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Melting of the surface of tungsten exposed to a pulsed electron beam has been simulated numerically. Comparison of the experimentally measured at BETA facility time dependence of the radius of the molten region with the calculated data has shown that the surface cooling caused by evaporation has a significant effect on the temperature distribution and melting of the material at sufficiently high densities of the surface heating power. This result validates the created theoretical model of the tungsten melting and evaporation in exposure to a pulsed electron beam. The studied mechanism of the limitation of the surface temperature is different from the well-studied vapor shielding. The presented model is a step to correct interpretation of the erosion caused by the melt motion and splashing in exposure to the ITER-relevant pulsed heating by electron beam.

A number of experimental studies of melt motion and droplet ejection caused by pulsed plasma load include the measurements of the shape of surface after the solidification of target. The measured shape may be different from the one during the heating stage because of melt motion. In present paper the evolution of this perturbations is treated as capillary waves on the melt surface. The dispersion relation for capillary waves taking into account viscosity and limited depth of liquid was used. The numerical estimations for the melt surface behavior are done for tungsten samples irradiated at BETA facility.

Experimental research on behavior of rolled tungsten plates under intense transient heat loads generated by a powerful (a total power of up to 7 MW) long-pulse (0.1–0.3 ms) electron beam with full irradiation area of 2 cm2 was carried out. Imaging of the sample by the fast CCD cameras in the NIR range and with illumination by the 532 nm continuous-wave laser was applied for in-situ surface diagnostics during exposure. In these experiments tungsten plates were exposed to heat loads 0.5–1 MJ/m2 with a heat flux factor (Fhf) close to and above the melting threshold of tungsten at initial room temperature. Crack formation and crack propagation under the surface layer were observed during multiple exposures. Overheated areas with excessive temperature over surrounding surface of about 500 K were found on severely damaged samples more than 5 ms after beam ending. The application of laser illumination enables to detect areas of intense tungsten melting near crack edges and crack intersections.

A new test facility for experimental simulation of transient heat load expected in the ITER divertor during unmitigated events is developed. Application of a long-pulse (0.1–0.3 ms) wide-aperture (up to 2 cm 2 ) electron beam as a heating device provides powerful energy loads at a tungsten target with F HF > 250 MJ m −2 s −0.5 . Dynamics of tungsten particles in the ablation plume is investigated with a novel for PSI experiments small-angle laser light scattering technique. The threshold of intense droplet generation and dynamics of particles sizes are estimated.

This paper presents a model for calculating deformations and mechanical stresses around a crack normal to surface that appeared under pulsed heat load. The model was applied to calculation of stresses that may lead to formation of cracks along the surface, which are observed when tungsten is exposed to ITER-relevant heat load. It was found that such stresses might be not negligibly small in comparison with the ultimate tensile strength, and thus the appearance of cracks normal to the surface may leads to development of cracks parallel to the surface. The calculated deformation of the region around a crack is in good agreement with the experimental data. The deformations calculated can be a basis for experimental detection of formation of cracks normal and parallel to the surface. Keywords: Pulsed heat load, Tungsten, Cracking, Theoretical simulation, Elasticity, Electron beam, Profilometry

Damage of divertor tungsten tiles as a result of transient plasma processes is one of the key problems of the ITER tokamak. Even if the temperature of the divertor tiles is maintained below the melting threshold, their thermal deformation, accumulation of residual stresses and cracking will occur. Cracking reduces the resistance of the metal to subsequent loads, therefore, the study of this process is important. In materials science, non-destructive methods for measuring mechanical stresses are very limited. In this work, a new method is proposed that allows non-contact investigation of stresses in polycrystalline tungsten plates by their bending. A corresponding diagnostic system and theoretical model are implemented. A number of experiments were carried out using this diagnostic system at the BETA facility at the Budker Institute to study the dynamics of tungsten plates bending with different thickness, subjected to pulsed heating of different intensities. The experimental results showed the qualitatively expected process of the plate bending, as well as good agreement with the theoretical model. This proved that the method described in the work is promising for use in order to study the residual stresses in a pulse-heated metal sample.

