Rf guns: bright injectors for FEL - Triangle Universities Nuclear ...

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C. Tracter / Rf guns- bright injectorsfor FEL
T
QUADRUPOLE
VERTICAL STEERING
-i `- HORIZONTAL STEERING
Fig. 3 . Stanford/Duk e rf gun layout (from ref. [44]) .
Cs 3Sb photocathode illuminated by a frequency doubled
Nd : YAG laser [4] .
The cavity shape was designed to minimize the nonlinear
components of the radial electric field, assuming
a do approximation . This gun produced a 1 MeV beam
of 70 ps pulses with a peak current of 200 A and a
normalized enuttance of 40m mmmrad [I1].
A second cavity independently powered and phased
was then added to the original one . Meanwhile, the
adjunction of a pulse compressor to the laser allowed to
generate 16 ps optical pulses . This new gun tested in
1987 and 1988, provided a 2.7 MeV beam . The maximum
extracted charge was 13 .2 nC corresponding to a
peak current of 600 A. No enuttance measurements
were possible due to the drift space between the second
cavity and the "pepper-pot" device that led to excessive
space-charge emittance growth [12] .
During this period, the photocathode lifetime was
significantly improved by replacing Cs 3Sb with CsK 2 Sb .
This lifetime remained very dependent though on the
operating conditions . Table 2 summarizes the performances
of the two experiments named for convenience
phase I and phase 11 .
Based on the gained experience, a new photoinlector
was built to serve as an electron source for the HIBAF
infrared FEL [13] . This new gun which replaced the
previous conventional injector provides a 350 A, 16 ps
and 351T mm mrad beam. It consists of six cells where
the first cell contains the CsK 2Sb photocathode (fig . 4) .
This photoinlector has been designed for maximum
flexibility in order to do parametrization studies for
LOS ALAMOS NATIONAL LABORATORY
ACCELERATOR TECHNOLOGY DIVISION
GROI.P AT-1
1300 Mi; PHOTOINJECTOR LINAC
Fig . 4 . Los Alamos HIBAF rf gun (from ref . [14]) .