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en wave plasma heating, turbulent transport suppression, H-like mode, wall conditioning, compact antenna for Alfv´ en heating. In the Uragan-3M (U-3M) and Uragan-2M (U-2M) torsatrons possibilities and prospects of Alfv´ en method utilization for wall conditioning, plasma production and heating are studied. In U-3M the effect of fast ion loss on H-like mode formation is investigated. In U-2M the wall conditioning associated with the chemical reactivity of the atomic hydrogen to create volatile substances is used. A compact four-strap antenna is proposed for

The method for measuring a material eroded from surfaces of inner elements subjected to strong plasma impact in a fusion reactor and falling down to the bottom of a vacuum chamber in the form of microparticles was suggested and approved. An open ultra high frequency (UHF) resonator fabricated of two spherical mirrors was used as a measuring tool. The working frequency of 37 GHz was sweeped linearly in the frequency range of 60 MHz. Single particles of a substance (teflon, paraffin, graphite) or fine-dispersed graphite dust falling free down through the central part of the resonator caused a decrease of the resonance amplitude and shift of its eigenfrequency. Experiments with controlled properties (specific density, size, and permittivity) of substances, such as teflon or paraffin, showed that using such a resonator these characteristics can be measured with a reasonable accuracy. The effect of external magnetic field and background plasma can be taken into account.

The results of local measurements of RF discharge plasma parameters in the process of internal transport barriers (ITB) formation in the vicinity of rational magnetic surfaces in the Uragan-3M torsatron are presented. The following phenomena were observed in the process of ITB formation: widening of the radial density distribution, formation of plateaus on radial density and electron temperature distributions, formation of regions with high shear of poloidal plasma rotation velocity and radial electric field in the vicinity of stochastic layers of magnetic field lines, decrease of density fluctuations and their radial correlation length, decorrelation of density fluctuations, and increase of the bootstrap current. After the ITB formation, the transition to the improved plasma confinement regime takes place. The transition moves to the beginning of the discharge with the increase of heating power. The possible mechanism of ITB formation near rational surfaces is discussed.

As an alternative to metal-made inner elements of MM and subMM diagnostics (antennae, reflectors, waveguides) we propose the elements fabricated of carbon–graphite materials (CGM). A comparative study of antennae and reflectors made of CGM and metal for 30–150 GHz in MM and for subMM ranges was carried out under the influence of pulsed plasma streams and heating up to 1000 °C. The effect of neutron irradiation on reflectivity at 337 μm was studied as well. The obtained data demonstrated that CGM can be used as material for fabricating the above mentioned inner diagnostic components in those large-scale fusion devices where similar materials are in use for the first wall protection. The Fabry–Perot type resonator with spherical CGM-made mirrors showed rather good characteristics, and can be used for measuring plasma properties in a divertor flow.

The paper presents data on radiotechnical characteristics of antennae and reflectors for mm and sub-mm wavelength ranges, made of carbon-graphite materials (CGM). It is shown that the reflectivity value of plane reflectors made of different kinds of CGM depends mostly on the surface roughness being close to that of metallic reflectors, and does not change significantly under neutron irradiation. Characteristics of CGM-made antennae are not much different from those for antennae made of metal. Pyramidal antennae made of CGM maintains its properties under imitated plasma disruption much better than a similar metallic one would.

Carbon-graphite materials are proposed for manufacturing inner elements of mm and submm diagnostics in a fusion reactor (i.e., reflectors and antennae of different kinds). The investigations of influence of pulsed power plasma streams with a power density ⩾ 1 kJ/cm2 on radiotechnical characteristics of these elements at frequencies 37.5, 75, 105, 694, 890 and 2520 GHz and neutron irradiation with a fluence ∼ 1022 n/cm2 on plane reflectors at a frequency 890 GHz were carried out. Results of our tests point out the prospect to use these materials for making the mentioned elements, especially in the case if similar materials will be used as the first wall protection.

