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)

11

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

14

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

17

Update II

07 December 2008

18

• 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

19

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

20

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

22

- 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