The results of preliminary experiments on measuring the spatial asymmetry of plasma flows in the GOL-NB device using movable Mach probe are presented and the diagnostics used is described. In the experiments, the high-field sections were mounted in the configuration with solenoidal magnetic field. The dynamics of plasma flows was recorded which was expected in the trap: the plasma flowed from the plasma gun along the magnetic field, accumulated in the GOL-NB central trap, and then after the plasma gun was switched off, flowed out from the central trap in two directions. At time of transition from the stage of plasma accumulation to the stage of its decay, the direction of plasma flow in the input high-field section was inverted. The balance of particles in the central trap is discussed. Experiments have shown that this technique can be used for studying the effects of improving plasma confinement after switching to the multiple-mirror configuration of high-field sections, in which, according to theory, under optimal conditions, a flow of b-ackscattered particles should arise, which will return them from the multiple-mirror sections to the confinement zone.

The physics of confinement of plasma rotating in the magnetic field with linear helical symmetry is studied at the SMOLA open trap at Budker Institute of Nuclear Physics of the Siberian Branch of the Russian Academy of Sciences. The factor characterizing the quality of plasma confinement in the system is its flow velocity. The paper describes the diagnostics applied, which is based on the Mach probe used under the conditions of nonmagnetized plasma; this diagnostics made it possible to determine the longitudinal flow velocity in the experiments. In different operating regimes of the device, the measured longitudinal flow velocity was (0.5−5) × 106 cm/s. It is discussed how the velocity depends on the magnetic field corrugation. The reverse flow of trapped particles returning to the confinement zone was detected.

The paper presents experimental results from the SMOLA device, which was built in the Budker Institute of Nuclear Physics for the verification of the helical mirror confinement idea. This concept involves active control of axial losses from the confinement zone in an open magnetic trap through the use of multiple mirrors that move in the plasma frame of reference. The discussed experiments focused on determining the cumulative effect of a helical mirror system in combination with a short segment of a stronger magnetic field. Combination of these two methods of axial flow suppression results in higher efficiency compared with each method individually. Different combinations of the mirrors were tested. The most effective flow suppression was observed if the short mirror was placed between the confinement region and the helical mirror. In this configuration, an effective mirror ratio of $R_{{\rm eff}} = 32.6\pm 7.8$ was achieved, along with a more than three-fold increase in plasma density within the confinement region. The possibility of a cumulative effect of different types of magnetic mirrors offers a way to improve the confinement performance of the reactor-grade mirror confinement devices.

The results on applying the technique of vortex plasma confinement in the GOL-NB facility are presented. The first experiments on optimizing the biasing the in-chamber electrodes demonstrated an improvement in the dynamics of trapping the injected fast hydrogen atoms, as well as a decrease in the fluctuations of local plasma parameters in the central trap and an increase in the plasma decay time. The geometry of in-chamber electrodes arrangement, as well as the polarity and magnitude of the supplied potentials, correspond to the theory of vortex confinement and to those in similar studies at other open traps.

The magnetic system of an open trap usually includes expansion sections located between high-field magnetic mirrors and end surfaces that receive plasma. In the GOL-NB device, an arc plasma gun is located in one of the expanders, which creates a low-temperature starting plasma in the confinement area. The parameters of the surface plasma sheath affect the electrical connection of the confinement area with the walls and, thereby, affect the contribution of the line-tying effect to the plasma stability and the longitudinal energy losses from the trap. The experiments with additional hydrogen injection into the plasma gun were carried out at GOL-NB. We observed a radiating plasma formation detached from the surface, which visually corresponds to that in radiating divertors in tokamaks. In both standard and detached modes, decaying plasma existed near the receiving electrodes during the entire observation time after the discharge current was terminated. In the central trap of GOL-NB, some structures in the Fourier spectrogram of magnetic fluctuations manifest earlier in the detachment mode than in the standard mode and have lower frequencies. We associate these structures with the onset of interchange-like modes due to the loss of plasma stabilization by the line-tying to the conducting ends. The observed plasma response to the additional gas supply confirmed our understanding of the line-tying effect as the main factor stabilizing the plasma core in the initial phase of density accumulation in the central trap.

The physics of confinement of a rotating plasma in a magnetic field with linear helical symmetry is being studied at the Institute of Nuclear Physics SB RAS on the open SMOLA trap. An indicator of the quality of confinement is the plasma flow velocity in the system. The paper describes the applied diagnostics based on the Mach probe under conditions of non-magnetized plasma, which made it possible to determine the longitudinal flow velocity in experiments. The measured longitudinal flow velocity was (0.5–5) · 106 cm/s in various operating modes of the installation. The dependence of the speed on the magnitude of the magnetic field corrugation is discussed. A reverse flow of trapped particles returning to the containment zone has been detected.

