ICRF – volume charge - antenna edge interactions in the U-3M and U-2M torsatrons. Part 3. ICRF − VSC interaction

The brief analysis of the volume-space charge (VCS) influence on the processes of plasma formation and plasma heating with thh use of high-power ion-cyclotron waves in the U-3M and U-2M torsatrons has been carried out. The results of investigations into this problems reached on other facilities were...

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Автор: Berezhnyj, V.L.
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Опубліковано: Національний науковий центр «Харківський фізико-технічний інститут» НАН України 2017
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Цитувати:ICRF – volume charge - antenna edge interactions in the U-3M and U-2M torsatrons. Part 3. ICRF − VSC interaction / V.L. Berezhnyj // Вопросы атомной науки и техники. — 2017. — № 1. — С. 10-13. — Бібліогр.: 21 назв. — англ.

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spelling nasplib_isofts_kiev_ua-123456789-1221132025-02-23T17:10:29Z ICRF – volume charge - antenna edge interactions in the U-3M and U-2M torsatrons. Part 3. ICRF − VSC interaction Краевые взаимодействия ICRF - объëмный заряд - антенна в торсатронах У-3М и У-2М. Часть 3. взаимодействие ICRF - объëмный заряд Крайові взаємодії ICRF - об’ємний заряд - антена у торсатронах У-3М та У-2М. Частина 3. взаємодія ICRF - об’ємний заряд Berezhnyj, V.L. Магнитное удержание The brief analysis of the volume-space charge (VCS) influence on the processes of plasma formation and plasma heating with thh use of high-power ion-cyclotron waves in the U-3M and U-2M torsatrons has been carried out. The results of investigations into this problems reached on other facilities were used in this paper. Проведен краткий обобщённый анализ влияния объёмного пространственного заряда (VSC) на процессы создания плазмы и эффективности её нагрева мощными ионно-циклотронными волнами в торсатронах У-3М и У-2М. Использовались также достижения исследований по этой проблеме на других установках. Проведено короткий узагальнюючий аналіз впливу об’ємного просторового заряду (VSC) на процеси створення плазми та ефективності її нагріву потужними іонно-циклотронними хвилями в торсатронах У-3М і У-2М. Використовувались також досягнення досліджень з цієї проблеми на інших установках. 2017 Article ICRF – volume charge - antenna edge interactions in the U-3M and U-2M torsatrons. Part 3. ICRF − VSC interaction / V.L. Berezhnyj // Вопросы атомной науки и техники. — 2017. — № 1. — С. 10-13. — Бібліогр.: 21 назв. — англ. 1562-6016 PACS: 52.55. Fa; 52.35. B https://nasplib.isofts.kiev.ua/handle/123456789/122113 en Вопросы атомной науки и техники application/pdf Національний науковий центр «Харківський фізико-технічний інститут» НАН України
institution Digital Library of Periodicals of National Academy of Sciences of Ukraine
collection DSpace DC
language English
topic Магнитное удержание
Магнитное удержание
spellingShingle Магнитное удержание
Магнитное удержание
Berezhnyj, V.L.
ICRF – volume charge - antenna edge interactions in the U-3M and U-2M torsatrons. Part 3. ICRF − VSC interaction
Вопросы атомной науки и техники
description The brief analysis of the volume-space charge (VCS) influence on the processes of plasma formation and plasma heating with thh use of high-power ion-cyclotron waves in the U-3M and U-2M torsatrons has been carried out. The results of investigations into this problems reached on other facilities were used in this paper.
format Article
author Berezhnyj, V.L.
author_facet Berezhnyj, V.L.
author_sort Berezhnyj, V.L.
