear terms in the expansion of the mo-mentum compaction factor or the orbit length as a power series in the momentum spread. With these nonlinear terms, par-ticles of different momenta cross transition at different times. The spread in crossing time is called the nonlinear time [l]