The ATF overview page provides information on the progress in the construction of
the Accelerator Test Facility at KEK.
The S-band Injector
The Damping Ring
It is essential to
produce a low emittance beam with a reasonable damping time,
since the spot size is limited by the emittance of the beam.
experimentally its feasibility, we are now constructing ATF (Accelerator Test
Facility) at KEK. The ATF consists of two major components:
a 1.54 GeV S-band injection linac and a damping ring.
The construction of the ATF is one of the
most important ingredients of the second 3-yesrs R&D program emphasizing the
practical aspects to build an e+elinear collider facility.
The ATF project is expected to achieve its goal in next two years.
The 1.54 GeV S-band Linac
The injector consists of a conventional thermionic gun, a 357 MHz subharmonic
buncher, and a 3 m long S-band buncher structure which is followed by 3 m long
constant gradient traveling wave structures.
The overall length of the linac is
75 m in which 18 structures, focusing magnets, and various monitors are placed.
The 100 MW S-band Klystron
Two structures are driven by one 100 MW klystron which can produce
4.5 micro-sec long RF pulse.
The 400 MW SLED Cavity
A newly developed SLED cavity compresses the output pulse from the
klystron and produces a 1 micro-sec long RF pulse with a peak power of 400 MW.
The compressed pulse is divided into two and fed to each structure providing an
average acceleration gradient of 40 MeV/m with beam loading.
In the present
design two of the 18 structures are operated with slightly different
frequencies, which compensates the energy differences and keeps the
bunch-to-bunch energy spread to about 0.15%.
The invariant emittance at the
entrance of the regular section is expected to be 3 x 10**-4 rad.m.
blow-up in the injector is required to be less than 50 % in order to match the
The lattice was designed to meet this requirement with a
minimum space factor.
Two triplets are only placed at the upstream end and
doublets are limited to the first half of the linac.
The remaining half of the
linac simply consists of singlets.
The alignment tolerance is 200 microns for both
structures and quads, which is expected to be achievable using a wire alignment
The Wire Alignment System
Needless to say, it is essential that a multi-bunch beam can be
extracted with sufficient intensity from the thermionic gun.
To this end we
have been developing a grid driver which consists of a fast ECL circuit and an
RF power amplifier.
The Electron Source
Recently the thermionic electron source is succeeded in producing 20 bunches
with 2.8 nsec bunch spacing.
The intensity of each bunch was measured to be
3 x 10**10 particles, which is required for the damping ring.
The Choke Mode Cavity
The choke mode cavity damps the higher order modes generaed by the bunch-cavity
intaraction. The multi-bunch beam has been accelerated by the accelerating
gradient of 52 MeV/m in the choke mode cavity.
The design goal of the ATF damping ring is to produce
a vertical invariant emittance of
5 x 10**-8 rad.m and
a horizontal invariant emittance of 5 x 10**-6 rad.m
under the multi-bunch operation
with up to 20 bunches spaced by 2.8 nsec.
The damping times are designed to be
as short as 6.3 msec horizontally
and 8.3 msec vertically by virtue of wigglers.
The damping ring has a racetrack shape and its circumference is 138.6 m.
The length of the straight section is 25.8 m, while the average arc radius is 13.8m.
A FOBO Cell
In order to achieve the
design emittances we apply a FOBO type lattice, in which the defocusing bend is
placed at the dispersion minimum point.
Each arc consists of 18 combined
function FOBO cells and the horizontal and vertical phase advances are designed
to be 140 and 52 in each cell, respectively.
In order to reduce the damping
times we use wigglers which are placed in the dispersion free space so that they
also help reducing the equilibrium emittance. The wigglers have an effective
field of 1.88 Tesla and a wiggler period of lw = 40.0 cm. A high field quality
is required since the non-linear fields reduce the dynamic aperture and cause a
beam loss at the injection. The wigglers are placed in one of the straight
sections and the total length of the wiggler section is 21.2 m. The lowest
allowable RF frequency of the damping ring is determined by the bunch spacing of
1.4 nsec in the X-band main linac case. On the other hand the highest is
limited by longitudinal coupled bunch instabilities. The RF frequency of the
ring is chosen to be 714 MHz which is a subharmonic of the S-band injector
Three septum magnets successively deflect the beam from the injector
by 165, 110, and 36 mrad. The beam is further deflected by about 4 mrad by a
kicker magnet to match the closed orbit of the ring. The injection error was
estimated to be 0.1 mm. To keep the beam loading of the cavity constant, the RF
bucket which has been occupied by an extracted bunch must be refilled
immediately by the injected bunch. This requires the beam extraction point, the
injection point, and the cavity section to be located in this order.
position monitor (BPM) system was designed to have a 10-micron resolution in order
to correct the 3-micron dispersion in the wiggler section.
The Beam Position Monitor
The BPM is chosen to be
of electrostatic type with four button-like electrodes, which has relatively low
coupling impedance and simple structure. Since the betatron coupling and the
dynamic aperture depend on alignment errors, it is fairly important to align
magnets and BPMs very accurately. As seen from the emittance ratio the betatron
coupling must be less than 1 %. The r.m.s. horizontal and vertical tolerances
for the magnets to meet this requirement are 60 microns and 30 microns,
the rotation errors of the magnets are required to be within 0.5 mrad. The
gradient errors of the quads must be less than 0.1 %. For this purpose the
components which comprise one normal cell are mounted on a one-piece rigid table
which is 2.4 m long and 1.0 m wide and has three legs, each having a vertical
and a horizontal mover with a stepping resolution of 5 microns.
The Alignment Table for a FOBO Cell
The mounting on the
table can be done with an accuracy better than 20 microns, since various alignment
techniques are available in the region extending over a few meters. The
alignment of the table along the design orbit is expected to be as accurate as
20 microns horizontally, 30 microns vertically, and 0.3 mrad in rotation or better.
Attentions were also paid to keep a rigidity of the floor.
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email@example.com Feb 09, 1995