Tungsten wire & VISAR Goran Skoro

24 October 2008

VISAR wire tests – Standard approach

02

We can measure radial or/and longitudinal displacement of the wire Wire Laser beam Laser beam

Issues:

VISAR signal?

(for 0.5 mm diameter, 3 cm long wire and peak current of 6 kA) Room temperature or high temperature (let’s say 1500K)?

Can we see a signal with 10m delay leg (we already have it) or we need a longer delay-leg (let’s say 30m)?

Radial or longitudinal oscillations?

Results of calculations -> following pages

Results Radial displacements VISAR signal: - flatline (for room and high temperature; for 10m and 30m delay-leg) Conclusion: We won’t see anything here

VISAR wire tests – Standard approach 03

Results Longitudinal displacements VISAR signal:

- very nice (decent) for

30m delay-leg at high

(room) temperature; - decent (low) for 10m delay-leg at high (room)

temperature; Conclusion: We have to focus on longitudinal oscillations

VISAR wire tests – Standard approach 04

VISAR wire tests – Standard approach 05

Sensitivity of VISAR signal on material parameters values Room temperature High temperature

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.

Another possibility Very thin wire (0.1 mm diameter) and (only) 1 kA current Beautiful VISAR signal at room temperature with 10m delay-leg

BUT…

VISAR wire tests – Standard approach 06

VISAR wire tests – Standard approach 07

A few words about VISAR’s laser beam spot size…

• • • • • •

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

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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).

08

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.

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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)

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Estimated temperature rise in the 0.2 mm diameter tungsten wire (at room temperature) ~ 130 K (similar to the NuFact target case) Results of calculations of wire stress, surface displacements and corresponding VISAR signal as a function of wire diameter are shown in following pages.

‘20 capacitors’ case - 0.2 mm diameter wire

VISAR wire tests – Alternative approach 09 end of wire First 2

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s First 2

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s 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…

‘20 capacitors’ case - 0.3 mm diameter wire

VISAR wire tests – Alternative approach 10 end of wire First 2

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s First 2

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s 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…

‘20 capacitors’ case - 0.4 mm diameter wire

VISAR wire tests – Alternative approach 11 end of wire First 2

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s First 2

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s Practically no signal for 20 capacitors but may look promising if we add more circuits (see slide 13)

‘20 capacitors’ case - 0.5 mm diameter wire

VISAR wire tests – Alternative approach 12 end of wire 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.

‘40 capacitors’ case - 0.4 mm diameter wire

VISAR wire tests – Alternative approach 13 end of wire First 2

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s First 2

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s ‘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…

Update I

12 November 2008 14

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

VISAR wire tests – Standard approach 15

‘20 capacitors’ case - 0.2 mm diameter wire

end of wire VISAR wire tests – Alternative approach 16 First 2

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s First 2

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s 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…

‘20 capacitors’ case - 0.3 mm diameter wire

end of wire VISAR wire tests – Alternative approach 17 First 2

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s First 2

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s 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…

Update II

07 December 2008 18

A few words about VISAR’s laser beam spot size… AGAIN

VISAR wire tests – Standard approach 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…

Results 0.5 mm wire, 8 kA VISAR signal: - 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…

VISAR wire tests – Standard approach 20

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

VISAR wire tests – Standard approach 21

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

VISAR wire tests – Standard approach 22

Something completely different VISAR @ ATF

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.

4mm thick Ti-6Al-4V

23 1.28 GeV electrons 0.7x10

^10 electrons per bunch 20 bunches per train 0.15 – 1.4 s between trains 10ps pulse length 1.44 J per bunch 70 microns x 7 microns spot size

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 George is rechecking the parameters used in initial calculation of surface velocity George Ellwood’s calculations