How can slow plasma electron holes exist?

TL;DR

This paper analyzes the conditions under which slow plasma electron holes can exist, showing they must be within a local minimum of ion velocity distribution and likely involve trapped electrons, with implications for plasma stability.

Contribution

It provides a one-dimensional analysis of slow electron holes, deriving criteria for their stable existence and clarifying their relation to trapped electrons and ion distributions.

Findings

Slow electron holes must lie within a local minimum of ion velocity distribution.

Stable equilibria depend on specific ion distribution and temperature ratios.

Observed slow potential structures are likely electron holes involving trapped electrons.

Abstract

One dimensional analysis is presented of solitary positive potential plasma structures whose velocity lies within the range of ion distribution velocities that are strongly populated: so called "slow" electron holes. It is shown that to avoid the self-acceleration of the hole velocity away from ion velocities it must lie within a local minimum in the ion velocity distribution. Quantitative criteria for the existence of stable equilibria are obtained. The background ion distributions required are generally stable to ion-ion modes unless the electron temperature is much higher than the ion temperature. Since slow positive potential solitons are shown not to be possible without a significant contribution from trapped electrons, it seems highly likely that such observed slow potential structures are indeed electron holes.