NEW PROJECT OF 1.5 GeV ELECTRON LINAC AND PULSE STRETCHER
T. TamaeLaboratory of Nuclear Science, Tohoku University
Mikamine, Sendai 982, Japan
High duty cycle beams are inevitable for Rew electron accelerators due to the rapid developement of nuclear physics.
After considering several types of accelerators to deliver
continuous beams, a conventional linac together with a pulse
stretcher has been chosen because of its existing technology
and adaptabilities for various research. Stretchers have beenstudied at several laboratories and noticed that no serious
difficulty exists for construction. Layout of the new linac
is shown in Fig. 1. The 210 m long linac is followed by anenergy compressing system(ECS) in order to improve an energy
spectrum of the beam from 2% to 0.2%. When a continuous beamis required, the pulse stretcher ring(STR) is excited toconvert a pulsed beam into continuous one with a duty factor ofbetter than 90%. The ring is also used for a storage ringcreating synchrotron radiation and monochromatic gamma rays.
A high resolution electron spectrometer(SP-E) and a protonspectrometer(SP-P) are set up in the electron scattering hall.
These spectrometers are to be used for coincident experiments
such as (e,e' p) in addition to single arm experiments. The
photonuclear experimental hall is equipped with a broad range
spectrometer and an apparatus for a tagged photon experiment
which is a very promissing field with use of continuous electron

TPBS;Broad Range Spectrometer
TP;Tagged Photon
ME;Meson Experimental Hall
SP-E;Electron Spectrometer
SP-P;Proton Spectrometer
SR;Synchrotron 'Radiation
MG;Monochromatic Gamma Ray
ECS;Energy Compressing System
STR;Pulse Stretcher
KM;Kicker Magnet
ND;Neutron DiffractionNtMESTRCD

FIG. 1.

Layout of the Next Tohoku Electron Linac-343-beams. The meson experimental
hall is devoted to the studies
concerning pion, kaon and muon
Experiments of neutronphysics.
diffraction are to be made in theND hall.

2.5

2.0 ~

1.5 GeV Linac'it5~;;;t::N:?ol.tw~... ~......

I',,
-'-s>
'An accelerator system consists'l?
' 'ccof an injection system, 32 accele'w
rator waveguides and ECS. Each
151.0of them is connected to one of the34 klystrons. Electron beams areaccelerated after being bunched at
0.52856 MHz by a prebuncher and a
Since rf cavity in STRbuncher.
operates at 476 MHz, the beam canbe bunched at 476 MHz by a sub300
200
1000
harmonic buncher before injected
PEAK BEAM CURRENT(mA)
waveguide,Ainto a prebuncher.
FIG. 2. Current-energy
which is 4.85 m long, consists ofrelation of 1.5 GeV linac.
131 cavities with a variation of
The numbers beside thedisk hole diameters to make alines indicate the peakquasi-constant gradient type.
of rf input powervalues
In order to prevent beam blowup
to an accelerator wave(BBU) , all 32 waveguides haveguide.
slightly different variations of
disk hole diameters and waveguides
with closest dimensions are arranged as not being neighbors. The repetition rate of the beam canbe varied from 50 pps to 600 pps. The beams with several kindsof parameters, such as pulse width and electron energy,
can be accelerated pulse to pulse so that two or three experiments are made simultaneously. The beams are separated at theend of the linac by a pulsed kicker magnet, for example, someportions of the beam is delivered to the ND experiment, andthe rest of the beam is used for other experiments. Fig. 2
shows energy-current characteristics of the linac and main
parameters of the electron beam are summerized in Table l.
Table lParameters of Electron Beam

Energy (unloaded)
Energy {loaded)
Current (peak) ;Ip
Current {average)
Power (average)
Pulse width;Tp
Energy spread
Pulse repetition rate
Acceleration frequency
Emittance;Et--'"'2 GeV

300 MeV - 1.5 GeV
10 - 1000 mA200 )lA {max.)
300 kW {max.)
0.1 - 0.5 )lsec
0.5 - 3.0 )lsec {I~ <250 mA)"'2 % {without ECST)"'0.2 % {with ECS)
50 - 300 pps
50 - 600 pps { Tp ~l \lsec)
2856 MHZ~1 mm mrad.

