When Dmitrii Kiramov and Boris Breizman were studying electron cooling by injecting a cryogenic pellet into a hot plasma, they noticed a peculiar property in their numerical simulation – the electron cooling rate slows down and eventually comes to a halt, leaving the system with a substantial part of its initial thermal energy. They investigate this process in a recent paper.

“Our initial numerical treatment revealed a counterintuitive behavior of the electron population,” said Kiramov. “We found that the hot electrons retained a significant part of their kinetic energy, and we also observed a collisionless phase space structure that attracted our attention.”

In their analysis, they consider a one-dimensional slab of hot, collisionless plasma between two cold walls. This set-up mimics two opposite sides of a dense pellet on a closed magnetic field line in which hot incident electrons are absorbed by the walls, while cold electrons – with much smaller thermal energies – are emitted.

They discovered that charge transfer from the hot electrons causes a potential well to form, confining the lower-energy, cooler particles. Once the plasma reaches a steady state – which is colder than its initial state, but not at thermal equilibrium – it remains there for a finite time before the confined electrons begin to leak from the well via collisions, further cooling down the plasma.

Though the analysis was done for a simplified system, the results reveal the hot electron population in a collisionless plasma slab can last much longer expected.

“Our paper presents a solution of an idealized stand-alone problem,” Kiramov said. “We believe that this problem is of interest in its own merits as it reveals nontrivial nonlinear structures that are linearly stable.”

Source: “Hot electrons between cold walls,” by Dmitrii I. Kiramov and Boris N. Breizman, Physics of Plasmas (2020). The article can be accessed at https://doi.org/10.1063/1.5134499.