Experiments on the effect of fast heat loads on the surface of tungsten were carried out on the BETA facility at the Budker Institute. Tungsten samples were uniformly heated by an electron beam with a heat flux factor below the melting threshold. During and shortly after exposure, the 2D surface temperature distribution was measured, as well as the temperature history on selected surface areas. Active diagnostics using the scattering of CW laser light on a surface exposed by the electron beam allowed us to monitor the damage dynamics. At the heating stage, an increase in the surface roughness occurred, caused by inhomogeneous elastic and plastic deformations of the heated layer. As the cooling progressed, the residual plastic deformations remained. Simultaneously with the modification of the surface, bending of samples with a thickness of 3-4 mm occurred. The bending dynamics of the sample was measured by the intensity of a converging laser beam reflected from the back surface of the sample, polished to a mirror state. The residual sag due to bending increases with the heat load similarly as residual roughness of the front surface of the sample. These data, together with simultaneously measured temperature dynamics and the spatial heating profile, can provide an experimental basis for the numerical calculation of the residual stresses in the sample. The data obtained in situ were compared with those measured outside the vacuum chamber with X-ray diffraction, optical profiler, and optical interferometer. At the stage of cooling, after a sufficient intensity of heating, the second stage of damage took place — the cracking of the surface layer. The time before the start of this relatively fast process usually exceeded the time to achieve a DBTT by 1–4 orders of magnitude.

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 dynamics of the diffraction peak shape during pulsed heat load on mosaic single-crystal tungsten were measured at the `Plasma' scattering station on the eighth beamline of the VEPP-4 synchrotron radiation source at the Budker Institute of Nuclear Physics. The observed evolution of the diffraction peak shape agrees with theoretical predictions based on calculations of deformation caused by pulsed heating. Three clearly distinguishable stages of the diffraction-peak evolution were found, correlating with the evolution of temperature and deformation distributions. The residual plastic deformation increased with subsequent heating pulses.

Hihg-power flows of plasma cause mechanical destruction of materials via non-uniform deformation and stresses. The possibility of synchrotron radiation scattering diagnostics of deformation is discussed. The experiments at the new scattering station “Plasma” on VEPP-4M demonstrated the measurements of change in the diffraction angle during pulsed heating.

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.

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.

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 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.

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...

Surface melting of tungsten under exposure to a pulsed electron beam was simulated numerically, the evaporation process taken into account. The calculation is based on the experimental time dependence of the total beam power. The model of the tungsten heating process is based on solving the two-phase Stefan problem. The position of the phase boundary depends on discontinuous time- and space-nonlinear coefficients and boundary conditions. The aim of the study is to provide a detailed resolution of the heat flow deep into the material with a fine spatial grid step. As compared with the size of the tungsten plate, the heating depth is very small. The problem statement under consideration is multiscale. Further expansion of the model involves taking into account microcracks. Micro-cracks occur during the cooling process after exposure and affect the temperature of the tungsten surface during the subsequent heating process. The article presents a modeling of cracks of different geometries typical for this process. The results of the calculations correlate with the experimental data obtained on the experimental test facility BETA at BINP SB RAS.

This paper describes the interferometer systems on the Large Helical Device (LHD). LHD is equipped with five interferometer systems, each of which has a different operational purpose and me...

Particle confinement processes are studied in detail on the Large Helical Device (LHD). Diffusion coefficients (D) and convection velocities (V) are estimated from density modulation experiments. T...

The Large Helical Device (LHD) is the world’s largest heliotron-type device with l = 2, m = 10 continuous superconducting helical coils and three pairs of superconducting poloidal coils. The major ...

A novel BETA test facility (Beam of Electrons for materials Test Applications) was developed at the Budker Institute to study the erosion of materials directly during the impact of intense thermal shocks. A powerful (up to 7 MW) long pulse (100–300 μs) electron beam is applied for experimental simulation of fast transient heat loads with the intensity probable in the ITER divertor. The heat flux parameter on a target can be widely varied (FHF = 10–300 MW m−2 s0.5) from a value significantly below the melting threshold to a value much higher, within the area of about 1 cm2. The use of an electron beam to simulate the thermal impact on the material surface makes it possible to employ a variety of optical diagnostics for in situ observations of the dynamics of surface erosion processes during intense thermal shocks. These distinctive features make BETA a promising tool in the research of material surface erosion mechanisms and for experimental verification of various analytical and numerical models associated with these mechanisms. The first results obtained with this facility include fast (10 μs exposure) imaging of the heated target in the near-infrared range and in the reflected light of 532 nm continuous wave (CW) laser, visualization of ejected tungsten particles using fast ICCD and CCD cameras with the minimal exposure of 2 μs and 7 μs respectively. The dynamics of dust particles ejected from the heated surface is investigated using a multichannel recording of the light of 532 nm CW-laser scattered on the dust particles. The present paper describes the first results of use of two new in situ methods: continuous recording of light scattered from the tungsten surface and three-dimensional tracking of tungsten particles using three viewing angles. The first method makes it possible to observe the dynamics of development of roughness and cracking of the polished tungsten surface, which manifest themselves as two successive processes separated by a large time delay. The second method allows us to track dust particles ejected from the melt layer, and thus determine the time and place of particle ejection from the surface.

