CURRENT ACTIVITIES FOR A NEUTRINO FACTORY AT CERN
R. Garoby, CERN, Geneva, Switzerland
for the Neutrino Factory Working GroupAbstract
Development is necessary in a large number oftechnologies before a Neutrino Factory complex can bebuilt. In the case of the CERN project [1, 2, 3], with thehelp of other laboratories, work is progressing on thesystems incorporated in the reference scheme. The
subjects under investigation, the status of the studies andthe trends of evolution of the scheme are described.

each transverse emittance is reduced by a factor of 4(conversely, the density in each transverse plane ismultiplied by 4). Solenoidal focusing is still used in thefollowing linear accelerator which operates at harmonics
of 44 MHz to increase the energy up to 2 GeV.
Bunchcompressorring
Target& Horn

1 CERN REFERENCE SCHEME
In a Neutrino Factory, neutrinos result from the decayof high energy muons circulating in a storage ring. Thesemuons are themselves decay products of the pions
produced by the interaction of a proton beam with theatoms of a target. In the CERN scheme (see Fig. 1), theproton beam is delivered, at 75 Hz, by a 2.2 GeVSuperconducting H Linac (SPL), followed by anaccumulator and compressor rings to obtain the required
time structure. A flux of 1.1×1016 p/s (or ~ 1023 p/year,taking 107 s/year) is sent onto the target, with thecharacteristics listed in Table 1.
Table 1: Proton beam on target
Parameter
Value
Mean beam power
4
Kinetic energy
2.2
Repetiton rate
75
Pulse duration
3.3
Number of bunches
140
Pulse intensity
1.51×1014
Bunch spacing
22.7(Bunch frequency)(44)
1
Bunch length (σ)
Relative momentum spread (σ)
5×10-350
Norm. horizontal emittance (σ)

2.2 GeV Superconducting H- Linac
Accumulator ring
44/88 MHz capture, cooling& acceleration
2-10 GeVrecirculator

10-50 GeV
recirculator

Muon decay ring(Bow-tie in inclined plane)
cdjp/pulsens(MHz)nsFigure 1: Conceptual layout of a Neutrino Factory atCERN
More than 1014 µ/s enter the cascade of 2 RecirculatingLinear Accelerators (RLA). Equipped with LEP-type 352
MHz superconducting RF cavities providing a total of 12
GeV of single-pass energy gain, these RLAs accelerate
the beam in 4 turns to 50 GeV.
The 3.3 µs duration burst of 50 GeV muons is injected
into the 2 km circumference muon storage ring, where it isleft to decay until the next burst is available, 13.3 mslater. Roughly 3×1020 neutrinos will then be generated
every year in each of the long straight sections aimed atremote experiments, thousands of kilometres away.

µm.rad2 PROTON DRIVER

Unit
MWGeV
Hz
µs

A liquid metal target is being used, inserted inside amagnetic horn that helps collect pions over a wide rangeof kinetic energy (100 to 300 MeV) and a large solid
angle. After a 30 m long decay channel where pions andmuons are kept focused by a 1.8 T solenoidal field,
44 MHz RF cavities rotate the bunches of muons in thelongitudinal phase plane and reduce their kinetic energy
spread by a factor of 2. The beam then passes throughliquid hydrogen cells for ionisation cooling, and 44 and
88 MHz RF structures to recover the loss of longitudinalenergy. At the end of this process, 250 m after the target,

2.1 Reference design
The present design makes extensive use of the large
inventory of RF equipment dismantled from LEP. The
800 m long superconducting Linac [4] that accelerates theH ions to 2.2 GeV re-uses all klystrons and 60 % of theLEP modules in its high-energy part (see Fig. 2). The low
energy part has to be of new construction, because theavailable cryostats can only be used above 390 MeV(from 390 MeV to 1 GeV, LEP cryostats are re-used,equipped with new β=0.8 cavities; above 1 GeV, LEP

modules are used without modification). Roomtemperature RF cavities are employed up to 120 MeV.
The beam is bunched at 352 MHz in an RFQ, and a fastchopper, at 3 MeV, provides the proper time structure for45 keV

7 MeV

120 MeV

1.08 GeV

13m

78m

334m

3 MeV

18MeV

237MeV 389MeV- RFQ1 chop. RFQ2 DTL
RFQ1chop.
CCDTL
RFQ2 RFQ1
0.52 chop.
0.7 RFQ2
0.8