title ICRF – volume charge - antenna edge interactions in the U-3M and U-2M torsatrons. Part 3. ICRF − VSC interaction
title_short ICRF – volume charge - antenna edge interactions in the U-3M and U-2M torsatrons. Part 3. ICRF − VSC interaction
title_full ICRF – volume charge - antenna edge interactions in the U-3M and U-2M torsatrons. Part 3. ICRF − VSC interaction
title_fullStr ICRF – volume charge - antenna edge interactions in the U-3M and U-2M torsatrons. Part 3. ICRF − VSC interaction
title_full_unstemmed ICRF – volume charge - antenna edge interactions in the U-3M and U-2M torsatrons. Part 3. ICRF − VSC interaction
title_sort icrf – volume charge - antenna edge interactions in the u-3m and u-2m torsatrons. part 3. icrf − vsc interaction
publisher Національний науковий центр «Харківський фізико-технічний інститут» НАН України
publishDate 2017
topic_facet Магнитное удержание
url https://nasplib.isofts.kiev.ua/handle/123456789/122113
citation_txt ICRF – volume charge - antenna edge interactions in the U-3M and U-2M torsatrons. Part 3. ICRF − VSC interaction / V.L. Berezhnyj // Вопросы атомной науки и техники. — 2017. — № 1. — С. 10-13. — Бібліогр.: 21 назв. — англ.
series Вопросы атомной науки и техники
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fulltext ISSN 1562-6016. ВАНТ. 2017. №1(107) 10 PROBLEMS OF ATOMIC SCIENCE AND TECHNOLOGY. 2017, № 1. Series: Plasma Physics (23), p. 10-13. ICRF – VOLUME CHARGE  ANTENNA EDGE INTERACTIONS IN THE U-3M AND U-2M TORSATRONS Part 3. ICRF − VSC INTERACTION V.L. Berezhnyj Institute of Plasma Physics of the NSC KIPT, Kharkov, Ukraine E-mail: berezhnyj@kipt.kharkov.ua The brief analysis of the volume-space charge (VCS) influence on the processes of plasma formation and plasma heating with thh use of high-power ion-cyclotron waves in the U-3M and U-2M torsatrons has been carried out. The results of investigations into this problems reached on other facilities were used in this paper. PACS: 52.55. Fa; 52.35. B INTRODUCTION In the case of the stationary plasma its state is deter- mined, in main, by the processes running in the con- finement volume. In rf discharges the ICRF-space vol- ume charge (VSC) edge interactions play a dominant role. At high rf power levels the ICRF systems can lead to the antenna surface deterioration, increase of heavy impurity emission from the antenna, change of the value and profiles of en and Те in the peripheral plasma, high rf power dissipation. These effects can cause the plasma degradation in the central region [1]. The loss of the rf power PRF may be related with a higher harmonic generation in the VSC layers. Nonlin- ear interactions between the pump waves and their har- monics lead to the excitation of waves with combination frequencies 1 2( )nf mf and side frequencies ( ), , ( )n n nf f f  ], which have been recorded in the U-3M and U-2M torsatrons [2-9]. The results obtained evidence on the strong nonlinear pump wave-VSC interaction accompanied by the para- metric decay processes. The ion-cyclotron range of frequencies (ICRF) was used in experiments for formation and heating of the plasma in the U-3M and U-2M torsatrons. This range includes fast waves (FW), slow ion-cyclotron waves (ICW) and ion Bernstein waves (IBW). For excitation of these waves one needs significant rf field components Ey and Ez and high pump powers PRF, i.e. a strong an- tenna-plasma coupling is required. This leads to the VSC formation, enhances the ICRF-VSC- antenna non- linear interactions. Excitation of FW, ICW and IBW waves and their propagation in the plasma, energy trans- fer to the plasma particles (heating efficiency) and trans- formation of these modes depend on the edge plasma parameters and their gradients. These parameters, in its turn, depend on the radial motions of the local density plasma as a consequence of formation of convective cells near the antenna and MHD structures at the plasma edge. All these phenomena are interrelated and therefore it is difficult to investigate them. Nevertheless an under- standing of these effects is a necessary condition for effective plasma heating in the ICRF range. 