Effect of controlled ion population on the evolution of a quiescent low aspect ratio toroidal pure electron cloud : A 3D PIC approach

A quiescent quasi-steady inhomogeneous, equilibrium state of axisymmetric toroidal electron cloud in a tight aspect ratio device, has been found¹. This numerical study was performed using a 3D3V PIC code PEC3PIC² where a maximum entropy mean field solution was used as “seed" solution to the solver. The resultant quasisteady equilibrium of the electron cloud satisfies full equations of motion with negligible presence of m = 1 Diocotron motion resulting in a quiescent toroidal electron plasma with superior confinement properties.

In experimental studies of toroidal electron clouds in large³ and small⁴ aspect ratio partial devices, small amount of backgound ions lead to ion resonance instability. In the present study, the effect of ions on the above-said quiescent quasi-steady equilibrium with nearly zero center of charge motion has been investigated. A minuscule or small ion fraction introduced in the device at a particular time, is found to result in systematic destabilization of the electron cloud. The resulting electron cloud exhibits systematically linear, followed by quasi-linear and nonlinear center of charge oscillation or toroidal Diocotron mode, with increasing ion fraction value upto a certain limit. Quantitative analysis of the wall probe current reveals interesting features⁵ of this phenomena. Initial observation shows that the electron cloud destabilization process depends on the mass of ion species as well as the strength of the toroidal magnetic field. These and other details will be presented in this paper.

1. S. Khamaru, R. Ganesh, and M. Sengupta, Phys. Plasmas 28, 042101(2021). 2. S. Khamaru, M. Sengupta, and R. Ganesh, Physics of Plasmas 26, 112106 (2019). 3. M. R. Stoneking et al, Phys. Plasmas 9, 766 (2002). 4. L. Lachvani et al, J. Phys.: Conf. Ser. 390, 012047 (2012). 5. S. Khamaru, R. Ganesh and M. Sengupta, "3D3V PIC Simulation of ions-driven destabilization of a toroidal electron cloud ", Manuscript Under Preparation.