HSource Low Energy section

DTL

2.2 GeV
357m

dump

LEP-II

Superconducting section
Stretching andcollimation line

PS / Isolde
Accumulator RingFigure 2: SPL block diagramme
an optimum longitudinal capture in the accumulator.
Protons are accumulated over 660 turns in theaccumulator ring, using charge exchange injection [5].
140 of the 146 buckets generated by the 44 MHz RF
system are progressively populated by up to
1.08×1012 p/b. At the end of accumulation, the bunches
are fast ejected and transferred into the compressor ring,where bunch compression takes place in 7 turns, with2 MV at 44 MHz and 350 kV at 88 MHz. The 1 ns rms
long bunches are then ejected onto the target.
Both the accumulator and the compressor rings aresituated in the ex-ISR tunnel. The location proposed onthe CERN site (see Fig. 3) also makes use of existingtunnels for the transfer of the SPL beam to the PS and toand compressor rings have not revealed any dramatic
problem, but more work is required concerning themicrowave instability and the effect of the electron cloud.
Table 2: Studies for the proton driver
Item
Main issue
DesignH source
Chopper
System design
RT Linac
Structures development
SC cavities
Pulsed test of cavities
Dev. of low β structures
Klystrons and supplies Pulsed operation
Servo-systems
Field stab. in pulsed mode
Beam dynamics
Optimisation
Alternatively, if the production of pions from a
2.2 GeV proton beam proves to be too unfavourable, or if
difficulties arise in the study of the neutrino complex,
linked to the choice of 23 ns spaced bunches, the designof the proton driver could change and make use of Rapid
Cycling Synchrotron(s) (RCS) [6].

2.3 Other Applications
Apart from its main use as a driver for a future
Neutrino Factory, this 2.2 GeV proton source could
benefit the rest of the accelerator complex at CERN,
replacing the present Linac 2 / PSB set-up with modern
equipment and providing improved beam characteristics
at the entrance of the PS. The present ISOLDE facility
could easily be supplied with 5 times more beam current
than nowadays, while removing constraints from the busy
PS supercycle. Potential benefits for the approved highenergy physics experiments are under study.
New proposals have also been made to exploit the2.2 GeV beam directly for generating a conventional
neutrino beam towards an experiment at medium distance(~ 100 km) [7], and for a second generation ISOLDE
facility [8].

3 TARGET AND CAPTURE
3.1 Reference DesignFigure 3: Proton driver complex on the CERN site
the ISOLDE experimental facility.

2.2 Ongoing Activities and Design Trends
Theoretical work is concentrated on the refinement of
the SPL design, in close relation with the development ofcritical hardware (Table 2).
Work is under way to improve on the reference design[4], by reducing the repetition rate to 50 Hz, generalising
the use of β=0.8 cavities up to 2.2 GeV, and increasing
the beam current during the pulse. Preliminary
investigations of the consequences for the accumulator

The target and pion capture set-up is one of the mostdifficult engineering challenges in the design of aNeutrino Factory, and no solution has yet been tested
enough to be considered as viable. In the case of theCERN reference scheme, preference is given to a liquid
metal target [9, 10] and horn focusing [11, 12], althoughsolenoid focusing is not discarded.
For convenience, Hg is being used for the preliminary
tests which have just begun. A safety accepted test set-up
is now available which will soon be used to analyse thepenetration of an Hg jet inside a 13 T magnet [10]. Beamtests will begin in the ISOLDE target area, with 1.4 GeV
protons on Hg in a trough [10].

Test of a prototype horn and power supply [13] arebeing prepared to try to quantify the horn lifetime.

140 cm unit length

The HARP experiment [14] will take data in 2001 andmeasure precisely the production of pions with 2 to 16
GeV protons on different types of target. Results willstrongly influence the target design and possibly also thecharacteristics of the proton driver.
The problems associated with a liquid metal target
being still unresolved, alternative solutions are being
pursued in different laboratories [15], and a new proposal
was recently made of a stationary target with metal
spheres [16].

E field lines in the
44 MHz RF cavity

sc solenoid

sc solenoid

240 cm104 cm
60 cm

3.2 Potential Design Evolutions

E field lines in the
44 MHz RF cavity

Beamaxis
sc solenoid

sc solenoid

Figure 4: Cooling channel modules, with 44 MHzcavities and superconducting solenoid

4 BUNCH ROTATION AND COOLING
4.1 Reference Design3530

rms normalised (mm.rad)

The preferred solution is adapted to the kind of small
bunch spacing (namely 23 ns) which can be obtained froma low-energy proton driver [17, 18]. The time structure of
the primary proton beam is preserved and no rebunchingis required. The essential parameters of the bunch rotationand cooling channel are summarised in Table 3. After adecay path of 30 m, the muon bunches are rotated toreduce their energy spread by a factor of 2. Two steps ofionisation cooling are then applied, the first one at themean energy of 200 MeV, with 24 cm long liquid
Hydrogen absorber cells and 44 MHz RF, and the second
one at 300 MeV with 40 cm absorber cells and 88 MHz
RF. The low frequency, the need for a high solenoid field
on axis and the large beam aperture due to the beamemittance contribute to make the equipment voluminous.
The design is modular, as illustrated in the sketch of the44 MHz part shown in Figure 4. The final acceleration to2 GeV is achieved with 88 and later 176 MHz cavities.
Figure 5 illustrates the expected reduction of transverse
emittance along this channel, and the final reduction by afactor of 4. The percentage of muons surviving perincident pion is shown in Figure 6.

