1. Feasibility Study of Time Resolved Diagnostics on the
Electrical Wire Explosion in the Water
using Optical Emission Spectroscopy
Kern Lee*, Jungmin Jo , Jeong-jeung Dang,  
Kyoung-Jae Chung and Y. S. Hwang
JK Seminar 2014 Kyoto, Aug 17~20
*PhD.course, Dept. of Nuclear Engineering, Seoul National University, San 56-1, Shillim-dong, Gwanak-gu, Seoul , Korea
[ ]

2. [1] A. Grinenko, Phys. Plasmas 13(042701), 2006
Abstract
In order to measure the thermodynamic properties(ne,Te) of metal plasma during electrical wire explosions in the water, Optical Emission Spectroscopy (OES) system has been considered.
Due to its high density and pressure of surrounding water, the line analysis, such as line-pair or broadening, was failed and black body (BB) radiation-like distribution was measured in the nanosecond time scale wire explosion in the water[1].
In this presentation, we measured emission spectra within visible range (340~700nm) emitted from the underwater electrical explosion of 40mm-long Al wires. The time integrated emission spectra was obtained using wide range spectrometer and time resolved light intensities of the sampled spectrum using photomultiplier tube (PMT).
Since the formation of plasma channel followed by current re-striking leads to significant deviation from the BB distribution, the BB fitting method results in relatively large error in deducing Te from such discharges. As an alternative method of deducing Te, one can estimate the radiation power from the discharge channel. The latter method requires the time evolution of the radius of discharging channel, the experimental measurement of that should be realized in close future.
[1] A. Grinenko, Phys. Plasmas 13(042701), 2006

3. Outline
Introduction
Experimental setup
Results and Discussion
Conclusion
Future work
References

4. I. Introduction (1/3) A. Pulsed electrical discharge of Wire
Related phenomena
Rapid phase transition (melting-vaporization-ionization)
Strong shockwave
Strong radiation emission
Very high plasma density hv hv hv
Surrounding media
Vacuum or Gas
Water hv hv hv
Main interest
Radiation (X pinch)
High energy density (Z pinch)
Material (nano-powder synthesis)
Shockwave (Extracorporeal SW lithotripsy, Discharge machining..)
Radical (sterilization)
Diagnostics
Image (channel & SW)
Current & voltage waveform
Spectroscopy
Pressure

5. I. Introduction (2/3) B. Black-body(BB) radiation distribution [1,2]
Light emitted from nsec time scale underwater electrical wire explosion
Two requirements for BB radiation
(i) Local thermodynamic equilibrium(LTE)
: Martin[3] estimated the equilibration time for underwater spark plasmas
.. electron-ion collision ~ 0.1ns
(ii) Ptl. Density to be high enough to make the channel opaque to the radiation
: Rosseland mean free path of photons
.. ni=1021cm-3 , T=few eV ~1μm
Deviation from the BB approximation increases significantly as current restrikes actively, especially for the Al wires, due to
Al exothermic reaction (combustion)
Existence of hot shell
Spectral line analysis is not applicable !
(lines cannot be resolved from continuum)
[1] A. Grinenko, Phys. Plasmas 13(042701), 2006
[2] A. Fedotov, Phys. Plasmas 15(082704), 2008
[3] E. A. Martin, J. Appl. Phys. 31, 2(255), 1960

6. I. Introduction (3/3) C. Typical VI waveforms in our system [4,5]
φ0.1mm cases (l=40mm)
Breakdown after current re-striking
φ0.25mm cases (l=40mm)
Vc ≤ 10kV : Single current pulse
Vc ≥ 11kV : Breakdown followed
Typical phase transition for a single pulse case
0~t3 : complete vaporization
t3~t4 : Initial plasma formation
[4] Q. Zhou, IEEE Transactions on Plasma Science, 39(7), 2011
[5] I.V. Lisitsyn, Pulsed Power Conference 1997, 11th IEEE International, 1(208), 1997

