The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

A Preliminary Study of the Proton Storage Ringfor the Neutron Science Project at JAERIMichikazu KINSHO, Fumiaki NODA, Joichi KUSANO, and Motoharu MIZUMOTO
Japan Atomic Energy Research InstituteTokai-mura, Naka-gun, Ibaraki-ken 319-11, JapanAbstract
The goal of the proposed Neutron Science Project( NSP ) at JAERI is to provide a short pulsed proton beam ofless than lr..ts with an average beam power of 5MW. Toachieve such purpose, a proton storage ring operated at 50Hz
with 4.17x1014 protons per pulse at 1.5GeV is required. The
preliminary study of the ring which satisfies specification of
the neutron science project is described in this paper.
1. Introduction
Japan Atomic Energy Research Institl)te, JAERI, hasbeen proposing the Neutron Science Project (NSP) which iscomposed of research facilities based on a proton linac and a
proton storage ring with an energy of 1.5GeV[l ]. The
proposed NSP is aiming at exploring new basic researches andnuclear technologies such as conoonsed matter physics andnuclear waste transmutation based on a proton accelerator. In
the proton storage ring, the pulsed beam from the linac isaccumulated, and high intensity pulsed beam is produced forthe neutron scattering experiment. The goal of the proton
storage ring is to provire a short pulsed proton beam of lessthan 1 !J.S with an average beam power of 5MW with two rings.
This paper describes the preliminary study of the 2.5MW
proton storage ring. Figure 1 shows a layout of the 2.5MW
proton storage ring including the High Energy BeamTransport (HEB1) line which connects the 1.5GeV linac to a
proton storage ring. The beam coming out of the linac is a
1.5GeV R beam and 1.86ms long with a peak current of30mA. The beam is chopped to the pulse length of 400ns with270ns gap. The beam is compressed by means of a multi-tum
charge exchange injection. When a harmonic number of thering is 1, a circumference and a revolution frequency are 185m
and 1.49MHz, respectively. The single bunch in the ring iscontained by rf resonant cavity. To achieve a beam power of
2.5MW with this beam structure, it is necessary to accumulate
2777 bunches. This corresponds to 2.08x1014 protons. Whenthe beam injection is completed, accumulated protons areextracted .from the ring during 1 tum. The average current
circulating in the ring with 1.49MHz revolution frequency
becomes 49.75A. At such a high average current, a beam lossof a very small fraction makes a very high radioactivity aroundthe ring. It is necessary to examine reduction and localizationof the beam loss with sufficient consideration of thedivergence of the beam by the space charge force, the resonancephenomena by the tune shift, longitudinal instability, e-pinstability and so on.
2. High Energy Beam Transport LineThe High Energy Beam Transport line (HEBI) connects
the 1.5GeV linac to a proton storage ring. A m:Yor
requirement of this line is to have low uncontrolled beam loss
in the ring in order to allow hands on maintenance. To achieve
such low beam losses in the ring, the beam must be preparedvery carefully before injection. The HEBr not only matches
the beam into the ring, but also determines the beam quality atinjection. The HEBr has following functions : (a) matchingof the beam from the linac into the transport line, (b)horizontal and vertical betatron and momentum collimation,(c) focusing the H" beam to the correct spot size for injection,and (d) halo eleanup. To reduce the probability of uncontrolled
beam losses, HEBT is equipped with many beam halo scrapers.
The ratio of aperture to rms beam size is kept greater than 10
throughout the line. The maximum magnetic field in dipoleand quadrupole is kept less than 1.8kG to keep H- stripping
losses to acceptable levels. More detail study of HEBT system,
for example the magnet lattice, components, etc., has been
performed.

Linac-341-Fig. 1 Layout of a 2.5MW Proton Storage Ring

The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

3. Storage Ring
Magnet Lattice

The magnet lattice is temporarily selected to be normal12
cell FBDO lattice. FBDO lattice is chosen for its smoothness
betatron function variation around the ring. This latticeensures the distance between defocusing quadruple magnet andfocusing quadruple magnet at 7.5m. The operation tune is inthe range where a phase advance per 1 cell becomes 80-110
degrees and provisionally selected to be (v x• v y) = (3.85,3.75).
The betatron function and energy dispersion function havebeen evaluated by using the lattice analysis program "MAD"developed at CERN. The calculated data are shown in Fig. 2
A large transverse beam emittance is required in the ring
to restrict the transverse space charge tune shift. The. chosen
values for an un-normalized 100% transverse emittance of an
injected H- beam, a ring acceptance and a collimator acceptanceare 2Jtmm-mrad, 530:rtmm-mrad and 200:rtmm-mrad,respectively .. This transverse emittance restricts the spacecharge tune shift to less than 0.1. Preliminary lattice function
and design parameter of the storage ring are summarized inTable 1.
3025..... 20E>. 15

Cl:l>< 10Cl:lTable 1 Preliminary lattice function and design parameters
Beam Average Power
2.5 MW I ring
Kinetic Energy
1.5 GeV
Average Current
1.67 rnA 1 ring
Repetition Frequency
50 Hz
Linac Peak Current
30 rnA
Linac Pulse Length3.72 ms
Number of Turns Injected2777 turns/ ring
Injected Beam Gap
270 nsInjected Pulse Length
400 nsHarmonic Number1Revolution Frequency
1.49 MHz
Circumference
185.4 mMagnetic Rigidity
7.51 Tm
Circulating Current
49.75 AI ring
No. of Circulating Protons 2.08xl0 14 protons I ring
Lattice
12 Cell FBDO
Ring Acceptance
530:rt mm-mrad
Collimator Acceptance
200:rt mm-mrad
Tunes(v.,vy) =(3.85,3.75)
Transition Energy
y t =3.54
Space-Charge Tune Shift< -0.1
Rf Cavity
Frequency
1.49 MHz
Harmonic Number
1Voltage
21 kV
Kicker Magnet
Reflection Angle
10 mradMagnetic Field Strength 0.02 TLength
3.8m

