Reducing shock in the Neutrino Factory target

Goran Skoro (University of Sheffield)

UKNF Meeting

11 January 2006

Contents:

Short reminder (Stress vs. macro pulse length)

New results (Effect of additional target material)

•Pbar target

•Neutrino Factory target

Summary, Plans

Short reminder

•The target is bombarded at up 50 Hz by a proton beam consisting of ~1ns long bunches in a pulse of a few micro-s length.

•The target material exposed to the beam will be ~ 20cm long and ~2cm in diameter.

•Energy density per pulse ~ 300 J/cc.

•Thermally induced shock (stress) in target material (tantalum).

Geometry: NF target

2cm

20cm

Boundary conditions: free

Thermal load of 100K

(equivalent energy density of ~ 300 J/cc)

Tinitial = 2000K

Material model used in the analysis

•Temperature Dependent Bilinear Isotropic Model

'Classical' inelastic model

Nonlinear

– Uses 2 slopes (elastic, plastic) for representing of the stress-strain curve

– Inputs: density, Young's modulus, CTE, Poisson's ratio, temperature dependent yield stress, ...

•Element type: LS-DYNA Explicit Solid

•Material: TANTALUM, Graphite (T2K)

~10-20% effect

< 3% effect

Stress vs. pulse length

micro-pulse

macro-pulse

Fixed: 1 us

Stress vs. pulse length

micro-pulse

macro-pulse

~ RAL proton driver

Important parameters: Energy deposition rate and shock transit time!

Stress vs. pulse length

micro-pulse

macro-pulse

10

“Old” Pbar test results

•In this case 'Damage' = reduction of pbar yields

•The tantalum target (disc, ~ few mm height) was bombarded by a proton beam (up to 5e12 ppp, pulse length = 1.6 us)

•The beam width was extremely small (down to sigma = 0.15 mm) -> Energy density per pulse up to 40.000 J/cc !!!!

• Expectation: 1 pulse should damage the target ...

•...but, tantalum survived thousands of pulses with 'no damage' !

•Here, only (relatively) small volume of the material was heated. Is that important?

NO DAMAGE for tantalum target (WHY?)

GraphiteTantalumGraphite

beam

Pbar target assembly

Simulations of Pbar target (tantalum cylinder case)

•Patrick Hurh (FNAL) about the beam parameters:

“the area where we could put a tantalum test target (before 2009) has beam parameters something like 0.5 mm sigma and 5e12 protons per pulse (120 GeV) 1.6 microsecond pulse length with a repetition rate of once every 2 seconds. .......If you need finer resolution on the pulse structure, I believe it is dependent upon the Booster batches per pulse and I'll have to find that out later.”

• I have simulated the target response keeping the same geometry as in NF target case (beam centreline = centreline of the cylinder) with target radius a = 3sigma = 1.5mm...

• ...and then repeated it with more material around the heated part of the cylinder

a

b

Approximation !!!But, stress is maximal along thebeam (centreline).

a

b

It looks like oscillation around'quasi-static' stress value(with much smaller amplitude than for b=0 case)

End of pulseCharacteristic time ~ 2b / speed of sound

Pbar energy density conditions

Temperature rise (peak) = 500 K

Origin of damage?Stress absolute value or “tension-compression” oscillations orboth?

Neutrino Factory case

'quasi-static' stress value +reflected “shock wave” every t_char

(for 2b = 10a = 10cm it is ~ 33 us)

this cannot be avoided...

...but the first peak (usually the highest one) is reduced

End of pulse

If both preferable situation when having more material!Let's say cross-section of toroid tobe bigger than 1cm in radius (if heatedarea has 1cm radius)

or smaller beam sigma!

Neutrino Factory case

stress is higher at a when additionalmaterial is present...

a

b

it is radially constrained

'plateau' – new equilibrium positioncorresponds to quasi-static stress

Neutrino Factory case

stress reduction because of longermacro-pulse (~5x)

stress reduction because of additionalmaterial (~ 2x)

'Elimination' of the first peak, definitely!

Maybe, tests at FNAL with differenttarget dimensions (heights)?

Is it a bigger toroid a problem?

Neutrino Factory case

Stress pattern dependenceon material parameters values (Yield stress in this case)

Need for experiments to determine material model data

Current pulse through wire!

fitting formula:

Details, progress, etc... see URL:http://hepunx.rl.ac.uk/ Target Studies Thermal Shock Simulations

Extraction of material data (first steps)

Summary:

• Simulations:

•Non-negligible stress reduction in the NF targets by varying the macro-pulse length and target/beam dimensions looks possible

• Experiments needed for better understanding of the material behavior (properties)

• Tests:

• Possible Pbar target tests have problems with diagnostics but some qualitative picture about the damage as a function of target dimensions/energy density can be obtained (hopefully)

• Current pulse – wire test