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 work reports the achievement of an energy content of 10 J per microsecond pulse in a directed flux of electromagnetic radiation in the frequency range of ~ 0.2–0.3 THz. The flux is generated by a fundamentally new method, which is realized through the pumping of upper-hybrid plasma oscillations in a magnetized plasma column with a relativistic electron beam (REB) and their subsequent transformation into a flux of electromagnetic radiation. In the described experiments at the GOL-PET facility, this method to generate THz radiation is implemented in the following way a beam of electrons with energy E ~ 0.5 MeV with a current density of (1–2) kA/cm2 is passing through a magnetized (4 T) plasma column with a density of 1014–1015 cm–3. By comparing the experimentally measured spectral composition of the radiation flux with the calculated spectrum, it is proved that this process is realized through resonant pumping of the branch of upper-hybrid plasma waves by such beam. A coordinated increase in plasma density and beam current density opens up the prospect of advancement in the generation of multi-megawatt radiation fluxes in the region of one terahertz.

One of the possible applications of high-current relativistic electron beams (REBs) is to generate electromagnetic waves at plasma frequencies due to the propagation of a beam through a magnetized plasma column. Research work in this direction, aimed at creating terahertz radiation sources at the BINP, is underway using the GOL–PET facility. We study the relaxation of a REB beam with a current density of (1–2) kA/cm2 in a magnetized plasma column with a density of 5 · 1014 cm–3. The purpose of these studies is to create a pulse radiation source with a power of tens of megawatts in the frequency range 0.1–1 THz. To date, a radiation flux with a power level of 10 MW and a maximum power spectral density in the frequency range 150–200 GHz has been achieved in the experiments. Further progress in these studies was related to the experimental establishment of the dependence of the power and spectral composition of the radiation flux on the parameters of the injected beam, in particular, its current density. The current density of the injected beam was varied due to the different compression of the beam cross section by the magnetic field. The results of measuring the characteristics of the radiation flux are presented in correlation with the results of measurements of the beam current density and plasma density.

This paper reports on the generation of a directed flux of electromagnetic radiation with an energy content of 10 J in the frequency range of 0.2–0.3 THz at a microsecond pulse duration in a beam–plasma system. The flux is generated when a relativistic electron beam (REB) pumps electron plasma waves in a magnetized plasma column. In the described experiments, this fundamentally new approach to generate terahertz radiation was carried out at the GOL-PET facility in the conditions of varying the beam current density and the plasma density in the appropriate ranges of 1–2 kA/cm2 and 1014–1015 cm–3. From the comparison of the flux energy spectrum measured experimentally in the frequency range 0.15–0.45 THz with the calculated one obtained using the previously proposed model of radiation generation in a beam–plasma system it was shown that this process occurs through resonant pumping by REB of precisely the branch of upper-hybrid plasma waves. Mastering this new method to generate terahertz radiation opens the prospect of its use to obtain multi-megawatt radiation fluxes in the frequency range up to 1 terahertz and higher. For such a development approach the most promising beam for pumping plasma oscillations seems to be a kiloampere REB generated in a linear induction accelerator.

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.

Research on one of the methods for generation of high-power radiation flows in the subterahertz/terahertz frequency range using the GOL–PET facility is underway at the G. I.Budker Institute of Nuclear Physics of the Siberian Branch of the Russian Academy of Sciences (BINP SB RAS). This method consists in using the mechanisms of conversion of the plasma waves excited by a relativistic electron beam (REB) into an electromagnetic radiation flow. In the course of research, a number of mechanisms by which plasma oscillations are converted into radiation were revealed, and a power level of 10 MW was experimentally achieved in a directed submillimeter radiation flow with a duration of a few microseconds. This paper is devoted to the study of radiation generation as a function of the degree of compression of the REB cross section. The compression degree was varied in the experiments by varying the magnetic field distribution along the axis of the facility. The magnetic field was varied uniformly along the entire facility axis. In addition, the configuration with an increased ratio of fields in the accelerator diode and in the plasma section of the facility was considered. As a result of the experiments, it was shown that a decrease in the strength of the guiding magnetic field by a factor of 1.6 does not lead to a change in the radiation generation mechanism.

