Like many discoveries, Alexey Shashurin’s came unintentionally.

Shashurin, an associate professor, has been studying nanosecond repetitively pulsed discharges (NRPs) or, more plainly, nanosecond sparks in air. Quickly depositing a large amount of energy, more than 1 megawatt of power for the duration of about 10 nanoseconds, into air gaps smaller than 1 millimeter, has broad aerospace applications. 

"These miniature discharges play a very important role in aerodynamics and combustion. For example, you can put these tiny plasma elements, known as plasma actuators, over the wing of the airplane to control the flow around the wing. This way, you can prevent undesired flow patterns and stimulate favorable ones," he says. 

"The sharp, nanosecond-scale rise of voltage to many-kilovolts leads to a very rapid and intense breakdown, energizing air particles to highly excited energy levels, producing a variety of reactive species and ionization. During the relaxation of stage after the spark, a large amount of heat is released, leading to extremely fast gas heating. This creates strong pressure gradients, resulting in a shock wave and induced vorticity that can be further used to fulfill a particular aerodynamic purpose.”

Plasma generators can also support combustion, which is especially useful in hypersonic vehicle engines like ramjets and scramjets. Air moves so quickly through those engines that it’s tricky to keep a fire going. A plasma discharge inside the flame can help recover combustion flow, Shashurin explains. 

His fundamental research involves the "diagnostics of these discharges and understanding discharge physics." He is characterizing plasma behaviors to inform computer models, making simulations of these applications more accurate.

With an initial grant from the U.S. Department of Energy in 2017, a team led by associate professor Sally Bane and Shashurin as co-principal investigators made a research breakthrough — but he noticed something else, too: "From that initial support, we characterized the nanosecond spark temperature using spectroscopy, something that was not done before. And on the way, we found out that this measurement approach can be used in general, just as an instantaneous thermometer for gas."

He had discovered a now-patented way to get a instantaneous and accurate temperature measurements of gas using these NRPs.

A novel approach 

A plasma is a gas so highly energized that it emits light. This can happen at a huge range of forms and conditions, from fluorescent lights to the surface of the sun. Breaking down the light emitted by plasmas into its component wavelengths, through spectroscopy, can indicate the temperature of the energized gas. 

This series of images shows the lifecycle of a plasma pulse, or spark, across a 3mm gap. Associate Professor Alexey Shashurin has patented a method for using high-speed spectroscopy on a plasma pulse like this one to take very precise and instantaneous temperature measurements.

The temperature of a spark is typically thousands of Kelvin. But doing spectroscopy at a very high framerate yielded a surprise. 

"When we were applying nanosecond pulse of energy, we saw there was a delay in the heating of the gas. For the first five to 10 nanoseconds of the discharge, the gas is not yet heating up. However, it’s already strongly glowing, which allows us to collect temperature information from it. So, these quick five-nanosecond pulses can be used to detect air temperature or essentially as a very fast thermometer," Shashurin says.

Measuring air temperature using a spark could be useful when it’s important to get an instantaneous precise reading, or where traditional temperature probes are impractical. Thermocouples can melt, perturb the flow, or underestimate the temperature due to cooling by the thermocouple wires themselves.

Shashurin’s patented approach works on gases in a large range of temperatures varying from room temperature to thousands of Kelvin, and provide response on the timescale of several nanoseconds. The technique is detailed in U.S. Patent No. 11,946,871, granted in April 2024.

A collaborative effort

Shashurin received additional funding from the U.S. Department of Energy in 2022, allowing him to test sparks created by a more powerful nanosecond pulse generator at Purdue’s Electric Propulsion and Plasma Laboratory. He says working at Purdue puts him near colleagues like Bane, who also does time-resolved plasma spectroscopy. Even when they’re not working on the same project, they often share ideas and equipment. Purdue’s reputation also helps attract the funding that makes these discoveries possible.

“We have a very solid experimental foundation for conducting advanced optical and microwave diagnostics work for combustion and aerodynamics applications. All these strengths are helping to win this research and to utilize all this unique equipment,” he says.

Intra-university collaboration can help fill gaps in Purdue’s vast capabilities. In the summer of 2023, Shashurin and his graduate student Won Joon Jeong packed equipment into the back of his car and drove to Princeton University in New Jersey. 

This diagram shows a typical experiment setup for doing high-speed spectroscopy on a plasma spark.

“We each have some unique pieces of equipment in our labs, but sometimes we need severalof them together in one place to make things work. We brought our unique nanosecond pulser and unique coherent microwave scattering system to Princeton. They already had the Raman spectroscopy system ready for us,” he says. 

Shashurin’s joy in working with other top minds is palpable when he talks about his work — “As expected, we saw a more energized, denser plasma with this new more powerful nanosecond pulser. Now we are putting together laser interferometry to monitor how high the electron density in these sparks actually is. Our gut feeling is that its ionization degree is very high, but we need to confirm experimentally,” he says.

The true thrill, Shashurin says, is that moment of discovery.

“The innovation part is what makes the research in general exciting to me. How to make something work, how to measure something that could not be measured before, or to suddenly understand something which was not yet understood. This project is very much like this.”