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—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 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.

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 notable feature of cadmium tungstate (CWO) is quite long ( ∼ 14. 5 μ s ) decay time of its brightest scintillation component. In this paper, the technique and results of high dynamic range measurements of CWO afterglow are discussed. Samples were irradiated by bremsstrahlung of a ∼ 0.5 MeV, 6 μ s electron beam. The afterglow tail of the scintillation curve was measured down to the relative amplitude

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.

Results, obtained during the last decade on linear traps in Novosibirsk, verified the original concepts and led to discovery of new ways of plasma confinement in linear traps. It turns out that the multiple-mirror confinement, once thought to be suitable for pulsed fusion only, can work in tandem with gas-dynamic traps. Its helical-mirror variant can provide additional advantages for confinement of rotating plasmas such as radial and axial plasma pinch fluxes. The specially designed gas-dynamic mirrors can be run in the high-β “diamagnetic” regime, closely resembling the field-reversed configurations both in shape and in the quality of axial confinement. These new ideas form the groundwork of the possible road for linear traps to fusion.

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.

Novel concept of the longitudinal plasma flow suppression by active plasma pumping in helicoidal magnetic field in mirror traps was proposed recently. Concept exploration device SMOLA is now being constructed to prove the possibility of the suppression and determine basic scalings of ist effectiveness. Technical solutions of the SMOLA device are described in the article. Vacuum system, magnetic system, plasma source and the source of the radial electric field are discussed. Estimations of the biasing required for optimal confinement are given. Physical program fort he first experiments is also proposed.

A new control system for capacitive energy storage with a rated energy content as high as 16 MJ is described. Capacitive energy storage has been used since 1987 to supply power for the pulsed magnetic system of the GOL-3 multiple-mirror trap. It has an operating voltage of 6 kV and consists of 6000 IK-6-150 capacitors that are located in six remote rooms together with the necessary infrastructure. The system for control and data acquisition was built using Arduino microcontroller boards operating as a distributed network of remote servers. The client part of the control system uses a personal computer with a standard configuration; communication with remote servers is effected by the TCP/IP protocol. The control system was put into regular operation at devices of the GOL-3 facility in the early 2016.

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.

The review presents experimental studies of plasma-surface interaction and materials behavior under plasma loads done in the multiple-mirror trap of the GOL-3 facility. In the experiments for the PSI, the energy density in the extracted plasma stream varies from 0.5 to 30 MJ/m2. Parameters of near-surface plasma measured by a set of diagnostics are reviewed. Surface patterns of targets exposed to the plasma are analyzed. The erosion depth depends on the energy loads—it rises from 0 to 600 μm at 0.5 and 30 MJ/m2, correspondingly. Cracking and evolution of graphite and tungsten surface morphology are discussed. The enthalpy of brittle destruction of graphite (10 kJ/g), which determines the threshold of bulk damage of targets irradiated with a charged-particle flux with large penetration depth, was determined. Comparison of different facilities for PSI studies are presented. Heat flux play a key role to the target surface erosion.

The study is devoted to tungsten surface modification after irradiation at the GOL-3 facility with plasma loads corresponding to the ITER type I ELMs. In order to emulate heating with a steady plasma flux in the ITER divertor, some of the tungsten samples were preheated up to 500 °C. It was found out that the behavior of the surface modification (the crack pattern and the number of bubbles) depends on the initial temperature of the targets. While the orientation of major crack networks correlates with the direction of machining of the samples. Afterwards we have observed the process of craters’ formation caused by dust particle impacts.

Currently, the GOL-NB multiple-mirror trap is being developed at the Budker Institute of Nuclear Physics, Siberian Branch, Russian Academy of Sciences. The main scientific goal pursued by building GOL-NB is direct demonstration of suppression of longitudinal losses of particles and energy from the trap by using sections with a multiple-mirror magnetic field, which can be attached to the central gas-dynamic trap. Plasma heating in GOL-NB will be accomplished by neutral beam injection with a power of up to 1.5MW. The paper presents the first results of modeling the dynamics of the plasma parameters and fast ions under the reference operating scenario of the trap in which traditional short magnetic mirrors, rather than multiple-mirror sections, are attached to the central trap. In such a configuration, the plasma lifetime in the trap is expected to be minimal. The modeling was performed by using the DOL kinetic code. As a result, the initial conditions of the experiments are refined and the requirements to the system of maintaining the particle balance in the trap are determined.