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 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.

In Budker Institute of Nuclear Physics (BINP), experimental and theoretical studies of generation of submillimeter electromagnetic radiation with characteristic frequencies 0.1–0.5 THz under conditions of intense plasma–beam interaction are carried out. Generation of radiation occurs in the specialized G-OL‑PET facility during collective relaxation of pulsed relativistic electron beam (REB) with the following parameters: electron energy 0.6 MeV, beam current 15 kA, and pulse duration 5 µs in a plasma column with plasma density (0.5–1) × 1015 cm–3, under conditions of its confinement in a vacuum chamber with a corrugated magnetic field (the ratio of the maximum and minimum values of magnetic induction in the corrugations is Bmax/Bmin = 4.5/3.2 T). An important feature of the carried out studies is the presence of specifically created inhomogeneities both in its cross section and along its length. In experiments carried out earlier under the same conditions, a high level of power (about 10 MW) was reached in the flux of submillimeter radiation going out into the atmosphere. Radiation energy of 7 J was reached at pulse duration of 0.5 µs, which is limited by the high-frequency breakdown at the vacuum side of the output window. This article is devoted to the problem of increasing the duration of the pulse of generated submillimeter radiation of such a high power. This is attained by suppressing high-frequency (HF) breakdown near the surface of the window through which the radiation flux is going out into the atmosphere and by increasing the duration of megavolt radiation pulse applied to the accelerator diode in which REB is generated.

Experiments on pulse generation of submm electromagnetic waves are carried out at GOL-PET facility in Budker Institute of Nuclear Physics. The generation is realized in the process of relativistic electron beam interaction with a plasma column. Recently, the experiments have demonstrated the multi-megawatt power generation of 1 mm wavelength radiation at pulse duration up to 3 µs [1] . Evolution of the energy distribution function of the beam electrons as a result of the beam-plasma interaction is very important characteristic of this process. Firstly, measuring this function permits to evaluate the efficiency of the beam energy deposition in plasma. Secondly, analyzing the evolution of the function in correlation with the time behavior of the power and the spectral composition of the radiation outgoing from the plasma column during the E-beam pulse at various experimental conditions allows one to understand the physics mechanism of the wave generation.

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.

Studies on the generation of electromagnetic radiation during beam-plasma interaction are of considerable scientific interest from the standpoint of analyzing the results of astrophysical observations [1] . Such studies look no less important from the standpoint of solving a wide range of practical problems using high-powerful submillimeter radiation. Experimental and theoretical studies on multimegawatt power submillimeter wave generation due to intense interaction of a relativistic electron beam (REB) with a magnetized plasma column are carried out in BINP SB RAS [2] . In the experiments performed at the GOL-PET facility, the beam with parameters 0.5 MeV/15 kA/6 μs was propagated in a magnetized (induction up to 4.7 T) plasma column with the diameter 6 cm, length 2 m and density (1÷2)x10 15 cm -3 . A radiation flux generated due to beam-plasma interaction was extracted in atmosphere through an output fluoroplastic window of 18 cm diameter. We measured the power and the spectral power density of this flux in the frequency band 0.1÷0.8 THz for various plasma density distributions. The radiation angular distribution of the output flux propagated in the hall atmosphere was also measured. The experiments have shown that the maximal spectral power density of the flux power is located in the frequency range 0.2÷0.35 THz that is the area of the upper-hybrid frequency of plasma oscillations. The value of the spectral power density in this frequency interval depends on the plasma density distribution over the plasma column diameter and on the law of density decrease along the axis at the beam output from the column. The experimental results are discussed in comparison with analytical and computer modeling.