targets for neutrino factories and muon colliders€¦ · james h. norem,b robert b. palmer,c ralf...

Targets for Neutrino Factories and Muon Colliders

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K.T. McDonaldPrinceton U.

Workshop on High-Power Targets forFuture Accelerators

Ronkonkoma, NY, September 10, 2003http://puhep1.princeton.edu/mumu/target/

Sketches of a 4-MW Target Station

Kirk T. McDonald High-Power Targetry Workshop, Sept. 10, 2003 1

The BNL E951 Collaboration

Audrey Bernadon,e David Brashears,k Kevin Brown,c Daniel Carminati,e Michael Cates,k John Corlett,h

Francois Debray,g Adrian Fabich,e Richard C. Fernow,c Charles Finfrock,c Yasuo Fukui,d Pei Feng,c

Tony A. Gabriel,k Juan C. Gallardo,c Michael A. Green,h George A. Greene,c John R. Haines,k

Jerry Hastings,c Ahmed Hassanein,b Michael Iarocci,c Colin Johnson,e Stephen A. Kahn,c

Bruce J. King,c Harold G. Kirk,c Jacques Lettry,e Vincent LoDestro,c Changguo Lu,l Ioannis Marneris,c

Kirk T. McDonald,l Nikolai V. Mokhov,f Alfred Moretti,f George T. Mulholland,a

James H. Norem,b Robert B. Palmer,c Ralf Prigl,c Yarema Prykarpatsky,c Helge Ravn,e Bernard

Riemer,k James Rose,c Thomas Roser,c Roman Samulyak,c Joseph Scaduto,c Peter Sievers,e

Nicholas Simos,c Philip Spampinato,k Iuliu Stumer,c Peter Thieberger,c Peter H. Titus,i James Tsai,k

Thomas Tsang,c Haipeng Wang,c Robert Weggel,c Albert F. Zeller,j Yongxiang Zhaoc

aApplied Cryogenics Technology, Ovilla, TX 75154bArgonne National Laboratory, Argonne, IL 60439

cBrookhaven National Laboratory, Upton, NY 11973dUniversity of California, Los Angeles, CA 90095

eCERN, 1211 Geneva, SwitzerlandfFermi National Laboratory, Batavia, IL 60510

gGrenoble High Magnetic Field Laboratory, 38042 Grenoble, FrancehLawrence Berkeley National Laboratory, Berkeley, CA 94720iMassachusetts Institute of Technology, Cambridge, MA 02139

jMichigan State University, East Lansing, MI 48824kOak Ridge National Laboratory, Oak Ridge, TN 37831

lPrinceton University, Princeton, NJ 08544


What is a Muon Collider?

An accelerator complex in which

• The initial state is even more precisethan e+e− (because muons radiate lessthan electrons).

• Muons (both µ+ and µ−) are collectedfrom pion decay following a pNinteraction.

• Muon phase volume is reduced by 106 byionization cooling.

• The cooled muons are accelerated andthen stored in a ring.

• µ+µ− collisions are observed over theuseful muon life of ≈ 1000 turns at anyenergy.

• Intense neutrino beams and spallationneutron beams are available asbyproducts.

TeV Colliders:

A light-Higgs factory:


A Neutrino Factory Based on a Muon Storage Ring

• Many elements in common with thefront end of a muon collider.

• Maximize π/µ/ν yield by collectinglow-energy (≈ 400 MeV) chargedsecondaries.

• Let pions decay to muons, thenaccelerate the muons and store themin a ring to obtain decay neutrinosof desired energy.

• Accelerator components moreeffective if only a few Hzof intense beam pulses.

• Study neutrino mixing, includingCP violation via CP -conjugateinitial states:

µ+ → e+νµνe, µ− → e−νµνe.

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Targetry for Muon Colliders and Neutrino Factories

• Targetry = the task of producing and capturing π’s and µ’s from proton interac-

tions with a nuclear target.

• At a lepton collider the key parameter is luminosity:

L =N1N2f

As−1cm−2,

⇒ Gain as square of source strength (targetry),

but small beam area (cooling) is also critical.

• At a neutrino factory the key parameter is neutrino flux, ⇒ Source strength

(targetry) is of pre-eminent concern.

