Fundamental Laboratory Plasma Physics Experiments—What Can They Tell Us About “Naturally” Occurring Phenomena in Space and Fusion?

Professor Earl E. Scime; Department of Physics and Astronomy, West Virginia University

Co-authors:
Mahmud Hasan Barbhuiya, Gustavo Elias Bartolo, Paul Cassak, Shane Cupp, Chloelle Fitz, Jacob Freeze, Tyler Gilbert, Maylay Guitard, Gabriela Himmele, Regis John, Matthew Lazo, Dan McDonald, John McKee, Ripudaman Singh Nirwan, Mitchell Paul, Thomas Rood, Peiyun Shi, Prabhakar Srivastav, Samuel Stalnaker, Thomas Steinberger, Katey Stevenson, Sonu Yadav.

One of the “Grand Challenges” of plasma physics is to understand the processes whereby energy stored in the magnetic fields of a plasma is converted into kinetic energy of the ions and electrons in the plasma. We see that this energy conversion happens in stars, in planetary magnetospheres, and in fusion plasmas. I will review the experimental evidence for the energy conversion process and highlight one particular possible mechanism—magnetic reconnection. I will then describe a new experiment at West Virginia University, the PHAse Space MApping (PHASMA) experiment that is designed to investigate the energy conversion process during magnetic reconnection at the kinetic, i.e., smaller than the electron gyroradius, scale in laboratory plasmas. PHASMA has a unique diagnostic complement that includes laser induced fluorescence diagnostics for ion velocity distribution function measurements, Thomson scattering diagnostics for electron velocity distribution function measurements, and a microwave scattering system for turbulence measurements. To create the conditions necessary for reconnection, PHASMA employs two plasma sources. First, PHASMA includes a 1 kW, steady-state helicon source capable of generating variable-density background hydrogen, helium, argon, krypton, and xenon plasmas with controllable plasma pressure (relative to the magnetic pressure), collisionality, and azimuthal flow shear. Second, PHASMA includes two pulsed plasma guns that create magnetized tubes of plasma, “flux ropes.” The flux ropes attract each other and merge through the process of magnetic reconnection. I will present experimental studies of the merger of two flux ropes through electron-only reconnection. We find that, consistent with theoretical predictions, the majority of incoming magnetic energy appears as electron thermal energy and that Ohmic processes are unlikely to be responsible for the measured increase in electron enthalpy. The electron velocity distribution function measurements include non-Maxwellian features, including beams that jet out from the X-point in both outflow directions. We observe that the electron beam speed scales with the local electron Alfvén speed. I will also describe the first three-dimensional measurements of electron velocity distribution functions in the laboratory and discuss them in terms of possible electron heating mechanisms during reconnection.

Work supported by NSF grants PHY-1827325 and PHY-1902111, NASA grant 80NSSC19M0146 and DoE grant DE-SC0020294

There will be a reception immediately preceding the colloquium at 3 p.m. in 316 VAN.