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Negative hydrogen ions (H⁻) are accelerated up to 400 MeV in the linac
and converted to protons via a charge-exchange foil before further
acceleration to 3 GeV in the Rapid Cycling Synchrotron (RCS) at J-PARC.
While the linac operates at RF frequencies of 324 MHz and 972 MHz, the
RCS acceleration frequency is approximately 1 MHz. Direct injection of
the continuous-wave (CW) linac beam into the RCS would result in roughly
half of the beam falling outside the RF bucket, leading to significant
beam losses. To avoid this, a beam chopper system located in the
upstream section of the linac modulates the CW beam into a comb-like
time structure matched to the RCS RF buckets.
However, choppers exhibit transient response and insufficient beam
chopping, which cause incomplete beam removal (a phenomenon commonly
referred to as “chop leakage”). This leakage contributes to beam losses
in the RCS and can potentially limit high-power beam operation. In this
paper, we report on systematic studies of RCS beam losses attributed to
chop leakage using beam diagnostics installed in the linac to
characterize the leakage behavior.
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