Wave Phenomena in Helicon Plasma
Employing of the electrodeless, low-pressure inductive discharges (ICPs) is preferable in many applications. A helicon discharge, which is known as the most efficient magnetized ICP [1], operates with excitation of electromagnetic helicon waves (whistlers) obeying the dispersion relation w=wcek₂kc²/...
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irk-123456789-1104192017-01-05T03:02:36Z Wave Phenomena in Helicon Plasma Virko, V.F. Kirichenko, G.S. Shamrai, K.P. Basic plasma physics Employing of the electrodeless, low-pressure inductive discharges (ICPs) is preferable in many applications. A helicon discharge, which is known as the most efficient magnetized ICP [1], operates with excitation of electromagnetic helicon waves (whistlers) obeying the dispersion relation w=wcek₂kc²/w²pe where wpe and wce are the electron plasma and cyclotron frequencies, and k = (k₂²+k₁²)¹/² is the total wave number. In a radially bounded plasma, the wave fields vary as exp[i(kzz-wt-mq)] where m is the azimuthal wavenumber. Helicons are excited by external rf antennas of various designs and propagate into plasma giving rise to various wave phenomena that considerably affect the discharge operation. Some of these phenomena are examined below experimentally and theoretically. Представлено результати експериментальних та теоретичних досліджень хвильових процесів у густій плазмі, які збуджуються ВЧ антенами різних конструкцій в діапазоні геліконних частот. Експерименти проведено в комбінованому ЕЦР-геліконному джерелі плазми. Виявлено, що виміряна багатопікова структура залежності поглинання від густини плазми задовільно узгоджується з результатами обчислень і пов’язана зі збудженням стоячих поздовжніх мод. Зі збільшенням ВЧ потужності досліджено перехід до самостійного геліконного розряду. В чисто геліконному розряді виміряні спектри шумових коливань, які включають НЧ смугу з шириною порядку 1 МГц та ряд ВЧ смуг поблизу основної частоти та її гармонік. Збудження НЧ коливань має поріг по потужності, тобто відбувається параметричним чином, а також по магнітному полю. Ці коливання ідентифіковано з іонно-звуковими хвилями, що розповсюджуються по азимуту у напрямку обертання електронів. Їхні кореляційні довжини зменшуються зі зростанням магнітного поля, а інтенсивність різко збільшується безпосередньо під антеною. Обчислення показують, що в умовах експерименту генерація звуку можлива як за рахунок параметричної нестійкості, так і внаслідок дрейфової течії електронів в полі пондеромоторної сили. Досліджено також геліконний розряд в неоднорідному магнітному полі та виявлено різке зростання генерації плазми порівняно з випадком однорідного поля. Представлены результаты экспериментальных и теоретических исследований волновых процессов в плотной плазме, возбуждаемых ВЧ антеннами различных конструкций в диапазоне геликонных частот. Эксперименты проведены в комбинированном ЭЦР-геликонном источнике плазмы. Обнаружено, что наблюдаемая многопиковая структура зависимости поглощения от плотности плазмы удовлетворительно согласуется с результатами вычислений и связана с возбуждением стоячих продольных мод. При увеличении ВЧ мощности исследован переход к самостоятельному геликонному разряду. В чисто геликонном разряде измерены спектры шумовых колебаний, которые включают НЧ полосу с шириной порядка 1 МГц и набор ВЧ полос вблизи основной частоты и ее гармоник. Возбуждение НЧ колебаний имеет порог по мощности, т.е. происходит параметрическим образом, а также по магнитному полю. Эти колебания идентифицированы с ионнозвуковыми волнами, распространяющимися по азимуту в направлении вращения электронов. Их корреляционная длина уменьшается с ростом магнитного поля, а интенсивность резко возрастает непосредственно под антенной.. Вычисления показывают, что в условиях эксперимента генерация звука возможна как за счет параметрической неустойчивости, так и вследствие дрейфового течения электронов в поле пондеромоторной силы. Исследован также геликонный разряд в неоднородном магнитном поле и обнаружено резкое возрастание генерации плазмы в сравнении со случаем однородного поля. 2003 Article Wave Phenomena in Helicon Plasma / V.F. Virko, G.S. Kirichenko, K.P. Shamrai // Вопросы атомной науки и техники. — 2003. — № 1. — С. 56-61. — Бібліогр.: 18 назв. — англ. 1562-6016 PACS: 52.35.-g http://dspace.nbuv.gov.ua/handle/123456789/110419 en Вопросы атомной науки и техники Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
