Multimirror Trap GOL-3 Research Articles

Experimental and Theoretical Investigations of High Power Sub-Millimeter Wave Emission at Two-Stream Instability of High-Current REB

Sub-mm radiation can be generated by conversion of plasma waves into electromagnetic (EM) radiation at strong Langmuir turbulence (LT) via the two-stream instability induced by a high current relativistic electron beam (REB). A plasmon scattering on plasma density fluctuations produces EM emission at the plasma frequency ωp (“ωp process”). Nonlinear plasmon-plasmon merging results in the generation of photons nearby the 2nd harmonic of the plasma frequency 2ωp (2ωp process”). For plasma densities 1020-1021 m-3, these frequencies are in the range of sub-mm waves: 180-566 GHz. The power density of sub-mm-wave emission from plasmas in the multi-mirror trap GOL-3 (BINP) during injection of a 10-μs-REB at plasma densities ne ≈ (1-5)·1020 m-3, electron temperatures Te ≈ 1-3 keV and magnetic field B ≈ 4 T was measured to be up to 1 kW/cm3 in the frequency band above 100 GHz.To calculate the second harmonic emission power from turbulent magnetized plasma we use the model of coalescence of two upper-hybrid waves. Results of these calculations and measured power are in good coincidence in the investigated area of plasma density.

Generation of High-Power Sub-THz Waves in Magnetized Turbulent Electron Beam Plasmas

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 2ωp (“2ωp-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 ne ≈ 5∙1014 cm−3, electron temperatures Te ≈ 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 ne ≈ 3∙1013 cm−3 (total power ≈ 30 kW). Theoretical investigations show that at a density of ne ≈ 3∙1015 cm−3 and a turbulence level of 5 % the generated sub-THz power can reach ≈ 1 MW/cm3.

Surface modification and droplet formation of tungsten under hot plasma irradiation at the GOL-3

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/m 2 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/m 2 correspondingly. Cracking, development of tungsten surface morphology and droplets splashing are discussed.

Two-pulse Thomson scattering system for measurements of fast fluctuations of electron density in multimirror trap GOL-3

Two-pulse modification of the Thomson scattering system that is used at GOL-3 for studies of fast large-amplitude local density fluctuations is described. Two 1054nm 20J pulses are generated with two master oscillators that share the same power amplification chain. First experiments indicate existence of shots with a significant change of electron density at adjacent radial locations at ∼100ns time scale.

Structure Modification of Different Graphite and Glassy Carbon Surfaces under High Power Action by Hydrogen Plasma

The studies of exposing of targets by hot-electron plasma exhaust from the multi-mirror trap GOL-3 are carried out. The results of Raman and SEM studies of surface modification of carbon materials with different ordering range of sp2 fraction (different graphite and glassy carbon) exposed by pulsed hot hydrogen plasma with energy density up to 8 MJ/m2 are presented. It is shown that on the depth up to ~ 80 nm due to this exposing there are obtained two effects – micro structural destruction of the surface and extension of crystallite size of sp2 phase, i.e. ordering of nanostructure.

Physical Basis for the Use of Long Pulse Electron Beam in Multi-Mirror Trap

Recent high plasma parameters were reached in experiments on plasma heating in the multi-mirror open trap GOL-3. In these experiments deuterium plasma with density 1014–1016 cm-3 in the 12-m trap with corrugated magnetic field of 4.8T/3.2T was heated by a relativistic electron beam of eU £ 0.8 MeV, I £ 20 kA, j ~ 1–2 kA/cm2, pulse duration ~ 9 ¼s and angular spread £ 0.2 rad. The electron temperature Te ~ 1-4 keV and ion one to Ti ~ 1-2 keV were reached. After the beam pulse the electron temperature of the plasma quickly (~20 ¼s) decreased to ~100 eV. In turn, this leads to an increase in the rate of cooling of ions through the ion-electron collisions, which, together with particle losses determines the energy confinement time of plasma as 0.5-1 ms.To increase the plasma parameters a prolonged heating of the plasma by the electron beam is proposed. The paper considers basic physical phenomena in the beam-plasma interaction and required parameters of the long pulse beam. A choice of long pulse beam parameters is based on the obtained experimental results and scalings. Then estimates of the expected plasma parameters under the influence of the new beam have been done.

