research

In a plasma, electrons can be accelerated to relativistic speeds (nearing the speed of light) when an electric field is large enough to overcome the frictional drag of collisions. These so-called “runaway” electrons can have individual energies over a thousand times their thermal counterparts, and together they can carry millions of amperes of current. In the worst case scenarios, runaways can seriously damage plasma facing components (PFCs) inside the tokamak. (Check out the video!)

We use various tools to study runaways in fusion plasmas – from their generation to termination, their impacts and resulting damage, and mitigation strategies. Kinetic modeling frameworks (like DREAM + STREAM) predict runaway currents and energy distributions during tokamak plasma ramp-up and disruptions, while high-fidelity 3D magnetohydrodynamics (MHD) simulations (with M3D-C1) self-consistently model runaway interactions with MHD activity. With codes like HEAT and GEANT4, we track runaway trajectories to realistic wall geometries and evaluate the resulting heating, melting, and vaporization. We validate these models with local electron beam experiments and using previous tokamak data (like that from Alcator C-Mod), providing confidence in predictions for future devices, like SPARC and ARC. 

This work is funded by Commonwealth Fusion Systems and the US Department of Energy. At MIT, we are supported by the broader Disruptions and MHD teams. We also collaborate closely with Columbia University and the Princeton Plasma Physics Laboratory in the US, as well as the Chalmers University of Technology and KTH Royal Institute of Technology in Sweden.

Neutrons are often the most energetic products of fusion reactions, produced in both deuterium-deuterium (DD) and deuterium-tritium (DT) fusion. As neutral particles, neutrons freely stream out of the strong magnetic field confining the plasma and only infrequently interact with surrounding materials. Thus, neutrons are valuable information carriers about when, where, and how much fusion is occurring. What is more, by carefully measuring the neutron energy spectrum, we can deduce properties of the reacting fuel ions, such as their composition, temperature, and even more exotic non-thermal features.

To this end, we design and build advanced neutron diagnostics which resolve neutron emission in time, space, and energy for next-generation high-performance fusion devices, like the SPARC tokamak. These include slow and fast neutron counters, a high-resolution magnetic-proton-recoil spectrometer, as well as novel deuterium-based liquid organic scintillators and single-crystal diamond detectors as part of a neutron camera. We characterize our prototypes with in-house DD and DT neutron generators and other radiation sources in extreme environments, complemented by high-fidelity modeling of advanced nuclear reactions (CQL3D/DRESS), neutron transport (OpenMC), magnetic ion optics (COSY), detector response (GEANT4), spectrum unfolding (TREVISO), tomographic reconstruction (ToFu), and more!

This work is funded by Commonwealth Fusion Systems and the US Department of Energy. Within the PSFC, we collaborate closely with the Neutronics, Fusion Materials & Technology, and High Energy Density Physics groups. We have external collaborators at the Princeton Plasma Physics Laboratory in the US, National Research Council (CNR) and University of Milan-Bicocca in Italy, and Uppsala University in Sweden.

Photons are perhaps the most ubiquitous and cross-cutting of our energetic particles as they are intimately connected to fast electrons, ions, and neutrons. By modeling and measuring these electromagnetic waves/particles, we can infer important information about the other three.

Runaway electrons radiate. Their spiral-like cyclotron motion in the plasma’s strong magnetic field leads to forward-directed synchrotron radiation; from the light spectrum and camera images (modeled with SOFT), we can infer runaway energy and spatial distributions. As runaways collide with much heavier ions, their braking deceleration (bremsstrahlung) leads to hard x-ray (HXR) emission. At MIT, we prototype novel inorganic crystals as HXR spectrometers, characterizing their response to HXRs with naturally radioactive photon sources and measuring the resulting scintillation light. 

Higher energy gamma rays are emitted in nuclear reactions. Both prompt and delayed gammas  (modeled with OpenMC/FISPACT) can result as fusion neutrons bombard and activate surrounding structures. Even fusion reactions themselves, like the deuterium-tritium (DT) reaction, can produce extremely energetic gammas. Other nuclear reactions among fast ion species, such as that between DT-fusion alphas and boron impurities, can generate photons with well-known energies. Gamma measurements can thus convey valuable, independent information about fusion energy, power, and products. At MIT, we are pursuing a novel magnetic-electron-recoil spectrometer for this purpose.

These projects are supported by Commonwealth Fusion Systems and Eni S.p.A.

Fusion plasmas need to be hot (over 100 million degrees!) to ignite fusion reactions, and the leading heating strategy is via “fast” ions. Some fast ions are accelerated by radio waves from external antennas, while others are generated directly in the fusion reactions themselves, such as the 3.5 MeV alpha particle (helium nucleus) in the deuterium-tritium fusion reaction. In future fusion power plants, alpha particle heating will be the key component for sustaining these “burning” plasmas. Understanding alpha particles’ and other fast ions’ interaction with the background plasma is therefore crucial for advancing fusion technology.

In the en.pa group, we investigate the evolution of fast ion populations, the growth of magnetohydrodynamic (MHD) instabilities, and the resulting fast ion-MHD dynamics. With detailed kinetic solvers (like CQL3D/DRESS), we self-consistently model the heating of multiple fuel ion species and the enhancement of many fusion reactions. High-fidelity extended-MHD simulations (with M3D-C1) couple the kinetic fast ion species with 3D nonlinear MHD activity, leading to fast ion transport, possible loss of confinement, and even reduction in plasma performance. Of particular interest is the destabilization of Alfven eigenmodes, explored with reduced models (like FAR3D). At MIT, we also prototype high frequency magnetic coils to measure these MHD instabilities in harsh fusion environments, such as that in the SPARC tokamak.

This work is funded by Commonwealth Fusion Systems and the US Department of Energy. We collaborate with researchers from the Princeton Plasma Physics Laboratory, Oak Ridge National Laboratory, University of Texas Austin, and Columbia University in the US, as well as the University of Seville in Spain.