1. RECTIFICATION, RF POWER DISSIPATION, HARMONIC GENERATION In Part 1 of this study the basic physics of space- volume charge (VSC) formation near the antenna sur- face in the rf discharges are described. As a result of many experimental, theoretical and simulation works a modern concept of a VSC (rf sheath) role in the initia- tion of many effects has been developed. A basic rf sheath physics is described in [1]. The primary cause for many of the observed phe- nomena is the rf sheath which exists at the “end-plates” where the field line contacts a conductor. A schematic sketch of the rf sheath is presented in Fig. 1 [1]. Fig. 1. Basic sheath physics schematic representation. The sheath is formed to equalize electron and ion loss rates. The resulting potential enhances the electron con- finement by forming a potential barrier for electrons, i.e. the sheath of a width . The same potential acceler- ates ions into the plates and causes the sheath power dissipation. For the rf-sheath, the driving voltages ±V0 at each end oscillate in time and the central potential (≈ 3Te) remains above the maximum voltage at either ends Both species, electrons and ions, initially try to leave at their respective thermal velocities. In response to the growing charge inbalance, the plasma develops a poten- tial to confine the electrons and to restore the charge ambipolatity. This potential, which must be higher than the applied voltage at either of the ends, reflects almost all the electrons at the sheath entrance. In addition to the reflecting electrons, this large sheath accelerates ions into the plates, creating a fast ion propagation that en- hances physical sputtering. The energy for this accelera- tion comes from the circuit and appears as lost power to mailto:berezhnyj@kipt.kharkov.ua ISSN 1562-6016. ВАНТ. 2017. №1(107) 11 the sheath. The whole process is driven by the need for the charge ambipolarity. This basic sheath physics ex- tends immediately into an ICRF sheath, where an oscil- lating voltage is applied to each of plates (Fig. 1). Elec- trons leave alternately out from the one end, then the other, escaping from the end where the applied voltage is the highest. This gives rise to an oscillating parallel electron current. The central voltage oscillates up and down at twice the applied frequency but always remains higher than the applied voltage at either of ends. The net effect is that there is reflection of both the applied volt- age and a large second harmonic. Harmonics with the frequencies 0n  (n=1, 2, 3) were recorded in the spectrum of plasma potential oscillations [2] and at the initial stage of rf plasma formation [3] in the U-3 torsatron. In the both papers the rf field harmonic gener- ation is not explained by the rf sheath formation near the antenna. However in [3] it is noted that the excitation of electrostatic plasma oscillations at frequencies 0n (n=2, 3) can be related with parametric instabilities in the electrostatic branch of Alfven waves which exert essential influence on the pumping wave energy dissipa- tion. For the first time the three first harmonics of the rf field in the U-3M torsatron have been obtained from the spectrum of a reflected X-wave of the reflectometer (F=19.1 GHz) [4]. It is noted that the cause might be the formation of a space discharge layers with nonlinear characteristics near the rf antenna. Later we have suc- cessfully confirmed this assumption in the simulation study of the interaction between the high-power elec- tromagnetic radiation of ICRF with a nonlinear element (rf sheath) [5]. The ICRF can penetrate into the dis- charge volume only through the rf sheath layer. Taking this into account the nonlinear character can be present- ed as a sum outx (t)=k[ inx (t)+ 2 inx (t)]. (1) Then, the pump mode A1cos( t ) at the rf sheath output will be described by the relation outx (t)=A1cos( t)+ /2  2 1A cos(2 t )+ /2  2 1A , (2) when k=1 is taken for simplification. Thus, at the output of VSC (sheath) not only the main component cos( )t but also its second harmonic cos (2 t ) and the time independent term ε/2 A1 2 appear; the latter indicates that the rectification effect takes place (the net sheath power dissipation). The results of ICRF-sheath interaction are very clearly demonstrated in the plots of Fig. 1 of our paper [5]. The results obtained fully coin- cide with above mentioned basic rf sheath physics [1]. Besides, our notion about the ICRF-VSC interaction points to their quantitative relation: the second harmonic amplitude and the rectification level are directly propor- tional to the rf pumping intensity. The latter is in accord with the results of [6]. If it is supposed that not only the first harmonics but the second harmonics too enters at the VSC input (due to the rerefraction from the chamber wall and from the antenna) then according to formulas (1) and (2) there are four harmonics with frequencies nf (n=1, 2, 3, 4) at the output. This process can be extend- ed in the same way. This version is of interest as in the U-3M torsatron in the spectra of different diagnostics the harmonics up to the eleventh harmonic fn=nf0 (n=2, 3, 4…11) were observed [5]. The amplitudes of the se- cond harmonic and the third harmonic were comparable with the amplitude of the fundamental harmonic [7]. In principle, the second harmonics and the third harmonic can be used for plasma heating. If the discharge condi- tions do not permit this process then in this case the generation of these harmonics can be considered as one of the rf power loss mechanism. Together with the ef- fect of ICRF part rectification into the constant potential difference [8] the PRF loss will be already appreciable during interaction with VSC. In the U-2M torsatron with the use of the both rf systems К1 and К2 the higher harmonics fn=nf1 (n=1, 2, …6) and fm=mf2 (m=1, 2, …5), respectively, were observed [9]. For the first time we have observed half-integer harmonics of 0( ) 2n f n   (n = 1, 3, 5…13) type, so-called inhar- monic obertones. In the case of simultaneous operation of К1 and К2 in the U-3M torsatron recorded were the (f1 - f2)-type difference frequencies from the spectra of re- flected rf signals [10], as well as, the intensities of the line H [5]. In the U-2M torsatron the combination fre- quencies were observed not only between the pumping frequency (f1 ± f2) but between their harmonics (nf1 ± mf2) too [9]. The frequency upper limits were restricted by the recording capabilities. At the initial stage of the discharge the (fn – F, fn, fn + F)-type oscillation fre- quencies were recorded. If F << fn, they are determined as side frequencies [9]. Similar results were obtained using the toroidal facility ACT-1 [11] where the har- monic generation and side frequencies were observed. The harmonic amplitudes have had the intensity suffi- cient for the parametric decay. 2. EDGE WAVE INTERACTIONS The ion-cyclotron range of frequencies (ICRF) is widely applicable for plasma heating. It includes fast waves (FW), slow ion-cyclotron waves (ICW) and ion Bernstein waves (IBW) [1]. For excitation of FW a high electron density (ne>5·10 13 cm -3 ) and a poloidal component Ey of the pumping wave are required. Usually FW vanishes in the low-density edge plasma as a consequence of the right- handed screw cutoff. Therefore a good antenna-plasma coupling is necessary, i.e. the antenna should be closer to the plasma. Thus during the VSC-antenna interaction the nonlinear effects are enhanced that is noted in Part 2 of this study. In the peripheral transforming layers FW can be transformed into other waves with nonlinear properties (ICW and IBW). In the U-3M and U-2M torsatrons commonly ICW (Alfven waves) are used for plasma formation and heating [12]. Because of low