A novel concept of helical mirror confinement with active axial and radial plasma flow control is discussed. The idea relies on the retarding force that appears when biased plasma experiences rotation in crossed electrical and helical mirror magnetic fields. Preparations for its experimental study started in Budker Institute of Nuclear Physics, Novosibirsk. Possible parameter space for a concept-exploration-class device SMOLA is identified. Details of numerical optimization of the magnetic structure of the device are presented. The main physical task for the SMOLA experiment will be direct demonstration of the helical mirror performance in simple experimental conditions at reasonably low plasma parameters.

The program of the deep upgrade of the GOL-3 multiple-mirror trap is presented. The upgrade is aimed at creating a new GOL-NB open trap located at the GOL-3 site and intended to directly demonstrate the efficiency of using multiple-mirror magnetic cells to improve longitudinal plasma confinement in a gasdynamic open trap. The GOL-NB device will consist of a new central trap, adjoint cells with a multiple-mirror magnetic field, and end tanks (magnetic flux expanders). Plasma in the central trap will be heated by neutral beam injection with a power of up to 1.5 MW and duration of 1 ms. At present, physical experiments directed at developing plasma technologies that are novel for this facility are being carried out using the 6-m-long autonomous part of the GOL-3 solenoid. The aim of this work was to develop a method for filling the central trap with a low-temperature start plasma. Transportation of a plasma stream from an arc source over a distance of 3 m in a uniform magnetic field with an induction of 0.5–4.5 T is demonstrated. In these experiments, the axial plasma density was (1–4) × 1020 m–3 and the mirror ratio varied from 5 to 60. In general, the experiments confirmed the correctness of the adopted decisions for the start plasma source of the GOL-NB device.

This paper describes results of GOL-PET experiments on subterahertz (0.1–0.5 THz) emission from a magnetized plasma column during passing of a high-current $(\sim \hbox{10 kA})$ relativistic $(\sim \hbox{0.6 MeV})$ electron beam. The subterahertz radiation is generated in the plasma due to high-level turbulence induced by two-stream instability of the electron beam. These experiments are carried out at the specialized GOL-3T device. For the plasma density range 10 $^{14}\hbox{–10}^{15}~\hbox{cm}^{-3}$ , we have studied emission dynamics from the plasma column in its transverse and longitudinal directions. The spectral composition and polarization of the radiation was also investigated.

A mathematical model of surface cracking under pulsed heat load was developed. The model correctly describes a smooth brittle–ductile transition. The elastic deformation is described in a thin-heated-layer approximation. The plastic deformation is described with the Hollomon equation. The time dependence of the deformation and stresses is described for one heating–cooling cycle for a material without initial plastic deformation. The model can be applied to tungsten manufactured according to ITER specifications. The model shows that the stability of stress-relieved tungsten deteriorates when the base temperature increases. This proved to be a result of the close ultimate tensile and yield strengths. For a heat load of arbitrary magnitude a stability criterion was obtained in the form of condition on the relation of the ultimate tensile and yield strengths.

The plasma system and diagnostics of the new facility GOL-3T are described. This facility is the final result of the first stage in the deep upgrade of the GOL-3 multiple-mirror system, which has operated at the Budker Institute of Nuclear Physics since 1988. The upgrade project supposes creation of two new independent facilities at the site of GOL-3. The GOL-3T facility is intended to study the physics of beam—plasma interaction and generation of subterahertz electromagnetic radiation during the collective relaxation of a high-power relativistic electron beam with a duration of 5–10 μs. Studies on the physics of multiple-mirror plasma confinement in axisymmetric magnetic systems will be continued in a new range of experiment parameters at the second facility, named GOL-NB.