[Beam cooling important mainly to be sure the beam fits in the pipe.]

• The exciting results from atmospheric and reactor neutrino programs (Super-K,

SNO, KamLAND) reinforce the opportunity for neutrino physics with intense

accelerator neutrino beams, where targetry is a major challenge.


Targetry Challenges for Intense Muon and Neutrino Beams

• Use of a multimegawatt proton beam for maximal production of soft pions→muons.

• Capture pions in a 15-20-T solenoid, followed by a 1.25-T

decay channel (with beam and target tilted by 100 mrad w.r.t. magnetic axis).��

• A carbon target is feasible for 1.5-MW proton beam power.

• For Ep>∼ 16 GeV, factor of 2 advantage with high-Z target.

• Static high-Z target would melt, ⇒ Moving target.

• A free mercury jet target is feasible for beam power of 4 MW (and more).


A “Conventional” Neutrino Horn + Target

If desire secondary pions with Eπ<∼ 0.5 GeV (neutrino factories), a high-Z target is

favored, but for Eπ>∼ 1 Gev (“conventional” neutrino beams), low Z is preferred.

A conventional neutrino horn works betterwith a point target (high-Z).

Small horn ID is desirable ⇒ challenge toprovide target cooling for high beamintensity.

Aggressive design: carbon-carbon targetwith He gas cooling:


A Solenoidal Targetry System for a Superbeam

• A precursor to a Neutrino Factory is a Neutrino Superbeam based on decay of pions

from a multimegawatt proton target station.

• 4 MW proton beams are achieved in both the BNL and FNAL (and CERN) scenarios

via high rep rates: ≈ 106/day.

• Classic neutrino horns based on high currents in conductors that intercept much of

the secondary pions will have lifetimes of only a few days in this environment.

• Consider instead a solenoid “horn” with conductors at larger radii than

the pions of interest – similar to the Neutrino Factory capture solenoid.

• Pions produced on axis inside the solenoid have zero

(canonical) angular momentum, Lz = r(Pφ + eAφ/c) = 0,

⇒ Pφ = 0 on exiting the solenoid.

• If the pion has made exactly 1/2 turn on its helix when it reaches the end of the

solenoid, then its initial Pr has been rotated into a pure Pφ, ⇒ P⊥ = 0 on exiting

the solenoid,

⇒ Point-to-parallel focusing.


Narrowband Neutrino Beams via Solenoid Focusing

• The point-to-parallel focusing occurs for Pπ = eBd/(2n + 1)πc.

• ⇒ Narrowband neutrino beams of energies

Eν ≈ Pπ

2=

eBd

(2n + 1)2πc.

• ⇒ Can study several neutrino oscillation peaks at once (Marciano, hep-ph/0108181),

1.27M 223[eV

2] L[km]

Eν[GeV]=

(2n + 1)π

2.

• Get both ν and ν̄ at the same time,

⇒ Must use detector that can identify sign of µ and e,

⇒ Magnetized liquid argon TPC (astro-ph/0105442).


Thermal Shock is a Major Issue in High-Power Pulsed Beams

When beam pulse length t is less than target radius r divided by speed of sound vsound,

beam-induced pressure waves (thermal shock) are a major issue.

Simple model: if U = beam energy deposition in, say, Joules/g, then the instantaneous

temperature rise ∆T is given by

∆T =U

C,

where C = heat capacity in Joules/g/K.

The temperature rise leads to a strain ∆r/r given by

∆r

r= α∆T =

αU

C,

where α = thermal expansion coefficient.

The strain leads to a stress P (= force/area) given by

P = E∆r

r=

EαU

C,

where E is the modulus of elasticity.


In many metals, the tensile strength obeys P ≈ 0.002E,

α ≈ 10−5, and C ≈ 0.3 J/g/K, in which case

Umax ≈ PC

Eα≈ 0.002 · 0.3

10−5≈ 60 J / g.

How Much Beam Power Can a Solid Target Stand?

How many protons are required to deposit 60 J/g in a material? What is the maximum

beam power this material can withstand without cracking, for a 10-GeV beam at 10 Hz

with area 0.1 cm2.

Ans. If we ignore “showers” in the material, we still have dE/dx ionization loss,

of about 1.5 MeV/g/cm2.