252015105DECAY

ROTATIONCOOLING I

ACCELER COOLING IIATION

0050

100150Distance from target (m)200

250300Figure 5: Evolution of transverse emittance

4.2 Ongoing activities and design trends
Beam dynamics is concentrated on the optimisation ofthe channel architecture to try to increase the flux ofmuons delivered to the first RLA. An 88 MHz cavity is
being prepared with existing RF hardware removed fromthe PS after the end of LEP, to test high gradient
operation. First results are expected for the end of 2001.
Tests in the presence of a 2.6 T solenoid field will follow
in 2002, when the superconducting solenoids presently
under design will be available.

Table 3: Parameters of the bunch rotation, cooling and pre-acceleration sections
Decay
Rotation
Cooling 1Accel. 1
Cooling 2
Length (m)
30
30
46
32
112
Diameter (cm)
60
60
60
60
30
B-field (T)
1.8
1.8
2.0
2.0
2.6
Frequency (MHz)
44
44
4488
Cavities gradient (MV/m)
222
4
Kinetic energy (MeV)
200
200
280
300

Accel. 2~ 450
202.688 & 176
4 - 102000

5.2 Ongoing activities and design trends

10.9A satisfactory optical arrangement is still being soughtfor the spreaders and combiners. A single cell 200 MHz
superconducting cavity is being built at CERN for Cornell
University, using the “Niobium sputtered on Copper”
technology developed for LEP [22]. Such a lowfrequency cavity in RLA1, and possibly also in the highenergy part of the preceding linac, would increase theacceptances of these accelerators in all planes, and evenreduce their length if the gradient is appreciably higher
than 7.4 MV/m.

0.8
0.7
0.6
0.5
0.4
0.3
0.2acceleration

cooling
80MHz

acceleration

cooling
40MHz

rotationdecay0production

0.1

6 MUON STORAGE

Figure 6: Percentage of muon/initial pion
The proposal of a Muon Cooling test facility is
actively being prepared [19], which should demonstratefull-scale engineering solutions and single-particle coolingwithout ambiguity.

5 MUON ACCELERATION
5.1 Reference design
Two racetrack-shaped recirculating linear accelerators
accelerate the muon beams from 2 to 10 GeV (RLA1) andfrom 10 to 50 GeV (RLA2) respectively. Both RLAs are
isochronous, and the muon bunches are accelerated on thecrest of the RF wave. The main parameters aresummarised in Table 4 [20]. This part of a Neutrino
Factory complex has been identified as one of the most
costly [21], so that it will deserve detailed engineering
optimisation in due time. 352 MHz LEP-type RF cavities
are installed in all straight sections. Spreaders at the endof each linac section, separate the beam into 4 different,
vertically-stacked arcs. After the 180° bend, combiners
merge the four beam energies into the input of the nextlinac section.
Table 4: Parameters of the RLAs
Parameter
RLA1
Injection energy (GeV)
2
Extraction energy (GeV)
10
Number of turns
4
Linac length (m)680
RF frequency (MHz)352
Mean arc radius (m)20
Circumference
806
Gradient in cavities (MV/m)
7.4
Normalised admittance (mm.rad)
16.5

RLA2
1050
4
3813
352
10044427.4
18.8

6.1 Reference design
A muon storage ring with a symmetrical bow-tie shapeand long straight sections aiming at remote neutrino
experiments has been studied [23]. Such a shape ispreferred for its advantages in terms of site layout andflexibility of orientation. The machine parameters arelisted in Table 5, and its geometry is sketched in Figure 7.
One third of the stored muons will decay in the straightsections, so that a total of ~ 3×1020 neutrinos will be sent
every year to each remote experiment. The Gran Sasso
cave will be one of them, and the location of the otherone, ~ 3000 km away, is the subject of an ongoing debate.
Although challenging, the alignment accuracy of 10-5 radrequired for the straight sections is considered feasible.
Table 5: Parameters of the muon storage ringParameterValueUnit
Beam energy
50GeV
Muon fluence1014
s-1Distances to far experiments
1000 &km
3000
Normalised divergence
0.1(γ×physical rms divergence)
Vertical slopes-78.6 &mrad-237.9Circumference
2007.9m% of useful muon decays per
28.7%
circulating muon per detector
RF frequency
352
MHz
Peak RF voltage
120
MV
Normalised emittance (mm.rad)
1.67mm.rady horizontal coordinate (m)REFERENCES

700

500

300

100-100-400-2000.0

200

400

Vertical coordinate (m)x horizontal coordinate (m)500-50-100-15001000

2000

Position on the circumference (m)Figure 7: Geometry of the muon storage ring

6.2 Ongoing activities and design trends
Engineering developments are needed to help optimise
the performance and cost of the storage ring, but noactivity has started yet. Beam dynamics studies continue.

7 CONCLUSION
Neutrino Factories have been seriously considered as
feasible since slightly more than 3 years. Studies in viewof a European facility of this kind have started with thefirm support of ECFA, and in close collaboration with US
groups working on muon machines for several years. In
the CERN context, where resources are diminishing andthe LHC project has top priority, it is quite encouraging toreport how much has been done in such a short time.
Developments are launched which should help improve
the quality of the present design and give confidence inthe feasibility of the components. An important goal in thenear future is to elaborate a proposal for a test facility
with clear objectives.

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