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

Abstract

The paper presents experimental results from the SMOLA device on the testing of the helical mirror confinement hypothesis. Helical mirror confinement is the technique of an active control of axial plasma losses from a confinement zone by multiple magnetic mirrors that move along the axis in the reference frame of the plasma that experiences $\boldsymbol{E} \times \boldsymbol{B}$ rotation due to an applied radial electric field. Theory predicts that a helical mirror will provide an axial force that modifies the plasma flow and, simultaneously, density pinching to the axis. The force direction depends on the plasma rotation direction. Experimental data on the axial plasma losses at different direction of the magnetic mirror movement are presented. If the trapped ions move in the direction opposite to the direction of the axial losses, then the particle flux reduces in the broad range of the plasma density. The confinement improves with the increase of the fraction of the trapped particles (effective mirror ratio was up to $R_{{\rm eff}}=5.8\pm 1.4$ ). If the trapped ions move in the same direction as the axial losses, then the flux depends on density. At intermediate densities, the integral flux through the transport section rises compared to the plasma flowing through the straight magnetic field. The effective mirror ratio is lower and does not significantly depend on the fraction of the trapped particles (effective mirror ratio at intermediate density was $R_{{\rm eff}}=3.3\pm 0.8$ ).

Electromagnetic oscillations and anomalous ion scattering in the helically symmetric multiple-mirror trap

“…These observations show that the mechanism of the anomalous scattering described in § 2 is not prohibited by the wave structure or its amplitude. This mechanism may play a role in the previously observed improved confinement at lower densities (Sudnikov et al 2022b).…”

“…This mechanism may play a role in the previously observed improved confinement at lower densities (Sudnikov et al. 2022 b ).…”

“…2021; Sudnikov et al. 2022 b ). The density in the entrance region of the transport section was roughly half that in all regimes, giving .…”

“…The plasma density between the mirrors in the confinement region was determined by the hydrogen flow rate from the gas feeding system into the plasma source. The density was in the range n = (0.5-5.5) × 10 18 m −3 for the confinement region, the average ion temperature was T i = 3.5-4.5 eV and the electron temperature on the axis was T e = 20-30 eV (Ivanov et al 2021;Sudnikov et al 2022b). The density in the entrance region of the transport section was roughly half that in all regimes, giving n = (0.2-2.5) × 10 18 m −3 .…”

“…One of the most interesting effects was the effectiveness at low plasma densities, when the ion mean free path is an order of magnitude larger than the corrugation period (Sudnikov et al. 2022 b ). The confinement is also associated with the electric potential fluctuations.…”