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Secondary Particle Production and Capture for
Muon Accelerator Applications
S.J. Brooks, RAL, Oxfordshire, UK ([email protected])
Transmission of positive muons to the
end of the phase rotator for various
target materials.
Abstract
The main results in this paper derive the optimum proton driver energy for
a target of tantalum, mercury or copper, 1cm in radius with lengths given
by the table below. Targets are simulated using MARS15 then the
resulting particles are tracked through the UKNF decay channel and
phase rotator using the code Muon1. This is done for each energy and
material, and the muons leaving the channel with the desired energy of
180±23 MeV are counted in units of muons per “proton.GeV” on target,
which is actually proportional to the muon yield rate for a fixed power.
Intense pulsed muon beams are required for projects such as
the Neutrino Factory and Muon Collider. It is currently
proposed to produce these from a high-Z target using a multimegawatt proton driver. This paper examines the effect of
proton energy on the yield and distribution of particles
produced from tantalum and mercury, with further analysis
using a tracking code to determine how these distributions
will behave downstream, including a breakdown of loss
mechanisms. Example ‘muon front end’ lattices are used
from the UK Neutrino Factory design.
Near Detector
Our design includes several unique features such as a solid, rapidly-moving target, and split extraction of pulses from
the main proton synchrotron to alleviate thermal shocks in the target. This proton machine could be realised for instance
via staged upgrades of ISIS at RAL. The area of interest in this poster and paper is the “front end” highlighted in blue.
FFAG III
(20-50GeV)
180MeV DTL
(Drift Tube Linac)
Achromat for
removing beam halo
Transforms longitudinal phase space
as shown in the diagram (right).
(in which pions decay to muons)
Solenoidal Decay Channel
Proton Beam Dump
Two Stacked Proton
Synchrotrons (boosters)
• 1.2GeV
• 39m mean radius
• Both operating at 50Hz
Two Stacked Proton
Synchrotrons (full energy)
• 6GeV
• 78m mean radius
• Each operating at 25Hz,
alternating for 50Hz total
(produces pions from protons)
(H- to H+/protons)
(muons decay to neutrinos)
Target enclosed in 20Tesla
superconducting solenoid
Stripping Foil
FFAG II
(8-20GeV)
Muon Decay Ring
RF Phase Rotation
Beam Chopper
(Radio Frequency
Quadrupole)
RFQ
(Low Energy
Beam Transport)
LEBT
H− Ion Source
Schematic of the current UK neutrino factory design (under study).
R109
FFAG I
(2-8GeV)
Muon Linac to 2GeV
(uses solenoids)
Proton bunches compressed to 1ns duration at extraction
• Mean power 5MW
• Pulsed power 16TW
The pion distribution does not change
radically enough with proton energy to
affect the optimal lattice.
Comprehensive optimisations of all parameters of the decay channel
and phase rotator were conducted on a distributed computing network.
Two independent runs, one starting with secondary particles from a
2.2GeV proton beam and the other from a 10GeV beam, each produced
an “optimal” lattice after several months. The table above shows their
yields and what happens when they are run on each others’ beams: in
fact, the lattices are almost identical, as shown by the parameter graphs
below, so not “specialised” for pions coming from one energy of beam.
The current phase rotator captures one muon
sign, with the other falling between RF buckets.
1000
900
Normalised Genetic Parameter
800
700
Drifts 2.2
Drifts 10
Fields 2.2
Fields 10
Lengths 2.2
Lengths 10
Radii 2.2
Radii 10
600
500
400
300
Analysis of how beam loss mechanisms change
as proton energy varies.
200
100
0
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15 16 17 18 19 20 21 22 23
The effect on the opposite sign can be seen in the blue particles in the longitudinal phase
space diagram above. This means that when all the muons are counted, as seen in the
diagram below, only a few of the negative muons are in the correct energy band, but the
target always produces both signs anyway so there is no way of getting rid of these.
Decay Channel Cell
1000
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
Phase Rotator Cell
Relative performance of a channel for
capturing negative particles.
By changing the phases of the RF systems by 180° negative particles
will be treated analogously to positive ones in the optimised channel.
The graph below shows the difference in performance between the two.
1000000
800000
100GeV
120GeV
1
75GeV
0
1200000
40GeV
50GeV
100
30GeV
200
1400000
20GeV
300
15GeV
400
8GeV
10GeV
500
4GeV
600
5GeV
6GeV
Drifts 2.2
Drifts 10
Phases 2.2
Phases 10
Voltages 2.2
Voltages 10
Fields 2.2
Fields 10
Lengths 2.2
Lengths 10
3GeV
700
2.2GeV
Normalised Genetic Parameter
800
For higher primary beam momentum, even the more forward-directed secondary
particles can have a transverse momentum sufficient to be outside the channel’s
acceptance. Here we see the higher energies experiencing losses further down the
decay pipe from smaller-angled particles.
Power Dissipation (Watts, normalised to 5MW
incoming beam)
900
600000
Proportion of muons leaving the channel in the
correct energy band.
400000
200000
0
1
10
100
1000
Proton Energy (GeV)
Primary energy (heat) deposition in rod.
Proton beams deposit heat directly in the target as well as by the particle reabsorption
mechanism shown in red (top figure). The graph above shows how much power out
of a 5MW beam would be converted directly into heat in the target. This is one of the
driving factors of the solid target design so it is fortunate that this optimum of minimum
heating (8GeV) coincides with the optimum of muon capture in the phase rotator.
This seems to be functionally independent of the proton energy, so although losses may
redistribute in the early-mid decay channel, by this stage they are simply proportional to yield
as proton energy varies, thus the phase rotator decouples from the proton energy issue.