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.

A test facility developed at the Budker Institute is designed for experimental simulation of effects of ITER-scale transient heat loads on plasma facing materials. Employing a long-pulse electron beam for creation of transients has advantages of low direct pressures on surface of material and lesser limitation of power density by vapor shielding. In addition, relative modest attendant background light allows using a set of diagnostics for observation of erosion dynamics. The set includes fast imaging in the near infrared, scattering of light of continuous wave laser on target surface and on droplets ejected from the melt layer.

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.

This article describes the diagnostic complex for investigation of plasma electromagnetic emission during beam-plasma interaction. Multichannel polychromator is used for measurements of spectral power density in frequency band of 100 ÷ 500 GHz. Two-channel polarimeter is used for obtaining information about emission polarization. The new cryogenic detector will be used for measurements of electromagnetic radiation over 600 GHz. Calorimetric detectors can also be used to increase the number of possible spatial registration points. Because experiments data interpretation depend on local plasma parameters such as plasma density or temperature diagnostic, complex include system of registration of density based on CO2-interferometry and Nd-laser scattering. The information about plasma temperature is obtained from diamagnetic loops.

Wide-area (2 cm2) high-power (up to 7 MW) submillisecond electron beam source was applied for generation of intense pulsed heat loads on tungsten sample. Various diagnostics sets were used for in-situ research of the surface damage: IR imaging of the front view of the target, capturing of the reflection pattern of continuous wave laser radiation, recording of intensity of thermal radiation from several spots of the surface. Sample was exposed to a various heat loads near and above melting threshold. Formation of the crack network on the surface and its development with successive pulses was observed. Two-dimensional temperature distribution was obtained after heating at the cooling stage. Melting and spalling of material were shown on severe damaged target. Propagation of the cracks along the surface at a depth of 0.1 – 0.2 mm was revealed with transverse microsections. Circular motion of the molten layer was found with subsequent exposures.

The paper presents an overview of plasma-material interaction studies at the Siberian Synchrotron and Terahertz Radiation Centre. The measurements of recrystallization and surface texturing are demonstrated. The growth of grain size after the exposure by electron beam was detected using two-dimensional diffractometry. The orientation of crystal structure was detected in the tungsten irradiated by deuterium plasma. The residual stresses in irradiated tungsten were measured. The predicted structure of deformations after pulsed heat load is confirmed. First in-situ experiments confirmed the presence of crystal plane rotation effect during a pulsed heat load.

A test facility for experimental simulation of transient heat load expected for ELMs type I events in ITER is developed at BINP SB RAS. Dynamics of tungsten particles in the ablation plume is investigated by small-angle light scattering technique and using fast CCD and ICCD cameras. The threshold of intense particle generation, sizes and velocities of particles ejected from the surface are estimated.

A test facility for experimental simulation of transient heat loads in ITER divertor with the use of high power electron beam is developed at the Budker Institute of Nuclear Physics. This report presents an experimental study of the absorption of the electron beam with an incident heat power flux of 10-50 GW/m2 on the area of about 2 cm2 of a tungsten target. The electron beam has duration of 0.1-0.3 ms and electron energy of 80-95 keV, Diagnostics for measuring of the beam parameters on the target are briefly discussed. Results of measurement of the beam profile and calorimetry of beam energy deposited in tungsten sample at ELM-like heat load are presented.

Electron beam appears to be a good instrument for the simulation of heat loads because of low pressure to material and large depth of shielding layer, allowing for the application of higher loads. This work is focused on theoretical and numerical modeling of heating tungsten by plasma flow and electron beam heating to clarify differences in the heating process. The time necessary for vapor shielding development is found for both of them. Essential time differences between electron beams with various energies were found. It was explained by significant cooling by evaporation at one of the heating stages. The surface temperature is almost stabilized and the maximal temperature location shifts under the surface at this stage.

The one-dimensional numerical simulations of the deformation during and after the pulsed heat load are carried out for tungsten manufactured according to ITER specifications. The calculations for stress-relieved tungsten demonstrated the cracking at temperatures above ductile-to-brittle transition temperature. The features of the stresses caused by thermal expansion are discussed.