group velocity ICW and IBW require higher electric field E0 of pumping waves for excitation and mainte- nance of a considerable power flux. It leads to the same result, namely to the increase of PRF and, as a conse- quence, to the enhancement of nonlinear interactions with plasma. In U-3M the excitation of IBW was rec- orded under conditions when the relative electron and ion velocity e iU V V  became comparable with the thermal ion velocity TiV [13]. In [14] the excitation of 12 ISSN 1562-6016. ВАНТ. 2017. №1(107) IBW in U-3M was interpreted as a consequence of the parametric instability. In [13] IBW and its harmonics with n=1, 2, 3 were recorded in the spectra of capacitor probes. In the both cases the IBW harmonics were ob- served near the ion cyclotron frequency harmonics. However, the excitation and propagation of these waves in the edge plasma, as well as, the energy transfer to the plasma particles (heating efficiency) are strongly de- pendent on the local plasma parameters (ne and Te) and their gradients due to the ICRF-VSC interaction. The local density depletion ne at the antenna is consistent with density pump-out by strong E X B convection [15]. The peripheral conditions undergo the influence of periodic MHD instabilities at the plasma edge (or ELM). They arise on the side of a lower value of В0 and are filamentary-like structures with increased ne and Te. In the course of time they elongate along the В0 lines with a characteristic size (5-10 cm) and then they accel- erate outwards into SOL [16]. These radial plasma mo- tions change the antenna-plasma coupling that leads to the change of the PRF dissipation in VSC and changes the value of E0 with corresponding consequences. The size and frequency of ELM’s depend on the plasma form, in particular, on its triangularity: the higher the plasma triangularity the less is the recurrence rate and the larger are the ELM sizes. The plasma geometry in- fluence on the ICRF-antenna connection was observed on many facilities [16]. The plasma heating conception in U-3M and U-2M with the use of ICRF is presented in [3, 12, 17]. As noted above the VSC formation begins with electron escape from the plasma to the antenna. Some of the authors have proposed a mechanism of energetic electron augmentation in the plasma [18, 19, 20]. The mechanism is based on the principle of stochastic Fermi acceleration that is experienced by the particle which comes occasionally in time into the layer with rf field. Just VSC’s are such layers in the rf discharges, as is indicated earlier. The particles are reflected from the oscillating layers and gain an additional energy as solid particles from the vibrating wall. Probably this mecha- nism can lead to the two-temperature state of electrons in U-3M. At least, the two-temperature state of ions in U-3M was observed experimentally [21]. CONCLUSIONS To use successfully the ion-cyclotron waves for plasma heating a complete understanding of ICRF- plasma interaction is necessary. A key feature of rf dis- charges is the presence of near-surface volume ion charges (VSC, rf sheath), Their linear and nonlinear numerous effects substantially change the structure and dynamics of rf discharges, determine the rf plasma heat- ing efficiency. In many tokamaks ICRF is applied as an additional method of plasma heating and can be used for the current excitation. Taking everything into account, ICRF is expected to be used in the thermonuclear facility ITER that stimu- lates a wide investigation of linear and nonlinear effects (not all of them) mentioned in this paper. To study these effects is a matter of some difficulty because most of them are interrelated and, moreover, the possibilities of diagnostics are limited. The design features of U-3M and U-2M torsatrons consist in the fact that in experi- ments on these facilities neither the preliminary ioniza- tion (plasma formation), nor the plasma heating by other methods, unless ICRF, are not applied. Thus, there is a good chance for the program investigations of the vol- ume charge-antenna edge interactions in pure relic form. This certainly requires upgrading rf systems, introduc- ing advanced diagnostics, providing many manipula- tions (periodic boronization, changing and varying of diagnostics, antenna displacement etc.) without vacuum condition deterioration. REFERENCES 1. I.R. Myra, D.A. D’Ippolito, D.A. Russel, et al. Nonlinear ICRF plasma interactions // Nuclear Fusion. 