Beam dynamical behaviour of the MESA SRF structures under recirculating operation
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Abstract
Energy Recovery Linacs (ERLs) are a novel type of electron accelerators which
circumvent certain problems with beam quality that arise in circular machines,
while also limiting the amount of power needed for acceleration compared to
linear accelerators (Linacs). The idea was first proposed by M. Tigner in [1].
In an ERL machine, the electrons are fed from the source into the accelerating
cavities and are then used for example in minimally invasive nuclear physics
experiments [2] or to drive a radiation source [3]. After the experimental use,
the electrons are recirculated back into the cavities with a phase shift of 180°
with respect to the accelerating field. This allows the particles to be decelerated
in the cavities’ electric field and to feed their kinetic energy back into the
cavities. The Mainz Energy-recovering Superconducting Accelerator (MESA)
is currently under construction at Johannes Gutenberg University Mainz. For
MESA stage 1 operation, a beam current of 1mA will be accelerated four times
in two cryomodules up to an energy of 105MeV. 100MeV of beam energy can
be recovered, and the beam will be dumped at the injection energy of 5MeV.
This leads to a significantly reduced power demand from the wall but requires
superconducting cavities operated at 1.8K with high quality factors to maintain
the accelerating field in the cavities. For MESA, stage 2 operation with up to
10mA is planned. High-current beams pose significant challenges to beam
dynamics, due to, for example, space charge at low energies and transverse
Beam Break Up (BBU) caused by dipole Higher Order Modes (HOMs) in the
superconducting cavities. The measurement of HOMs in the MESA cavities
performed at Helmholtz-Institut Mainz (HIM), their impact on beam dynamics,
and the limits of achievable beam currents imposed by BBU are presented in
this work.