New plans for next-step experiments on a multiple-mirror confinement in GOL-3 are discussed. The proposed changes in the hardware configuration include separation of the existing GOL-3 device into two independent plasma facilities. The first device will continue research on physics of highly turbulent electron-beam-heated plasma. It will use the existing generator of the electron beam and a shortened part of the existing solenoid. The second device will be devoted to a new experimental program on studies of efficiency of multiple-mirror end sections that should decrease power and particle losses from the trap. Details of the physics and upgrade plans for the new device, tentatively named GOL-NB, are discussed.

Heating and confinement of plasma in a multimirror magnetic configuration have been studied at the GOL-3 facility (Budker Institute of Nuclear Physics, Siberian Branch, Russian Academy of Sciences, Novosibirsk). The experiments are aimed at estimating the densities and charge states of the main impurities in the GOL-3 plasma and determining their contribution to the effective plasma charge. Plasma with a density of ∼1015 cm−3 was heated by a relativistic electron beam (1 MeV, 8 μs, ⩽200 kJ). At the end of electron beam injection, the plasma temperature reached 1 keV. The densities of impurities were determined using VUV and visible spectroscopy, as well as mass spectrometry of the residual vacuum. To determine the effective plasma charge, the experimental data were compared with the results of numerical simulations of the ionization balance of impurities. It is shown that the effective plasma charge calculated with allowance for the contributions from the main impurities does not exceed Zeff = 1.8, which cannot explain the experimentally observed improved confinement of low-density plasma.

This paper describes results of studies on application of high current (1–10 kA) relativistic electron beams to generate millimeter and subterahertz (sub-THz) radiation in two ways. The first one is realized at the ELMI-device where the scheme of a free electron maser (FEM) with a sheet electron beam is applied for the case of a planar two-beam-channel geometry. The second way is based on emission of sub-THz radiation from plasmas with turbulence induced by a strong relativistic electron beam. These experiments are carried out at the GOL-3 facility. We present experimental results obtained at these two devices and briefly describe their theoretical analysis.

A method for creation of a long magnetized column of dense hydrogen plasma in a metal chamber by means of a high-current linear discharge is considered. It is the main method for the formation of preliminary plasma in the GOL-3 multimirror trap, in which a plasma column with a length of up to 12 m and diameter of 8 cm, suitable for conducting experiments on the injection of a relativistic electron beam, was obtained. Conditions for stable discharge operation in the density range of 3 × 1019–1022 m−3 are determined, including a discharge with a uniform longitudinal plasma density profile and incomplete initial ionization of hydrogen. It is demonstrated that the system is capable of operating in a magnetic field with a variable configuration and strength of up to 6 T in the solenoidal section and up to 12 T in the end mirrors. It is shown that an important role in the development of a discharge is played by fast electrons with energies corresponding to the initial applied voltage (about 25 kV), which provide primary gas ionization. The properties of low-temperature plasma in such a discharge are discussed.

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 commissioning of two 25 keV, 0.75 MW neutral beams. Low-temperature start plasma was created by 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. Plasma parameters and operation regimes are reported.

The paper presents experimental investigations of tungsten surface modification after plasma loads at the multimirror trap GOL-3. Energy loads on tungsten surface varied from 2 up to 4 MJ/m2 per shot with sets from 1 to 9 repetitive irradiations that corresponds to loads higher than tungsten melting threshold and is close to ITER giant ELM loads. Targets surface modification and transverse microsections after irradiation was studied by optical microscopy, SEM and hardness tester. Formation on tungsten surface of three different crack networks with typical cell sizes of 1000, 10 and 0.3 μm and bubbles are identified. The network of large cracks extend perpendicularly to the irradiated sample surface to a depth of 50–350 μm. Erosion depth depends on energy loads – rises from 20 to 200 μm at 2 and 4 MJ/m2 correspondingly. Cracking, development of tungsten surface morphology and droplets splashing are discussed.

Development of a new linear device for confinement of fusion plasmas is under way in the Budker Institute of Nuclear Physics, Novosibirsk. The new device combines features of existing GOL-3 and GDT...