This work is devoted to the project of a new-generation open trap, gas-dynamic multiple-mirror trap (GDMT), proposed at the Budker Institute of Nuclear Physics, Siberian Branch, Russian Academy of Sciences. The aim of the project is to substantiate the possibility of using open traps as thermonuclear systems: a source of neutrons and, in the future, a thermonuclear reactor. The main objectives of the project are to develop technologies for long-term plasma maintenance in an open trap, optimize neutron source parameters based on the gas-dynamic trap, and demonstrate methods for improving plasma confinement. The magnetic vacuum system of the facility consists of a central trap, multiple-mirror sections that improve the longitudinal plasma confinement, and expanders designed to accommodate plasma flux absorbers. The facility is to be built in several stages. The starting configuration is broadly similar to the GDT facility and includes a central trap with strong magnetic mirrors and expanders. It solves two main problems: optimization of the parameters of the neutron source based on the gas-dynamic trap and study of the physics of the transition to the configuration of a diamagnetic trap with a high relative pressure β ≈ 1, which significantly increases the efficiency of the system. This work describes the technical design of the starting configuration of the facility and outlines the physical principles on which the GDMT project is based.

A multiple-mirror confinement is an alternative concept in fusion energy development that improves the particle and energy confinement times in open traps (linear magnetic systems). The paper discusses the development progress of the GOL-NB multiple-mirror experiment that was recently commissioned at BINP. The reference description of the GOL-NB hardware is presented. Properties of a low-temperature start plasma are discussed. The first results from test experiments with injection of one 25 keV neutral beam are shown. Methods of plasma stabilization in the non-min- B configuration are discussed. In general, the ongoing commissioning progress and results of the preliminary experiments are modestly optimistic for the expected device performance.

Abstract—A multi-chord beam diagnostic system for an open multiple-mirror trap GOL-NB is described. The system is based on a 10 keV ribbon beam of fast neutral hydrogen atoms generated by a wide-aperture ion injector with an arc plasma source. The plasma linear density profile is calculated from the attenuation of the beam after passing through the plasma. This paper discusses the design of the diagnostics and its capabilities, and also presents the measured profiles of the linear plasma density in the experiment on filling the trap with the starting plasma. The modernized version of diagnostics and the expected increase in its resolution are described. The prospects of using the Charge eXchange Recombination Spectroscopy (CXRS) method on the existing element base are evaluated.

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.

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 $R_{{\rm eff}}=3.3\pm 0.8$ ).

The results are presented from preliminary experiments on neutral beam injection with the power of approximately 1 MW into the central trap of the GOL-NB facility. The main technical task of the work was the integrated commissioning of all systems and basic diagnostics. The results are presented on the attenuation of neutral beams in plasma and the parameters of the fast ion population. In the regimes under discussion, the attenuation factor of the flow of injected particles of heating beams reaches 40% at the simultaneous start of the initial plasma accumulation in the trap and its heating by neutral beams. The dynamics of accumulation and energy spectrum of the fast ion population formed in plasma as a result of neutral beam injection are discussed, and the physical mechanisms are analyzed that can provide for the observed spectrum of fast ions.

The SMOLA open magnetic trap was created at the Budker Institute of Nuclear Physics of the Siberian Branch of the Russian Academy of Sciences (BINP SB RAS) for studying the physics of suppression of plasma longitudinal losses from the system in which rotating plasma is confined in a magnetic field with helicoidal symmetry. The possibility of controlling the plasma rotation velocity is crucial in this concept. Methods of spectroscopic and magnetic diagnostics that allowed obtaining parameters of plasma rotation in the experiments are described. The angular plasma rotation velocity was found to be (0.5–1) × 106 s–1 in different regimes of system operation. The dependences of angular plasma rotation velocity on various parameters of the experiment are discussed.

The system is described for the formation of the low-temperature starting plasma flow in the GOL-NB trap. The starting plasma is a target for capturing heating neutral beams. The plasma flow is formed in the arc plasma gun installed in a relatively weak magnetic field. Next, it is compressed in the increasing magnetic field and then transported to a distance of approximately 4 m. The design of the plasma gun is described. Optimization of the operating regimes and scenarios of the system for creating the starting plasma made it possible to reduce the gas load onto the vacuum system of the facility, which resulted in reducing the losses associated with the presence of gas dragged along together with the plasma. The plasma flow at the outlet from the high magnetic field section is increased approximately four times, as compared to the results of the first plasma campaign. It is discussed how the limiters and other intrachamber electrodes affect the plasma flow formation. The achieved plasma flow parameters will be sufficient to start the experiments on plasma heating with the help of neutral beams at the GOL-NB multi-mirror trap in its full-design configuration.

A system of four-electrode Langmuir probes developed for the GOL-NB multiple-mirror trap is discussed. The system is used for studies of a low-temperature start plasma (1019–1020 m-3, 5 eV) that fills the device during the initial phase of the experiment. The probe allows simultaneous measurements of plasma density, electron temperature and radial electric field. The accuracy of the probe measurements is also discussed.