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

Magnetic Horn for a Long-Baseline Neutrino Oscillation Experiment at KEK
Masaharu IEIRI, Yutaka YAMANOI, Yoshihiro SUZUKI, Michifumi MINAKAWA, Hiroyuki NOUMI, Yoji KATO,Harumi ISHII, Kazuhiro TANAKA, Minoru TAKASAKI, Kenzo NAKAMURA, Takahiro INAGAKI*,
Mitsuhiro KOHAMA**, Takasumi MARUYAMA***, Shinichi MIYAMOTO****, KoichiroNISHIKAWA
Physics Division III, lPNS, KEK, 1-1 Oho, Tsukuba, Ibaraki 305, JAPAN*Physics Department, University of Tokyo, 7-3-1 Hongo, Tokyo 113, JAPAN**The Graduate school of Science and Technology, Kobe University, Kobe, Hyogo 657, JAPAN*** Bubble Chamber Physics Laboratory, Tohoku University, Aoba, Sendai, Miyagi 980-77, JAPAN****The Central Research Laboratories, NEC Corp., Miyamae, Kawasaki, Kanagawa 216, JAPAN

Abstract

1 Introduction

Two magnetic horns have been designed and constructed
for the long-baseline neutrino oscillation experiment fromKEK to SuperKarniokande. The horns, which produce thetoroidal magnetic field along the beam axis, are used toenhance the neutrino flux. Pulsed current of 250 kA (2 msec)
which is synchronized with a fast extracted proton beamfrom the accelerator will be supplied every 2 sec. for bothhorns. A trial run of horns has been started.

A long-baseline neutrino oscillation experiment has been
proposed [1]: neutrinos of a few GeV produced at KEK 12
GeV PS will be measured at SuperKamiokande which islocated 250 km away from KEK [2]. A 400 m long neutrino
beam line, extended from EP1-A primary beam line of thenorth counter hall towards the direction of theSuperKamiokande, is now under construction. Figure 1
shows a plane view of the neutrino beam line at KEK. It
consists of the straight section, the arc section, the target
section and the decay tunnel. Magnetic horns will be
installed at the target section in order to maximize neutrino
flux, i.e., to focus produced pions (parent particle of v) tothe forward direction.

Beam Dump
To SK_j•r
Muon
Monitor

Production
TargetTarget Section:! --'~<' ,: __;DecayTunnel

2nd Horn

1st Horn(Target)

EPl
050 m
KEK 12-GeV PSFig. 1

Plane view of the neutrino beam line-377-The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

2nd Magnetic Horn
12GeVprotons

I= 250 kA (2 msec.)
I = 250 kA (2 msec.)~--------t=o=.5~m~----~)H;:(Fig. 202 (m)I

I

Cross sectional drawing of the 1st and 2nd magnetic horns

2 Magnetic Horn

10 2X-2.1 Principle

.....
0D..

A magnetic horn consists of cylindrical inner and outerconductors and end-plates as shown in figure 2. A single
current, passed through these conductors, produces a toroidalmagnetic field along the beam axis between the inner andouter conductors as(1)B [T]=I/5r
where r is the distance from the symmetry axis in mm and I
is current in kA. Positive pions produced at the ltarget, which
diverge from the beam axis, are bent back to forward
direction.

~0,....

><

1000
800
600
400i.i:200;;-.

E(.).......

:IHorns ONOFF02

3

54(GeV/c)
Fig. 3

2.2 Design

The horns are designed by optical and mechanical study.
The production target is integrated in the 1st horn as a partof the inner conductor in order to increase the acceptance forthe secondary pions. The 1st horn will gather produced pions
to the forward direction and the 2nd horn will make them
parallel toward SuperKarniokande for as wide an energy.
region as possible.
Current to the horns, shapes of inner and outer conductorsof horns and the distance between two horns are determined
to maximize the neutrino flux at SuperKamiokande. Figure
3 shows a neutrino momentum distribution expected at
SuperKarniokande for 1020 protons on target. The neutrino
flux will be enhanced by a factor of 14 when the horns areoperated properly.
Conductors of the horns are expected to be thin as
possible, since produced pions pass through them. However,Lorentz force by 250 kA, e.g., on the end-plate of the 1st
hom is estimated to be more than 2 tons at maximum.
Moreover, pulsed current will be supplied to horns morethan 107 times (2 sec. repetition for 3 years).

