STUDIES OF ENERGY RECOVERY LINACS AT ... - CASA

3.2 Injector and Injection Line
The injector region consists of a DC photocathode gun, two solenoids, a buncher,
quarter cryomodule (two 5-cell niobium cavities), a matching and a merger section
[50]. A view of the region is given in Fig. 3.2.
The electron source is a DC photocathode gun which consists of a Gallium
Arsenide (GaAs) cathode illuminated by a 527 nm laser with up to 6 W of power.
A thin layer of Cesium on the surface of the cathode acts to decrease the work
function. The photoelectrons are then quickly accelerated across a 350 kV voltage
gap.
The first solenoidal lens focuses the rapidly diverging electron beam to a waist
in the buncher. The buncher is a 1497 MHz copper cavity whose gradient is set
to minimize the longitudinal emittance. The second solenoid matches the beam
transversely into the quarter cryomodule (cryounit) where the bunch is accelerated
from 350 keV to 7 MeV. Typically the charge per bunch is 135 pC but can be varied
continuously up to this value by changing the attenuation of the drive laser beam.
Following the cryounit the beam travels to the matching section which consists
of four quadrupoles and is used to generate upright phase spaces in both transverse
planes. Nominal injection conditions are βx = βy = 10 m and αx = αy = 0.
Longitudinally, the injector must produce a relatively long bunch length (2 ps rms)
with a small momentum spread (0.1% rms) at the entrance to the linac. Accelerating
a long bunch through the linac minimizes longitudinal HOM excitation and the
attendant single bunch instabilities. Additionally, it reduces longitudinal emittance
growth due to longitudinal space charge [51].
The merger section consists of a three bend achromatic geometry with each
dipole providing a bending angle of 20 ◦ . The final dipole is common to the reinjection
chicane which is used to merge the recirculated (high energy) beam with the injected
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