Pion Production from Water-Cooled Targets

Stephen Brooks / [email protected]

UKNF meeting, Warwick, April 2008

Pion Production fromWater-Cooled Targets

Stephen Brooks / [email protected] meeting, Warwick, April 2008

Contents

1. Recap of current UKNF solid target

2. Pros and cons related to water cooling

3. Results from MARS 15.07– Including water in & lengthening the target– Pion yield (reabsorption)– Additional heating– Pion temporal distribution

• Conclusions


1. Recap of current UKNF solid target


Traditional UKNF Solid Target

• p+ target +,− +,− neutrinos

• Radiation-cooled solid rods of tungsten– Replaced every 50Hz beam pulse by chain or

wheel (~200 in whole loop)

• 2-3cm diameter, 20-30cm length

• Inside initial 20T solenoidal capture field– Usable bore ~20cm diameter– Pions tend to spiral in magnetic field


Pion Motion


Main Design Problems

• Direct heating of the target from energy deposition

• Pions becoming reabsorbed– Target being too wide– Re-entering the target due to magnetic field

• Reabsorption produces more heating!

• A total of 20-30cm of high-Z target material thickness seems optimal


Where does the proton beam power go?

12.9%

24.3%

62.8%

Heat deposited in target

Kinetic energy of protons within5° of forward axis

Kinetic energy of other (diffuse)secondaries, including pions

Figures from a 10GeV proton beam (ISS baseline) hitting a 20cm long, 2cm diameter tantalum rod target


Muon Transmission from MARS15-Generated Pions

3

GeV

4

GeV

8

GeV

4

0GeV

5

0GeV

7

5GeV

1

00G

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120

GeV

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1 10 100 1000

Proton Energy (GeV)

Pio

ns p

er P

roto

n.G

eV (e

st. C

hica

ne/L

inac

)

pi+/(p.GeV)

pi-/(p.GeV)


Energy Deposition in Rod (heat)

• Scaled for 5MW total beam power

2.2G

eV

3GeV

4GeV

5GeV

6GeV

8GeV

10G

eV

15G

eV

20G

eV 30G

eV

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eV50

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75G

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eV12

0GeV

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Proton Energy (GeV)

Po

wer

Dis

sip

atio

n (

Wat

ts,

no

rmal

ised

to

5M

W

inco

min

g b

eam

)


Not Multi-Megawatt Heating

Machine Beam power Proton energy Heating power

ISS beam 4MW 10GeV 514kW

UK neutrino factory scenarios

4MW 8GeV 512kW

5MW 10GeV 643kW

5MW 8GeV 640kW

neutron source

169kW 800MeV211A

169kW ×1

×3

×4


2. Pros and cons related to water cooling


Why it “wouldn’t work”

• Additional water would reabsorb too many pions– It would also increase heating in itself

• Increasing the target length would increase longitudinal time-spread of pions– 1m length = 3ns > 1ns RMS of proton beam

• Water-cooling has a maximum of 1MW– Would require 50% water in the small target


False Assumptions Corrected

• Additional water would reabsorb too many pions– Water = 1.0 g/cc, tungsten = 19.2 g/cc

• Increasing the target length would increase longitudinal time-spread of pions– Pions of interest >250 MeV/c momentum

– > 0.87, protons = 0.996, only lag matters

• 1MW is enough capacity


Advantages of Water Cooling

• Conventional technology– Many examples in operation

• Including elsewhere in the target assembly! E.g. cooling for the normal-conducting solenoids

– No solid moving parts (apart from pumps)– Radioactive flow loop isn’t liquid mercury

• Although there is still some tritium to deal with

• All parts of target stay below about 200°C


Disadvantages of Water Cooling

• Water will cavitate if the instantaneous temperature rise is too high, erode walls– Also if the flow rate is too high for pressure

• Flow manifold has to be somewhere and enter/exit the target

• Pressures may have to be large to induce sufficient flow rate

• Relies on fluid dynamics, requires much more careful design than in this talk


Naïve Flow Rate Calculation

• Assumes perfect:– Conductivity of metal target pieces– Thermal conduction from target to water– Mixing of water

• Pin = 700kW; T = 50K

• Flow rate 3.34 kg/s

• Speed 10.6 m/s for 2cm diameter pipe– Will be more than this realistically


3. Results from MARS 15.07


Simulation Geometry

Protons

20cm, 50cm, 1m, 2m

2,3,4,5cm

Bz = 20T 10cm radius

Particles logged at end-plane

Particles removed outside bore

Parallel beam, circular parabolic distribution, 2cm diameter

WaterTantalum “coin”, 2mm thick 100 coins in target for 20cm total Ta thickness

4×4 = 16 runs


Three Figures of Merit

• Useful pion yield– Weighted depending on (pL,pT) momenta

• Amount of heating in the system– How much does the water contribute?

• Time-spread acquired from long target– Only interested in “useful” pions here too


0

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pi+/(p.GeV)

pi-/(p.GeV)

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pi+/(p.GeV)

pi-/(p.GeV)

Useful Pion Yield1000 protons10000 protons50000 protons

3x50cm gives 94.4% of 2x20 (solid)3x100 gives 90.8%


Amount of Heating

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Energy deposition proportion intantalum

Energy deposition in water

Total heating for 4MW beam

3x50cm gives 644kW heating3x100 gives 488kW


Arrival Time Distribution2m length

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Arrival time (10 bins = 1ns)

All pions

Useful pions

1m length

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All pions

Useful pions

50cm length

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All pions

Useful pions

20cm length

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All pions

Useful pions

(2cm diameter rod)


RMS of Useful Arrival Times

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RM

S t

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ns)

Instantaneousbeam

Combined with1ns RMS protontime-spread



Modified Geometry

Protons

20cm, 50cm, 1m, 2m

2mm thickness stainless steel outer tube

WaterTantalum “coin”, 2mm thick 100 coins in target for 20cm total Ta thickness

4×4 = 16 runs

3mm thickness additional water outside main target


Modified Pion Yield

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pi+/(p.GeV)

pi-/(p.GeV)

pi+ with tube

pi- with tube

Ratio(pi+)

Ratio(pi-)

3x50cm: 94.4% 91.9% of 2x20 (solid)3x100: 90.8% 82.7%


Amount of Heating

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Energy deposition proportion intantalum

Inner water

Water tube

Steel tube

Total water

Total heating for 4MW beam

Total without tube

3x50cm: 644 737kW heating3x100: 488 616kW


RMS of Useful Arrival Times

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RM

S t

ime-

spre

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Instantaneousbeam




Conclusions

• The neutrino factory requirements do not seem to preclude a water-cooled target– Fast particle production targets can have a

much lower % heat load than slow targets

• Does this also mean an SNS-style enclosed mercury target might work?

• Need to investigate in more depth to either verify or exclude these options

Documents

Pion Production from Water-Cooled Targets