7. II. Experimental setup Spectrometer : HR4000 CG (Ocean Optics. Inc.)
Monochromator : Acton2500i (Princeton instruments Inc.)
𝑽 𝑹 𝒕 = 𝑽 𝒎 𝒕 − 𝑹 𝒆 𝑰 𝒎 𝒕 − 𝑳 𝒆 + 𝑳 𝒑 𝒅 𝑰 𝒎 𝒕 𝒅𝒕
Equivalent circuit
Al (ϕ 0.1, 0.25) wires
Wire length : 40mm
Light intensity measured
Al (0.1mm )- Vc=8kV : breakdown case
Al (0.25mm )- Vc=10kV : single pulse case

8. III. Results and Discussion (1/6) A
III. Results and Discussion (1/6) A. Time integrated emission spectra : Spectrometer (HR4000 CG)
Exposure time = 2 sec (open shutter)
( discharge time ~ 20μs)
Emission spectra in the range either
𝜆≤370 𝑜𝑟 𝜆≥700
are considerably attenuated
Spectral characteristics of the surrounding water at the extreme pressures during the discharge have not been studied well
Even in the single current pulse case, severe deviation from BB radiation is observed
Al (ϕ 0.25mm) -10kV
Single pulse case

9. III. Results and Discussion (2/6) B
III. Results and Discussion (2/6) B. Time resolved light intensities of the sampled spectrum
: monochromator - Acton2500i
detector - PMT
Al (ϕ 0.25mm) -10kV
Single pulse case
Discharge time ~ 20μs;
Radiation emission lasts much longer than electrical energy deposition
Exothermic reaction
(combustion of
Al micro-particle )
Sampled spectrum (total 11 wavelength)
340
396.2(Al-I)
430
466.3(Al-II)
500
559.3(Al-II)
590
624.3(Al-II)
645
669.6(Al-I)
700
𝟒𝐀𝐥+𝟑 𝐎 𝟐 ⇒𝟐𝐀 𝐥 𝟐 𝐎 𝟑 +∆𝐇(62kJ/g)

10. : monochromator - Acton2500i
III. Results and Discussion (3/6) B. Time resolved light intensities of the sampled spectrum
: monochromator - Acton2500i
detector - PMT
Typical spectra of the exploding wire radiation

11. III. Results and Discussion (4/6) B
III. Results and Discussion (4/6) B. Time resolved light intensities of the sampled spectrum
: monochromator - Acton2500i
detector - PMT
Al (ϕ 0.10mm) -8kV
Breakdown case
Much stronger light intensity than that of
ϕ 0.25mm wire
Electrical energy deposition after breakdown :
(ϕ 0.25mm : 1.57 kJ/g,
ϕ 0.1mm : 132 kJ/g )
Combustion of
Al micro-particle
plays an major role on the radiation emission especially for the 0.25mm wire explosion
Sampled spectrum (total 10 wavelength)
340
396.2(Al-I)
430
466.3(Al-II)
500
559.3(Al-II)
590
624.3(Al-II)
645
669.6(Al-I)
700

12. III. Results and Discussion (5/6) B
III. Results and Discussion (5/6) B. Time resolved light intensities of the sampled spectrum
: monochromator - Acton2500i
detector - PMT
Typical spectra of the exploding wire radiation
Significant deviation from the BB radiation
Plasma channel developed → outer (hot) shell formation / (cold) core
: two BB curves overlapped[1]

