Plasma flow suppression by the linear helical mirror system

Abstract

The paper presents experimental results from the SMOLA device that is the first facility with a helical mirror section of the magnetic system. This device was built in the Budker Institute of Nuclear Physics for the verification of the helical mirror confinement idea that is the technique of an active control of axial losses from a confinement zone. Theory predicts that, with rotating plasma, a helical mirror will provide suppression of the axial plasma flow and, simultaneously, density pinching to the axis. Experiments demonstrated the increase in plasma density in the entrance trap by a factor of 1.6 in the helical configuration. The integral axial flux from the transport section drops severalfold. The effective mirror ratio of the helical section was $R_{eff} > 10$ . Particle flux returning by the helical mirror section towards the confinement zone was observed. At high corrugation ratios, the axial flux direction is different at the magnetic axis and in the periphery of the plasma in the helical section. All axial fluxes scale linearly with the plasma density, even if the ion mean free path is comparable to the total length of the helical section. Good agreement of the experimental results with theoretical predictions is found.

Helical magnetic mirror performance at up- and downstream directions of the axial force

“…This growth is consistent with the previous experimental observations (Sudnikov et al. 2022). In contrast, the density does not depend on the mirror ratio in the transport section at pumping out (figure 5 b ).…”

“…In previous experiments in the confinement regime at a plasma density between the mirrors of (Sudnikov et al. 2022), no statistically significant difference was found between low- and high-density cases. In the described experiments, the density range was broader, and the level of the data uncertainty was reduced.…”

“…We should note here that in the confinement regime, the plasma potential and rotation velocity were limited compared to the previous experiments (Sudnikov et al. 2022), thus limiting the confinement effectiveness.…”

“…The increase in plasma density in the entrance trap by a factor of 1.6 in the helical configuration was demonstrated (Sudnikov et al. 2022). This paper presents the latest experimental results on the axial plasma flows in the helical mirror system at different directions of the plasma rotation and therefore different directions of the axial force that the trapped particles experience.…”

“…Here we should note that the decrease of the particle flux through the exit from the transport section is balanced by the generation of the return flux in the middle part of the transport section close to the plasma axis (Sudnikov et al. 2022). This return flux reduces or even reverses the average velocity in the entrance part of the transport section.…”