loss problems associated with the acceleration of radioactive beams and what we can do about it...
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Loss problems associated with the acceleration of radioactive beams
and what we can do about it
A.Janssonf fermilab
Loss issues (and ideas for solutions) in the CERN beta-beam baseline scenario.
A. Jansson, Rencontres de Moriond 2003
Loss issues
• Activation of machine – Hands-on maintenance possible?
• Heat load for super-conducting magnets?– Size of decay ring.
A. Jansson, Rencontres de Moriond 2003
Decay losses• The daughter nucleus
will carry most of the initial kinetic energy.
• Because of the change in magnetic rigidity (p/q), it will be lost somewhere in the machine.
6He2+
6Li3+
e-
e
A. Jansson, Rencontres de Moriond 2003
Activation limit
• 1 W/m generally accepted limit from spallation sources ( ~1 GeV protons)
• Large spread in experimental values (red circles).
[ R. Hardekopf, in Fermilab-Conf-00/185]
US limit for hands-on maintenance
A. Jansson, Rencontres de Moriond 2003
Decay losses / power dissipation
rest
0
rest
0
lab
kin )1(
ENENEN
W
labN
dt
dN
L
fENLW
rest
0/
Decay rate:
Power dissipation:
Average power per unit length:
W independent of energy!
(lab decreases with energy)
(f is duty factor)
A. Jansson, Rencontres de Moriond 2003
CERN baseline scenario
PS
Decay
RingISOL target & Ion source
SPL
Cyclotrons
Decay ring
Brho = 1500 Tm
B = 5 T
Lss = 2500 m
SPS
Storage ring and fast cycling synchrotron
PS
Decay
RingISOL target & Ion source
SPL
Cyclotrons
Decay ring
Brho = 1500 Tm
B = 5 T
Lss = 2500 m
SPS
Storage ring and fast cycling synchrotron
A. Jansson, Rencontres de Moriond 2003
Estimated losses–CERN scenario6He
18Ne
Machine Ions extracted Batches Loss power Power/lengthSource+Cyclotron 2 e6 /s 52.5 msStorage ring 1.0 e12 1 3.0 W 19 mW/mFast cycling syncrotron 1.0 e12 16 7.4 W 47 mW/mPS 1.0 e13 1 765 W 1.2 W/mSPS 0.9 e13 inf 3.63 kW 0.41 W/mDecay ring 2.0 e14 * 157 kW 8.9 W/m
Machine Ions extracted Batches Loss power Power/lengthSource+Cyclotron 8 e11 /s 52.5 msStorage ring 4.1e10 1 0.18 W 1.1 mW/mFast cycling syncrotron 4.1 e10 16 0.46 W 2.9 mW/mPS 5.2 e11 1 56.4 W 90 mW/mSPS 5.9 e11 inf 277 W 32 mW/mDecay ring 9.1 e12 * 10.6 W 0.6 W/m
These numbers assumes 8s rep rate and only include decay losses from the beta beam!
(lost on inside)
(lost on outside)
limit
* denotes equilibrium intensity in decay ring
A. Jansson, Rencontres de Moriond 2003
How bad is 9 W/m?• For comparison, a 50 GeV muon storage ring
proposed for FNAL would dissipate 48 W/m in the 6T superconducting magnets. Using a tungsten liner to – reduce peak heat load for magnet to 9 W/m.– reduce peak power density in superconductor
(to below 1mW/g)– Reduce activation to acceptable levels
• Heat load may be OK for superconductor.• What about activation?
[ N.V. Mokhov / C.J. Johnstone, Fermilab-Conf-00/207]
A. Jansson, Rencontres de Moriond 2003
Machine activation• Scaling of spallation
source limits to higher energies?
• Effect of material thickness/range (what gets activated)?
• Simulations needed!
[ R. H
ardekopf, in Ferm
ilab-Conf-00/185]
A. Jansson, Rencontres de Moriond 2003
How reduce energy deposition?
• Decay losses can not be avoided, only redistributed (to some degree).
• The energy deposited in the injectors is minimized by accelerating as fast as possible (reduced duty cycle).
• New machines (e.g. decay ring) should be built with loss containment in mind.
