Effects of RF field rectification and accelerated electron beam generation in the torsatron U-3M during plasma production

The presented experimental results show that during the interaction between the RF field and a spatial charge of positive ions having the nonlinear volt-ampere characteristic a part of the RF field is rectified into the direct component. This direct electric field accelerates the electrons emitting...

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Datum:2011
Hauptverfasser: Berezhnyj, V.L., Berezhnaya, I.V., Voitsenya, V.S., Pinos, I.B., Filippov, V.V.
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Sprache:English
Veröffentlicht: Національний науковий центр «Харківський фізико-технічний інститут» НАН України 2011
Schriftenreihe:Вопросы атомной науки и техники
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Zitieren:Effects of RF field rectification and accelerated electron beam generation in the torsatron U-3M during plasma production / V.L. Berezhnyj, I.V. Berezhnaya, V.S. Voitsenya, I.B. Pinos, V.V. Filippov // Вопросы атомной науки и техники. — 2011. — № 1. — С. 26-28. — Бібліогр.: 17 назв. — англ.

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spelling irk-123456789-906122016-01-04T15:41:58Z Effects of RF field rectification and accelerated electron beam generation in the torsatron U-3M during plasma production Berezhnyj, V.L. Berezhnaya, I.V. Voitsenya, V.S. Pinos, I.B. Filippov, V.V. Магнитное удержание The presented experimental results show that during the interaction between the RF field and a spatial charge of positive ions having the nonlinear volt-ampere characteristic a part of the RF field is rectified into the direct component. This direct electric field accelerates the electrons emitting from the antenna surface. Показано, що при взаємодії ВЧ-поля з об’ємним просторовим зарядом позитивних іонів з нелінійною вольт-амперною характеристикою частина його випрямляється у постійну складову. Кулонівське поле позитивних іонів прискорює електрони, що емітуються з поверхні антени. Показано, что при взаимодействии ВЧ-поля с объемным пространственным зарядом положительных ионов с нелинейной вольт-амперной характеристикой часть его выпрямляется в постоянную составляющую. Кулоновское поле положительных ионов ускоряет эмиттирующие из поверхности антенны электроны. 2011 Article Effects of RF field rectification and accelerated electron beam generation in the torsatron U-3M during plasma production / V.L. Berezhnyj, I.V. Berezhnaya, V.S. Voitsenya, I.B. Pinos, V.V. Filippov // Вопросы атомной науки и техники. — 2011. — № 1. — С. 26-28. — Бібліогр.: 17 назв. — англ. 1562-6016 PACS: 52.55.Fa; 52.35.B http://dspace.nbuv.gov.ua/handle/123456789/90612 en Вопросы атомной науки и техники 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.
Berezhnaya, I.V.
Voitsenya, V.S.
Pinos, I.B.
Filippov, V.V.
Effects of RF field rectification and accelerated electron beam generation in the torsatron U-3M during plasma production
Вопросы атомной науки и техники
description The presented experimental results show that during the interaction between the RF field and a spatial charge of positive ions having the nonlinear volt-ampere characteristic a part of the RF field is rectified into the direct component. This direct electric field accelerates the electrons emitting from the antenna surface.
format Article
author Berezhnyj, V.L.
Berezhnaya, I.V.
Voitsenya, V.S.
Pinos, I.B.
Filippov, V.V.
author_facet Berezhnyj, V.L.
Berezhnaya, I.V.
Voitsenya, V.S.
Pinos, I.B.
