Tungsten wire & VISAR
Goran Skoro
24 October 2008
VISAR wire tests – Standard approach
VISAR signal?
(for 0.5 mm diameter, 3 cm long wire and peak current of 6 kA)
WireLaser beam
Laser beam
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?
We can measure radial or/and longitudinal displacement of the
wire
Issues:
Results of calculations -> following pages
02
VISAR wire tests – Standard approach
Results
Radial displacements
VISAR signal:
(for room and high temperature; for 10m and 30m delay-leg)
- flatline
Conclusion:We won’t see anything here
03
VISAR wire tests – Standard approach
Longitudinal displacements
VISAR signal:- 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
04
VISAR wire tests – Standard approach
Sensitivity of VISAR signal on material parameters values
High temperatureRoom 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.
05
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
Another possibility
BUT…
06
• 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
A few words about VISAR’s laser beam spot size…
*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).
VISAR wire tests – Standard approach
07
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.
• 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)
Results of calculations of wire stress, surface displacements and corresponding VISAR signal as a function of wire diameter are shown in following pages.
08
First 2 s
First 2 s
‘20 capacitors’ case - 0.2 mm diameter wire
end of wire
VISAR wire tests – Alternative approach
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…
09
end of wire
First 2 s
First 2 s
VISAR wire tests – Alternative approach
‘20 capacitors’ case - 0.3 mm diameter wire
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…
10
end of wire
First 2 s
First 2 s
VISAR wire tests – Alternative approach
‘20 capacitors’ case - 0.4 mm diameter wire
Practically no signal for 20 capacitors but may look promising if we add more circuits (see slide 13)
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end of wire
VISAR wire tests – Alternative approach
‘20 capacitors’ case - 0.5 mm diameter 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.
12
end of wire
First 2 s
First 2 s
VISAR wire tests – Alternative approach
‘40 capacitors’ case - 0.4 mm diameter wire
‘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…
13
Update I
12 November 2008
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VISAR wire tests – Standard approach
Previous calculations:
- wire length = 3 cm
Conclusion:No difference (as expected)
- we plan to use a longer wire (~ 5 cm) in tests with our ‘old’ power supply
VISAR signal as a function of wire
length
15
‘20 capacitors’ case - 0.2 mm diameter wire
First 2 s
First 2 s
end of wire
VISAR wire tests – Alternative approach
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…
16
‘20 capacitors’ case - 0.3 mm diameter wireVISAR wire tests –
Alternative approach
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…
end of wire
First 2 s
First 2 s
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Update II
07 December 2008
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• 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…
A few words about VISAR’s laser beam spot size… AGAIN
VISAR wire tests – Standard approach
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VISAR wire tests – Standard approach
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…
Results
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VISAR wire tests – Standard approach
0.3 mm wire, 8 kA
VISAR signal:
- very nice signal at room temperature for 10 m delay-leg
We should have nice signal even for lower current…
Results
21
- 1500K case not shown -> stress too high (here ~ 450 MPa)
VISAR wire tests – Standard approach
0.3 mm wire, 6 kA
VISAR signal:
Results
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- very nice signal at room temperature for 10 m delay-leg- 1500K case not shown -> stress too high (here ~ 250 MPa)
Something completely different
23VISAR @ ATF
1.28 GeV electrons0.7x10^10 electrons per bunch
20 bunches per train0.15 – 1.4 s between trains
10ps pulse length 1.44 J per bunch
70 microns x 7 microns spot size
George Ellwood’s calculations
4mm thick Ti-6Al-4V
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.
Maybe, interesting for us for testing the tungsten foils
First results based on George Ellwood’s AUTODYN simulations
shown hereAt least 3x higher velocity is needed to have decent VISAR
signal
George is rechecking the parameters used in initial
calculation of surface velocity