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VISAR & Vibrometer results

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VISAR & Vibrometer results

Goran Skoro (University of Sheffield)

UK Neutrino Factory Meeting

Lancaster, April 2009

02

Part I: VISAR

Idea was to measure the VISAR signal and to extract the longitudinal oscillations of the pulsed wire

VISAR tests have been performed with 0.3 mm diameter tungsten wire

Analysis shown on the following slides will try to address this problem

VISAR signal obtained for the very first time - nice agreement with simulations results -

Two characteristic results (shots 3 and 5) shown on the left*

VISAR tests

But, noise is an issue here!!!

VISAR signal

Yellow – Current pulse

VISAR signal

VISAR signal

Current pulse

Current pulse

VISAR signal

3

* Note the different time scale

Green, Purple – VISAR signal (2 channels)

5

Laser

beam

Wire

03

FFT method (MATLAB) has been used

– from time to frequency domain -

Dominant signal frequency clearly seen in frequency spectrum (noise - negligible)

We need this region too in order to describe the wire motion properly (noise ~ signal)

Frequency analysis of the VISAR signal

3

Current pulse

VISAR signal

VISAR signal

~ 16ms

~ 60 kHz

04

Real effect (friction of wire’s end) or noise?

Compare with previous plot – looks like a noise

(here: not enough data points for noise)

Dominant signal frequency (not so clearly) seen in frequency spectrum

FFT method (MATLAB) has been used

– from time to frequency domain -

Frequency analysis of the VISAR signal

Really powerful method; we need more data from VISAR

5

Current pulse

VISAR signal

VISAR signal

~ 60 kHz

~ 16ms

~ 8ms

~ 125 kHz

05

Frequency analysis of the VISAR signal - Filtering

A few examples how we can filter the data

Filter set 2

2

Filter set 1

1

06

Dominant frequency that corresponds to longitudinal motion of the wire is clearly present in both frequency spectra

Frequency analysis of the VISAR signal and LS-DYNA results

shot 3

~ 60 kHz

~ 60 kHz

07

New tests have been performed with a shorter wire

But, interesting ‘coincidence’ in frequency spectrum…

VISAR tests with a shorter wire

A few characteristic results

First conclusion: no signal here!

Current pulse

VISAR signal

VISAR signal

Difference between green and purple

VISAR signal

VISAR signal

Current pulse

VISAR signal

VISAR signal

Difference between green and purple

Current pulse

Difference between green and purple

Laser

beam

Wire

08

FFT analysis of each shot – no signal seen (expected, if we look at previous slide)

But, by averaging the frequency spectra, the ‘structure’ starting to appear exactly at the right position

We could expect to see this dominant frequency (~ 80 kHz)

Frequency analysis of the VISAR signal

averaged

Tests with a shorter wire

Experiment

Small statistics – so we can say it’s a coincidence (but will be interesting to collect more data)

09

The measurements have been performed at different temperatures as a function of applied current.

First wire has bent during the test at 1750 C. Only single shot at this temperature has been recorded (shown on the left)

Each temperature (and current value) – between 5 and 20 shots (FFT analysis of each shot then averaging)

Wire has been replaced and measurements continued – excellent consistency between two sets of data!

VISAR tests have been performed with 0.5 mm diameter tungsten wire

More than 130 shots have been taken between 17 and 19 March

Scripts* have been written for a quick analysis of such a large ammount of data

VISAR tests – data collection

*Particular thanks to Jelena Ilic for her help.

VISAR signal

VISAR signal

1750 C

Difference between green and purple

Current pulse

Laser

beam

Wire

10

Frequencies seen in spectra:

~ 20 kHz;

~ 40 kHz;

~ 60 kHz;

~ 80 kHz;

~ 100 kHz;

~ 140 kHz (weak).

We expect* to see 80 kHz as a result of wire thermal expansion

What about the others?

Answer (part I):

20, 60, 100, 140 kHz are the fundamental frequencies of the wire vibrations

Peak current = 7.5 kA except for T = 850 C where it was 8.1 kA

VISAR tests – Results (Part I)

- as a function of temperature -

C (for tungsten) ~ 4 km/s

L (wire full length) ~ 5 cm

f0 = 20 kHz,

f1 = 60 kHz,

f2 = 100 kHz,

f3 = 140 kHz…

*See: FFT_Visar_ver2.ppt (Slide 9)

11

Legend:

1 = ff (1st harmonic)

2 = ff (2nd harmonic)

3 = ff (3rd harmonic)

4 = ff (4th harmonic)

b = bending frequency

Arrow = 80 kHz (thermal expansion)

Only the magnitude of the 80 kHz spectral structure increases with increasing current!!!

VISAR tests – Results (Part III)

- as a function of current (for T = 1100 C) -

12

Legend:

1 = ff (1st harmonic)

2 = ff (2nd harmonic)

3 = ff (3rd harmonic)

4 = ff (4th harmonic)

b = bending frequency

Arrow = 80 kHz (thermal expansion)

Again, higher current –> higher magnitude of the 80 kHz spectral structure!!!

VISAR tests – Results (Part IV)

- as a function of current (for T = 1300 C) -

80 kHz thermal expansion

13

r - density; n – Poisson’s ratio;

l – length of wire (heated part)

P R E L I M I N A R Y

- Young’s modulus of tungsten wire -

VISAR tests

Errors estimated from the widths of the characteristic spectral ‘line’.

f – measured frequency

14

15

Part II: Vibrometer

Laser Doppler Vibrometer tests have been performed with 0.5 mm diameter tungsten wire

Only a few shots taken - shown on the left and below*

Vibrometer tests

But, noise is an issue here!!!

Velocity

“Displacement”

Yellow – Current pulse

Green, Purple – “Displacement”, Velocity

* Note the different time scale

6

Current pulse

Velocity

5

Current pulse

“Displacement”

Velocity

4

Current pulse

“Displacement”

Laser beam

Wire

16

FFT method (MATLAB) has been used to analyse velocity signal

– here: short time scale -

Frequency analysis of the Vibrometer signal

Current pulse (and reflections)

Radial oscillations of the wire

We expect to see radial oscillations

~ 1ms

~ 1 MHz

~ 160 ns

~ 6 MHz

6

Current pulse

“Displacement”

Velocity

17

FFT method (MATLAB) has been used to analyse velocity signal

– here: medium time scale -

Expected frequency of longitudinal oscillations should be in this region (even lower)

Very noisy here – is the previous results (see slide 3) in this frequency range realistic?

Frequency analysis of the Vibrometer signal

We ‘see’ radial oscillations of the wire

We expect to see ‘longitudinal oscillations’

~ 150 -300 kHz

~ 6 MHz

4

Current pulse

“Displacement”

Velocity

18

FFT method (MATLAB) has been used to analyse velocity signal

– here: long time scale -

We expect to see violin modes of wire oscillations

Frequency analysis of the Vibrometer signal

They are here

5

Current pulse

“Displacement”

Velocity

19

Velocity decoder VD02 has been used to extract the surface velocity of the wire

- but it’s upper frequency limit is 1.5 MHz -

Frequency analysis of the Vibrometer signal - Filtering

What is ‘wrong’ with these pictures?

6 MHz signal seen in both frequency spectra

To answer this question we need a decoder which works in higher frequency regime (DD300)

~ 6 MHz

~ 6 MHz

20

Frequency analysis of the Vibrometer signal and LS-DYNA results

Radial oscillations of the wire!!!

VISAR – noisy but a decent level of information can be extracted (FFT, filtering, …)

Vibrometer has been purchased (just arrived to RAL)

More results soon…

~ 6 MHz

~ 6 MHz

21