Sub-THz radiation can be generated by conversion of plasma waves into electromagnetic (EM) radiation in a plasma with strong Langmuir (LT) turbulence produced via a two-stream instability of a high current relativistic electron beam (REB). Nonlinear plasmon-plasmon merging results in the generation of photons nearby the 2nd harmonic of the plasma frequency 2ω p (“2ω p -process”). For plasma densities of 1014 − 1015 cm−3, these frequencies are in the range of sub-THz waves at 370–570 GHz. The specific power density of sub-THz-wave emission from plasmas in the multi-mirror magnetic trap GOL-3 (at BINP) during injection of a 10-μs-REB with a current density of about 1 kA/cm2 at plasma densities n e ≈ 5∙1014 cm−3, electron temperatures T e ≈ 1.5 keV and magnetic induction B ≈ 4 T was measured to be approx. 1 kW/cm3 in the frequency band around 300 GHz. In the case of a weakly relativistic 100-μs-electron beam (90 keV) with 250 A/cm2 the corresponding results are 700 W/cm3 around 90 GHz with an efficiency of 1–2 % at n e ≈ 3∙1013 cm−3 (total power ≈ 30 kW). Theoretical investigations show that at a density of n e ≈ 3∙1015 cm−3 and a turbulence level of 5 % the generated sub-THz power can reach ≈ 1 MW/cm3.

The MHD activity of plasma in the GOL-3 facility was studied experimentally. The complicated azimuthal structure of the profile of the longitudinal current is revealed by magnetic measurements and high-speed imaging. The azimuthal mode composition of magnetic perturbations, as well as the frequency of their rotation in different regimes, is determined. The signs of differential plasma rotation in the facility are described. Possible mechanisms responsible for the relation of differential rotation to current filamentation and magnetic reconnection in the GOL-3 plasma are proposed. These mechanisms explain the previously observed experimental evidences of these processes.

The design principles and construction of the subterahertz radiometric spectral systems developed for the GOL-3 facility are described. The spectral systems are designed according to the quasi-optical scheme and use multilayer filters based on frequency-selective surfaces. The design and manufacturing technology of such elements are discussed. The results of measuring subterahertz radiation of plasma at the frequency close to the double plasma frequency are presented.

A simple Michelson CO2 interferometer for measuring the high-temperature plasma density within one interference fringe (n e l = 1016 cm−2) at two points (0.8 and 9.0 m) of the long (L = 12 m) GOL-3 corrugated trap was used. A piezoelectric element that provides displacements of the mirror in the reference arm of the interferometer is used to calibrate the interferometer and perform remote control of the initial measurement phase. The interferometer is manufactured from dielectric materials, thus excluding a mechanical action of stray magnetic fields on its elements. The time resolution of the interferometer is determined by a HgCdTe diode and equals ∼1 ns. The sensitivity of the interferometer is ∼5 × 10−4 of a fringe (n e l = 2 × 1013 cm−2).

The paper summarizes recent advances in physics of multiple-mirror confinement. GOL-3 in Novosibirsk is the only existing large-scale device of this type. Achieved plasma parameters are: n ~ 1021 m...

The paper presents experimental investigations of plasma-surface interaction and materials behavior under plasma loads relevant to type I ITER ELMs. The experiments were performed with quasi-statio...

Investigation results for electromagnetic radiation emission induced by strong Langmuir turbulence driven by a microsecond relativistic electron beam are presented. The radiation is associated with...

The latest experimental campaign at the GOL-3 multiple-mirror trap was mainly aimed at features of heating and stability of the electron-beam-heated turbulent plasma. The discussed experiments feat...

Multimirror approach to plasma confinement for fusion is studied at GOL-3 facility in the Budker Institute of Nuclear Physics (Novosibirsk, Russia). The presented work is devoted to diagnostics of heavy impurities in plasma of this facility. The main purpose is quantitative measurement of density, degree of ionization of the main heavy impurities in plasma and their contribution to an effective charge of plasma at the GOL-3 facility. Density of interested elements was measured by optical spectroscopy and mass-spectroscopy of residual vacuum. A number of spectroscopic diagnostics has been developed for this purpose. Experimental data were compared with results of numerical calculations of impurities dynamics. As a result it is revealed that the contribution of heavy impurities to effective charge of plasma does not exceed 20%; thus the presence of impurities in plasma of the GOL-3 facility does not influence essentially the plasma confinement at the multimirror trap.

