Intensity Doubler: A Proposal for a Major Upgrade of the KEK 12GeV-PS
Ken Takayama, Dai Arakawa, Yoshio Arakida, Susumu Igarashi, Yoshiro Irie, Masahiko Uota,Tadamichi Kawakubo, Junichi Kishiro, Hikaru Sato, Yoshito Shimosaki, Masashi Shirakata,
Takeshi Toyama, Eiji Nakamura, Katsumi Marutsuka, Isao Yamane, Koichiro Nishikawa, Makoto
Sakuda, and Masaharu Numajiri
High Energy Accelerator Research Organization (KEK), Tsukuba, Japan
Abstract
A proposal for a major upgrade of the KEK 12GeV-PS
is presented. The proposal involves a combination ofbeam stacking in a 500MeV accumulator ring (AR) andacceleration of a superbunch in the 12GeV main ring(MR), referred to as the Intensity Doubler (ID). A
super-bunch is created in the AR by barrier-bucket
stacking of 12 bunches injected from the booster ring(BR), and is transferred to the MR in one turn. By
introducing the AR in the 12GeV-PS accelerator complex,
it is possible to reduce the present injection time of about
500msec and increase the machine duty-cycle.
Superbunch acceleration is achieved by rapidly switching
induction units. Through the final two years of the K2K
experiment, the ID could supply proton beams that would
be two times higher than the present levels in terms ofaveraged intensity. The scheme is based on the conceptof induction synchrotron [1]. A quite unique features ofsuperbunch acceleration such as bunch-stacking in thebarrier bucket is discussed with actual parameters of theKEK-PS. The present status of R&D works for the ID
are reported.

1 INTRODUCTION
Since June 1999, the MR has been delivering proton
beams to the target in excess of 6x1012ppp for the K2K
experiment. The extensive effort to increase the beam
intensity is reviewed in the reference[1]. A beam intensityof 8x1012ppp was achieved after acceleration in 2000,
March, and the MR is currently operating with a nominal
beam intensity between 7.2 to 7.5x1012ppp. However,from a statistical accuracy perspective, further increases inbeam intensity are clearly desirable. If the ID were to
begin operating in the beginning of 2004, it is estimatedthat the number of neutrino events would increase by 40%
before the scheduled run-end. It is essential to upgrade
the KEK 12GeV-PS before the MINOS and theCERN-Gran Sasso experiments commence with higher
energy and appropriate beam intensities.
According to our extensive machine studies[2], it seems
that proton beams with substantially higher local densities
than those under the present operation are not acceptablewith the aperture of the MR. The only way to achieve
more protons is to stack them in the longitudinal phasespace, in other words, along the time coordinate. An
accumulator ring placed between the BR and MR is quite
effective for this, and effectively reduces the acceleration
period by about 500msec. As a result, the acceleration
period can be reduced from 2.2sec to 1.6sec, with further
effort to eliminate another 100msec. The accumulator will
be installed in the MR tunnel. Taking advantage of thefact that the ring operates at a fixed momentum, themagnets are built using permanent strontium ferritemagnets. In conventional synchrotrons, a number ofprotons being accelerated is simply in proportional to thestacking number of pulses in one acceleration cycle, withthe maximum stacking number being a function of theharmonic number of RF. Each pulse delivered from theBR is injected into the central position of the RF bucketwith temporal gaps between adjacent bunches, which mustbe sufficient to allow the injection kicker-fields to reachthe flat-top level. In the MR, this temporal gap represents
half of the total time of 1.5µsec required for 9 injected
Booster bunches (80nsecx9=720nsec), with remaining
time being unavailable for stacking. However, it ispossible to overcome this fundamental restriction
associated with the conventional RF acceleration byapplying the concept of induction synchrotron (IS)[3],
which employs induction accelerating modules instead ofRF cavities. The combination of dual-harmonic RFacceleration in the BR with superbunch stacking in theAR, together with acceleration of the superbunch in theMR should increase beam intensity per pulse by at least
60% and eventually makes the ID attainable.

