STUDIES OF ENERGY RECOVERY LINACS AT ... - CASA

ever, linacs are limited to accelerating small amounts of average beam current due
to the prohibitively expensive radio-frequency (RF) power required.
An energy recovering linac (ERL) is a powerful alternative accelerator concept
which combines the desirable characteristics of both storage rings and linacs, by
having the potential to accelerate hundreds of milliamperes of average current to
several giga-electron volts in energy while maintaining excellent beam quality.
1.1 Energy Recovering Linear Accelerators
The idea of energy recovery was first proposed in 1965 for use in a collider
[1]. While such a collider has yet to be realized, within the last decade energy
recovery has found a niche in drivers for light sources. A schematic for a generic
ERL based light source is given in Fig. 1.1. Electrons are generated in a high
brightness injector, accelerated through a linac and then transported to a region
where the desired radiation is generated (e.g. an undulator or a wiggler). After
performing their intended purpose, the electrons are returned to the linac 180 ◦ out
of phase with respect to the RF accelerating field for energy recovery. At the exit of
the linac, the energy of the decelerated beam is approximately equal to the injection
energy and the beam is directed to a beam dump. In ERLs the decelerated beam
cancels the beam loading effects of the accelerated beam. Therefore ERLs can, in
principle, accelerate very high average currents with only modest amounts of RF
power.
Because the net RF current seen in the linac is negligible, high average currents
can be accelerated economically. Furthermore, since the electron beam only exists
in the accelerator for a short time (typically two passes), the equilibrium that is
unavoidable in a storage ring does not have time to develop. Thus the beam quality
in an ERL is determined, to a large extent, by the injector. This combination of
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