JAERI-Conf 95-021

1p-3
1.54 Ge V ATF Injector Linac
S. Takeda, M. Akemoto, T. Asaka*,
H. Hayano, M. Kagaya*, S. Kashiwagi**, T. Naito and H. Matsumoto
KEK, National Laboratory for High Energy Physics, Tsukuba 305*Department of Applied Physics, Tohoku-Gakuin University, Tagajo 985**Department of Physics, Faculty of.Education, Yokohama National University, Yokohama 240
Abstract
This paper describes the present status of 1.54 GeV Shand injector linac of the ATF(Accelerating Test Facility)
for JLC.

1INTRODUCTION

The ATF (Accelerator Test Facility) project was started
in 1988 in order to stimulate the R&D work for the JLC
project [1]. On the first stage, the accelerating gradientof 93 MeV /m has been achieved by an S-hand linac [2, 3].
The project has been extended to construct an accelerator
system as a proto-type machine of the JLC [4].
As shown in Figure 1, ATF consists of five major
accelerator-parts: an S-hand injector linac [5], a beamtransport-line [6], a damping ring [7], a bunch compressor, and electron/positron sources. ATF generates, accelerates, damps, and compresses a train of 20 buncheswith 2 x 10 10 electrons/bunch and 2.8 ns bunch spacing.
The amount of total number of electrons in a bunch-train
is approximately half that of the JLC-1 machine. The
multi-bunches are accelerated by the accelerating gradient of 33 MeV /m which is approximately same that of
the JLC-1 machine on the initial stage. The goals of thevertical and horizontal beam emittance to be achieved are
3 x 10-8 m·rad and 5 x 10-6 m·rad, respectively. The goalof the bunch length is 100 p,m with a one-stage compressor with a large (1/50) compression ratio. The amountsof both the beam emittance and the bunch length aresame those of the JLC-1 machine. ATF will verify themulti-bunch scheme of linear colliders in all parts fromthe injector linac to the bunch compressor.

2

1.54 GeV ATF Linac

The 1.54 GeV ATF injector linac was designed to accelerate multi-bunch electrons for the injection to a lowemittance damping ring [8]. The injector linac consists ofan 80 MeV preinjector linac, 8 units of regular accelerator
section, and an energy compensation system. The parameters of the 1.54 GeV ATF injector linac are summarizedin Table 1.

380 MeV Preinjector Linac

The required specification of preinjector results from theenergy acceptance and dynamic aperture of the damping
ring [9]. Although the energy spread among the bunches
will be compensated by a special accelerating structurein the 1.54 GeV ATF Linac, the energy spread within abunch is determined by the bunch length at the exit ofthe preinjector. Also, the dynamic aperture of the damping ring determines the maximum emittance of the Linac
beam. Assuming no emittance growth in the linac, thespecification of the maximum emittance will be applied
to the preinjector. As shown in Figure 2 the preinjector
consists of a thermionic electron gun, two sub-harmonicbunchers, four single cell bunchers, an accelerating structure, a matching section of beam lattice, an energy analyzer and beam instrumentations.

Figure 2: Schematic drawing of the 80 MeV preinjector.

The thermionic gun system consists of a triode with
type EIMAC Y646-E or Y796 grid-cathode assembly [10].
The HV pulser p~;ovides maximum pulse voltage of 240 kV
with 3 p,s pulse duration. Grid pulses with pulse separation of 2.8 ns are produced by gating the outputof a 357 MHz signal generator synchronized with subharmonic bunchers. The multi-bunch from the gun has abunch length of 1 ns FWHM. Each bunch contains 3 x 10 10electrons. The bunch population is larger than the specification of the linac beam because of an about 70 % transmission efficiency in the buncher section.
To avoid a phase shift of the bunching voltage due tothe beam-loading, low . R/Q cavities, which reduce thebeam-loading voltage, are used. To obtain more flexibility for buncher tuning, four single-cell standing-wave
cavities with low R/Q for the buncher are applied as well
as low R/Q Sub-harmonic bunchers.
The accelerator section of the 80 MeV preinjector is
a 3 m-long constant-gradient accelerating structure. The
geometrical dimension and rf specification are identical to
the structures of the 1.54 GeV linac. The energy spreadof the multi-bunch due to the beam-loading is evaluatedto be 5 %in full width at the bunch population of 2 x 10 10 •The OTR (Optical Transition Radiation) monitor is used
to observe the bunch-by-bunch profile and length. Thebunch-by-bunch energy spread can be observed by theOTR monitor and a bending magnet as shown in Figure 3.
The energy spread is evaluated to be 1.24% in full width
at the bunch population of 0.5x10 10 electrons [11].
A wire scanner is routinely used for bunch-by-bunch
emittance measurement in the ATF preinjector. A MCPPMT with gate technology detects gamma generated fromthe fire by colliding the beam electrons.

4Accelerator Section of 1.54 GeV LinacThe role of the accelerator section of the injector linac is
the acceleration of multi- bunch from 80 MeV to 1.54 GeV
with a minimum energy spread and a minimum emittance
growth. It consists of eight rfunits, two energy compensation units, a linac lattice, and beam monitors. The beamtransport line is located between the 1.54 GeV ATF linac
and an injection kicker of the damping ring.

