STUDIES OF ENERGY RECOVERY LINACS AT ... - CASA
STUDIES OF ENERGY RECOVERY LINACS AT ... - CASA
STUDIES OF ENERGY RECOVERY LINACS AT ... - CASA
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ABSTRACT<br />
An energy recovering linac (ERL) offers an attractive alternative for generating<br />
intense beams of charged particles by approaching the operational efficiency<br />
of a storage ring while maintaining the superior beam quality typical of a linear<br />
accelerator. Two primary physics challenges exist in pushing the frontier of ERL<br />
performance. The first is energy recovering a high energy beam while demonstrating<br />
operational control of two coupled beams in a common transport channel. The<br />
second is controlling the high average current effects in ERLs, specifically a type of<br />
beam instability called multipass beam breakup (BBU). This work addresses both<br />
of these issues.<br />
A successful 1 GeV energy recovery demonstration with a maximum-to-injection<br />
energy ratio of 51:1 was carried out on the Continuous Electron Beam Accelerator<br />
Facility at Jefferson Laboratory in an effort to address issues related to beam quality<br />
preservation in a large scale system. With a 1.3 km recirculation length and<br />
containing 312 superconducting radio frequency (SRF) cavities, this experiment has<br />
demonstrated energy recovery on the largest scale, and through the largest SRF<br />
environment, to date.<br />
The BBU instability imposes a potentially severe limitation to the average<br />
current that can be accelerated in an ERL. Simulation results for Jefferson Laboratory’s<br />
10 kW free electron laser (FEL) Upgrade Driver predict the occurrence of<br />
BBU below the nominal operating current. Measurements of the threshold current<br />
are described and shown to agree to within 10% of predictions from BBU simulation<br />
codes. This represents the first time the codes have been benchmarked with experimental<br />
data. With BBU limiting the beam current, several suppression schemes<br />
were developed. These include direct damping of the higher-order mode using two<br />
different cavity-based feedbacks and modifying the electron beam optics to reduce<br />
the coupling between the beam and mode. Specifically the effect of implementing<br />
(1) point-to-point focusing (2) a reflection of the betatron planes about 45 ◦ and<br />
(3) a rotation of the betatron planes by 90 ◦ is measured. Each method increased<br />
the threshold current for stability. Beam optical control methods proved to be so<br />
effective that they are routinely used in the operation of the 10 kW FEL Upgrade.<br />
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