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.

The paper presents the results of numerical simulations of the collective relaxation of an electron beam in a magnetized plasma at the parameters typical to experiments on the ignition of a beam-plasma discharge in the Gas Dynamic Trap. The goal of these simulations is to confirm the ideas about the mechanism of the discharge development, which are used to interpret the results of recent laboratory experiments [Soldatkina et al 2021 {\it Nucl. Fusion}]. In particular, a characteristic feature of these experiments is the localization of the beam relaxation region in the vicinity of the entrance mirror. A strong mirror magnetic field compresses the beam so that its transverse size becomes less than the wavelength it excites. In addition, near the mirror, the electron cyclotron frequency is much higher than the plasma one, which can significantly affect the possibility of propagation of the most unstable waves outside the beam. Particle-in-cell simulations make it possible not only to find how efficiently intense plasma oscillations penetrate the rarefied periphery, but also to prove that the turbulent zone in a realistic nonuniform plasma has regions dominated by transverse electric fields. This creates the necessary conditions for efficient acceleration of the trapped particles to energies much higher than the initial beam energy.

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