17-JAERI-Conf 95-021

Bunch compraooor

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Wiggler magnet_Li---+--....11DC ,.;.-supply for modulator~~----------------------------1~m------------------------~
Figure 1: Schematic diagram of ATF (Accelerator Test Facility).
TOSHIBA, E3712

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!!' 1 42MV/mFigure 3: Bunch-by-bunch energy and spread observed

52 IIV/m~. 42MV/m

52 IIV/m~i j---ll___j----l--...1-1___j~by an OTR monitor. The bunch population is 0.5x10 10 •~!

""l•t-:-:12-:-0.,-Me-V--..-i•lI• 120 MeV •I

Energy Gain with Beam L011dlng

4.a

RF System

The rf unit of regular section consists of an E-3712
klystron, a modulator [12], a dual-iris SLED cavity [13],rf waveguides, two 3 m-long accelerating structures [14]and rf dummy-loads as shown in Figure 4. The klystronproduces the rf peak power of 80 MW with a pulse duration of 4.5 ps and supplies to a dual-iris SLED cavity.
The rf phase is reversed at 3.5 ps and the peak power of
400 MW is extracted from the SLED cavity w.ith a pulse
duration of 1.0 ps . The rf power is divided into two rf
waveguides in order to supply a peak power of 200 MW
into a 3 m-long accelerating structure.
The accelerating field distributes from 52 MV /m at thedownstream of the accelerating structure to 42 MV /m atthe upstream of the structure. The energy gain of the firstbunch among twenty bunches is 119 MeV in an structure.
The energy spread is evaluated to be ±2.6 % at the bunch
population of 2.0x10 10 , since the energy gain of the lastbunch is 112.54 MeV in an structure with beam-loading.

4.b Alignment System
In order to ·avoid any emittance growth in the linac,
the accelerating structures should be aligned to less than
200 pm r.m.s. of the vertical and horizontal directions.
The support tables of the accelerator section of the linac
have an active mover mechanism and wire-position sensors to align the linac components with a tolerance of lessthan 20 pm r.m.s. of the vertical and horizontal direc-Figure 4: RF system for the regular section of the, linac.

tions. The 91 m-long wires are stretched in both sidesof the linac from the preinjector stage to the end of thelinac. One end is fixed to the preinjector stage, which
does not have an active mover mechanism; the other end
is stretched by a tension weight of 33.5 kg. Each position sensor consists of a pair of induction coils electricallyconnected in series, and mounted on a vertically movable offset stage fixed at a support stage. The sensors
are installed at four corners of the support table for Qmagnets and beam monitors, and a short support table
for an accelerating structure. As for the long support
table for the two accelerating structures, six sensors areinstalled at four corners, and both sides of the center ofthe table. The wire position is detected by a synchronous
detection of the signal from the differential coils using
60 kHz, 100 rnA AC current on the wire. The resolution of position sensor is 2.5pm. The dynamic range ofthe sensors is ±2.5 mm, which is determined by the gap
length between two induction coils. The linac support tables are machined with an accuracy of less than ±lOpm.
These sensors are aligned along the sag of the wire witha vertical offset. As a result, the support tables are vertically and horizontally aligned with an accuracy of lessthan 20pm r.m.s ..

18--

JAERI-Conf 95-021

quencies; 2, 856±4.32727 MHz [15]. The energy spread ofthe multi-bunch compressed by the ECS would be smaller
than that of a single bunch.

Table 1: Parameters of 1.54 GeV ATF injector linac
Beam Energy
Bunch Population
Bunches/Train
Bunch Spacing
Repetition Rate
Energy Spread (Full Width
Beam Emittance
Total length
Pre-injector
Linac80 MeV Pre-Injector
Gun Voltage
Beam Energy
Number of Bunches
Bunch Population
Bunch Separation
Population tolerance
Bunch length (FWHM)
Normalized emittance
Regular Accelerating Sections
Accelerating Structure
Total length
Total number
Accelerating Field
Maximum Field
with Beam-loading
RF Frequency
Feed Peak Power
Klystron
Klystron Peak Power
Klystron Pulse Length
Number of Klystrons
RF Pulse CompressionPower Gain
Klystron Modulator
Total Number
Energy Compensation System
Accelerating Structures
RF Frequency
RF Frequency
Klystron
Total Number
Klystron Peak Power
Klystron Pulse Length
Klystron Modulator
Total Number

4.d Lattice
The lattice of the 1.54 GeV linac has been designed byusing SAD simulation code. The beam acceptance of thelinac is set to 7 x 10- 3 m, which is ±4.8u of the incoming beam from 80 MeV preinjector. The result of a simulation of emittance blow-up due to the wakes of cavities under misalignment of accelerator components withan orbit correction using the beam position monitors.
The simulation is performed by assuming the injection
error .6.x=.6.y=l mm, and skew rotation error of quads is0.2 mrad. The single bunch energy spread at the exit isu, = 0.3 %, and l.6.p/pl $0.75 %. These results showthat a misalignment of less than 500 p.m r.m.s. is notserious after a simple orbit correction.

