Loss problems associated with the acceleration of radioactive beams and what we can do about it...

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.


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


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


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)


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


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


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]


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]


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.


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)


Decay in FODO lattice

0 10 20 30 40 50sm0

20

40

60

80

100

m Dm0 200 400 600 800 1000

sm0

25

50

75

100

125

150

m

0 200 400 600 800 1000sm0

25

50

75

100

125

150

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)


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

12

14

16

18

20

0 20 40 60 80 100 120

10

15

20

25

30

35

40

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)


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


0 100 200 300 400 500sm0

50

100

150

200

250

300

350

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!


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.


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?


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]


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!!!!


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!!!!


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?


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)


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).

Loss problems associated with the acceleration of radioactive beams and what we can do about it...

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