Abstract
Occasionally, it is possible to bring together experiment, theory, and simulation in detail. Such an occasion occurred during a high intensity beam physics study in the Spallation Neutron Source (SNS). A transverse dipole instability in the vertical direction has been observed in the accumulator ring for a coasting beam that was stored for 10 000 turns. This instability was observed at a beam intensity of about $12\text{ }\ensuremath{\mu}\mathrm{C}$ and was characterized by a frequency spectrum peaking at about 6 MHz. The probable cause of the instability is the impedance of the ring extraction kickers. We carry out here a detailed benchmark of the observed instability, uniting an analysis of the experimental data, a precise ORBIT code tracking simulation, and a theoretical estimate of the observed beam instability.
Highlights
The Spallation Neutron Source (SNS) accelerator consists of a 1 GeV HÀ linear accelerator followed by an accumulator ring of length 248 meters
It is possible to bring together experiment, theory, and simulation in detail. Such an occasion occurred during a high intensity beam physics study in the Spallation Neutron Source (SNS)
A simple theoretical analysis using Eq (3) was applied to the observed growth rate of the dominant n 1⁄4 12 harmonic to give an estimate of the extraction kicker impedance that was in reasonable agreement with the measured value [5,6]
Summary
The Spallation Neutron Source (SNS) accelerator consists of a 1 GeV HÀ linear accelerator followed by an accumulator ring of length 248 meters. Because the SNS ring is required to operate at this extremely high beam intensity, transverse instabilities have been a concern. The approach was twofold: we studied analytic coasting beam models [1,2] in the SNS parameter regime and applied the results of these studies to benchmark [3] the transverse stability model in the ORBIT code [4]. Transverse stability was calculated for the full injection process using the measured values of the dominant extraction kicker impedance [5]. These studies predicted that, under normal operation with bunched beams, SNS would be stable. Circumference Kinetic energy Bunch population , Relativistic gamma , relativistic beta T, gamma transition , phase slip factor x, y tunes Revolution frequency Ring rf harmonics Ring rf voltages Ring bunch length rms energy spread x, y, chromaticity "x, "y rms emittances h yi Average beta-y ring, kickers
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