The characteristics of particle transport in three different magnetic configurations are studied from density modulation experiments in the Large Helical Device (LHD). These three configurations are represented as different magnetic axis positions (R ax ) of the vacuum field. Experiments were carried out in a range of different heating powers for each configuration with almost constant density. The experimental values of particle diffusion coefficients (D) and particle convection velocities (V) are compared with neoclassical estimates. The value of D is found to be anomalously large compared to neoclassical values in both the core and edge in all configurations. At low collisionality, this anomaly tends downward. The core convection velocities are comparable with neoclassical estimates. In more-outward-shifted configurations, particle transport is enhanced. The electron temperature and electron temperature gradient are the determinate parameters for D and V, respectively, in each configuration. The effective helical ripple is one of the important parameters for particle transport in the LHD; however, other hidden parameters exist. The role of fluctuations in particle transport is investigated from turbulence measurements using a two-dimensional phase contrast interferometer. Three kinds of fluctuation having different locations, propagation direction, and peak wave number are observed. One of these, which exists in the outermost edge region and propagates in the ion diamagnetic direction in the laboratory frame, plays a possible role in edge anomalous diffusion. The amplitudes of ion diamagnetic fluctuation components are compared with the linear growth rate of the ion temperature gradient mode.

Electromagnetic emission from plasma in the vicinity of the fundamental plasma frequency and at its double value during a relativistic electron beam (REB) relaxation is studied at the GOL-3 facility [1]. In our paper we will present results of experiments carried out on the specialized GOL-PET device for the emission of terahertz radiation from plasma at its density ∼ 1015 cm−3 and the magnetic field strength 2–4 T. The plasma column created by a high-voltage discharge in the GOL-PET device has its diameter 6 cm and the length about of 250 cm. The injected electron beam has the following parameters: ∼0.8 MeV, ∼20 kA, ∼10 μs. The beam current density in the plasma is up to 3kA/cm2. The radiation in the frequency interval from 0.1 up to 0.8 THz is analyzed by fast eight-channel polychromator and polarimeter based on frequency selection quasioptical elements.

Two modifications of imaging interferometry: heterodyne (HI) and phase contrast interferometers (PCI) are designed for observation of plasma density profiles and density fluctuations respectively. Besides, spatial distributions of plasma velocities, velocities fluctuations and related electrical fields can be obtained from the analysis of HI and PCI data. New sensitive phase counters, developed at Budker Institute of Nuclear Physics, enable HI to include some capabilities of the PCI. In addition to well recognized transversal spatial resolution of imaging technique, progress in deconvolution of line-of-sight-integrated data was recently made. Computer simulation, bench-test experiments and recent experimental results from the Large Helical Device illustrate the potentials of the imaging interferometry for investigation of plasma. Application of the imaging interferometry with spatial resolution along the viewing line to mirror machines is finally considered.

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 1054 nm 20 J 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 ∼100 ns time scale.

First experimental results on microwave radiation at double plasma frequency from plasma heated by ~100 keV, 2-10 MW electron beam of ~100 μs duration in GOL-3 are presented. Measurements were done...

A multichannel imaging heterodyne interferometer is installed on Large Helical Device. Two branches of the interferometer: one with CO2 laser (wavelength λi=10.6 μm) and other with diode pumped yttritium–aluminium–garnet (YAG) laser (wavelength λi=1.06 μm) are used for electron density measurements and vibration compensation. Two slab beams and one circular beam are injected vertically. Liquid nitrogen cooled linear detector arrays are used for detection of slab beams and room temperature detector is employed for single circular beam for CO2 beams. Avalanche photodiodes record signals for YAG beams. Chordal resolution is from 15 to 22.5 mm, phase resolution is 10−3 of CO2 fringe, which is determined by the electrical noise of phase counter. About 5×10−3∼10−2 of CO2 fringe uncompensated signal remains after vibration compensation. Combining existing 13-channels far infrared (wavelength λi=119 μm) laser interferometer, profile measurements at high density (>1020 m−3) become possible.

An incoherent Thomson scattering system for observing local unsteady cavities that form in a turbulent plasma in the course of Langmuir collapse is described. Using this diagnostics, the density cavities in a laboratory plasma with developed strong Langmuir turbulence were directly observed for the first time and their spatial and temporal characteristics were determined.