Filippov, V.V.
author_sort Berezhnyj, V.L.
title Effects of RF field rectification and accelerated electron beam generation in the torsatron U-3M during plasma production
title_short Effects of RF field rectification and accelerated electron beam generation in the torsatron U-3M during plasma production
title_full Effects of RF field rectification and accelerated electron beam generation in the torsatron U-3M during plasma production
title_fullStr Effects of RF field rectification and accelerated electron beam generation in the torsatron U-3M during plasma production
title_full_unstemmed Effects of RF field rectification and accelerated electron beam generation in the torsatron U-3M during plasma production
title_sort effects of rf field rectification and accelerated electron beam generation in the torsatron u-3m during plasma production
publisher Національний науковий центр «Харківський фізико-технічний інститут» НАН України
publishDate 2011
topic_facet Магнитное удержание
url http://dspace.nbuv.gov.ua/handle/123456789/90612
citation_txt Effects of RF field rectification and accelerated electron beam generation in the torsatron U-3M during plasma production / V.L. Berezhnyj, I.V. Berezhnaya, V.S. Voitsenya, I.B. Pinos, V.V. Filippov // Вопросы атомной науки и техники. — 2011. — № 1. — С. 26-28. — Бібліогр.: 17 назв. — англ.
series Вопросы атомной науки и техники
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fulltext EFFECTS OF RF FIELD RECTIFICATION AND ACCELERATED ELECTRON BEAM GENERATION IN THE TORSATRON U-3M DURING PLASMA PRODUCTION V.L. Berezhnyj, I.V. Berezhnaya, V.S. Voitsenya, I.B. Pinos, V.V. Filippov Institute of Plasma Physics, NSC “Kharkov Institute of Physics and Technology”, Kharkov, Ukraine The presented experimental results show that during the interaction between the RF field and a spatial charge of positive ions having the nonlinear volt-ampere characteristic a part of the RF field is rectified into the direct component. This direct electric field accelerates the electrons emitting from the antenna surface. PACS: 52.55.Fa; 52.35.B THE EFFECT OF THE RF VOLTAGE RECTIFICATION In a large body of measurements the existence of a high positive spatial potential in the near-electrode space of RF discharges was shown [1,2]. Immediately in the torsatron U-3M this effect has not been studied. However, in the experiments on studying the behavior of impurities during plasma RF-heating the flows of metal impurities into the plasma from the RF-antenna and the helical coil casings were found out, Fig.1 [3]. A main cause for the ingress of impurities Ti and Fе is the sputtering from the surfaces of the antenna and the helical winding casings. It has been suggested that the surface sputtering occurs because of bombarding the surfaces with ions accelerated by a quasi-constant positive potential up to the energies of 100 eV. Such a potential can arise due to the formation of a spatial charge (SC) of positive ions near the electrodes of RF discharges [1,2]. The potential direct component is determined 222 22 0 0 6 dp UneU m i ⋅⋅ ⋅ ⋅⋅= ω μπ . (1) Here – the value of a positive space charge, ine ⋅=ρ 0μ - electron mobility at pressure , - peak value of RF voltage, - the length of RF discharge. As follows from (1) the amplitude is determined by Torrp 1= mU d 0U ρ and the RF field amplitude ω μ p mEA 0= . The total Coulomb field of positive ions in the SC usually is 106 V/cm [4]. According to the version of [2] the RF discharges possess valvular properties. The direct component of the potential difference U0 between the plasma and the RF electrode is the result of the rectification of a part of RF voltage: π/~0 VU = . The rectification of RF voltage occurs similarly to the linear detection mode. In many experiments the value of U0 reached 400 V [2]. A process of SC formation in U-3M can be represented by the following way. As the RF-generator is brought into operation, the field is localized near the surface of an unscreened frame antenna