2006, v. 46, № 7, p. 455-468. 2. V.L. Berezhnyj, V.N. Bondarenko, M.P. Vasil’ev, et al. Behaviour of impurities during rf heating in the Uragan-3 torsatron. Reprint (IAEA-CN-50/C-V-1) // Proceedings of the 12 International Conference on Plasma Physics and Controlled Nuclear Fusion Re- search, International Atomic Energy Agency in Nice, 12-19 October. 1988, v. 2, p. 599-609. 3. Yu.G. Zalesskij, P.I. Kurilko, N.I. Nazarov, V.V. Plusnin, O.M. Shvetz. Investigation of the rf start phase of plasma generation in the torsatron “Uragan-3” // Fizika Plazmy. 1989, v. 15, № 12, p. 1424-1429 (in Russian). 4. V.L. Berezhnyj, V.L. Ocheretenko, O.S. Pavlichenko, et al. Identification of harmonics of rf field, which is used for production and heating of plasma in the torsatron “U-3M” // Problems of Atomic Science and Technology. Series “Plasma Physics”. 2006, № 6(12), p. 53-55. 5. V.L. Berezhnyj, I.V. 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Spectral analysis of antenna rf current in the torsatron U-2M // Problems of Atomic Science and Technology. Series “Plasma Physics”. 2014, № 6, p. 8-11. 10. O.S. Pavlichenko A.I. Skibenko, E.D. Volkov, V.L. Berezhnyj, et al. Peculiarities of plasma fluctua- tions during rf heating in Uragan-3M torsatron by means of two alfven waves. // Problems of Atomic Sci- ence and Technology. Series “Plasma Physics”. 2005, № 2(11), p. 17-19. ISSN 1562-6016. ВАНТ. 2017. №1(107) 13 11. F.N. Skiff, K.L. Wong, M. Ono. Harmonic genera- tion and parametric decay in the ion-cycloron frequency range // Phys. Fluids. 1984, № 27(9), p. 2205-2207. 12. V.E. Moiseenko, V.L. Berezhnyj, V.N. Bondaren- ko, et al. RF plasma production and heating below ion- cyclotron frequencies in Uragan-3M torsatrons // Nuclear Fusion. 2011, № 51, v. 083036, p. 8. 13. A.I. Skibenko, O.S. Pavlichenko, V.L. Berezhnyj, et al. 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Collec- tion of papers. 1993, p. 277-281. 18. V.A. Godyak. Statistic electron heating at the oscil- lating plasma boundary // Zhurnal Technicheskoj Fiziki. 1971, № 7, v. 41, p. 1364-1368 (in Russian). 19. A.I. Akhiezer, A.S. Bakaj. On the theory of stochas- tic particle acceleration // Doklady Akademii Nauk, USSR, Series “Mathematics, Physics”. 1971, v. 201, № 5, p. 1074-1077 (in Russian). 20. A.I. Akhiezer, A.S. Bakaj. On the stochastic plasma heating by high-frequency fields // Fizika Plasmy. 1976, v. 2, № 4, p. 654-657 (in Russian). 21. A.M. Pankratov, A.A. Beletskii, V.L. Berezhnyj. Behavior of RF discharge plasmas in the Uragan-3M and Uragan 2-2M torsatrons // Contrib. Plasma Phys. 2010, v. 50, № 6-7, p. 520-528. Article received 22.12.2016 КРАЕВЫЕ ВЗАИМОДЕЙСТВИЯ ICRF  ОБЪËМНЫЙ ЗАРЯД  АНТЕННА В ТОРСАТРОНАХ У-3М И У-2М ЧАСТЬ 3. ВЗАИМОДЕЙСТВИЕ ICRF  ОБЪËМНЫЙ ЗАРЯД В.Л. Бережный Проведен краткий обобщённый анализ влияния объёмного пространственного заряда (VSC) на процессы создания плазмы и эффективности её нагрева мощными ионно-циклотронными волнами в торсатронах У-3М и У-2М. Использовались также достижения исследований по этой проблеме на других установках. КРАЙОВІ ВЗАЄМОДІЇ ICRF  ОБ’ЄМНИЙ ЗАРЯД  АНТЕНА У ТОРСАТРОНАХ У-3М ТА У-2М ЧАСТИНА 3. ВЗАЄМОДІЯ ICRF  ОБ’ЄМНИЙ ЗАРЯД В.Л. Бережний Проведено короткий узагальнюючий аналіз впливу об’ємного просторового заряду (VSC) на процеси створення плазми та ефективності її нагріву потужними іонно-циклотронними хвилями в торсатронах У-3М і У-2М. Використовувались також досягнення досліджень з цієї проблеми на інших установках.