JAERI-Conf 95-021

PA03
Helium Beam Acceleration in The KEK Proton Synchrotron!.Sakai, A.Takagi, Y.Mori*, S.Machida, M.Yoshii, T.Toyama, M.Shirak:ata, Y.Shoji and H.SatoNational Laboratory for High Energy Physics, Oho 1-1, Tsukuba Ibaraki 305 Japan
A helium beam has been successfully accelerated inthe KEK 12 GeV Proton Synchrotron up to 23 GeV of thelimiting energy of the main ring. Although a charge
exchange injection of negative hydrogen ions has been usedin the booster synchrotron, it cannot be applied to theinjection of positive ions, such as helium ions and otherheavy ions. A new type of septum magnet for a beam
injection system which can realize both negative andpositive ion injections, has been developed. The septum
magnet has three conductor plates which combine the bumpmagnet and the septum magnet in a body. A newly developed
injection system realized alternative use of both negativeand positive ion injection schemes.

I. INTRODUCTION
The charge exchange injection system has been
widely used for the injection system of proton
synchrotrons, because of its excellent efficiency of multitum injection of proton beams. This system is also used
to inject proton and deuteron beams into the KEK
booster synchrotron [1]. This system, however, cannot
be applied to the injection of positive ions, such as
helium ions and other heavy ions. Therefore, aconventional multi-tum injection scheme must take theplace of the charge exchange scheme. There is no space
for another injection system because of the booster
designed well-compact. To replace the injection devices
every operation cycle is not practical.
To avoid this, a certain technical break-through isindispensable. A new injection system has beendeveloped to realize both negative and positive ioninjection. With this system, in April, 1994, a helium
beam was successfully accelerated, and a physics
experiment has been carried out [2]. Also, a highintensity proton beam has been accelerated by using thissystem.
Until now negative ion charge-exchange injection
system and positive ion multi-turn injection system
regarded to be a distinct system. But this new injection
system has opened the multi-purpose uses of the proton
synchrotron, i.e. from high intensity proton beam
acceleration to high energy heavy ion beam acceleration .
The principle of the newly developed positive/negative
ion injection system and the first helium acceleration issummarized in this paper.

helium-ion acceleration is shown in Figure 1. Fourbump magnets having single-turn conductor plates arearranged asymmetrically so as to enlarge the angle
between the injection orbit and the circulating orbit undera limited magnetic field. Bump 2 is a newly developed
magnet, which is combined with the septum magnet for
positive ion injection.
During a charge-exchange injection, only thebump magnets are excited. This situation is quite
equivalent to that in the conventional charge-exchange
injection system. These bump magnets are connected inseries, and are excited up to 0.39 T in order to change theclosed orbit so that the beam impinges on a carbon
stripping foil. In Figure 1 the dotted line indicates thecharge-exchange injection. Negative hydrogen ions are
fully stripped at the foil, and start to circulate in the ring.
In order to prevent an emittance blow-up due to multiple
scattering at the foil, the bump magnets must be turned
off just after the completion of injection.
B. Multi-Turn Injection by a Septum Magnet
The solid line indicates positive-ion multi-tum
injection which overlaps the charge-exchange by bump 2.
A cross-sectional view of bump 2 is shown in Figure 2.
This has been newly developed and plays the role tocombine the bump magnet and the septum magnet. This
combined magnet bump 2 has three single-turn conductor
plates in the pole faces [3]. The electrical currents
flowing through these conductors produce two dipole
magnetic fields having opposite signs, which areseparated by a middle thin conductor plate (1 mm thick),which forms a septum.
During positive-beam injection, the closed orbit
must be shifted by 60 mm toward the septum magnet inparallel which is achieved by exciting two additional
bump magnets (not shown in Figure 1). Their positions
are one quarter of the betertron wavelength upstream anddownstream from the injection point. Beams aregradually injected from the center of the phase space tothe outside by changing the field excitation of theadditional bump magnets.
Bump 1

Bump2

Bump 3

Bump 4

II. NEW INJECTION SYSTEM
A. Charge -Exchange Injection
A schematic layout of the combined injection
system which is presently being used for proton andSeptum Conductor*Present address: Institute for Nuclear Study, Universityof Tokyo, 3-2-1, Midori-cho, Tanashi, Tokyo 188 Japan

Stripping Foil

Fig. I. New injection system for the KEK-PS booster-29-JAERI-Conf 95-021

Central Orbit
Negative Ion Injection Point
Positive Ion Injection Pointr-r.,.....,"'*..,......,.+r~~

1-81Magnetic
Field:!Septum Conductor
H ltmm thick)(a) bump magnet (b) septum magnet (c) bump + septum:··

Fig.4. Excitation current on the central septum conductor.

·:
or---L-~~----_.--n··~---

IV. ACCELERATION OF HELIUM IONS

Bump Magnetic
Field 1--l
Septum Magne:ic
Field
Fig. 2. Cross sectional view of Bump 2 and the exited field
pattern for positive beam injection.

III. POWER SUPPLY SYSTEM

Most of the problems in RF acceleration of lightions of Q/A =0.5 at the KEK-PS had been cleared at thetime when deuteron acceleration succeeded in 1992.
However, in the acceleration of helium ions, there
remains difficulties with respect to multi-tum injectionof the positive ions.