because its geometrical dimensions (l ≈ 1 m) are much less than the generated wave length in space (λ ≈ 34 m). Thus, in the initial moment of time the antenna can be represented as a cold cathode. First, the plasma is created in this local RF field near the antenna surface. Therewith, in this plasma layer a spatial charge of positive ions is formed [1,2]. The alternating voltage increasing on the antenna leads to increase of the direct component U0 [2]. Thus, the energy of the flow of ions bombarding the antenna surface increases, enhancing the effect of antenna sputtering (see Fig. 1). Fig. 1. Intensity of the CrI line near the helical winding casing and TiI line near the RF antenna with TiN coating The ingress of high Z impurities into a central plasma volume influences L to H regime transition, development of internal and external transport barriers of RF discharges; even insignificant impurity influx can be problematic for a hot plasma [5]. As was found in [5], impurities were generated due to sputtering of RF antenna surface caused by bombardment with ions accelerated in the field of space charge (other terms used are ‘RF sheath’, ‘plasma sheath’ or ‘near field sheath’). An attempt was made to suppress the influx of impurities by protection of antenna with Mo tiles or by boronization. In the first case the influx of impurities decreased noticeably but not fully. Much more effective for suppression of impurity influx was boronization of antenna surface; however, the boron-containing film was practically completely eroded during a pulse by plasma impact (with the rate ~20 nm/s) when RF power was PRF=3 MW. Taking into account that RF heating of plasma is planned to be used in ITER, solution of the problem of impurities is very actual. In the review paper [6] more than 100 papers relating different aspects of nonlinear effects in ICRF-plasma are analyzed, including generation of heavy impurities. At the quasi-stationary stage of RF discharge 26 PROBLEMS OF ATOMIC SCIENCE AND TECHNOLOGY. 2011. № 1. Series: Plasma Physics (17), p. 26-28. the field frequency is significantly less than the electron plasma frequency ω0<<ωре. Therefore, the condition of RF field localization near the antenna surface is substantially held. Consequently, the RF field, propagating from the antenna, must overcome SC. SC has a nonlinear volt-ampere characteristics [2], therefore, the signal at its output can be represented as xout(t) = k[xin(t)+εx (t)]. Then the pump wave A2 in 1cos(ωt) at the SC output is converted to xout(t) = A1cos(ωt)+ε/2⋅A1 2cos(2ωt) + ε/2⋅A1 2. Besides the pump wave and its second harmonics, a constant term Δ= ε/2⋅A1 2 appeared. This term corresponds to the shift of the average value of the function A1cos(ωt). It evidences on the presence of the alternating current rectification similar to that shown in [2]. If, e.g., there is simultaneous interaction between the SC and the first harmonic with every next one up to the tenth one, then the shift value appreciably increases [7]: )]2 10 2 1()2 3 2 1()2 2 2 1(2 1[ 2 10 1 AAAAAAA n n ++⋅⋅⋅+++++=∑ = Δ ε (2) apparent. In the present paper rectification of some part of RF field become we demonstrate that due to interaction of the pump mode with the nonlinear element, both the effects of higher harmonics generation and The rectification efficiency is determined by the SC nonlinearity coefficient ε and by the RF harmonic amplitudes participating in the interaction. GENERATION OF THE ELECTRON BEAM First works on generation of currents by powerful electromagnetic waves were provided when laser beams interacted with solids. In [8], the current AI 100= , was registered when metallic target was irradiated by the ruby laser beam ( , kWW 10= MWP 300 = ns60=τ , mμλ 7.0= ), with coefficient of efficiency . In [9] the currents were generated by impact of laser radiation ( %103 2−⋅≅ 2CO MWP 8.90 = , mμλ 6.10= , sμτ 4...3= ) with the target ( Cu ). The current was characterized by , UAI 14= V780= , and coefficient