"Liquid Metal and Stellarator Research at Columbia University"
Abstract: Nuclear fusion power-plants will need to be operated in steady state, and are expected to generate large heat and neutron fluxes, challenging for most solid plasma-facing materials. This seminar will provide an overview of recent and ongoing research at Columbia University in the areas of stellarators –inherently steady-state magnetic confinement fusion devices- and liquid metal walls. It will also discuss the potential relevance of such exploratory studies to larger facilities worldwide, as well as to future reactors. In particular, overdense plasma heating, error-field studies, the inversion of stellarator images, and plasma start-up techniques will be reported from the CNT stellarator. It will be argued that the small, low-field CNT could reach very high pressures, opening the way to unexplored territory in the Magnetohydrodynamic stability of stellarators. Numerical predictions and experimental results from CIRCUS will also be presented. This is a table-top device expected to generate 3D helical equilibria by only deploying tilted planar coils. Finally, a spin-off, non-fusion concept will be described: a small classical stellarator plasma not of hydrogen isotopes but of, say, bismuth, gold or lead, might improve the production-rate and charge-state of ions in electron-cyclotron-resonant ion sources for particle accelerators. An analogy will be cast between the plasma conditions needed in fusion and in ion sources. The second part of the talk will cover liquid metal experiments conducted in the absence of plasmas, initially. Building on Columbia experience with plasma stability and stabilization at the HBT-EP and DIII-D tokamaks, we studied the stability and stabilization of a free-surface Gallium-Indium-Tin alloy flowing in a magnetic field. The inclination is adjustable, to simulate the floor, wall and “ceiling” of a reactor. In all three cases, the free-surface flow adhered to the underlying solid wall by means of electromagnetic forces. Strong d.c. magnetic fields, either alone or in combination with d.c. electrical currents, had a stabilizing effect on the flow. In the presence of a plasma, however, undesired currents could be induced in the liquid walls, and pose the need for their feedback stabilization. To that end, resistive sensors of liquid metal thickness were developed and successfully tested, as well as simple jxB actuators. Future work will consist in “closing the loop” between them. Furthermore, a nearly complete cylindrical setup mimicking the interior of a tokamak will be devoted to the first experiments of full coverage by liquid metals, by means of electromagnetic and centrifugal forces.
Bio: Francesco Volpe conducts research on magnetized fusion plasmas and specializes in microwave heating and magnetohydrodynamic stabilization. He holds a a “Laurea” in Physics (1998, Univ. of Pisa, Italy) and a PhD in Experimental Physics (2003, Univ. of Greifswald, Germany), with theses respectively at ENEA Frascati and IPP Garching. He carried out post-doctoral research at the Culham Science Center in the U.K. and at General Atomics in San Diego, CA, and was an Assistant Professor at the University of Wisconsin, Madison, from 2009 to 2011. In 2012 he joined Columbia University, where he is now Associate Professor. Volpe worked on several tokamaks, spherical tokamaks, stellarators and reversed field pinches in the EU, US and Japan and made contributions to the physics of Electron Bernstein Waves and of disruptive locked modes. He is currently operating two stellarators at Columbia and collaborating with the DIII-D tokamak in San Diego. Recently he started a liquid metal wall stabilization program at Columbia. He authored or coauthored 60 journal articles, and is member of various national and international committees. Volpe received the 2003 Otto Hahn Medal (thesis prize of the Max Planck Society), the 2011 DOE Early Career Award, a visiting professorship at Kyoto University in 2012, and the 2015 Excellence in Fusion Engineering Award by Fusion Power Associates.