Stellarators, Microwaves & MHD

Group photo, July 2016. From l. to r.: Taha, Francesco, Ken Yumou, Rui, Jessica.
Click here for earlier group photos.

Our group is part of the Plasma Physics Laboratory in the Department of Applied Physics and Applied Mathematics at Columbia University in New York, a laboratory founded in 1961.

Our group was established in 2012 and conducts experimental research on the tokamak and stellarator approaches to magnetic confinement fusion. Both approaches have advantages and disadvantages. We study and try to minimize the disadvantages.

Stellarator plasmas, for example, are stable and steady-state, but have complicated 3D shapes. We investigate whether the elegant physics of 3D equilibria between forces that expand and compress the plasma can be satisfactorily reproduced through simpler coils, or relaxed mechanical tolerances. The main physics question is: what is the impact of the resulting "field errors" on plasma confinement, transport, and stability? We also study whether (1) stellarators are still stable at very high plasma pressures, (2) the exhausted power spreads on a broader area than in tokamaks, (3) we can use our understanding of fusion plasmas to optimize plasmas of, say, gold or lead, for non-fusion purposes such as ion sources for particle accelerators.

Tokamak plasmas, on the other hand, are unstable and tend to reorganize themselves in different topologies, often featuring "magnetic islands". Islands interact with the wall surrounding the plasma and with imperfections or “errors” in the confining field. As a result, islands can “lock” in a specific toroidal position, grow, and cause a rapid loss of confinement known as disruption. In our group we study (1) the physics of locked and near-locked islands, (2) their stabilization and avoidance of the consequent disruption by means of applied 3D fields and wave-driven currents and (3) their use in, for example, quantifying the field errors mentioned above.

In analogy with stabilization of plasmas, we are pioneering the feedback stabilization of "liquid walls". These are flowing films of liquid metals, envisioned to coat the inner walls of future fusion reactors.

Last but not least, we use microwaves to start up and heat stellarators, and to stabilize and diagnose tokamaks. An ink-jet printed metamaterial lens of our invention -a special array of metallic and dielectric elements- is predicted to exhibit reverse chromatic aberration. This is equivalent to generating a "reverse rainbow", and will maximize resolution in diagnostics collecting higher frequency components ("colors") from farther locations. Furthermore, a new numerical method based on the old Huygens' principle shows promise to quickly model wave propagation in plasmas while retaining diffraction effects.

Top: MHD delegation in San Diego: Wilkie (l) and Ryan (r).
Click here for earlier photos from San Diego.

Right: Taha (r) and Francesco with Taha's liquid metal setup.

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