For most people, the night sky conjures a sense of stillness, an occasional shooting star the only visible movement. A conversation with Rishabh Datta, however, unveils the supersonic drama crashing above planet Earth. The PhD candidate has focused his recent study on the plasma speeding through space, flung from sources like the sun’s corona and headed toward Earth, halted abruptly by colliding with the planet’s magnetosphere. The resulting shock wave is similar to the “bow shock” that forms around the nose cone of a supersonic jet, which manifests as the familiar sonic boom.
The bow shock phenomenon has been well studied. “It’s probably one of the things that’s keeping life alive,” says Datta, “protecting us from the solar wind.” While he feels the magnetosphere provides “a very interesting space laboratory,” Datta’s main focus is, “Can we create this high-energy plasma that is moving supersonically in a laboratory, and can we study it? And can we learn things that are hard to diagnose in an astrophysical plasma?”
Datta’s research journey to the bow shock and beyond began when he joined a research program for high school students at the National University Singapore. Tasked with culturing bacteria and measuring the amount of methane they produced in a biogas tank, Datta found his first research experience “quite nasty.”
“I was working with chicken manure, and every day I would come home smelling completely awful,” he says.
As an undergraduate at Georgia Tech, Datta’s interests turned toward solar power, compelled by a new technology he felt could generate sustainable energy. By the time he joined MIT’s Department of Mechanical Engineering, though, his interests had morphed into researching the heat and mass transfer from airborne droplets. After a year of study, he felt the need to go in a yet another direction.
The subject of astrophysical plasmas had recently piqued his interest, and he followed his curiosity to Department of Nuclear Science and Engineering Professor Nuno Loureiro’s introductory plasma class. There he encountered Professor Jack Hare, who was sitting in on the class and looking for students to work with him.
“And that's how I ended up doing plasma physics and studying bow shocks,” he says, “a long and circuitous route that started with culturing bacteria.”
Gathering measurements from MAGPIE
Datta is interested in what he can learn about plasma from gathering measurements of a laboratory-created bow shock, seeking to verify theoretical models. He uses data already collected from experiments on a pulsed-power generator known as MAGPIE (the Mega-Ampere Generator of Plasma Implosion Experiments), located at Imperial College, London. By observing how long it takes a plasma to reach an obstacle, in this case a probe that measures magnetic fields, Datta was able to determine its velocity.
With the velocity established, an interferometry system was able to provide images of the probe and the plasma around it, allowing Datta to characterize the structure of the bow shock.
“The shape depends on how fast sound waves can travel in a plasma,” says Datta. “And this ‘sound speed’ depends on the temperature.”
The interdependency of these characteristics means that by imaging a shock it’s possible to determine temperature, sound speed, and other measurements more easily and cheaply than with other methods.
“And knowing more about your plasma allows you to make predictions about, for example, electrical resistivity, which can be important for understanding other physics that might interest you,” says Datta, “like magnetic reconnection.”
This phenomenon, which controls the evolution of such violent events as solar flares, coronal mass ejections, magnetic storms that drive auroras, and even disruptions in fusion tokamaks, has become the focus of his recent research. It happens when opposing magnetic fields in a plasma break and then reconnect, generating vast quantities of heat and accelerating the plasma to high velocities.
Onward to Z
Datta travels to Sandia National Laboratories in Albuquerque, New Mexico, to work on the largest pulsed power facility in the world, informally known as “the Z machine,” to research how the properties of magnetic reconnection change when a plasma emits strong radiation and cools rapidly.
In future years, Datta will only have to travel across Albany Street on the MIT campus to work on yet another machine, PUFFIN, currently being built at the Plasma Science and Fusion Center (PSFC). Like MAGPIE and Z, PUFFIN is a pulsed power facility, but with the ability to drive the current 10 times longer than other machines, opening up new opportunities in high-energy-density laboratory astrophysics.
Hare, who leads the PUFFIN team, is pleased with Datta’s increasing experience.
"Working with Rishabh is a real pleasure,” he says, “He has quickly learned the ins and outs of experimental plasma physics, often analyzing data from machines he hasn't even yet had the chance to see! While we build PUFFIN it's really useful for us to carry out experiments at other pulsed-power facilities worldwide, and Rishabh has already written papers on results from MAGPIE, COBRA at Cornell in Ithaca, New York, and the Z Machine."
Pursuing climate action at MIT
Hand-in-hand with Datta’s quest to understand plasma is his pursuit of sustainability, including carbon-free energy solutions. A member of the Graduate Student Council’s Sustainability Committee since he arrived in 2019, he was heartened when MIT, revising their climate action plan, provided him and other students the chance to be involved in decision-making. He led focus groups to provide graduate student input on the plan, raising issues surrounding campus decarbonization, the need to expand hiring of early-career researchers working on climate and sustainability, and waste reduction and management for MIT laboratories.
When not focused on bringing astrophysics to the laboratory, Datta sometimes experiments in a lab closer to home — the kitchen — where he often challenges himself to duplicate a recipe he has recently tried at a favorite restaurant. His stated ambition could apply to his sustainability work as well as to his pursuit of understanding plasma.
“The goal is to try and make it better,” he says. “I try my best to get there.”
Datta's work has been funded, in part, by the National Science Foundation, National Nuclear Security Administration, and the Department of Energy.