Spark Discharge

A false-color microscope image of 10 bar xenon spark discharge. The photograph was backlit with a 3 ns laser pulse (532 nm) at the moment of peak plasma emission. The laser pulse is unimpeded by the surrounding gas (green background) and enters the imaging system, passing through a 532 nm line filter before exposing the camera. Laser light incident the blackbody spark plasma is completely absorbed (shadow in center) and does not make it to the camera. The shadows produced by the sharpened electrodes (center top and bottom) have been removed for clarity, although their outline
can still be resolved. Broadband plasma emission is blocked by the line filter, thus giving the visual appearance of a “blackbody”.  – Bataller, Alexander William. Exploring the Universality of Sonoluminescence. University of California, Los Angeles, 2014.

Another method of creating dense plasma is by way of spark discharges.  Grab a pair of rubber-soled shoes, run across the carpet for a few minutes, and then slowly bring your finger toward a metal doorknob.  If done on a particularly dry day, you will witness a flash of light emerging from your finger followed by a snapping sound and a rather unpleasant sensation.  As Benjamin Franklin would say, you have just drawn off the “Electric Fire” (i.e. electricity) and created a spark.  Although the experiment just described can be an annoying one, its origins are rich in physical processes: the charge transfer from insulating surfaces (triboelectricity), the storage of charge on your body (static electricity), the transfer of charge along your salty skin to your finger (electrodynamics), the heating and ionization of air (discharge physics), the spark’s luminescence (plasma physics), and the snapping sound heard from the spark’s expansion (hydrodynamics and acoustics).

If the initial voltage and gas pressure is high enough, a highly energetic state of matter known as dense plasma can be created from spark discharges.  The final research component of my thesis at UCLA used the spark platform for recreating the same conditions observed in ultrafast laser breakdown and Sonoluminescence, and measuring the same blackbody radiation.  The name “blackbody” itself implies that all incident light is absorbed, which was experimentally verified for dense conditions (see photograph above).  This creates a unique opportunity to explore spark discharges for potential uses as a high power optical switch.

Would you like to know more?

• http://acoustics-research.physics.ucla.edu/dense-microplasmas/ – Research website of Professor Seth Putterman at UCLA showing more goodies on spark discharges.
• Benjamin Franklin Letter – Highly recommended reading that was written by an American founder and scientist!  My favorite statement from this letter that relates to my work with spark discharges is: “…the wonderful effect of pointed bodies both in drawing off and throwing off the Electrical Fire.”
• Z Machine – World’s biggest discharge (200 Trillion Amps/Second = 5000x lightning strikes).  This experiment has already achieved the hottest temperature ever created on earth (2 billion Kelvin), and researchers are now gunning for fusion ignition.
• Bataller, A., Putterman, S., Pree, S., and Koulakis, J. “Observation of Shell Structure, Electronic Screening, and Energetic Limiting in Sparks.” Physical Review Letters, 117, 085001 (2016).
• Bataller, A., Koulakis, J., Pree, S. and Putterman S. “Nanosecond High-Power Dense Microplasma Switch for Visible Light.” Applied Physics Letters, 105, 223501 (2014).