A. Jansson, Rencontres de Moriond 2003
Decay ring
High beta (small divergence)production straight. FODO lattice (or drift)
Matching section Matching section
Dispersion suppressorDispersion suppressor
Dispersion suppressorDispersion suppressor
Return straight (with injection, RF etc)FODO lattice
neutrinos
Arc(FODO lattice)
Arc (FODO lattice)
A. Jansson, Rencontres de Moriond 2003
Decay in FODO lattice
0 10 20 30 40 50sm0
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m Dm0 200 400 600 800 1000
sm0
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m
0 200 400 600 800 1000sm0
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mExample lattice for 6He2+ (top traces)and 6Li3+ (bottom traces).
When beam decays, it will be mismatched relative to its periodic lattice, causing beta beating to occur (right)
A. Jansson, Rencontres de Moriond 2003
FODO stability• Stable motion limits
6He phase advance• Minimal aperture at
~40 degrees per cell• Ne is not a problem
(focussing is reduced for decay product)
0 20 40 60 80 100 120
10
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0 20 40 60 80 100 120
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Phase advance per cell
6He
18Ne
Max lattice betaMax propagated beta of secondary beam
Bet
a (a
rb u
nits
)B
eta
(arb
uni
ts)
A. Jansson, Rencontres de Moriond 2003
Options for straight sections
• Add absorbers at quadrupole locations
• Design straight sections to contain both primary and secondary beam.– Stability of motion– Sufficient aperture/low beta beating– “Clean” separation scheme needed in the end of
straight sections (56 kW secondary beam power produced in each straight)
small phaseadvance
A. Jansson, Rencontres de Moriond 2003
0 100 200 300 400 500sm0
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m
Matching section aperture• The matching section
following the production straight usually feature a high beta location
• Should have sufficient extra aperture to allow for beta beating of secondary beam, in order not to become an aperture restriction!
A. Jansson, Rencontres de Moriond 2003
0 100 200 300 400 500sm0
1
2
3
4
Dm
Dispersion suppressor• Zero dispersion required in
straights • The dispersion suppressor will
naturally act as a separator at the end of the straight– Secondary beam appears to have
large energy offset. – Place septum where beams are
sufficiently separated.– Dispose of secondary beam
cleanly.
A. Jansson, Rencontres de Moriond 2003
Beam separation in the arcs
• For efficient and clean separation of decay products, need short magnets separated by drifts.
• Contradicts keeping the arcs short relative to straight sections.
• Absorb most of the energy in magnets?
A. Jansson, Rencontres de Moriond 2003
Custom SC magnet• Dipoles can be built
with no coils in the path of the decay particles to minimise peak power density in superconductor (quench stability).
[ S. Russenschuck, CERN]
A. Jansson, Rencontres de Moriond 2003
Conventional magnet alternative• Longer machine, lower
losses/length, higher cost
• C-magnet open to the inside.
• Minimal thickness for ions to traverse.
• But, decay products bend inwards!
• Decay ring the size of LHC!
6Li3+ 6He2+
Need more study!!!!
A. Jansson, Rencontres de Moriond 2003
Larger ring SC alternative• Increasing machine size
reduces local losses (for constant intensity).
• Arc drifts may allow some degree of separation – Inward bend!
– Absorbers in front of magnets?
• Cost!Need more stu
dy!!!!
A. Jansson, Rencontres de Moriond 2003
Non-decay losses in decay ring
• All injected beam is lost in the machine!
• Most losses not linked to decay will be due to the stacking scheme (limited by longitudinal acceptance).
• At equilibrium, particles are continuously being pushed out of the bucket.– Remove DC beam/satellites (using a kicker)?– Momentum collimation?
A. Jansson, Rencontres de Moriond 2003
Conclusions
• 6He is the problematic ion species
• Decay products from straights could be collected and disposed of rather cleanly.
• Arc decay losses more difficult to control (activation likely to be a serious issue)
• Dissipated energy in arcs is rather high, but not worse than for muon-based scenarios (seems possible to use SC magnets)
A. Jansson, Rencontres de Moriond 2003
Open questions…
• Will SC magnets actually take the heat?• Detailed loss distribution• Translate losses into activation levels
– Will SC survive?– Maintenance possible?
• Detailed simulations needed!• Requires machine design (iterative process).