13. III. Results and Discussion (6/6) C
III. Results and Discussion (6/6) C. Temperature estimation from Plank’s equation[1]
: monochromator - Acton2500i
detector - PMT
1st method : BB fitting
𝑰 λ 𝑻,𝑨 =𝑨 𝟐𝝅𝒉 𝒄 𝟐 λ 𝟓 𝟏 𝒆𝒙𝒑 𝒉𝒄 λ𝒌𝑻 −𝟏
𝑰 λ 𝑻,𝑨 : BB intensity
A : Amplitude that depends on geometric factors
𝑬𝒓𝒓𝒐𝒓 𝑻 𝒕 , 𝑨 𝒕 = 𝟏 𝑵 𝒏=𝟏 𝑵 [ (𝑰 𝒆𝒙 ( λ 𝒏 ,𝒕)− 𝑰 λ 𝒏 ( 𝑻 𝒕 , 𝑨 𝒕 ))/ 𝑰 λ 𝒏 ( 𝑻 𝒕 , 𝑨 𝒕 ) ] 𝟐
𝑰 𝒆𝒙 λ 𝒏 ,𝒕 : measured intensity at λ 𝒏
𝑻 𝒕 , 𝑨 𝒕 : fitting parameters which minimizes error
-Optical calibration is not necessary
2nd method : radiation power calculation
𝒍 𝒘 , 𝑹 𝒘 : length and radius of discharging channel
D : distance b/w detector and wire
𝒉 𝒅𝒆𝒕 : length of detector edge
𝑹 𝒓𝒆𝒇 : attenuation due to reflection (water-glass interface)
𝑰 𝒆𝒎𝒊𝒕𝒕𝒆𝒅 λ 𝒏 ,𝒕 ~𝟐 𝑹 𝒓𝒆𝒇 𝑫 𝒉 𝒅𝒆𝒕 𝟐 𝟏 𝒍 𝒘 𝑹 𝒘 (𝒕) 𝟏 ∆λ × 𝑰 𝒆𝒙 ( λ 𝒏 ,𝒕)
𝑻 λ 𝒏 𝒕 = 𝟏.𝟐𝟒𝟏𝟓×𝟏 𝟎 𝟑 λ 𝒏 𝒍𝒏( 𝟑.𝟕𝟒𝟑𝟓×𝟏 𝟎 𝟐𝟗 𝑰 𝒆𝒎𝒊𝒕𝒕𝒆𝒅 λ 𝒏 ,𝒕 λ 𝒏 𝟓 +𝟏)
𝑻 λ 𝒏 𝒕 : Temperature in eV
λ 𝒏 : wavelength in nm
-Channel radius is required
-Applicable to each wavelength

14. IV. Conclusion Time integrated emission spectra
Even in the case of single current pulse, severe deviation from the BB curve is observed
Light emitted after the termination of electrical current is so strong that the accumulated spectrum cannot be used to deduce the plasma property during the wire explosion
Time resolved light intensities of sampled spectrum
Time evolution of light intensities are measured at each sampled wavelength
Plasma channel formation followed by electrical breakdown is combined with the exothermic reaction, the radiation distributions from explosion of thin wire significantly deviated from BB radiation
Two methods deducing temperature by using Planck’s equation
Although 1st method is easily applicable without any optical calibration, an attempt to fit the full range of the sampled spectrum becomes inaccurate
Since 2nd method requires the radius of discharging channel, it should be obtained via simulation or the shadow imaging
: Both methods should be realized to cross check their accuracy

15. V. Future work
Radiation emission from different wire materials (e.g. Cu or W)
Exothermic reaction of Al in high temperature makes the measured spectrum deviates from the BB radiation
: similar measurement should be repeated to verify this effects
Design of polychromator using interference filters within visible range
Strong light intensity eases the concerns related to the electrical noise in the signal
Measurement of the discharging channel radius as well as the shock front location
Combining with the analysis based on the Planck’s equation, further estimation of plasma density can be possible not only the temperature estimation

16. VI. References [1] A. Grinenko, Phys. Plasmas 13(042701), 2006
[2] A. Fedotov, Phys. Plasmas 15(082704), 2008
[3] E. A. Martin, J. Appl. Phys. 31, 2(255), 1960
[4] Q. Zhou, IEEE Transactions on Plasma Science, 39(7), 2011
[5] I.V. Lisitsyn, Pulsed Power Conference 1997, 11th IEEE International, 1(208), 1997

17. Backup : The visible and UV spectra of liquid H2O