1.54 GeV2 X 10 10 electrons/bunch
20
2.8 ns25 pps
<1.0%<3xlo- 4 m·rad (la)88 m
18m
70 m (active length: 48 m)
200 kV [240 kV achieved]
80 MeV [105 MeV achieved]20
2xl0 10 electrons
2.8 ns:$ ±1.0%<10 ps<3xlo- 4 rad m (rms)5211" /3 mode constant gradient3m
1643 MY /m [52 MY /m achieved]
33 MeV /m [40 MeV /m achieved]
2.856 GHz
200 MW /Structure
80 MW [85 MW achieved]
4.5 /-IS

Acknowledgement

The authors would like to thank Prof. Y. Kimura andProf. K. Takata for their encouragements and discussions
during the work. They wish to thank to Mr. S. Morita forhis help during the construction of the linac. They also
wish to thank to the ATF group for their useful discussions and kind collaborations.

REFERENCES[1] Y. Kimura, "Electron-Positron Linear Collider R&D Programat KEK" , Proceedings of the 6th Symposium on Accelerator
Science and Technology (held at Tokyo, Oct. 1987).

8

Dual-iris SLED
5.Dat peak[2] K. Takata, "The Japan Linear Collider", in Proceedings ofthe 1990 Linear Accelerator Conference (held at Albuquerque,U.S.A., Sep. 1991) pp 18-20.
[3] K. Takata, The JLC Project and ATF, Proc. of the 3rd Workshop on Japan Linear Collider (JLC) (held at KEK, Feb.
1992), KEK Proceedings 92-13, pp 1-6.

82856 + 4.32727 MHz2856 - 4.32727 MHz[4] S. Takeda et al., "ATF (Accelerator Test Facility)", Conference Record of the 1991 IEEE Particle Accelerator Conference(held at San Francisco, May 1991) pp 2047.
[5] S. Takeda et al., "1.54 GeV S-band Linac for Accelerator TestFacility", Proc. of 15th International Conference on High Energy Accelerators (held at Hamburg, July 1992) pp 839-841.

250MW
1.0 /-IS
2[6] S. Kuroda et al., "ATF Beam Transport Line", Proc. of the1994 International Linac Conference (held at Tsukuba, Aug.
1994) pp 83-85.
[7] J. Urakawa, KEK Proceedings 91-10, 1991, pp 18-33.

4.c

Energy compensation System

In the damping ring the variation of bunch spacing is
not acceptable, the energy compensation system by using four dipole magnets is not applicable. The proposed.6.f energy-compensation system (ECS) is a new idea tocompensate for the multi-bunch energy by keeping thebunch separation synchronized with the rf frequency. By
passing the multi-bunch through an accelerating structure
driven at an rf frequency which is slightly larger or smaller
than the fundamental frequency, the multi-bunch would
be obtained the different energy gain caused by the phaseshift. As a result, the energy spread of the multi-bunch iscompressed to a small value, which is required from thedamping ring. The compensation energy depends on theZ-position of the electrons in a bunch, since the bunch
has a bunch length and the compensated field has a slopeof the part of sinusoidal wave. If the bunch is compensated by both a negative slope and a positive slope, theeffect of the slope is canceled and the bunch would be accelerated or decelerated by a flat-top field. The system
has high flexibility for bunch populations from zero to
4x10 10 electrons/bunch by adjusting the rf power of theklystrons. The system consists of two klystrons and two
3 m-long accelerating structures designed at two rf fre-[8] H. Hayano et al., "1.54 GeV Injector Linac for ATF Damping
Ring", Proc. of the 1994 International Linac Conference (heldat Tsukuba, Aug. 1994) pp 50-52.
(9] H. Hayano eta!., "An 80 MeV Injector for ATF Linac", Proc.
of the 1994 International Linac Conference (held at Tsukuba,Aug. 1994) pp 236-238.
[10] T. Naito et al., "Multi-bunch Beam with Thermionic Gun forATF", Proc. of the 1994 International Linac Conference (heldat Tsukuba, Aug. 1994) pp 375-377.
[11] T. Naito et al., "Bunch by Bunch Monitors for ATF Injector Linac", Proc. of the 1994 International Linac Conference(held at Tsukuba, Aug. 1994) pp 887-889.
[12] M. Akemoto et al., "Pulse Modulator for 85 MW Klystron in
ATF Linac", Proc. of the 1994 International Linac Conference(held at Tsukuba, Aug. 1994) pp 415-418.
[13] H. Matsumoto et al., "High Power Test of a SLED Systemwith Dual Side-Wall Coupling Irises for Linear Collider",Nucl. Instr. and Meth. A330 (1993) pp 1-11.
[14] H. Matsumoto et al., "High Power Test of a High GradientS-band Accelerator Unit for the Accelerator Test Facility",
Proc. of the 1992 International Linac Conference (held at Ottawa, Aug. 1992) pp 62-64.
[15] T. Korhonen et al., "R&D of the ATF Timing System", Proc.
of the 1994 International Linac Conference (held at Tsukuba,Aug. 1994) pp 831-833.

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