Xabier Sarasola

Plans for the creation and studies of electron–positron plasmas in a stellarator

Electron-positron plasmas are unique in their behavior due to the mass symmetry. Strongly magnetized electron-positron, or pair, plasmas are present in a number of astrophysical settings, such as astrophysical jets, but they have not yet been created in the laboratory. Plans for the creation and diagnosis of pair plasmas in a stellarator are presented, based on extrapolation of the results from the Columbia Non-neutral Torus stellarator, as well as recent developments in positron sources. The particular challenges of positronium injection and pair plasma diagnostics are addressed.

CONSTRUCTION AND INITIAL OPERATION OF THE COLUMBIA NONNEUTRAL TORUS

Abstract We report on the results from initial testing and operation of the Columbia Nonneutral Torus, a new stellarator experiment constructed at Columbia University to study the confinement of nonneutral plasmas, electron-positron plasmas, and stellarator confinement in the presence of strong electrostatic fields. A new algorithm for automatic identification of good magnetic surfaces, island chains, and stochastic regions in Poincaré maps is also described. We present some of the details of the design of the interlocked in-vessel coils and the vacuum system and report on initial vacuum performance. Magnetic surface mapping and visualization results are also presented, confirming the existence of ultralow aspect ratio magnetic surfaces with excellent quality and good agreement with numerical calculations.

First experimental studies of the physics of plasmas of arbitrary degree of neutrality

The first detailed experimental characterization of the physics of plasmas of arbitrary neutrality is reported. It is shown that the degree of neutralization of the plasma in the Columbia Non-Neutro Torus stellarator can be varied continuously from quasi-neutral to pure-electron plasma, and that the plasma can be sustained indefinitely at any degree of neutralization. The physical phenomena change significantly as the degree of neutralization is varied. Quasi-neutral plasmas exhibit a single mode low-frequency flute instability. This mode aligns almost perfectly with the magnetic field lines, presenting a resonant m?=?3 poloidal structure. For weakly magnetized ions, and moderate degrees of neutralization, multi-mode behaviour is observed, in some cases fully turbulent. At the other extreme of the neutralization scale, electron-rich plasmas with a small but non-negligible amount of ions exhibit single mode fluctuations. Essentially identical phenomena were previously reported by Marksteiner et al (2008 Phys. Rev. Lett. 100 065002). In addition to the already known poloidal mode number m?=?1 of these fluctuations, the toroidal mode number is now known: n?=?0. This not only confirms the previous observations that the electron fluid force balance appears to be broken, but the frozen-in flux condition is also violated.