Measurement and simulations of intensity-dependent effects in the Fermilab Booster synchrotron.

摘要

The Fermilab Booster is a nearly 40-year-old proton synchrotron, designed to accelerate protons from 0.4 to 8 GeV kinetic energy for extraction into the Main Injector, and currently operating with a typical intensity of 4.5 x 1012 particles per beam, roughly twice the design value, because of requirements for high particle flux in various experiments. Its relatively low injection energy provides certain challenges in maintaining beam quality/stability under such demands. Quantification of effects limiting intensity could provide enhanced beam stability and reduced downtime. Future accelerator design may also benefit from this better understanding of intensity-limiting effects near injection.;Chapter 1 summarizes 20th-century accelerator research up to modern synchrotrons. Chapter 2 introduces some accelerator-physics terminology, and briefly describes the Booster. Synergia, a space-charge modeling tool, is presented with relevant benchmarks.;Emittance is discussed in Chapter 3. Space-charge fields couple particle motion, leading to interplanar emittance exchange, necessitating simultaneous measurements to obtain adequate descriptions at higher intensities. Measurements are described and results are given. RMS emittances agree with known values at nominal intensities and emittance exchange is accounted for. Correlation terms between the planes are quantified using Synergia, and shown to be at most an 8% effect.;Studies of coherent and incoherent betatron-frequency intensity dependence near injection are presented. In Chapter 4 coherent frequency shifts are shown to be from dipole- and quadrupole-wakefield effects. Asymmetry of the laminated, magnetic chambers are responsible for the magnitudes and opposing signs of horizontal and vertical wakefield tune shifts.;Chapter 5 details procedures for obtaining a coherent-shift intensity dependence, yielding -0.009/1012 and +0.001/10 12 in the vertical and horizontal planes respectively, accumulating to maximal values over several hundred turns. Two independent, corroborating studies are compared.;In Chapter 6, a measure of the incoherent tune shift with intensity puts an upper limit on direct space-charge effects. 0.004/1012 is predicted for representative incoherent particle tune shifts from realistic Gaussian distributions, considering beam-envelope growth with intensity. Tune-spread dependence was 0.005/1012 from quantification of resonant stopband widths through beam-extinction, similar to predicted values, one order of magnitude smaller than the space-charge term from Laslett tune shifts for a fixed, uniform beam.