2 SUPERBUNCH ACCELERATION
Unlike a conventional RF synchrotron, acceleration andlongitudinal focusing in an IS are independently provided
by two kinds of induction cell (IC), as schematicallyshown in Fig.1. A dc-like induction acceleration isprovided by the IC, which is energized with a long
voltage-pulse and a short reset-pulse. The other IC
which is excited by short-pulses generates a moving
barrier bucket (BB) in the longitudinal phase-space,which, rather than being fish-like, is almost rectangular inshape.
The moving rectangular bucket canaccommodate particles to its full capacity. The bucket
shape tends to create a uniformly diffused longitudinal
distribution of the particles apart from both edges of thebucket. The uniformity is important in diminishing thespace-charge effects in the transverse and longitudinal
directions. The frequency or phase feed-back in RF
acceleration, which makes tracking against the ramping
magnetic guide-field possible, is replaced by an induction
voltage feed-back and a programmable change in thetrigger-timing. These feedbacks should be rather simple,
unlike that in RF acceleration where the feedback gain
depends on the beam intensity and requires carefuladjustment.
In order to create a superbunch in the AR, two kinds ofbarrier buckets are employed, as illustrated in Fig.2.
The bunches injected from the BR into the AR arecaptured by a matched capture bucket. Each bunch ismoved adiabatically toward the edge of the stacking
bucket and is then released into the stacking bucket insuch a way that the reset timing for the edge voltage of thestacking bucket is delayed by the bunch-length of the
fresh bunch. Thus the fresh bunch merges into thestacked beam-core. After this process is repeated 12times, resulting in a 1 µsec-long superbunch, this is
extracted in one turn and injected into the MR. This isimmediately trapped in the BB in the MR, as describedabove, and is quickly accelerated by an integrated
induction-voltage of 25kV. An important point to notehere is the fact that a bunch in the BR is accelerated withdual harmonic RF so that they are well-matched to thecapture bucket, which, in tern, helps keeping theline-density low.

3 500MEV ACCUMULATOR RING
The AR is to be located in the existing 12GeV-PS
tunnel, which is almost circular in order to match the12GeV-PS with a super-periodicity of 4, each consistingof 7 FODO cells. Although the tunnel is 4 m wide, muchof this considerably large cross-sectional space isoccupied by the MR lattice magnets, together with theircable racks and cooling-water pipes along the inner wall.
This means that the only space available to accommodate
the AR is in the passage between the lattice magnets andthe outer wall, but given the limited space too, it will benecessary for the AR lattice to closely mimic the12GeV-PS lattice.
Two sets of induction devices
generating 140kV per set are required to generate
the BBs for capture/stacking. Sufficient space mustalso be reserved for these devices.
A combined FODO lattice is being considered toensure that (1) gradient magnets, the cross section ofwhich is given in Fig.3., are almost uniformly distributed
so as to satisfy the constrains discussed above, (2) thetotal cell number is fixed at 28(4x7), and that (3) thebetatron tunes are nearly identical to those of the MR.
The 1.5m-long gradient magnet has a bending angle of
3.75 degrees. A super period with mirror symmetry
consists of 5 regular FODO cells and 2 FODO insertion
cells (long straight section), as seen in Fig.3. Thesemissing-bend cells are used to inject, extract, and capture

the proton beams. The AR must be capable of being
adjusted in order to correct for the COD and to adjust thebetatron tunes. This will be achieved by utilizing
electromagnetic dipole and quadrupole magnets which
will be positioned between the split gradient magnets. The
lattice functions are shown in Fig.4.
Because the magnetic field is permanent, no power
supplies, cooling systems, power distribution cables, orelectrically safety systems are required. From theextensive experience at Fermilab[4], it is also clear thatthe permanent magnets are very stable over time andagainst temperature fluctuations. The basic design of thepermanent magnet was done. This is a 1.586kG gradientdipole with a 7cm gap at the center, a 26cm horizontal
aperture and a 10cm horizontal good-field aperture. The
overall dimensions are 30cm in height by 45cm in widthby 1.5m in length. The magnets are straight and the sagittaof the beam inside a magnet is typically 1.23cm. In thedesign of the so-called "hybrid" permanent magnet, thefield is driven by permanent magnet material and theshape of the field is determined mainly by steel poleelements.
This is similar to the gradient magnet
employed in the Fermilab Recycler Ring. The field
quality is designed to give a vertical magnetic field errorof less than 1x10-3 across the designated good-fieldregion.

