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 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.

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.

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 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.

The GOL-NB project is a physics demonstration experiment on multiple-mirror plasma confinement that is currently under development in the Budker Institute of Nuclear Physics [1-3]. The final configuration of the device will include a 2.5-m-long central gasdynamic trap with two attached multiple-mirror sections of 3 m each, and two end magnetic flux expanders that house a start plasma creation system, plasma receiver endplates and a system of biased electrodes for plasma stabilization. Plasma will be heated by two 0.75 MW, 25 keV neutral beams. GOL-NB will be a scaled-down physical model of a future fusion-grade open trap [4]. The main scientific task of the project is to demonstrate improved confinement with activation of the multiple-mirror configuration.

The first plasma campaign was completed in a start configuration of the GOL-NB multiple-mirror trap. The device included a 4 m long high-field section, two end sections of magnetic flux expanders and a short temporary section for the commissioning of two 25 keV, 0.75 MW neutral beams. Low-temperature start plasma was created using a coaxial arc plasma gun. The main objective of the first experiments is to develop and characterize the low-temperature plasma stream that will be used later for filling the central trap of GOL-NB with the start plasma. The plasma parameters and operation regimes are reported.

The challenge of creation of an open trap with the reactor-grade plasma is achievable if such trap will use specialized sections of the magnetic system for suppression of particle and energy losses along the magnetic field. Currently, two new experimental devices are under construction in the Budker Institute for studies of physics of plasma confinement in magnetic systems with multiple-mirror configurations. Linear topology of the traps enables early start of experiments with plasma before the completion of the magnetic and vacuum systems. In the paper, we will report experimental results on the transport of a low-temperature start plasma flow through a section with a multiple-mirror magnetic field as well as the direct comparison with the case of solenoidal magnetic field. In the final configuration of GOL-NB, that plasma stream will be used as the target for the capture of heating neutral beams. In 2017, new SMOLA helical multiple-mirror trap achieved the first plasma. In this trap, plasma rotation is used for creation of moving magnetic mirrors in the rotating frame of reference. An active plasma pumping by the moving magnetic mirrors can deliver an exponential dependence of the confinement efficiency on the system length. Modification of the plasma flow profile at helical mirror confinement was demonstrated in the experiment. Main results from the first experimental campaign will be discussed.

Results of spectral and magnetic diagnostics of plasma differential rotation in the GOL-3 multiplemirror trap are presented. It is shown that the maximum frequency of plasma rotation about the longitudinal axis reaches 0.5 MHz during the injection of a relativistic electron beam into the plasma. The data of two diagnostics agree if there is a region with a higher rotation frequency near the boundary of the electron beam. Plasma differential rotation can be an additional factor stabilizing interchange modes in the GOL-3 facility.

New experimental results on magnetic compression and transport of low-temperature arc plasma are presented. In the experiments, the transport efficiency of the plasma stream in a multiple-mirror configuration was compared with the same in a uniform solenoidal field. The magnetic compression ratio of the plasma stream varied from 5 to 60 at the initial diameter of 5 cm. As the result, the plasma with (1 – 4)×1020 m-3 density at the axis was obtained at ∼3 m distance from the plasma source. Theory predicted that a multiple-mirror magnetic field should not significantly decelerate and weaken collisional plasma. The experiment confirmed this prediction. In the presented work, we simulated the baseline scenario of GOL-NB filling by cold start plasma.

Physical and technical details of the GOL-NB project are presented. GOL-NB is a medium-scale multiple-mirror trap that is under development in the Budker Institute, Novosibirsk, Russia. This device will be created in several years as a deep conversion of the existing GOL-3 facility. It will consist of a central trap with two 0.75 MW neutral beams, two multiple-mirror solenoids, two expander tanks and a plasma gun that creates the start plasma. The central trap with the neutral beam injection-heated plasma is a compact gas-dynamic system. The multiple-mirror sections should decrease the power and particle losses along the magnetic field. The confinement improvement factor depends on plasma parameters and on the magnetic configuration in the multiple mirrors. The main physical task of GOL-NB is direct demonstration of the performance of multiple-mirror sections that will change equilibrium plasma parameters in the central trap. In this paper we discuss results of the scenario modeling and progress in the hardware.

A spectral system with the spatial resolution intended for diagnosing rotation of the plasma in the GOL-3 facility is described. The system is capable of measuring the radial dependences of the Doppler shift and broadening of spectral lines emitted by the plasma from various points in it. A DFS-24 double focusing spectrometer with a reciprocal linear dispersion of 0.45 nm/mm is the basis for the spectral system. If a radiation source is axially symmetric and extended, it is possible to obtain the intensity of spectral lines on the surface of the photosensitive area of a CCD array with the spatial resolution across the plasma by forming an image of the plasma in the sagittal focal plane of the entrance collimation lens, though the spectrometer astigmatism is strong. As a result, a spatial resolution of 0.8 mm has been attained in the developed system at a 55-mm field of view.

In 2015, the existing GOL-3 multiple-mirror trap has been converted into three specialized devices, each dedicated to a specific scientific problem. Experiments on studies of plasma mechanisms of sub-THz radiation generation use the GOL-3T device. A device with a sub-ms electron beam was isolated for further studies of ITER-grade transient heat loads to tungsten. Research of multiple-mirror physics will continue in a new GOL-NB device that will consist of a central trap with 0.3 – 0.6 T field, 2.5 m length and 1 m diameter and two attached multiple-mirror sections with 4.5 T field. The plasma will be heated by 1.5 MW NBI. Operation of the first stage of GOL-NB started in 2015. It includes 6-m-long 4.5 T solenoid of variable configuration with a plasma gun. The (1 – 4)×10 m plasma stream transport through the system was studied. A new helical mirror confinement idea and the first concept exploration device SMOLA will be presented as well.