of efficiency . The direct conversion of energy of microwaves was observed in [10] (metal target). At pulsed rate of flow %10 1−≅ 2100 cmkWq ≅ ( cm5=λ ) the current reached , the positive potential , and coefficient of efficiency exceeded . AI 200= kVU 5.1= %10 Two important conclusions follow from these early results on generation of currents by electromagnetic waves. First, in spite of difference in wavelengths (lasers and RF) and powers, the observed phenomenon of current generation is very similar what indicates that identical physical processes are realized (formation of spatial charge). When high power radiation is focused on a target, some amount of material is vaporized and ionized, thus plasma is created which expands from the target surface. Because of difference in velocities of electrons and ions, the space charge of positive ions forms. Second conclusion: the efficiency of current increases with increasing the wavelength of the power source. For example, in paper [11] the “effective temperature” of fast electrons emitted from plasma target due to strong microwave field ( 280 cmkWqm = ; cm5=λ ) was in a good correlation with the scaling law 3210 )(1075.7)( λmh qkeVT ⋅⋅= − , with )( 2cmWqm ; λ (μm). Taking into consideration that RF power in U-3M experiments is close to that used in [11] and the wavelength ( m34≅λ ) is significantly longer, one can suppose the generation of accelerated electrons in the case of U-3M is also possible. The formation of a near-cathode SC layer is a necessary condition for the electron emission from the cold cathode [12,13]. In general, the field emission occurs in the direct electric field due to bombardment of cathode by positive ions and by tunneling electrons due to positive ion fields arising as a result of excited atom ionization at the antenna surface. The electron beam moves perpendicularly to the antenna surface against the beam of ions which bombard it. By their way electrons ionize gas atoms thus maintaining the concentration of positive ions in the SC, and go on into the main plasma. In the direct electric field some electrons pass into the fast electron regime. In the acceleration process the electrons appeared near the antenna surface as a result of gas ionization are also involved. If the intensity of the direct electric field exceeds the Dreicer field )( )( 10/ln2 3 012 eVT сmn TnEE e eeD − − − =Λ=> π , then in the acceleration process a significant part of antenna-emitting electrons and plasma electrons are participating [14]. For the mode of U-3M operation with ne = 2⋅1012 сm-3 and Те of several hundreds of electrons this condition is well- fulfilled. The presence of an accelerated electron beam is qualitatively confirmed by the experimental fact of a Hβ line intensity increasing up to the initial level, 9 µs after fall down to zero since the RF pulse finish in U-3M, Fig.2 [15]. The ms9≅ time delay can characterize the time for relaxation of the beam energy in collisions with hydrogen molecules, with start to come freely into the confinement volume after the density of periphery plasma (mantle) decreased and its screening effect disappeared. The second rise of the divertor plasma density cam also be explained by this reason. Fig. 2. Time behavior of Hß line intensity CONCLUSIONS 1. So, the processes of RF field harmonic generation, RF field rectification and electron acceleration occur as a result of SC formation and SC-RF field interaction. These nonlinear processes are the main sources of RF power 27 4. R.V. Paul. Study of electricity. Moscow: “Fizmatgiz”, 1962, 516 p. loss on the fundamental harmonic, the frequency of which is selected to the condition of plasma heating in U-3M ω = (0.8…1)×ωβi. As an example, let Р0 = 150 kW. One third of this power is rectified into the direct component of the potential difference U0. The rest 100 kW we should roughly distribute between the first four harmonics and thus obtain Р(ω0) ≈ 25 kW. This figure roughly coincides with estimations of the plasma energy density <nT>. 