My research interests include the physics of…
- Relativistic particles in plasmas—“runaway” electrons in tokamaks
- Tokamak plasma disruptions—prediction, avoidance, mitigation, and impact
- High magnetic field, compact fusion devices—concepts and diagnostics
- Transformation optics and optical black holes—new!
My PhD thesis work focuses on the physics of “runaway” electrons (REs) in tokamak plasmas. Interestingly, in a plasma, the probability of one particle colliding with another decreases as the particle’s speed increases. This means that, given a strong enough electric field in a plasma, a fast electron can overcome friction and “run away” to relativistic energies! While REs are a neat plasma phenomena, they can also hit the wall of the plasma chamber and cause serious damage. Thus, the ultimate goal of this field of research is to know enough about RE evolution to avoid them in future fusion devices.
We generated REs in the Alcator C-Mod tokamak, which has a strong enough magnetic field for REs to emit visible synchrotron radiation. In close collaboration with the Plasma Theory group at the Chalmers University of Technology, I have studied how synchrotron spectra can indicate RE energies and how synchrotron images—like those shown below—can give insight into RE spatial and temporal dynamics. Currently, I am looking at what information can be gained from polarization measurements of synchrotron emission.
Tokamak Plasma Disruptions
Fusion-grade tokamak plasmas can have temperatures over 100 million Kelvin and carry currents over 1 million Amps! Therefore, they carry a lot of thermal and magnetic energy. Confining a donut-shaped plasma in a “magnetic bottle” can sometimes lead the plasma wriggling out of control and expelling its energy over milliseconds; this is called a plasma disruption. For future power-generating tokamaks, disruptions need to be predicted in advance and avoided—or their effects mitigated if avoidance is impossible.
To better understand the physics of disruptions, I have studied radiation asymmetries from mitigation of “healthy” and “sick” C-Mod plasmas, measured profiles of “halo” current as disrupting plasmas touch the vacuum vessel wall, and built databases for new machine learning applications to disruption prediction algorithms. Runaway electrons—mentioned above—can also be caused by disruptions. See the impact of runaways with the vacuum vessel wall in the figure below.
High Field, Compact Fusion
A future fusion power plant will have to confine a plasma with a high enough pressure for a long enough time in order to produce net energy, i.e. more power output (from fusion reactions) than input (to run the device). In the past, the main focus of the fusion community was to make the device really big—more plasma means more power. However, big machines are costly. Recently, advancements in high temperature superconducting (HTS) magnets allow another path: compact (cheaper) devices with high magnetic fields, strong enough to balance high plasma pressure. This was the inspiration for the conceptual ARC pilot plant, as well as MIT’s newest innovation, the net-energy SPARC tokamak.
During my time at MIT, I got to work on—and inside!—the Alcator C-Mod tokamak. With the highest magnetic field in a tokamak, C-Mod broke the world record for stored plasma pressure on its last day of operation, 30 September 2016. Since then, I have helped update the ARC design—see the rendering—to include a novel divertor design and robust heat exhaust management system. In addition, I have explored the feasibility of neutron diagnostics for a high-field, compact, SPARC-like device.
Transformation Optics for Black Holes
The mathematical existence of black holes is one of the most amazing consequences of Einstein’s general theory of relativity. Astronomical evidence suggests that there are many super massive objects in our universe, sitting at the centers of galaxies and colliding to produce gravitational waves; these are very likely black holes. However, we will not likely have the chance to visit one in our lifetimes. A field of research called transformation optics aims to study black holes in the lab by tailoring optical media to have black-hole-like properties.
My good friend Andrew Turner and I submitted two short pieces to Harvard’s Black Hole Initiative essay competition. Andrew’s brainchild, “Black Holes, Entropy, and the Arrow of Time,” won fourth place! Check out this article in the magazine Nautilus and Elon Musk’s tweet about it. And my hope of “Building a better black hole demonstration” led to our newfound interest in and exploration of optical black holes. Hopefully another publication is coming soon!