Experimental results of the ATF in-line injection system Page: 1 of 5
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Submitted to the 1995 Particle Accelerator Conference,
Dallas,, Texas, May 1-5, 1995.
Experimental Results of the ATF In-line Injection System *
X.J. Wang, T. Srinivasan-Rao, K. Batchelor, M. Babzier, I. Ben-Zvi, R. Male
I. Pogoreisky, X. Qui, J. Sheehan and J. Skaritka
Brookhaven National Laboratory, Upton, NY 11973 USA
The initial experimental results of the Brookhaven
accelerator test facility (ATF) in-line injector is presented.
The ATF in-line injector employed a full copper RF gun with
a pair of solenoid magnets for emittance compensation. The
maximum acceleration field of the RF gun was measured to
be 130 MV/m. The electron yield from the copper cathode
was maximized using p- polarized laser and the Schottky
effect. The quantum efficiency under optimum conditions was
measured to be 0.04%. The measured electron bunch length
was less than 11 ps, which agreed with the laser pulse length
measurement using a streak camera. The normalized rms.
emittance for 0.25 nC charge is 0.9 0.1 mm-mrad, which
is almost four times smaller than the emittance predicted by
the space-charge effect for a non-emittance compensation
photocathode 1F gun. The normalized rms emittance for 0.6
nC charge was measured range from 1 to 3 mm-mrad. This
measurement was first experimental demonstration of
emittance compensation in a high-gradient, S-band
photocathode HF gun.
A new photocathode RF gun injector based on the
emittance compensation technique  was installed at the
Brookhaven accelerator test facility (ATF).
The ATF is a facility dedicated for FEL and laser
acceleration research  , consisting of a laser driven RF
gun injector , 70 MeV linac and experimental beam lines.
We will present experimental results of photoemission and
electron beam emittance measurement for the emittance
compensation RF gun. Comparison of experimental results
with simple analytical predictions show that the measured
emittance smaller than the uncompensated emittance. This
indicates that the emittance compensation actually works.
II. THE ATF IN-LINE NJECTOR
The design of the injector  optimized the distance
between the RF gun and the linac. The small emittance
produced by the emittance compensation was frozen through
the acceleration in the linac. The cell photocathode RF gun
was followed by a solenoid magnet. A second solenoid
magnet was placed behind the HF gun to buck the first
solenoid magnet. Following the solenoid magnet is a six-way
cross for vacuum pumping port, and a 45 degree aluminum
mirror mounted on an actuator for monitoring the laser beam
profile and optical transition radiation (OTR). There were
beam profile and charge measurement devices located before
and after the ATF two sections linac. A group of quadrupole
magnets positioned subsequently can be used for emittance
*This work was performed under the auspices of US DOE
under Contract No. DE-AC02-76CH00016.
measurement or beam matching for the experimental lines.
There are two beam profile monitors with five meter
separation after the linac. A pop-up beam profile monitor and
momentum slit were installed after the dipole magnet for
energy spread measurement and energy selection.
The ATF diode-pumped Nd:YAG oscillator can generate
81.6 MHz pulses with 14 ps FWHM pulse length and 100
mW power. The IR was frequency quadrupled to UV (266
nm) on the laser table. The UW laser pulse was transported to
the RF gun hutch via 20 meter long evacuated pipe. The
optics in the gun hutch was designed to compensate for the
ellipticity of the emitting area caused by the oblique
III. RF GUN CHARACTERIZATION AND
ELECTRON EMISSION MEASUREMENTS
It took about a week of conditioning for the RF gun to reach
the designed acceleration field 100 MV/m. A four inch
mirror was mounted on a precision optical rotation stage
outside the window of the six-way cross for angular
distribution measurement of optical transition radiation
(OTR). A PMT was used to detect angular distribution of
OTR while a CCD camera was used to measure the beam
profile from OTR. Fig.2 plotted OTR experimental data and
theoretical fittings for two different RF power levels. The
data showed that the highest electron beam energy is about 6
MeV, which corresponds to the peak acceleration field 130
MV/m. The peak surface field in the RF gun cavity is about
20 % higher than the acceleration field.
Figure 1 Angular distribution of OTR.
The effect of the coupling slot between the waveguide and
RF gun cavity was investigated experimentally. The main
effect of the coupling slot is introduction of TM i mode
D:SrRIBuTnoN OF t dOCUMENT is UNUME MASTER
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Wang, X.J.; Srinivasan-Rao, T. & Batchelor, K. Experimental results of the ATF in-line injection system, article, May 1, 1995; Upton, New York. (digital.library.unt.edu/ark:/67531/metadc702933/m1/1/: accessed April 22, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.