Now, 1 MeV = 1.6× 10−13 J, so 60 J/ g requires a proton beam intensity

of 60/(1.6× 10−13) = 1015/cm2.

Then, Pmax ≈ 10 Hz · 1010 eV · 1.6× 10−19 J/eV · 1015/cm2 · 0.1 cm2

≈ 1.6× 106 J/s = 1.6 MW.

Solid targets are viable up to about 1.5 MW beam power!


Window Tests (BNL E-951, 5e12 ppp, 24 GeV, 100 ns)

Aluminum, Ti90Al6V4, Inconel 708, Havar, instrumented with fiberoptic strain sensors.

[Thanks to our ORNL colleagues.]

Fabry-Perot cavity length

Incoming optical fiberGauge length

Measured Strain (500 KHz) in the 10-mil Aluminum Window Beam Intensity = 2.5 TP

-40

-30

-20

-10

0

10

20

30

40

-8 -4 0 4 8 12 16 20

micro-secs

mic

ro-s

trai

ns

Predicted Strain in the 10-mil Aluminum Window Beam Intensity = 2.5 TP with 1mm RMS sigma

-40

-30

-20

-10

0

10

20

30

40

-8 -4 0 4 8 12 16 20

micro-secs

mic

ro-s

trai

ns


A Carbon Target is Feasible at 1-MW Beam Power

A carbon-carboncomposite withnear-zero thermalexpansion is largelyimmune to beam-inducedpressure waves.

A carbon target in vacuum sublimates awayin 1 day at 4 MW.

Sublimation of carbon may be negligible ina helium atmosphere.

Tests underway at ORNL to confirm this.1E-10

1E-09

1E-08

1E-07

1E-06

1E-05

0.0001

0.001

0.01

0.1

1

10

100

0 20 40 60 80 100

Power Deposited in Target (kW)

Su

blim

atio

n R

ate

of G

raph

ite T

arge

t (m

m/d

ay)

Nominal Value

- 15 mm diameter x 800 mm length target- Radiation cooling

For 1.5 MW beam

Radiation damage is limiting factor: ≈ 12 weeks at 1 MW.

Will radiation damage ruin the low thermal expansion coefficient?Kirk T. McDonald High-Power Targetry Workshop, Sept. 10, 2003 13

Effects of Radiation on SuperInvar

SuperInvar has a very lowcoefficient of thermalexpansion (CTA),⇒ Resistant to “thermalshock” of a proton beam. 0

5

10

15

20

25

30

0 40 80 120 160

The

rmal

Exp

ansi

on (

dL),

mic

rons

Temperature, deg C

Non-irradiated sample B6Irradiated sample S42Irradiated sample S46

However, irradiation at theBNL BLIP facility show thatthe CTA increases rapidlywith radiation dose.

CTA vs. dose ⇒

shielded enclosure for seven months to allow for the radio-activity to decline to more manageable levels for the sub-sequent measurements. Nonetheless, our measurements ofthe CTE and tensile properties had to be performed withinthe confines of a hot cell equipped with remote handlingcapabilities.

The samples, with holder, were immersed in a water tankfor target cooling purposes. In addition, water was directedto flow through each sample holder. The unobstructed wa-ter flow rate is 6 GPM. We estimate that the actual waterflow rate through the sample was reduced to the order of2 GPM. Given this flow rate and the peak proton currentof 108 µA experienced during the exposure, we calculatethat the peak temperature within the interior of a samplerod was on the order of 200◦ C.

Upon removal of the samples from the target holder, theindividual cylinders were washed in an acid bath to removecorrosion from the rods. Samples were then sorted by po-sition in the the target, making use of identifying marks oneach cylinder and nickel wire.

MEASUREMENTS

Activation Measurements

The samples were placed individually into an ATOM-LAB 100 dose calibrator in order to measure the integratedactivation levels. The first (entrance) plane (Fig. 1) con-sisted of unnecked-down rods and wire positioned in a hor-izontal orientation, while the the fourth (exit) plane had asimilar arrangement but with a vertical orientation. Theactivation levels of the front plane could then be used toextract information as to the vertical profile of the incidentproton beam, while the exit plane could be used for obtain-ing the horizontal profile of the proton beam (Fig. 2). Thenickel wire and Invar rods have different volumes as wellas composition, hence overall normalization for each dataset differ. However, the beam rms widths extracted fromeach set of material agree well.