URAGAN-3M (U-3M) is an l=3, m=9 torsatron with R0=1 m, ā ≈ 12 cm and (ā) ≈ 0.4. The entire magnetic system is enclosed in a large vacuum chamber and there is no material limiter, so that an open helical divertor is realized in this device. A plasma is RF produced in 100% hydrogen gas and heated in the multimode Alfven resonance regime (ω ωci) at a continuous working gas leak-in. With an RF power of ~200 kW at B = 0.45 T, a quasi-steady (up to 50 ms) state of the plasma parameters sets in with e 2 × 1018 m-3, Ti(0) 130 eV and Te(0) equivalent to 300 eV. By using microwaves and Langmuir probes, the time and space behaviours of the electron density/ion saturation current are studied in the confinement volume, near the boundary of the confinement region, and in the divertor flux region. On the basis of ion saturation current profile measurements near the boundary of the confinement region the values of local particle flux density across the boundary are estimated: Γ⊥ ≈ 7 × 1020 m-2.s-1 far from the antenna and Γ⊥ approximately=1021 m-2.s-1, Γ⊥ ≈ 2 × 1021 m-2.s-1 near the antenna. Such flux densities and the corresponding diffusion coefficients are anomalously high, exceeding the neoclassical values by 2 to 3 orders of magnitude. On the other hand, these values are comparable with the flux densities and diffusivities obtained in edge plasma with similar parameters on other stellarators under NBI and ECR heating conditions. Therefore, it is claimed that the method of plasma heating that has been chosen in U-3M does not result in any substantial deterioration of particle confinement (with the possible exception of the region close to the antenna) as against other methods. Presumably, the anomalous particle transport across the boundary in U-3M is caused by destruction of magnetic surfaces and/or electrostatic drift wave turbulence. It is found that increasing the RF power results in an increase of the diverted plasma flow and of the hydrogen consumption necessary to maintain the average plasma density at a given level. This is considered as a manifestation of a common regularity, according to which particle losses grow with heating power, this effect having been observed recently on a stellarator type device under NBI and ECR heating conditions. An effect specific to U-3M, the increase of the average electron density, being observed within 1 to 2 ms after RF pulse termination, is attributed mainly to particle transport reduction due to plasma cooling, while the intensity of neutral particle ionization still remains sufficiently high in the confinement volume at the initial phase of cooling under continuous hydrogen leak-in conditions.

Numerical and experimental results of Alfven wave heating of plasmas in the frequency range below the ion cyclotron frequency ( ω ω ci ) are presented. Two different types of antenna were used for plasma production and heating: a frame type antenna (FTA) conventionally used in the URAGAN-3M device and a three-half-turn antenna (THTA) proposed recently to avoid the deleterious effects of conversion of fast wave to slow wave in the plasma periphery and to perform plasma core heating more effectively. Numerical modeling of electromagnetic field excitation in the URAGAN-3M plasma by the FTA and THTA was performed using a one-dimensional code. The results of calculations showed better performance of the compact THTA compared with the FTA for the case of a high density plasma (approximately 10 13 cm −3 ). When using the THTA, the experiments performed showed the possibility of dense plasma production (more than 2 × 10 13 cm −3 ) and heating, which had not been obtained earlier in the URAGAN-3M. Shifting the power deposition profile deeper inside the plasma body with the THTA resulted in modification of the plasma density profile and an improvement in plasma confinement.

The comparative study of working characteristics of some graphite and metallic components of SHF diagnostics, such as several kinds of antennae, plane reflectors and cylindrical waveguides are presented. To simulate the consequence of a plasma disruption in large tokamaks on graphite-made antenna, we tested two antennae (MPG and ARV kinds of graphite) on the influence of power plasma flow: W ≥ 1.10 MJ/m2 with the time duration Δt ≈ 0.1 ms. All results show that the prospects to use graphite elements in a reactor-scale fusion device are rather promising.

The authors study the mechanisms by which light (oxygen, carbon) and heavy (metal atoms) impurities are released from the walls in the Uragan-3 torsatron, in which radiofrequency (RF) heating is the only way of heating (and producing) the plasma. Having investigated the relationship between the quantity of impurities in the peripheral plasma, as well as the mean density, the confining magnetic field and the power input, they show that metal impurities appear as a result of sputtering from metal surfaces. Carbon, too, like the metal atoms, enters the plasma as a result of physical sputtering of carbon bearing materials. However, oxygen production occurs in a different way ? through chemical interaction between hydrogen and metal oxides on the surface. By comparing impurity concentrations in the edge plasma and the fluxes of charge-exchange atoms as functions of n?e, power input and magnetic field, the authors show that these fluxes do not play a decisive role, either in surface sputtering of the antenna and the magnet casing or in the chemical interaction responsible for the release of oxygen from the surfaces. Sputtering from the surface of the magnet casing that faces the confinement volume is shown to be governed mainly by the high frequency current associated with RF potential oscillations of the peripheral plasma. Sputtering from the surface of the RF antenna is due to its interaction with the quasi-constant ion current, with an energy of ? 100 eV produced during RF heating. The formation of water molecules on the surface is due to interaction of surface oxides with hydrogen atoms formed during the dissociation of H2 molecules.