A. Booster Injection

A block diagram of the power supply for thebump and septum magnets is presented schematically in
Figure 3. The wave form of the bump field (excitation
current of the bump magnet) for charge-exchange
injection is shown in Figure 4 (a). The bump magnets
must be turned off just after completion of injection, inorder to prevent any emittance blow-up due to multiple
scattering at the stripping foil. This can be realized byusing a PFN-type pulsed power supply. As for theseptum magnet, the pulse width, since it depends on thenumber of possible turns at beam injection, would beenough for 30 jJ.Sec. Although it may be possible to use
DC current, in order to eliminate any cooling problems,pulsed-mode operation is necessary. The wave form of
the septum field (excitation current of the septummagnet) is shown in Figure 4 (b). This can also be
realized with a PFN-type pulsed power supply insulated
from ground, whose electric power is provided by aninsulating transformer. The PFN and its power supply,
floated on a high-voltage insulation, are controlled
through optical fibers [4].
As shown in Figure 4(c), in the middle thin
conductor which forms a septum, a current that is twiceas large flows instantaneously during positive ion-beam
injection by overlapping the bump current and theseptum current. However, since the pulse width of thecurrent is less than 40% of the bump current, it is notnecessary to add another cooling system to the ordinary
magnet system.

The 750kV He2+ ion beam is injected into thelinac. The helium beam is accelerated under the 47tmode
operation scheme[3]. Its velocity is half that in the caseof proton acceleration. As a result, at the end of the linac
it has 3% less momentum than that of proton beam. The
beam transport parameters and booster injection
parameters must be optimized according to the beammomentum.
During multi-tum injection the beam is painted
on the horizontal phase space. As shown in Figure 5,
trace (c), by thanging the decay current of additional
bump magnets, the circulating beam orbit during aninjection period was moderately shifted for efficiently
stacking the linac beam. This is also a new attempt inaccelerator technology. Four turns of the Iinac beam wereaccumulated with this technique.

Figure 5. Current wave forms of the injection magnets andbeam current monitor at the booster injection: (a) septum,(b) main bump, (c) additional bump with two time-decay
constants and (d) beam current.
Fig. 3. Block diagram of the power supply.

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We observed a very fast beam loss; only half of
the accumulated beam was extracted from the booster.
This beam loss was caused at the time that the mainbump magnet field fell sharply to zero. Field
measurements of the main bump magnets showed thatthe eddy currents induced in the magnets caused a total
error field, so as to cause closed orbit distortions, themaximum value of which was about 4 mm. The total
error field due to the septum magnet eddy current, as
well as the main bump magnet, was also measured.
However, both of the error fields induced by eddy
currents are of opposite sign and can cancel each otherby adjusting falling time of both magnets.
After the schedule of helium beam experiments,
the problem of beam loss at the fall period of main bumpmagnet field. has been solved by this cancellation, whichis confirmed by machine study using positive proton
beams. The acceleration of higher intensity helium ion
beams is expected in the next experiment.
B. Acceleration in the Booster and Main Ring
Synchrotrons
For heavy ion accelerations in both the boosterand main ring synchrotrons, the radio-frequencies of theaccelerating systems are lower during injection, andsweep more widely than in proton acceleration. The
tuning systems in both synchrotrons have been modified.
The injection frequencies can be changed by attaching
additional capacitors in both systems. Especially, in themain ring synchrotron, the bias current power supplies
for the ferrite loaded tuning cavities were upgraded.
The intensity of a helium beam was expected to
be at most on the order of 1010 particles per bunch. To
stable accelerate such a low intensity beam the preamplifiers of both the position and beam phase monitors
for the RF feedback systems have been improved.
During a commissioning run lasting one month,
the helium beam was very stable, it was also expected atvarious energies of 8 GeV (2 GeV/u) to 20 GeV (5GeV/u), based on the requirements of the physics
experiments.

V. SUMMARY
A new injection system which combines thebump magnet for charge-exchange injection and theseptum magnet for positive-ion multi-turn injection has
been developed to realize both negative and positive-ion
injection in the KEK PS. Although the "combined"
system is a rather novel one, it is not a fragile system asa last resort. The combined injection septum magnet canform a sharp, steep septum field, as shown in Figure 2,
rather than that of the conventional septum magnet used
to separate the circulating beam and injection beam.
Using this combined injection system in April, 1994,
operation to accelerate helium-ion beams (He++) for
physics experiments had been performed for one month.
After that, proton-beam acceleration by charge-exchange
injection has continued until now (May 1995) without
any trouble. The combined injection system has theperformance equal to a conventional charge-exchange
injection system.
This new injection system has opened up newuses of the KEK PS, i.e. from high-intensity proton
beam to high-energy heavy-ion beams.

VI. ACKNOWLEDGMENT
I would like to express sincere appreciation to theDirector General of KEK, H. Sugawara, as well as Profs.
M. Kihara, Y. Kimura, Y. Yamane and T. Kawakubo
for their continuous encouragement. I am also indebted toProf. Mori, University of Tokyo, T. Kurosawa, Y.
Arakida, and the staff of heavy ion acceleration group inthe KEK PS for their valuable discussions and technicalsupports.

VII. REFERENCES[1] Y. Mori, et. al., Acceleration of Deuteron Beam in
the KEK Proton Synchrotron, IEEE Particle AcceleratorConference, 1993, p.3754.
[2] I. Sakai, et. al. Helium Beam Acceleration in The
KEK Proton Synchrotron with A Newly Developed
Injection System for Positive/Negative Ions IEEEParticle Accelerator Conference, 1995, to be published.
[3] I.Sakai, Magnet System for both Negative-Ion
Charge-Exchange Ingection and Positive-Ion Multi-Tum
Injection, 14-th International Conference on Magnet
Technology[4] I. Sakai, et. a!. A New System to Trigger Fast RiseIEEE Particle Accelerator Conference,
Thyristers,
1989, p.2005.

Figure 6. Helium acceleration in the main ring: (a) a helium
beam intensity during a cycle, (b) a slowly extracted
intensity and (c) a main magnet field pattern.

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