4 CURRENT R&D WORKS
The key components of the ID are the gradient magnet,
the induction cell excited to MHz rates, and the MHz
repetition rate solid-state driver. The R&D gradientmagnet was manufactured according to their basic design
specifications and subject to field measurements.
Results is reported in an accompanied paper [5]. The
original design for the power system was based on thesimple idea of using an array of field effect transistors(FETs) or static induction thyristor (SIT) to switch energy
from a pre-charged capacitor bank to an induction cell.
The performance of the induction modules required foracceleration and BB trapping is common, apart from thepulse length. The output voltage is 2.5-3kV per cell andthe repetition rate is 670kHz at 500MeV and 880kHz at
12GeV. The induction cell for acceleration is excitedwith a 0.5 µsec-long pulse and is reset with a 100
nsec-long pulse, while the induction cell for BB trapping
is excited and is reset with a 30-40nsec short pulse. In
order to produce a net 1 µsec-long accelerating voltage,
0.5 µsec-long pulses generated independently on two
accelerating units are combined into a 1 µsec-long pulse.
As both Finemet and Co-Fe ferrite are possible candidates
for the core-material of the induction units, core-loss
measurements have been conducted for minor-loop
operation at the test bench. From these results, it turned
out that the core-losses in both materials are dominated by
the eddy current loss, but that their magnitude are withinmanageable. Although technical details of a prototype of a

2.5-3kV, 100nsec induction unit have been presented
elsewhere [6], typical examples of pulse shapes of
induced voltage and a sigle pair obtained by employing
SITs as switching elements are shown in Fig. 5.

5CONCLUSION

The AR can be constructed at quite a low cost by
utilizing well-established permanent magnet technology.
Although superbunch acceleration represents a challenge,
the technology needed for this is steadily being
developed, and there is a high degree of confidence thatthe performance levels required for the ID are attainablewith the technology currently available. The results ofneutrino oscillations may become more solid with twice asmuch statistics. The physics impact of the ID will be
discussed in details at any other physics workshop.
This work is partially supported by the Grant-in-Aid for
Scientific Research in Priority Areas "Neutrino
Oscillation and Its Origin", No. 12047228.

REFERENCES[1] K.Takayama, “KEK PS beam-intensity upgrade
program and its result”, ICFA Beam Dynamics
Newsletter, No.20, 11 (1996).
[2] S.Igarashi, “An Experimental Study of the Intensity
Dependent Beam Loss Mechanism in The KEK-PS”,
Proc. of EPAC2000, 1300 (2000).
[3] K.Takayama and J.Kishiro, “Induction Synchrotron”,
N.I.M.A451/1, 304 (2000).
[4] Fermilab Recycler Ring, Technical Design Report
1996.
[5] E.Nakamura et al., “Permanent Magnets for the 500
MeV Accumulator Ring of the Intensity Doubler
Project in KEK-PS”, in this conference, P2oth11.
[6] J.Kishiro et al, “Rapidly Switched Accelerating
Devices for Induction Synchrotron”, Proc. ofEPAC2000, 1966 (2000).

Figure 2: How to create a super-bunch in the AR

Figure 3: Four cells in the super-periodicity. Gray: thegradient focusing and defocusing magnets, Black: thebending magnets, White: the normal Q-mag

Figure 4: The lattice functions. Solid line (upper): βx,Broken line: βy, Solid line (lower): D(s)FIGURESµsecµsec
Figure 1: Capture of a super-bunch in the BB and its
acceleration with induction voltage Vac(t)Figure 5: (upper) Pulse shapes of induced voltage(kV/div); (lower) a single pair