2000
1800
1600
1400
1200

at
distribution
momentum
Neutrino
20
SuperKamiokande for 10 protons on target.

Mechanical design of horns are optimized underconsiderations of buckling limit against Lorentz forces,
fatigue limit for alternate tractions, bending by self-weight,
and stress caused from thermal expansion of the 1st horn's
inner conductor. The aluminum alloy 6061-T651 is usedsince it shows good electrical conductivity and mechanicalproperties as shown in Table 1. The wall thickness, theshape of the rib, the size of bolts and other mechanicalparameters are designed to satisfy the safety factor more than
4 for the above limits. The inner conductor is cooled bycontinuously spraying water onto it. The sprayers areassembled on the outer conductor and cooling water of about
40 1/min is supplied to each horn.

-378-

Table 1

Characteristics of A6061-T651

Mass density
Thermal Conductivity
Tensile strength
Stress limit (107 alternate tractions)

2.70
0.17
379
144g/cm 3kW/m/°C
N/mm 2N/mm2

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

The power supply system for horns consists of a pulse
generator with 12 capacitors of 500 JlF and a transformer ofratio 10. Inductance and resistance for 1st horns are 1.03 JlH
and 187.1 11n, respectively and those of total circuit
including strip-lines and a transformer are 148 JlH and 36
mQ. The capacitor of 6 mF at 4.7 kV is estimated to supply
250 kA with 1.4 msec rise time for the 1st horn and that of
2 mF at 7.6 kV gives 0.83 msec rise time. We will
optimize the operating high voltage for longer capacitor
lifetime with acceptable heating of the inner conductor.

4 PlanThe long-baseline neutrino oscillation experiment will bestarted from January 1999. Before the experiment many tests
on the horn system will be done. Particularly, long term
operations must be performed at 250 kA with 2 sec
repetition at least one week. To find relative misalignmentof conductors and/or the electrical asymmetry of current path,azimuth dependence of magnetic field will continuously b~measured. Adjustable bases _of the horns and strip-lines at thesite between transformers and horns are now being designed.

3 Test Operation
Acknowledgement
The 1st horn was tested at DC 5000 A, which corresponds
roughly to pulsed current of 250 kA (2 msec) with 2 sec
repetition, to study thermal property. The temperature rise at
the target of the inner conductor was 10.4 °C with spraying
water. It corresponds 13 °C rise at 250 kA pulsed operation.
The thermal expansion of the inner conductor was 263 Jlm
and this result is consistent with the calculation by thetemperature rise for each part of the inner conductor and 23.6
Jlrnlm/°C for A6061.
The radial dependence of the magnetic field has beenmeasured at 130 kA at around the target and it was well
explained by eq. (1) as shown in figure 4. Figure 5 shows
the field strength as a function of supplied current to the 1sthorn and it is also consistent with eq. (1).
For long term operation, a trial run of the 1st horn has
been performed at 150 kA with 6.5 sec repetition for 24hours and 250 kA with 4 sec repetition for 12 hours with thecapacitor of 3 mF at 6.3 kV. There were no failures for
either the horn or the power supply so far.

References[1] K. Nishikawa et al., KEK-PS E362 Proposal,unpublished.
ofPositioning"Precisional.,[2] Noumiet
SuperKamiokande with GPS for a Long-Baseline
Neutrino Oscillation Experiment", H. KEK Internal
96-17 & to be published in NIMtj7:'a;

We have visited CERN several times since we started .todesign our horn and power supply. Dr. J. M. Maugain andhis group members showed and explained their horn systemvery kindly. Especially, Mr. G. Acquistapace, Mr. S.
Rangod, and Dr. V. Falaleev of CERN have visited KEK andmade many important remarks about our horn system. The
helpful discussions with Dr. A. E. Ball, Dr. F. Voelker, Dr.
J. M. Zazula, Dr. G. Voytek and Dr. A. Carroll were muchappreciated.

18tI

1.6!,4rF

I= 130 (kA)~[l1 .2 L

Ir

1Ff~

0806 [B = I (kA) /5 r (mm)0r= 20 (mm):: ~l50

75

100

125

150

175

200

225

250

I (kA)

r(mm)Fig. 4 Radial dependence of the magnetic field at 130 kA.

Fig. 5-379-Field strength as a function of supplied current.