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Figure 2: Measured specimen activity as a function of tar-get position.

This measured beam profile, along with the total pro-ton flux and incident energy, was then used as input into

the code MCNPX[2] and the results used to calculate theatomic displacements within each sample. Results of theactivation measurements of each sample correlate well withthe calculated values for the atomic displacements aver-aged over each rod.

Thermal Expansion Measurements

For the measurement of the coefficient of thermal expan-sion, we utilize an L75 dilatometer provided by LINSEIS,Gmbh. This device was specifically fabricated to allow usease of remote operation since the measurements were con-fined to a hot cell where remote manipulation of the equip-ment as well as the mechanical insertion of the samples wasrequired. Measurement of non-irradiated samples demon-strated that the stock material had the expected CTE of 0.6× 10−6 /◦K at room temperature while the base line forthe temperature range of 50◦C to 150◦C was 1.0 × 10−6

/◦K (see Fig. 3). This figure also demonstrates that irradia-tion dramatically alters the thermal expansion properties ofSuper-Invar. The results for all fourteen straight irradiatedspecimens are shown in Fig. 4.

0

5

10

15

20

25

30

0 40 80 120 160

The

rmal

Exp

ansi

on (

dL),

mic

rons

Temperature, deg C

Non-irradiated sample B6Irradiated sample S42Irradiated sample S46

Figure 3: Measured thermal expansions

0

1

2

3

4

5

6

0 0.05 0.1 0.15 0.2Coe

f. T

herm

al E

xpan

sion

, 10-

6/K

Displacements per Atom

Dilatometer Measurements

Plane 1Plane 4

Base

Figure 4: Measured coefficients of thermal expansion as afunction of calculated atomic displacements.

We also measured the CTE of the eight Inconel rods asSuperInvar is made strongerby moderate radiation doses(like many materials).

Yield strength vs. dose ⇒

well as two non-irradiated Inconel specimens. Since the In-conel rods were used as spacers at the edges of the target,their levels of activation and atomic displacement are typi-cally less that the Super-Invar samples. Nonetheless, we doobserve a small change in the CTE for Inconel (see Fig. 5).

13.2

13.4

13.6

13.8

14

0 0.02 0.04 0.06Coe

f. T

herm

al E

xpan

sion

, 10-

6/K

Displacements per Atom

Inconel Dilatometer Measurements

Figure 5: Measured coefficients of thermal expansion ofthe Inconel samples as a function of calculated atomic dis-placements.

Tensile Measurements

The effect of different levels of irradiation on the me-chanical properties of Super-Invar was assessed by per-forming a tensile test on specimens that have been speciallydesigned for that purpose. In particular, the two middleplanes of the target were formed by specimens which hadbeen necked-down to a diameter of 80 mils. The maximumirradiation levels reached during the exposure to the beamhas been calculated to be 0.25 dpa.

0

500

1000

1500

2000

2500

0 0.05 0.1 0.15 0.2 0.25

Extension (mm)

Lo

ad (

N)

Irradiated (0.25 dpa)

non-irrad #1

non-irrad #2

Figure 6: Load-displacement curves for irradiated and non-irradiated invar specimens.

The load-displacement curves of virgin as well as irra-diated specimens from the same block of material wereobtained. Particular care was taken to maintain the sameparameters of tensile test in order to avoid scattering of

the data. As a result very similar load-displacement curveswere achieved for the non-irradiated specimens. This pro-vided a reference for the mechanical properties (such asthe yield strength, the ultimate strength and the modulus ofelasticity) that are evaluated as a function of the irradiation.

500

520

540

560

580

600

620

640

660

680

0 0.05 0.1 0.15 0.2 0.25

dpa

MP

a

Figure 7: Yield vs atomic displacement for irradiated andnon-irradiated invar specimens.

While no effect was observed for the modulus of elastic-ity, irradiation effects are apparent. Specifically, the mate-rial becomes stronger but brittle. A 15% increase in tensilestrength was observed. The irradiated material, however,lost its post-yield strength (no ultimate strength) and frac-tured at smaller displacement (strain) levels.