5. S.J. Wukitch, B. LaBombard, Y. Lin, et al. // Jornal of Nuclear Matterial. 2009, p. 390-391. 6. J.R. Myra, D.A D’Ippolito, D.A. Russel, et al. Nucl Fusion. 2006, №46, p 455-468. 7. V.L. Berezhnyj, I.V. Berezhnaya, V.S. Voitsenya, et al. // Problems of Atomic Science and Technology. Series “Plasma Physics”. 2010, N 6, p. 31-33. 2. Effect of rectification U∼→U0 represents in [2] as π/~0 VU = and in [5] as RfPU =0 . We showed that the efficiency of rectification is determined by the sum of squares of amplitudes of all harmonics which interact with a RF-sheath. 8. T.U. Arifov, G.A. Askar’yan, I.M. Raevskiy, N.M. Tarasova // ZhETF, 1968, v 55 p 385-388 (in Russian). 9. W.T. Silfvast and L.H. Szeto // Apll. Phys. Letters. 1977, v. 31, № 11, p 726-728 (in Russian). 10. G.A. Askar’yan, G.M. Batanov, N.K. Berezeckaya, et al. // Pisma v ZhETF. 1979, v.29, № 11, p 706-709 (in Russian). 3. Formation of RF-sheath results in bombardment of RF antenna with accelerated ions, provoking flux of heavy particles into plasma. 11. D.M. Karfidov, N.A. Lukina, K.F. Sergeichev. XV International Conference on Phenomena in ionized gages. Minsk, USSD, 1981. Contributed Papers, Part II, p. 805-806. 4. Existence of RF-sheath is a necessary condition for the beam of accelerated electrons to be formed. This effect is used in investigations on the direct transformation of electromagnetic energy into the energy of current. Recently with the laser beam energy 10…15 J, laser wavelength λ = 810 nm, pulse duration τu = 40…50 fs the accelerating voltage reached ≈ 1 GV/сm and the energy of accelerated electrons up to 200 MeV [16]. For the conditions of U-3M experiments some estimations give the electron energy ≈ 170 keV. The accelerated electrons can be strongly weakened because of anomaly resistance due to excitation of current instabilities [17]. Also high energy electrons in U-3M can provide additional ionization of gas that comes into the confinement volume from the vacuum tank. 12. А. Engel. Ionized gases. Moscow: “Fizmatgiz”, 1959 (in Russian). 13. E.I. Lutsenko, N.D. Sereda. L.M. Koncevoi // Fizika Plazmy. 1976, v. 2, N 1, p. 72-81 (in Russian). 14. V.Е. Golant, et al. Osnovy fiziki plazmy. Moscow: „Atomizdat“, 1977. 384 p. (in Russian). 15. V.S. Voitsenya, E.D. Volkov, L.I. Grigor’eva, et al. // VIII Stellarator Workshop. Kharkov, USSR, 27-31 May, 1991, p. 269-273. 16. V.N. Ginzburg, V.V. Zelenogorsky, E.V. Katin, et al. // XXXVII International (ZVENIGOROD) conference on Plasma physics and fusion, 8-12 Feb. 2010. Book of abstracts, p.17 (in Russian). REFERENCES 17. D.M. Каrfidov, N.A. Akylina, K.F. Sergeychev, et al. // Pis’ma v Zhurnal Eksperimental’noi i Tekhnicheskoi Fiziki. 1981, v. 34, N 9, p. 489-493 (in Russian). 1. S.M. Levitsky // Iournal of Technical Phycsycs. 1957, v. 27, N 5, p. 1001-1009, (in Russian). 2. V.A. Godyak, A.A. Kuzovnikov // Fizika Plasmy. 1975, v. 1, N 3, p. 496-503. Sov. J. Plasma Phys. 1975, v. 1, 276 p. (in Russian). Article received 29.10.10 3. E.D. Volkov, V.M. Zalkind, V.G. Konovalov, et al.: Preprint KIPT 89-11. Moscow-CISI Atominform. 1989, 15 p. (in Russian). ЭФФЕКТЫ ВЫПРЯМЛЕНИЯ ВЧ-ПОЛЯ И ГЕНЕРАЦИИ ПУЧКА УСКОРЕННЫХ ЭЛЕКТРОНОВ В ТОРСАТРОНЕ У-3М В.Л. Бережный, И.В. Бережная, В.С. Войценя, И.Б. Пинос, В.В. Филиппов Показано, что при взаимодействии ВЧ-поля с объемным пространственным зарядом положительных ионов с нелинейной вольт-амперной характеристикой часть его выпрямляется в постоянную составляющую. Кулоновское поле положительных ионов ускоряет эмиттирующие из поверхности антенны электроны. ЕФЕКТИ ВИПРЯМЛЕННЯ ВЧ- ПОЛЯ ТА ГЕНЕРАЦІЇ ПУЧКА ПРИСКОРЕНИХ ЕЛЕКТРОНІВ У ТОРСАТРОНІ У-3М В.Л. Бережний, І.В. Бережна, В.С. Войценя, І.Б. Пінос, В.В. Филіппов Показано, що при взаємодії ВЧ-поля з об’ємним просторовим зарядом позитивних іонів з нелінійною вольт- амперною характеристикою частина його випрямляється у постійну складову. Кулонівське поле позитивних іонів прискорює електрони, що емітуються з поверхні антени. 28