Plasma-Surface Interaction during ITER Type 1 ELMs: Comparison of Simulation with QSPA KH-50 and the GOL-3 Facilities

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-stationary plasma accelerator QSPA Kh-50 and multi-mirror trap GOL-3 devices located in Kharkov (Ukraine) and Novosibirsk (Russia) respectively. QSPA generated repetitive plasma streams of duration 0.25 ms and the energy density up to 2.5 MJ/m2. In GOL-3 multi-mirror trap plasma was heated up to temperature of 2-4 keV by a high power relativistic electron beam. Energy density in the exhaust plasma stream vary from 0.5 to 30 MJ/m2. Surface patterns of the targets exposed by QSPA and GOL-3 plasma are analyzed. Cracking, development of tungsten surface morphology and droplets splashing are discussed. It is shown that under an applied energy density loads (>1 MJ/m2) the evolution of surface morphology due to plasma irradiation are similar for two devices in spite of the qualitative differences of particles energy of the impact plasma streams. Formation of three different crack networks with typical cell sizes of 1000, 10 and 0.3 μm are identified after irradiation of tungsten surface. Experiments show that major cracks (cell size of 1000 μm) are attributed to a ductile-to-brittle transition. The key role of heat loads magnitude on development of surface due to powerful plasma impacts is demonstrated.

Multichannel Radiometric System for Registering Submillimeter-Wave Radiation at Beam-Plasma Interaction

We describe a 4-channel radiometric system developed for registering pulses of submillimeter-wave radiation in the frequency band 210-530 GHz. The system employs a quasi-optical scheme with polarization demultiplexed channels, wherein subsequent frequency filtering and spectral signal detection are provided by quasi-optical bandpass filters and high-speed Schottky detectors respectively. The system has frequency resolution of the channels ~30-50 GHz and response time better than 2 ns and destined for registering emission of electromagnetic radiation in the vicinity of a double plasma frequency, which is produced at turbulent heating of dense plasma in the multimirror magnetic trap GOL-3 by a powerful relativistic electron beam of 10 microsecond duration.

First Experiments on Neutral Injection in Multimirror Trap GOL-3

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.

Use of Pellet Injection Technology at GOL-3 for Plasma Fueling and Plasma-Surface Interaction Research

Two separate topics on a pellet injection activity at the multimirror trap GOL-3 are presented. The first topic is connected with studies of influence of a high-energy-density plasma on solids. A carbon pellet of 2-mm diameter was injected into the plasma for this purpose. The second discussed topic is a status of works on a cryogenic pellet injector for plasma fueling at GOL-3.

Anomalous Fast Heating of Ions in GOL-3 Facility

Phenomenon of fast heating of plasma ions in the multimirror trap GOL-3 is discovered and explained. An essence of effect is that ion component of the plasma is heated during electron beam injection much faster (at 100-1000 times) than due to binary electron-ion collisions.An analysis of experimental results is presented and discussed. A model of fast non-Coulomb ion heating is suggested.

Detection of Fusion Neutrons on the Multimirror Trap GOL-3

Recently GOL-3 has been reconfigured to a multimirror trap with improved confinement and high ion temperature. A dense plasma is created with a life time in the millisecond range. BTI neutron bubble detectors, a stilbene scintillation crystal, a BC501A liquid scintillator, and a silver-activation counter have been used for measurements of the neutron emission from GOL-3. The results are in agreement with charge-exchange (CX), spectral broadening of the Dα line, and diamagnetic measurements.