SUMMARY

Our results indicate that selecting a target material basedon it’s attractive coefficient of thermal expansion shouldbe proceeded by a consideration of the effects that radia-tion damage can impart on this property. Super-Invar canbe considered a serious target candidate for an intense pro-ton beam only if one can anneal the atomic displacementsfollowed by the appropriate heat treatment to restore its fa-vorable expansion coefficient. On the other hand, the moremodest influence of radiation damage on the Inconel sam-ples suggests that targetry material selection based on yieldstrength rather than low thermal expansion coefficient maylead to a more favorable result.

REFERENCES

[1] H.G. Kirk, TARGET STUDIES with BNL E951 at the AGS,Proceedings of the 2001 Particle Accelerator Conference,Chicago, Il., March 2001, p.1535.

[2] MCNPX Users Manual-Version 2.1.5, L.S. Waters, ed., LosAlamos National Laboratory, Los Alamos, NM. TPO-E83-G-UG-X-00001. (1999)


High-Performance Titanium Alloy Developed by Toyota

T. Saito et al., Science 300, 464 (2003)

“We describe a group of alloys that

exhibit super properties, such as

ultralow elastic modulus, ultrahigh

strength, super elasticity, and super

plasticity, at room temperature and

that show Elinvar and Invar behav-

ior. These super properties are at-

tributable to a dislocation-free plas-

tic deformation mechanism. In cold-

worked alloys, this mechanism forms

elastic strain fields of hierarchical

structure that range in size from the

nanometer scale to several tens of

micrometers. The resultant elastic

strain energy leads to a number of

enhanced material properties.”

Cold working of this Ti alloy [Ti-23Nb-0.7Ta-2Zr-1.2O] is favorable 3 ways:

higher tensile strength, lower Young’s modulus, and lower thermal expansion coefficient.

Are these advantages robust against radiation damage? Will test at BLIP in FY04.


A Liquid Metal Jet May Be the Best Target

for Beam Power above 1.5 MW

Mercury jet target inside a magnetic bottle:20-T around target, dropping to 1.25 T inthe pion decay channel.

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Mercury jet tilted by 100 mrad, protonbeam by 67 mrad, to increase yield of softpions.


Pion/Muon Yield

For Ep>∼ 10 GeV,

more yieldwith high-Z target.[MARS calculations:]

Mercury target radiusshould be ≈ 5 mm,with target axis tiltedby ≈ 100 mrad to themagnetic axis.

0 50 100 150 200 250Atomic mass A

0.0

0.2

0.4

0.6

0.8

1.0

Mes

on y

ield

(0.

05<

p<0.

8 G

eV/c

) pe

r pr

oton

Proton beam (σx=σy=4 mm) on 1.5λ target (r=1 cm)20 T solenoid (ra=7.5 cm) MARS13(97) 8−Dec−1997

π+ + K

+

π− + K

−

C AlCu

GaHg

Pb

30 GeV

16 GeV

8 GeV

PtO2

0 3 6 9 12 15Target radius (mm)

0.15

0.20

0.25

0.30

Yie

ld a

t 9 m

+, 50 mrad−, 50 mrad+, 100 mrad−, 100 mrad+, 150 mrad−, 150 mrad

16 GeV on Hg

RT = 2.5σx,y

0 50 100 150 200Tilt angle (mrad)

0.25

0.26

0.27

0.28

0.29

0.30

0.31

Yie

ld a

t 9 m

16 GeV on Hg

RT = 2.5σx,y = 5 mm

π−+µ−

π++µ+

Can capture ≈ 0.3 pion per proton with 50 < Pπ < 400 MeV/c.


20-T Capture Magnet System

Inner, hollow-conductor copper coils generate 6 T @ 12 MW:

Bitter-coil option less costly, but marginally feasible.

Outer, superconducting coils generate 14 T @ 600 MJ:

Incoloy Alloy 908 Conduit >1000 superconducting wires Supercritical helium flows in interstices

and central channel

Cable-in-conduit construction similar to ITER central solenoid.

Both coils shielded by tungsten-carbide/water.


