Abstract
The rapid cycling synchrotron of the intense pulsed neutron source at Argonne National Laboratory normally operates at an average beam current of 14 to 15 � A, accelerating protons from 50 to 450 MeV 30 times per second. The beam current is limited by a single-bunch vertical instability that occurs in the later part of the 14 ms acceleration cycle. By analyzing turn-by-turn beam position monitor data, two cases of vertical beam centroid oscillations were discovered. The oscillations start from the tail of the bunch, build up, and develop toward the head of the bunch. The development stops near the bunch center and oscillations remain localized in the tail for a relatively long time (2–4 ms, 1–2 � 10 4 turns). This vertical instability is identified as the cause of the beam loss. We compared this instability with a head-tail instability that was purposely induced by switching off sextupole magnets. It appears that the observed vertical instability is different from the classical head-tail instability.
Highlights
The intense pulsed neutron source (IPNS) accelerator system at Argonne National Laboratory consists of an HÀ ion source, a Cockcroft-Walton preaccelerator, a 50 MeV Alvarez linac, a rapid cycling synchrotron (RCS), transport lines, and ancillary subsystems
After integration of the raw beam position monitor (BPM) data, we should be able to calculate the centroid and intensity profile on a turnby-turn basis, but we found that there is a large-amplitude, low-frequency random noise, which shifts the baseline of the integrated BPM signal and makes the directly integrated BPM data unusable for comparing the centroid and intensity profiles between different turns
We need to further process the integrated BPM data before we use it: first, after integration, local minima and maxima are identified for intensity profile of each turn; with these minima and maxima points, boundaries of each turn can be obtained; compensations are made to remove the contributions from the baseline shifting for each turn
Summary
The intense pulsed neutron source (IPNS) accelerator system at Argonne National Laboratory consists of an HÀ ion source, a Cockcroft-Walton preaccelerator, a 50 MeV Alvarez linac, a rapid cycling synchrotron (RCS), transport lines, and ancillary subsystems (controls, diagnostics). The accelerator normally operates at an average beam current of 14 to 15 A, delivering pulses of approximately 3:0 Â 1012 protons at 450 MeV to the target, 30 times per second. At an injected charge of 3:6–3:7 Â 1012 protons (0:58–0:59 C), a vertical instability starts to grow approximately 10 ms after injection. We found that the vertical oscillations of the bunch centroid start at the tail and are localized in this region for the remaining 4 ms until extraction. This phenomenon is quite different from what is observed during a classical head-tail instability [1], which had been considered as one of the possible mechanisms of the instability at the RCS
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