Tungsten wire & VISAR

1. Tungsten wire & VISAR Goran Skoro 24 October 2008

2. Results of calculations -> following pages VISAR wire tests – Standard approach Can we see a signal with 10m delay-leg (we already have it) or we need a longer delay-leg (let’s say 30m)? Room temperature or high temperature (let’s say 1500K)? We can measure radial or/and longitudinal displacement of the wire (for 0.5 mm diameter, 3 cm long wire and peak current of 6 kA) Issues: Laser beam Laser beam Wire VISAR signal? Radial or longitudinal oscillations? 02

3. VISAR wire tests – Standard approach (for room and high temperature; for 10m and 30m delay-leg) Conclusion: We won’t see anything here Results Radial displacements VISAR signal: - flatline 03

4. VISAR wire tests – Standard approach - very nice (decent) for 30m delay-leg at high (room) temperature; Conclusion: We have to focus on longitudinal oscillations - decent (low) for 10m delay-leg at high (room) temperature; Results Longitudinal displacements VISAR signal: 04

5. VISAR wire tests – Standard approach If we have a nice signal, VISAR is sensitive to material parameters values. Here shown changes of VISAR signal for +-10% changes of material parameters (E, CTE). Change of E is responsible for time-shift of the signal. High temperature Room temperature Sensitivity of VISAR signal on material parameters values 05

6. VISAR wire tests – Standard approach Very thin wire (0.1 mm diameter) and (only) 1 kA current Beautiful VISAR signal at room temperature with 10m delay-leg BUT… Another possibility 06

7. VISAR wire tests – Standard approach A few words about VISAR’s laser beam spot size… 07 • VISAR signal intensity has been tested as a function of the wire diameter • Laser beam has been pointing at the end of wire (end of wire has been polished) • Nice signal has been observed for 0.5 mm diameter wire • Very low signal has been observed for 0.3 mm diameter wire • Problem: Laser beam spot size is too big (=> 0.5mm diameter) • Consequence: We can hardly see a thing for wire diameters smaller than 0.4 mm • So, the only chance to do the test with existing (10m) delay leg is to pulse a wire until it reaches high temperature* and then try to measure the VISAR signal • *The difference in a wire surface displacements at room and high temperature (see upper plots in Slides 3 and 4) is a result of very low tungsten resistivity at room temperature (10x lower than at 1500 K).

8. Results of calculations of wire stress, surface displacements and corresponding VISAR signal as a function of wire diameter are shown in following pages. VISAR wire tests – Alternative approach While waiting for ‘refurbishment’ of our power supply, there is Roger’s idea to shock a wire by discharging the number (n~20) of capacitors. 08 • Parameters: • Voltage applied to capacitor ~ 50 kV; peak current ~ 950 A • Very short pulse (20 ns rise time, 30 ns fall time) • ‘n’ circuits in parallel (n=20) • Estimated temperature rise in the 0.2 mm diameter tungsten wire (at room temperature) ~ 130 K (similar to the NuFact target case)

9. Decent signal for 10m delay-leg; radial movement that affects longitudinal one -> clearly seen at the beginning (see inset plot); shame that our laser-beam spot size is so big so the amount of reflected light is so small… VISAR wire tests – Alternative approach end of wire ‘20 capacitors’ case - 0.2 mm diameter wire First 2 s First 2 s 09

10. As expected, situation is much worse than for 0.2 mm diameter; temperature rise is only ~ 35 K; Lorenz force induced pressure wave starts to dominate… VISAR wire tests – Alternative approach end of wire ‘20 capacitors’ case - 0.3 mm diameter wire First 2 s First 2 s 10

11. Practically no signal for 20 capacitors but may look promising if we add more circuits (see slide 13) VISAR wire tests – Alternative approach end of wire ‘20 capacitors’ case - 0.4 mm diameter wire First 2 s First 2 s 11

12. We could see a signal here without any problems if there is any. Unfortunately, we have a flatline – the wire is ‘dead’ (from the VISAR’s point of view). More (but reasonable number of) circuits in parallel won’t change the results. VISAR wire tests – Alternative approach end of wire ‘20 capacitors’ case - 0.5 mm diameter wire 12

13. ‘Doubling the number of capacitors will give us a beautiful signal for 0.4 mm diameter wire during the first 2 micro-s. And we could see it (even with the laser-beam spot size we have at the moment). But this ‘huge number of circuits’ scenario has its disadvantages… VISAR wire tests – Alternative approach end of wire ‘40 capacitors’ case - 0.4 mm diameter wire First 2 s First 2 s 13

14. 14 Update I 12 November 2008

15. VISAR wire tests – Standard approach - we plan to use a longer wire (~ 5 cm) in tests with our ‘old’ power supply VISAR signal as a function of wire length Conclusion: No difference (as expected) Previous calculations: - wire length = 3 cm 15

16. VISAR signal has been calculated for shorter delay-legs (1m, 3m). It seems that 10m delay-leg is optimal if want to measure the wire oscillations on short time scale… VISAR wire tests – Alternative approach end of wire ‘20 capacitors’ case - 0.2 mm diameter wire First 2 s First 2 s 16

17. VISAR signal has been calculated for shorter delay-legs (1m, 3m). It seems that 10m delay-leg is optimal if want to measure the wire oscillations on short time scale… VISAR wire tests – Alternative approach end of wire ‘20 capacitors’ case - 0.3 mm diameter wire First 2 s First 2 s 17

18. 18 Update II 07 December 2008

19. VISAR wire tests – Standard approach A few words about VISAR’s laser beam spot size… AGAIN 19 • Laser beam spot size is not an issue anymore • End of wires have been properly polished • Huge signals for 0.5 mm and 0.3 mm diameter wires • Decent signal for 0.1 mm diameter wire • All options are possible (except 0.1 mm -> too thin, bending is a problem) • Forget the conclusions given in Slide 7 • Now we can look what is the best choice of wire diameter to start tests with…

20. VISAR wire tests – Standard approach - a small improvement comparing to 6 kA case (Slide 4) at room temperature for 10 m delay-leg Conclusion: Maybe we should start tests with 0.3 mm diameter wire because… Results 0.5 mm wire, 8 kA VISAR signal: 20

21. VISAR wire tests – Standard approach We should have nice signal even for lower current… - very nice signal at room temperature for 10 m delay-leg - 1500K case not shown -> stress too high (here ~ 450 MPa) Results 0.3 mm wire, 8 kA VISAR signal: 21

22. VISAR wire tests – Standard approach - very nice signal at room temperature for 10 m delay-leg - 1500K case not shown -> stress too high (here ~ 250 MPa) Results 0.3 mm wire, 6 kA VISAR signal: 22

23. George is rechecking the parameters used in initial calculation of surface velocity Maybe, interesting for us for testing the tungsten foils First results based on George Ellwood’s AUTODYN simulations shown here At least 3x higher velocity is needed to have decent VISAR signal Something completely different VISAR @ ATF 4mm thick Ti-6Al-4V George Ellwood’s calculations 23 ATF at KEK has the same configuration as the ILC injector, i.e. ATF is composed from electron-gun, 1.5 GeV electron linac, 1.5 GeV damping ring(circular accelerator), and beam extraction diagnostic line. The design work was started in 1990. The beam operation began in 1997. 1.28 GeV electrons0.7x10^10 electrons per bunch 20 bunches per train 0.15 – 1.4 s between trains 10ps pulse length 1.44 J per bunch70 microns x 7 microns spot size