Pair plasma traps

Pair plasma traps

Because the properties of a plasma system are often inextricably linked to its magnetic topology, being able to compare and contrast the behavior of pair plasmas in highly distinct but complementary magnetic field configurations is expected to significantly extend our understanding beyond what we will learn from each experiment individually.  Both dipoles and stellarators have been shown to effectively confine both non-neutral and quasi-neutral plasmas without requiring internal plasma currents.  We are designing and building two different tabletop-sized, superconducting traps for our electron-positron pair plasmas.

Levitated dipole: Dipole magnetic fields occur copiously in nature, from permanently magnetized minerals, to planets like the Earth, to stellar objects.  In the laboratory, dipole magnetic field lines that touch no material surfaces can be achieved by levitating a current-carrying, superconducting coil.  The "floating coil" is suspended in the vacuum chamber by a feedback-controlled "lifting coil" above it, providing an attractive force to balance gravity.   The confining magnet field lines are purely poloidal, with large mirror ratios and short connection lengths; the magnetic field gradient and curvature cause particles to drift toroidally around the trap on surfaces of constant magnetic flux.

Optimized stellarator: In a stellarator, magnetic field lines are predominantly toroidal but also have a poloidal component, so that they twist around the torus; this "rotational transform" is generated by external coils.  The design of those coils, the vacuum magnetic field they produce, the properties of the plasma that will be confined by the magnetic field, and the required engineering tolerances are determined through the sophisticated process of stellarator optimization, the forefront of which our collaborators in Stellarator Theory at IPP and the Simons Hidden Symmetries in Fusion Energy Collaboration are actively developing.  While optimizing a stellarator for a laboratory electron-positron plasma is not the same as optimizing a stellarator for a fusion plasma, it uses many of the same computational tools and techniques; in return, a pair plasma will offer a highly sensitive test of the effectiveness of the optimization.

Selected publications:

1.
Saitoh, H.; Stoneking, M.; Pedersen, T. S.: A levitated magnetic dipole configuration as a compact charged particle trap. Review of Scientific Instruments 91, 043507 (2020)
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