ATF Overview

The ATF overview page provides information on the progress in the construction of the Accelerator Test Facility at KEK.

Introduction

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. To confirm 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 Injector

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. The emittance blow-up in the injector is required to be less than 50 % in order to match the ring acceptance. 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 method.

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 Damping Ring

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 frequency. 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. The beam 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, respectively, and 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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