Role of q Profile for Plasma Confinement in the Multimirror Trap GOL-3

Role of q Profile for Plasma Confinement in the Multimirror Trap GOL-3

Application of Thomson Scattering System at 1.06 μm for Study of Plasma Density Dynamics at Multimirror Trap GOL-3

Thomson scattering system for measurements of radial profile of plasma density (range 0.5-5×1021 m-3) with temperature up to 2 keV was developed at the GOL-3 facility. First harmonics (λ=1.06 μm) of Nd glass laser is used. Scattered light from different points of plasma cross-section is imaged to a set of quartz optical fibers and detected by avalanche photodiodes.During the first 10 μs after start of the relativistic electron beam injection the intense light emission from plasma is observed. Single powerful laser pulse is used for providing of good signal-noise ratio in this period. Later the plasma radiation intensity decreases and the less powerful laser oscillator operated in multiple-pulsed regime is used.Description of the diagnostics, methodical aspects of operation, and results of the density dynamics measurements are presented in the paper.

Role of q Profile for Plasma Confinement in the Multimirror Trap GOL-3

Dense plasma heating by a relativistic electron beam and its confinement are studied in the multiple mirror trap GOL-3. Axial currents, which exist in the system, cause helical structure of the magnetic field. The safety factor q is shown to be below unity on the axis. Experimental data on the distribution and evolution of currents, structure of the magnetic field, and their influence on confinement and on MHD activity are discussed.

Study of Charge-Exchange Neutrals Emission from Hot Plasma at the Multimirror Trap GOL-3

Tentative experiments on registration of the energy spectrum of fast charge exchange (CX) neutrals emitted from the high-density hot plasma of the GOL-3 facility were carried out. Experimental data provided by used 5-channel CX neutrals analyzer are presented and the procedure of determining of the energy distribution of registered CX neutrals is discussed. From calculated data of the neutrals energy distribution the estimated temperature is 1.5 ± 0.5 keV.

Investigation of Impurity Dynamics at GOL-3 Facility

Dynamics of light impurities at multimirror trap GOL-3 was studied using imaging spectroscopy diagnostics in VUV and visible spectral range. The results of impurity balance and transport simulation as well as comparison with measurements are presented in this paper. Concentration of impurities in plasma and diffusion coefficient are found out from the measurements. Suitability using VUV and XUV spectroscopy for measuring main plasma parameters is considered.

Progress on the Multimirror Trap GOL-3

Main results of researches on plasma heating and confinement of dense plasma in the multimirror trap GOL-3 are presented.Recently magnetic system of the facility was converted into completely multimirror one. This results in further improvement of energy confinement time of plasma with ion temperature ~1 keV. Collective plasma heating by ~120 kJ (~8 ɷs) relativistic electron beam results in Te ~ 2 keV at ~1021 m-3 density. High Te exists for ~10 μs. To this time Ti reaches ~2 keV. Ion temperature keeps at the high level during ~1 ms. The energy confinement time sufficiently increases and a value of nτE = (1.5 [divide] 3)·1018 m-3s.

Multimirror open Trap Gol-3: Recent Results

Review of the experimental results for the last two years on study of dense plasma heating and confinement in a long multimirror trap GOL-3 is presented. This facility is an open trap for confinement of hot (0.1-1 keV) dense (1015-1016 cm−3) plasma. The plasma heating is provided by a high-power electron beam (1 MeV, 30 kA, 8 μs) with energy content of up to 200 kJ. The upgrade to full-scale corrugation of a magnetic field was completed at the facility during last two years. In the 12-meter solenoid the multimirror sections of 4-m-length were made at the both ends of the solenoid (Bmax/Bmin = 5,2 / 3,2 T, cell length is 22 cm). The modified source of preliminary plasma was put in operation for improvement of macroscopically stable beam transport through the plasma column. New diagnostics were developed for the experiments. Search of optimal conditions for confinement of plasma with ~1015 cm−3 density and high ion temperature, and also for macroscopically stable system “electron beam - plasma” was carried out in the new configuration of facility. As a result of the experiments the plasma with density of (1-2)·1015 cm−3, neTe+niTi =(0.5-2)·1015 keV/cm3 and confinement time of 100-200 microseconds in a multimirror trap is obtained. The observations of high ion temperature and mechanism of ion heating is discussed in the paper.