50 ~~~~~~~~~~~~~02468101214 160s [m]Fig. 2 Betatron and Dispersion Function of 12Cell FBDO
Lattice
Beam Injection

There is a main source of beam loss in an injection areaand hence the most critical item for the ring because the main
loss is due to H- or H+ beam intersection with a stripping foil
during and after injection. There are two types of injection
method for charge exchange. One is the direct R injection
method in which H- beam is converted to H+ beam with thestripping foil located in the injection magnet. The othermethod is the Jrl injection method in which H- beain is firstlyconverted to Jrl beam in a stripping magnet outside of the ringand then Jrl beam is injected to the stripping foil insi<k thering. In both method, it is very important to minimize delayed
stripping of ffl atoms in the ring. The lifetime of ffl atoms
depends on the Stark state in the magnet [2]. The lifetime ofl.SGeV Jrl atoms is shown in Fig. 3. In case of H0 injection
method, because there is no injection magnetic, the Jrl atoms
excited by the foil become H+ ions in a bending magnet. Whenthe magnetic field strength of bending magnet is lT, the life
time of the atoms whose principal quantum number n is high(>3) becomes less than lOps. An injection magnet is normallyused for injecting H- ions to the ring in H- injection method
When the magnetic field strength of the injection magnet isO.lST, atoms with n~4 remain as Jrl and may be removed
from the ring, andatoms with n>S rapidly become H+ andare
accepted in the ring. At present it is under consideration which
method should be adopted to reduce the beam loss in the ring.
A large transverse beam emittance is required not only torestrict the transverse space charge tune shift but also to reducethe circulating proton beam intersection with a stripping foil.
To obtain the large transverse emittance the phase-space
painting is also considered [3].
Radio Frequency Cavity-342-

Though the acceleration of the beam is not carried out inThe II th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

10·7'"'
0when injection of all bunches is completed, accumulated beam
is extra0ed from the ring during 1 tum. An interval between
bunches is 270ns from the injection beam pulse structure
which is chopped 670ns bunch width with 60% duty cycle. It
is necessary that the magnetic field.of a kicker magnet isenough to extract the beam from the ring to less than 270ns
and is kept the strength to more than 400ns. In fact, therequired field rise time of kicker magnet is less than 150ns
considering the increase of the beam bunch due to synchrotron
oscillation and divergence by the space charge effect duringmulti-tum ring injection. When the un-normalized 100%
extraction emittance, beta function, dispersion function, andmomentum spread are 200nmm-mrad, 15m, 3m, and 0.41 %,respectively, the reflection angle becomes about 10 mrad A
kicker magnet of 0.02T and3.8m is required in order to realize
this extraction process.

10·8(])(/)'-'(])

10·9

E+=(])::; 10·10

Magnetic Field (Tesla)
Fig. 3 Stark State Lifetimes in Lab. Framethe rf cavity, the rf cavity is required for maintaining the bunch
structure in the storage ring. The injection beam is chopped atthe bunch revolution frequency of 1.49MHz. The necessary
cavity voltage is the sum of voltage VP which is proportional
to the momentum spread of the injection beam and voltage V,cwhich supplements the decreasing voltage by the space charge
effect. The VP voltage is about 13.5kV by assuming that themomentum spread is to be ± 0.41 %. The necessary V,.,
voltage changes according to the accumulated number of beam
bunches. When all beam bunches are accumulated, the V,.,
voltage becomes about 7.5kV under the condition that thelongitudinal distribution is parabolic. The rf voltage is raised
from 13.5kV to 21kV to maintain the beam bunches duringinjection.
The rf cavity consists of capacities, resistors, and ferrite.
Because the circulating current is about 49. 75A, it is likelythat the cavity characteristics should be changed due to thehigh frequency magnetic field induced by circulating current.
Therefore, the rf cavity is important issue to be developed.
Beam Extraction
The fast extraction method is used from the request of theneutron scattering experiment. In this extraction method,

4. Upgrade Path
The phase I construction of the NSP project is a 1.5MW
spallation neutron source. A systematical upgrade path has
been proposed The upgrade from 1.5MW to 2.5MW will beachieved by increasing a peak current or a pulse length of thelinac beam. The upgrade from 2.5MW to 5MW will beaccomplished by providing a second ring to aim the beams
from both rings at the same target. In this case, the beamtransport system has to combine the beam bunches from thetwo rings into a single pulse of less than 1!1-s duration.
References[1] M.Mizumoto et al, A High Intensity Proton Linac
Development for Neutron Science Research Program
XVlli International Linac Conference, LINAC96, Geneva,Switzerland 26-30 August 1996, p662-664[2] R. Damburg and V. V. Kolosov : 'Theoretical studies ofhydrogen Rydberg atoms in electric field>", e<E. R. F.
Stebbings and F. B. Dunning, "Rydberg states of atoms
and molecules", Cambridge University Press, 1983, p3171[3] F. Noda et al, : A Beam Simulation of the Proton Storage
Ring for the Neutron Science Project, in these proceedings-343-