[SLAC Pubs and Reports]
Low Emittance Guns for the ILC Polarized Electron Beam
Polarized electron beams generated by DC guns are routinely available at
accelerators including JLAB, Mainz and SLAC. These guns operate with a cathode
on the order of -100 kV. To minimize space charge effects, relatively long
generated at the gun and then compressed longitudinally external to the gun
and during initial acceleration. For linear colliders, this compression is
using a combination of rf bunchers. For the basic design of the International
Collider (ILC), a 120 kV DC photocathode gun is used to produce a series of
nanosecond bunches that are each compressed by two sub-harmonic bunchers (SHBs)
followed by an L-band buncher and capture section. The longitudinal bunching
results in a significantly higher emittance than produced by the gun alone.
energy experiments using polarized beams are not generally sensitive to the
emittance, there are several benefits to a lower source emittance including a
efficient injector system and a lower radiation load during transport
especially at bends as
at the damping ring. For the ILC, the SHBs could be eliminated if the voltage
of the gun
is raised sufficiently. Simulations using the General Particle Tracer (GPT)
indicate that a cathode bias voltage of >200 kV should allow both SHBs to be
433 or even 650 MHz, while >500 kV would be required to eliminate the SHBs
altogether. Simulations can be used to determine the minimum emittance possible
injector is designed for a given increased voltage. A possible alternative to
the DC gun is
an rf gun. Emittance compensation, routinely used with rf guns, is discussed
voltage DC guns.
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