Preliminary experimental scaling of the helical mirror confinement effectiveness

Abstract

The paper presents experimental results from the SMOLA device that is the first facility with a helical mirror section of the magnetic field. This device is built in the Budker Institute of Nuclear Physics for the verification of the helical mirror confinement idea that is the recently introduced technique of the 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 that plasma density at the exit from the transport section is suppressed with activation of the helical field, the effect is significant and highly reproducible. The most pronounced effect is observed on the plasma periphery, where the mirror ratio is the highest. The integral suppression ratio reaches 2–2.5 in the discussed experiments. Experimental results are compared with simplified theoretical estimates. The integral suppression ratio matches the simple theoretical estimates even if the transversal diffusion is neglected.

Plasma flow suppression by the linear helical mirror system

“…The first systematic studies of the axial plasma flow through a linear helical mirror magnetic system in the SMOLA device successfully demonstrated the validity of the main theoretical predictions for a system of this kind. Comparing with our preliminary results (Sudnikov et al 2020), minor improvements of the device parameters and operation regimes allowed clearer demonstration of the confinement improvement in the helical mirror system comparing with the simple solenoidal field in the transport section. High rotation velocity and high mean corrugation ratio resulted in an observable density increase in the entrance trap by a factor of 1.6 compared with the solenoidal field.…”

“…If the magnetic field is high enough to prevent diffusion, the dependence of the plasma parameters on the magnetic field becomes insignificant (see figure 6). In previous experiments (Sudnikov et al 2020) the plasma stream degraded if the axial magnetic field was B z ≤ 40 mT. At higher fields the dependence of the flux suppression on B z in theory and in experiment was quite slow.…”

“…Axial plasma flux suppression by the helical sections was demonstrated in the first experimental campaign, an integral suppression ratio of 2-2.5 was achieved (Sudnikov et al 2019). An improvement of the suppression ratio with the increase of the magnetic field, corrugation ratio and plasma rotation velocity was demonstrated (Sudnikov et al 2020). This paper presents the latest experimental results on the axial plasma flows in the helical mirror system in a broad range of the plasma densities at high rotation velocity and high corrugation ratio.…”

“…the ratio of the mean free path to the period of the helical corrugation is λ/h ∼ 1. As in Sudnikov et al (2020), we take the characteristic length of the exponential decrease of the plasma density with the axial coordinate in the following form:…”

“…The effects of the helical corrugation and the radial diffusion were calculated numerically in a way similar to Sudnikov et al (2020). The transport section was split into 768 slices (64 slices in one period of the magnetic field variations).…”