VISAR & Vibrometer results Goran Skoro (University of Sheffield) UK Neutrino Factory Meeting Lancaster, April 2009

Part I: VISAR 02

VISAR tests 03 VISAR tests have been performed with 0.3 mm diameter tungsten wire Idea was to measure the VISAR signal and to extract the longitudinal oscillations of the pulsed wire Two characteristic results (shots 3 and 5) shown on the left* Yellow – Current pulseGreen, Purple – VISAR signal (2 channels)* Note the different time scale3Current pulseVISAR signal 5 Current pulse VISAR signal Wire Laser beam VISAR signal obtained for the very first time - nice agreement with simulations results - But, noise is an issue here!!! Analysis shown on the following slides will try to address this problem

04 3 Current pulse VISAR signal Frequency analysis of the VISAR signal FFT method (MATLAB) has been used – from time to frequency domain - ~ 16  s ~ 60 kHz 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)

5 Current pulse VISAR signal 05 Frequency analysis of the VISAR signal ~ 60 kHz Dominant signal frequency (not so clearly) seen in frequency spectrum ~ 16  s~ 8  s ~ 125 kHz Real effect (friction of wire’s end) or noise? Compare with previous plot – looks like a noise (here: not enough data points for noise) Really powerful method; we need more data from VISAR FFT method (MATLAB) has been used – from time to frequency domain -

06 Frequency analysis of the VISAR signal - Filtering A few examples how we can filter the data Filter set 2 2 Filter set 1 1

07 Frequency analysis of the VISAR signal and LS-DYNA results ~ 60 kHz Dominant frequency that corresponds to longitudinal motion of the wire is clearly present in both frequency spectra ~ 60 kHz shot 3

VISAR tests with a shorter wire 08 New tests have been performed with a shorter wire Current pulseVISAR signal Wire Laser beam Difference between green and purpleCurrent pulseVISAR signal Difference between green and purpleCurrent pulseVISAR signal Difference between green and purple A few characteristic resultsFirst conclusion: no signal here! But, interesting ‘coincidence’ in frequency spectrum…

09 FFT analysis of each shot – no signal seen (expected, if we look at previous slide) Small statistics – so we can say it’s a coincidence (but will be interesting to collect more data) Frequency analysis of the VISAR signal Tests with a shorter wire 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) Experiment averaged

VISAR tests – data collection 10 VISAR tests have been performed with 0.5 mm diameter tungsten wire More than 130 shots have been taken between 17 and 19 March The measurements have been performed at different temperatures as a function of applied current. Wire Laser beam First wire has bent during the test at 1750 C. Only single shot at this temperature has been recorded (shown on the left) Current pulseVISAR signal Difference between green and purple1750 C Wire has been replaced and measurements continued – excellent consistency between two sets of data! Each temperature (and current value) – between 5 and 20 shots (FFT analysis of each shot then averaging) Scripts* have been written for a quick analysis of such a large ammount of data *Particular thanks to Jelena Ilic for her help.

VISAR tests – Results (Part I) 11 Frequencies seen in spectra: ~ 20 kHz; ~ 40 kHz; ~ 60 kHz; ~ 80 kHz; ~ 100 kHz; ~ 140 kHz (weak). Peak current = 7.5 kA except for T = 850 C where it was 8.1 kA - as a function of temperature - 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 *See: FFT_Visar_ver2.ppt (Slide 9) C (for tungsten) ~ 4 km/sL (wire full length) ~ 5 cmf 0 = 20 kHz,f 1 = 60 kHz,f 2 = 100 kHz,f 3 = 140 kHz…

12 VISAR tests – Results (Part III)- as a function of current (for T = 1100 C) - Only the magnitude of the 80 kHz spectral structure increases with increasing current!!! Legend: 1 = ff (1 st harmonic) 2 = ff (2 nd harmonic) 3 = ff (3 rd harmonic) 4 = ff (4 th harmonic) b = bending frequency Arrow = 80 kHz (thermal expansion)

13 VISAR tests – Results (Part IV)- as a function of current (for T = 1300 C) - Again, higher current –> higher magnitude of the 80 kHz spectral structure!!! Legend: 1 = ff (1 st harmonic) 2 = ff (2 nd harmonic) 3 = ff (3 rd harmonic) 4 = ff (4 th harmonic) b = bending frequency Arrow = 80 kHz (thermal expansion) 80 kHz thermal expansion

14 P R E L I M I N A R Y - Young’s modulus of tungsten wire -  - density; – Poisson’s ratio; l – length of wire (heated part) f – measured frequency Errors estimated from the widths of the characteristic spectral ‘line’. VISAR tests

Part II: Vibrometer 15

Vibrometer tests 16 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* Yellow – Current pulseGreen, Purple – “Displacement”, Velocity* Note the different time scale6Current pulse“Displacement”Velocity Wire Laser beam But, noise is an issue here!!! 5Current pulse“Displacement”Velocity4Current pulse“Displacement”Velocity

17 Frequency analysis of the Vibrometer signal FFT method (MATLAB) has been used to analyse velocity signal – here: short time scale - ~ 1  s ~ 1 MHz Current pulse (and reflections) Radial oscillations of the wire ~ 160 ns ~ 6 MHz We expect to see radial oscillations 6Current pulse“Displacement”Velocity

18 Frequency analysis of the Vibrometer signal FFT method (MATLAB) has been used to analyse velocity signal – here: medium time scale - ~ kHz Expected frequency of longitudinal oscillations should be in this region (even lower) We ‘see’ radial oscillations of the wire ~ 6 MHz We expect to see ‘longitudinal oscillations’ 4Current pulse“Displacement”Velocity Very noisy here – is the previous results (see slide 3) in this frequency range realistic?

19 Frequency analysis of the Vibrometer signal FFT method (MATLAB) has been used to analyse velocity signal – here: long time scale - We expect to see violin modes of wire oscillations They are here 5Current pulse“Displacement”Velocity

20 Frequency analysis of the Vibrometer signal - Filtering What is ‘wrong’ with these pictures? ~ 6 MHz 6 MHz signal seen in both frequency spectra Velocity decoder VD02 has been used to extract the surface velocity of the wire - but it’s upper frequency limit is 1.5 MHz - To answer this question we need a decoder which works in higher frequency regime (DD300)

21 Frequency analysis of the Vibrometer signal and LS-DYNA results ~ 6 MHz 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…