Investigation of Solid Tungsten Targets for Neutrino Factory Applications

1. Solid Target Studies for NF Rob Edgecock On behalf of: J.Back, R.Bennett, S.Gray, A.McFarland, P.Loveridge & G.Skoro Tungsten wire at 2000K

2. Reminder • Solid means • tungsten bars, each ~2x20cm • 150-200 bars • changed between beam pulses • cooled radiatively or possibly by helium/water • Why? • lots of experience world-wide & safer • already have a license at RAL • Issues for solids: • shock – original show-stopper • radiation damage • target change • Focus has been on shock - but now moving on

3. Shock • Was solid show-stopper: one of main reasons for liquids • Impossible to lifetime test with proton beam, so Aims: measure lifetime validate LSDyna model understand W behaviour 60kV, 8kA PSU, 100ns rise time

4. I, II, III –> ‘chronology’. • We have got better with the tests over time (better clamping of the wire; better understanding of ‘violin modes’ –> better alignment of the wire) Better at lower temperature! More than sufficient lifetime demonstrated: > 10 years for 2cm diameter target > 20 years for 3cm diameter target Focus now: Measure stress; Confirm modelling. SEM imaging BegbrokeNano, Oxford Materials Characterisation Services The aim to observe any surface damage which might indicate the presence of thermal fatigue Results: inconclusive • Lifetime/fatigue tests results 200 targets

5. Laser Doppler Vibrometer • VISAR • Mis-informed that VISAR required at high temp • Large irreducible noise in our frequency range • Took a long time to get any (poor) results • LDV • Much, much, much, ................. better than VISAR • Works to >2300K • Used to measure wire surface velocity • Frequency analysis used to extract results • Compared with LSDyna other measurements

6. Laser beam Laser beam Wire Laser Doppler Vibrometer Longitudinal and radial mesurements possible • Longitudinal • Bigger oscillations: ~μm; lower frequency: ~20kHz • But.....temperature variation along wire • Wire fixed at one end, constrained at other • Oscillations more difficult to understand • Radial • Smaller oscillations: 50-100nm; higher frequency: ~12MHz • But......fixed temperature • Easier to model

7. Longitudinal oscillations Frequency analysis Longitudinal oscillations vs LSDyna

8. Radial oscillations Radial oscillations: frequency analysis vs LSDyna Radial oscillations vs LSDyna

9. Comparison with LSDyna • Oscillations are complicated • wire partially fixed to frame • frame also moves • violin modes, etc • Have pretty good agreement but studies continuing • Level of stress is correct • correct velocity is reproduced • wire is being stressed at above NF levels • Lifetime results are valid

10. ‘Fit’ – will be used later for comparison Comparison with Measurements Concern: low strength from static measuremts at high temp • J.W. Davis, ITER Material Properties Handbook, 1997, Volume AM01-2111, Number 2, Page 1-7, Figure 2

11. If we know the Poisson’s ratio , density , root of corresponding Bessel function , wire radius r and measure the frequency f as a function of temperature then: Illustration Laser beam Wire Measurement in Wire Tests

12. Tungsten Young’s Modulus

13. Young’s Modulus of Tungsten • Results are very encouraging • But…… • E is not tensile strength • Tensile strength is what really matters • Expectation is E would fall rapidly once plastic • Can we check this? • Difficult with tungsten....having a go with Ta (see GS talk)

14. Young’s Modulus Conclusions • Measurements ~ comparable with existing • Details largely understood, but more work required • Bottom line: Strength remains high at high temperature and at high stress! • Reason why measured lifetime higher than some expected • Plans: • Continue detailed studies • Repeat lifetime tests, but measure with LDV over time • Use beams and measure with LDV to cf LSDyna - request made to ISIS - use Ilias’s facility at CERN

15. Radiation Damage • ISIS: • used tantalum for > 10 years, tungsten ~5 years • targets changed after ~12dpa • ~2-5 years at NF, depending on # of targets • no signs of swelling or embrittlement • Ta examined in detail; W still to be done • Still to be done • tensile strength after irradiation • hope to form consortium to measure 12dpa ISIS target • but……..

16. Radiation Damage NB Static measurements.

17. Target Change • Current option: a wheel – being investigated now • Several already used, but most relevant: design study Horizontal for compatibility with baseline target station

18. Target Change • For us, major issue is capture solenoid • Helmholtz coil looks difficult due to forces • Placing wheel up-stream of solenoids: ~60% of Study II muon capture rate

19. Target Change • Arranging visit to discuss collaboration with ESS • But.....problem being studied by ISIS TS2 engineers • Given free reign, uninhibited by physicists • Studying an idea with minimal impact on solenoids • More if it works out

20. Conclusions • Shock: • We’ve done this to death! • Don’t believe it is a problem • Tests with beams to come • Radiation damage: • Lots of local experience exists • Needs to be applied to our case • But existing data are encouraging • Target change: • Wheel looks feasible – detailed study underway • Alternatives being investigated