:§_nergy f_ompression §_ystem-344-

Pulse Stretcher
The pulse stretcher ring is 160 m in circumference andconsists of 4 straight sections and 4 quadrats. Each of thequadrats is composed by 8 bending magnets of 8 m radius ofcurvature and 8 quadrupole magnets. Each quadrat forms an·achromatic system so that the value of the 0ff-energy function
vanishes in all straight sections. The third integer resonanceof 19/3 in the horizontal direction is used to extract acontinuous beam from the ring.
The betatron frequencies are
6.3 and 6.2 in the horizontal and vertical planes respectively.
At the time of injection, the equilibrium orbit is brought near
the septum magnet by bumper magnets and the betatron frequency
is changed to v =6.25 by pulsed quadrupole magnets. After athree turn injeetion is over, the bum~er magnets and quadrupole magnets are turned off during the fourth turn and thetrajectory is returned to the central orbit. Circulating
electrons lose their energy through synchrotron radiation.
STR is operated withour rf acceleration as much as possible,
because it breaks uniformity of the beam bunched. The beam
extraction is made bydifferent modes fordifferent ranges ofelectron energy. The
beam extraction modes
3%
0·1%
are shown in Fig. 31000
RF Accelerakn
Achromatic
When electrons lose over
Monochromatic
3% of their energy throughextraction
extraction
Achromaticsynchrotron radiation inextraction
the ring, rf acceleration
has to conpensate the--600 pps500energy. The achromatic
extraction is adopted inthis energy range, chang- - - - 300 pps
ing the betatron frequency
in the horizontal direction asymptotically close
to 19/3 by means of thequadrupole magnets. When
1.So.s
1·00Ee (GeV)
the energy loss is under
0.1% of the injected
electron energy, no rf
FIG. 3. Beam extraction modes of s·:·::;,.
acceleration is needed.
Different modes are adopted accordin9
The achromatic extraction
to the electron energy and the repeti "is also used in thisrate.
Table 2

Parameters of Pulse Stretcher

Circumference
Radius of curvature
Betatron frequency
Injection mode
Extraction mode
Extracted beam emittance
Dilatation factor
RF frequency
RF peak voltage

160 m

8.0 m
6.30 (horizontal)
6. 20 (vertical)
3 turn horizontal oscillation
third integer resonance
0.8 mm mrad (horizontal)
1.0 mm mrad (vertical)
0.045
476 MHZ

200 kV-345 __:_

range. Electrons losing their energy between 0.1% and 3% in thering can be extracted by the monochromatic extraction. the beam
is injected with energy spread equal to the maximum energy loss.
The lowest energy of the injected electrons corresponds to thethird integer resonance. Electrons, losing their energy
through synchrotron radiation down to the energy of theresonance, are extracted consecutively. An average current of
96 VA will be obtained, which is equal to the average current
from the linac when the linac is operated with the repetition
rate of 300 pps, 1.6 vsec in pulse width, and a peak current of
200 rnA. The main parameters of STR are listed in Table 2.
Test Stretcher Ring
A small stretcher ring is under construction, which is used
for studying technical problems and supplies continu.ous beams to
the present electron spectrometer. A two turn injection puts aelectron beam of 100 nsec in pulse width into the ring ofaround 15 m circumference. The maximum energy of stored
electrons is 150 MeV. The betatron frequencies are 1.3 and 1.2
in the horizontal and vertical planes respectively. The monochromatic extraction by the third integer resonance of 4/3 inthe horizontal plane is introduced, which has been stated
before. A 3 vA electron beam is expected to be obtained witha duty factor of 90%, when a pulsed electron beam of 100 rnA
peak is injected with the repetition rate of 300 pps. Fig. 4
shows the layout of the test ring.
MQFQ
PQ
HM
PHM
KM
SMISMESM
SE
LDM

Bending magnet
Quadrupole magnet
Quadrupole magnet for fine tuning
Pulsed quadrupole magnet
Hexapole magnet for high order corrections
Pulsed hexapole magnet
Kicker magnet
Septum magnet for injection
Septum magnet for extraction
Septum magnet
Septum electrode
Electron spectrometer~-()LDM

Ml2

M9QFIG. 4.

Layout of the test pulse stretcher.

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