Target System Support Facility

Extensive shieldingand remote handlingcapability.[Spampinato et al.,Neutrino FactoryFeasibility Study 2(2001)]


Lifetime of Components in the High Radiation Environment

Component Radius Dose/yr Max allowed Dose 1 MW Life 4 MW life

(cm) (Grays/2× 107 s) (Grays) (years) (years)

Inner shielding 7.5 5× 1010 1012 20 5

Hg containment 18 109 1011 100 25

Hollow conductor coil 18 109 1011 100 25

Superconducting coil 65 5× 106 108 20 5

Some components must be replaceable.

[MARS calculations:]

FS−2 24 GeV Target Station: MARS14 02/19/01R,

Z,

Z

X

0

35

70

105

140140

cm

0 200 400 600600

cm

105 104 103 102 101 100 10−1 10−2 10−33.6e+06 0.0e+00

Azimuthally averaged absorbed dose (MGy/yr)


Viability of Targetry and Capture For a Single Pulse

• Beam energy deposition may disperse the jet. [FRONTIER calculations]

• Eddy currents may distort the jet as it traverses the magnet.


Beam-Induced Cavitation in Liquids Can Break Pipes

ISOLDE:

BINP:

SNS:

Snapping shrimp stun prey viacavitation bubbles.


The Shape of a Liquid Metal Jet under a Non-uniform Magnetic Field

S. Oshima et al., JSME Int. J. 30, 437(1987).


Studies of Proton Beam + Mercury Jet (BNL)

Proton

Beam

Mercury

Jet

1-cm-diameter Hg jet in 2e12 protons at t = 0, 0.75, 2, 7, 18 ms.

Model (Sievers): vdispersal =∆r

∆t=

rα∆T

r/vsound=

αU

Cvsound ≈ 50 m/s

for U ≈ 100 J/g.

Data: vdispersal ≈ 10 m/s for U ≈ 25 J/g.

vdispersal appears to scale with proton intensity.

The dispersal is not destructive.

Filaments appear only ≈ 40 µs after beam,

⇒ after several bounces of waves, or vsound very low.


Tests of a Mercury Jet in a 20-T Magnetic Field

(CERN/Grenoble, A. Fabich, Ph.D. Thesis)

Eddy currents may distortthe jet as it traverses themagnet.

Analytic model suggestslittle effect if jet nozzleinside field.

4 mm diam. jet,v ≈ 12 m/s,B = 0, 10, 20 T.

⇒ Damping ofsurface-tension waves(Rayleigh instability).

Will the beam-induceddispersal be damped also?


Issues for Further Targetry R&D

• Continue numerical simulations of MHD + beam-induced effects.

• Continue tests of mercury jet entering magnet.

• For solid targets, study radiation damage – and issues of heat removal from solid

metal targets (carbon/carbon, Toyota Ti alloy, bands, chains, etc.).

• Confirm manageable mercury-jet dispersal in beams up to 1014 protons/pulse

– for which single-pulse vaporization may also occur. Test Pb-Bi alloy jet.

• Study issues when combine intense proton beam with mercury jet inside a high-field

magnet.

1. MHD effects in a prototype target configuration.

2. Magnetic damping of mercury-jet dispersal.

3. Beam-induced damage to jet nozzle – in the magnetic field.

• ⇒ We propose to construct a 15-T pulsed magnet,

that can be staged as a 5-T and 10-T magnet.


A 15-T LN2-Cooled Pulsed Solenoid

• Simple solenoid geometry with rectangular coil cross section and smooth bore

(of 20 cm diameter)

• Cryogenic system reduces coil resistance to give high field at relatively low current.

– Circulating coolant is gaseous He to minimize activation, and to avoid need to

purge coolant before pulsing magnet.

– Cooling via N2 boiloff.

• Most cost effective to build the 4.5-MW supply out of “car”

batteries! (We need at most 1,000 pulses of the magnet.)


Possible Sites of the Beam/Jet/Magnet Test

E-951 has existing setupin the BNL A3 line – butbeam may be no longeravailable there.

J-PARC 50-GeVfast-extracted beam:(LOI 30)

CERN PS transfer line:


targets for neutrino factories and muon colliders€¦ · james h. norem,b robert b. palmer,c ralf...

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