Results of experiments on injection of a sequence of several electron beam pulses (80–90 keV, 20–75 A, ∼30 μs) into 1019–1021 m−3 deuterium plasma are presented. The first beam pulse was injected into deuterium gas, the following pulses propagated through residual plasma that survived the inter-pulse periods. A typical pulse duration was of the order of 106 ω p − 1 that is much longer than most of the interaction-related processes. However, we noticed significant differences in plasma processes between the first pulse and the rest ones. A suggested physical factor behind those differences is longitudinal plasma current that develops during the first beam pulse.

The paper reviews recent experimental results from GOL-3. Currently efforts are focused on further development of a physical database for multiple-mirror confinement systems and also on an upgrade of plasma heating systems of GOL-3 device. In general, current GOL-3 parameters demonstrate good prospects of a multiple-mirror trap as a fusion reactor.

New operation regime with a high-mirror-ratio corrugation at first 2 meters of GOL-3 plasma was studied. Five corrugation periods of 44 cm length were created with mirror ratio R ≈ 2.7 by means of special configuration of a power feed of existing solenoid coils. First data on the plasma heating, confinement and features of neutron emission from the strongly corrugated plasma in the above conditions are presented.

First stage of neutral beam heating system was developed at the GOL-3 facility. Configuration of magnetic field and initial gas distribution were adopted for operation with NBI system. As a result the neutral beam with 0.5 MW power and 0.8 ms pulse duration was achieved and stable regimes suitable for beam injection experiments were found. The neutral beam system was used in first experiment as diagnostic tool for measuring the plasma density. Dynamics of plasma density and its radial distribution were studied in experiments.

Experiments involving irradiation of carbon targets with a hot electron plasma stream performed on the GOL-3 facility were aimed at the simulation of the effect of hot plasma of a thermonuclear reactor on plasma collectors in regimes close to those expected in the ITER. In these experiments, the emphasis was on the study of the surface-erosion processes exposed to the plasma stream. The graphite evaporation and chemical-erosion processes were studied with absolutely calibrated spectroscopic systems of the visible region. Spectroscopic experiments yielded data on the flow of carbon atoms from the surface of a plasma collector and on the contribution of chemical erosion to the surface-damage process. The obtained values are compared with the results of direct measurements of the erosion depth and the results of numerical calculations.

The processes of plasma wave transformation into electromagnetic radiation during strong interaction of a relativistic electron beam with magnetized plasma are studied in BINP in collaboration with NSU. Since 2010, experimental studies on such transformation processes were carried out on the GOL-3 facility by measuring the radiation emitted from beam-plasma system. Then after 2014 the GOL-PET device is used for these studies. Results of these experiments on emission in the frequency interval 0.1-0.5 THz from a long plasma column due to E-beam injection are described in the presented paper. The experimental results have been compared with theoretical ones obtained for EM-wave generation by the beam-plasma system.

Results from different facilities for plasma-surface interaction (PSI) studies are compared and their relevance to each other and conditions of ITER transient events are discussed. For PSI studies different types of devices were used including open traps, tokamaks, linear plasma machines including quasi-stationary plasma accelerators, electron beams and lasers. The same damage thresholds were demonstrated with different techniques. At energy loads corresponding to ITER transient events (edge localized modes (ELMs), disruptions, etc.) only macroscopic erosion mechanisms are important. They are cracking, droplet formation and macroscopic mass losses of the target. Cracking occurs during cooling stage after irradiations. Details of plasma stream parameters are of secondary importance for cracks formation; only target temperature plays a role. Macroscopic mass losses strongly correspond with melting of a thin surface layer and heat loads play a key role for melting. Heat flux defines the targets surface erosion.

Technical solutions of the concept exploration device SMOLA for longitudinal plasma flow suppression in linear plasma trap by active plasma pumping in helicoidal magnetic field are described in the article. Vacuum system, magnetic system, plasma source and the source of the radial electric field are discussed.