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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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 |
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Магнитное удержание Магнитное удержание |
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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 |
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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 |
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2011 |
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Магнитное удержание |
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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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first_indexed |
2025-07-06T18:48:19Z |
last_indexed |
2025-07-06T18:48:19Z |
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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
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