A CO2 laser (λ=10.6 μm) imaging interferometer was developed for detailed and precise electron density profile measurements and was installed on the large helical device in 2001. A CO2 laser is adopted to avoid fringe loss due to the short wavelength. An imaging scheme is employed that uses three slab-like beams and multichannel detector arrays to measure with fine spatial resolution. As an initial trial we used one slab-like beam that passes through the plasma edge. Phase distributions were measured without fringe loss even after pellet injection. Density profiles can be obtained by the slice and stack method with a spatial resolution of 16 mm (about 3% of the plasma minor radius). The electron density was found to rise to about 6×1019 m−3 just after pellet injection even near the last closed flux surface. Mechanical vibration effects would be negligible in such a case. A bench top experiment of a coaxial YAG laser (λ=1.06 μm) interferometer for vibration compensation was carried out.

A phase contrast interferometer employing a CO_2 laser (wavelength lambda_i = 10.6 ?m) is designed and installed in order to study density fluctuations on a large helical device. A 250×50 mm slab beam passes the edge of the plasma, where rho= r/a>0.65, and provides observations of edge density fluctuations. A spatial image of the integrated fluctuations is measured by a multichannel detector array, of which the effective spacing is 5 mm in the plasma. Measured wave number components are dominated by radial components, and within the range of 7.2×10^?2

Experiments on strong Langmuir turbulence (LT) driven by electron beam are reported. The technique of cold high-current relativistic electron beam (REB) permits to set up experimental conditions that are practically important but difficult for theoretical treatment of LT. These conditions include strong kinetic effects of plasma non-Maxwellian electrons, ion-acoustic oscillations which are weakly damped due to plasma non-isothermality and dispersion of Langmuir waves that are considerably modified by external magnetic field. A relatively dense plasma permits the use of the Thomson scattering method for observation of spectra of plasma fluctuations, electron distribution function, and local dynamics of plasma density. LT is studied in two operating modes that are characterized by moderate and increased current of REB. The experimental results with moderate REB current do not support the widely accepted picture when most of the Langmuir oscillations are trapped in density cavities. The energy flow through turbulence to plasma electrons is explained without major contribution of fully developed collapse, whereas, with increased REB current dynamic density cavities of spatial scale much less than the size of turbulent region are directly observed.

The regime of strong Langmuir turbulence characterized by the plasma nonisothermality and by the presence of an appreciable non-Maxwellian hot-electron component was experimentally studied. Turbulence was excited in the preliminary produced plasma by the relativistic electron beam. Thomson scattering of laser IR radiation served as the main diagnostic method. The spatial spectra of the Langmuir turbulence and of the attendant ion-sound turbulence were studied using Thomson collective scattering. Thomson incoherent scattering was used for studying the plasma electron distribution function and searching for the local dips of plasma density. Stark spectroscopy of turbulent microfields and the method of observation of plasma radiation at the double plasma frequency were also used. Based on the experimental data, the mechanism of Langmuir oscillation damping by plasma electrons was analyzed. The Langmuir wave conversion induced by the ion-sound turbulence is the most probable channel for energy transfer from the turbulence to plasma electrons, the low-frequency fluctuations being the direct consequence of the strong Langmuir turbulence.

Surveys of the ion and electron heat transports of neutral beam (NB) heating plasma were carried out by power balance analysis in He and H rich plasma at LHD. Collisionality was scanned by changing density and heating power. The characteristics of the transport vary depending on collisionality. In low collisionality, with low density and high heating power, an ion internal transport barrier (ITB) was formed. The ion heat conductivity (χi) is lower than electron heat conductivity (χe) in the core region at ρ

Thomson scattering diagnostics has been updated to allow measurements at two instants of time for either of two spatial points in plasma during a single plasma discharge. Laser produces now two 15J...

A CO2 laser (λ=10.6 μm) imaging interferometer is designed for electron density profile and electron density fluctuation measurements on large helical device (LHD). The purposes of this diagnostic are reliable electron density monitor in high density operation (especially in pellet injection discharge), precise measurement of electron density profiles and observations of density fluctuations. By using a CO2 laser, refraction effect becomes negligible, and the interferometer promises to be free from fringe jumps at high density. We plan to get 5 mm spatial resolution, (56 ch for each of two 280 mm slab beams, 50 ch for one 250 mm and 16 ch for 80 mm crossed beam), 3×10−3 rad phase resolution and 200 kHz frequency response for the above purpose. And fluctuation will be studied within the wave number range of 7×10−3⩽k⩽0.6 mm−1.