Mechanisms of light (oxygen, carbon) and heavy (metal atoms) impurity production have been studied in the URAGAN-3 torsatron, where the RF method is the only one to produce and heat the plasma. The study of the impurity amount in the edge plasma versus the average density n e and the input RF power has shown that the metal and carbon impurities arise due to physical sputtering of metal and carbon-containing materials. Yet, the mechanism of oxygen release is different, and originates from a chemical hydrogen-metal oxide interaction on the surface. The comparison of the impurity concentration in the edge plasma and the charge-exchange atom flux as functions of n e and the magnetic field demonstrates that the charge exchange atoms are not determinative either in surface sputtering of the antenna and the helical winding casing or in the chemical processes of oxygen atom production. The sputtering of the casing surface facing the confinement region is shown to be mainly due to an oscillating charge particle current caused by the RF oscillation of the edge plasma potential. The surface sputtering of the antenna and the casing areas crossed by the divertor flux is caused by a quasisteady flux of ≥100 eV ions that appear during RF heating. The H 2 O molecule formation on the surfaces is attributed to the hydrogen atom-surface interaction, where the H-atoms are produced due to H 2 molecule dissociation.

In Uragan-3M torsatron one or two high-power waves are used for plasma production and heating. Powerful RF waves excite different types of plasma fluctuations. In this work, two kinds of oscillations caused by Alfven waves in plasma are presented. These oscillations were observed by UHF reflectometry. Spectral analysis of reflected microwaves showed an existence of plasma density fluctuations when two heating waves were used simultaneously. The ion Bernstein waves (IBW) manifested themselves as plasma density fluctuations and have been observed by microwave backscattering in the vicinity of ion gyro frequency harmonics. Experimental observation of IBW gives a tool for studying predicted link between IBW amplitude and ion temperature. Measurement of ion cyclotron harmonics frequency for different values of probing frequency allows the determination of their cut-off radius and radial profile of electron density.

This paper consists of two parts and concerns the influence of fusion reactor conditions on inner elements of some diagnostic systems. The first part is devoted to results of imitation experiments on the long-term operation of plasma-viewing mirrors, and in the second part there are discussed the results on a possibility to use carbon-graphite materials for fabricating the active elements of mm and sub-mm diagnostics (radiating and receiving antennae, reflectors). It is evident that results of the second part can be useful only in the case that carbon-based materials but not a beryllium will be chosen as the first wall protection.

In the IPP-Kharkiv there are two torsatrons (stellarators) in operation, and in both of them Alfvén resonance heating under high-k ∥ conditions is used. This method of heating is advantageous for small-size devices, since in contrast to the minority and second-harmonic heating it can be realized at lower plasma densities. A series of experiments has been performed at the Uragan-3M torsatron with an aim to investigate the features of the discharge with a three-half-turn antenna. Electron temperatures in the range are achieved at plasma densities . The plasma energy content has increased by a factor of 2 with respect to the plasma produced with the frame antenna. A new four-strap shielded antenna has been manufactured and installed in the Uragan-2M. A high-frequency discharge for wall conditioning is introduced in the Uragan-2M torsatron. The discharge is sustained by a specially designed small frame antenna, and efficient hydrogen dissociation is achieved. A self-consistent model has been developed for simulation of plasma production in ICRF. The model includes a set of particle and energy-balance equations for the electrons, and the boundary problem for the Maxwell equations. The first calculation results on RF plasma production in the Uragan-2M stellarator with the frame-type antenna are presented.

A modified method of dual polarization single line microwave backscattering is proposed for correlation and spectral studies of density fluctuations in plasma with known geometry of magnetic field (stellarator with low β plasma, reflex discharge). Simultaneous launching of microwaves of both ordinary (O) and extraordinary (X) polarization via the same antenna has an advantage for correlation measurements of fluctuations in adjacent plasma layers because a procedure of separation of reflected waves is simplified.

Possible mechanisms responsible for metal impurity release during ICRF production and heating of the plasma are studied with the URAGAN-3 torsatron. It is shown that the edge plasma potential oscillations at the pumping frequency and its harmonics produce an impurity influx from the surface of the helical winding casings (Fe, Cr). The impurity release from the antenna surface (Ti) is caused by a quasisteady ~ 100 eV ion flux arising in the divertor magnetic flux region during the RF pulse.