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Basic plasma physics Basic plasma physics Virko, V.F. Kirichenko, G.S. Shamrai, K.P. Wave Phenomena in Helicon Plasma Вопросы атомной науки и техники |
| description |
Employing of the electrodeless, low-pressure inductive discharges (ICPs) is preferable in many applications. A helicon discharge, which is known as the most efficient magnetized ICP [1], operates with excitation of electromagnetic helicon waves (whistlers) obeying the dispersion relation w=wcek₂kc²/w²pe where wpe and wce are the electron plasma and cyclotron frequencies, and k = (k₂²+k₁²)¹/² is the total wave number. In a radially bounded plasma, the wave fields vary as exp[i(kzz-wt-mq)] where m is the azimuthal wavenumber. Helicons are excited by external rf antennas of various designs and propagate into plasma giving rise to various wave phenomena that considerably affect the discharge operation. Some of these phenomena are examined below experimentally and theoretically. |
| format |
Article |
| author |
Virko, V.F. Kirichenko, G.S. Shamrai, K.P. |
| author_facet |
Virko, V.F. Kirichenko, G.S. Shamrai, K.P. |
| author_sort |
Virko, V.F. |
| title |
Wave Phenomena in Helicon Plasma |
| title_short |
Wave Phenomena in Helicon Plasma |
| title_full |
Wave Phenomena in Helicon Plasma |
| title_fullStr |
Wave Phenomena in Helicon Plasma |
| title_full_unstemmed |
Wave Phenomena in Helicon Plasma |
| title_sort |
wave phenomena in helicon plasma |
| publisher |
Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
| publishDate |
2003 |
| topic_facet |
Basic plasma physics |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/110419 |
| citation_txt |
Wave Phenomena in Helicon Plasma / V.F. Virko, G.S. Kirichenko, K.P. Shamrai // Вопросы атомной науки и техники. — 2003. — № 1. — С. 56-61. — Бібліогр.: 18 назв. — англ. |
| series |
Вопросы атомной науки и техники |
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AT virkovf wavephenomenainheliconplasma AT kirichenkogs wavephenomenainheliconplasma AT shamraikp wavephenomenainheliconplasma |
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2025-07-08T00:35:41Z |
| last_indexed |
2025-07-08T00:35:41Z |
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1837036934499663872 |
| fulltext |
BASIC PLASMA PHYSICS
WAVE PHENOMENA IN HELICON PLASMA
V. F. Virko, G. S. Kirichenko and K. P. Shamrai
Institute for Nuclear Research, NAS of Ukraine, Kiev
Employing of the electrodeless, low-pressure inductive discharges (ICPs) is preferable in many applications. A helicon
discharge, which is known as the most efficient magnetized ICP [1], operates with excitation of electromagnetic helicon
waves (whistlers) obeying the dispersion relation 22
pezce /kck ωωω = where peω and ceω are the electron plasma and
cyclotron frequencies, and 2122 )( /
z kkk ⊥+= is the total wave number. In a radially bounded plasma, the wave fields
vary as exp[i(kzz−ωt−mθ )] where m is the azimuthal wavenumber. Helicons are excited by external rf antennas of
various designs and propagate into plasma giving rise to various wave phenomena that considerably affect the discharge
operation. Some of these phenomena are examined below experimentally and theoretically.
PACS: 52.35.-g
1. CAVITY MODES
Helicon waves in short devices are excited as discrete,
axially standing modes (i.e., the cavity modes). As the
helicon wavelength depends on density, these
geometrical resonances result in a non-monotonic
dependence of the absorbed power on plasma density,
with maxima related to excitation of particular modes.
However, it is difficult to reveal such dependence in
discharge where the density establishes in a self-
consistent way. For this reason, we conducted modeling
experiments on absorption of a weak rf signal in
independently prepared plasma whose density could be
varied ad arbitrium [2].
The left part of experimental device (see Fig. 1) was
a helicon source: a 14 cm diam quartz tube placed in
magnetic field about 100 G and excited by an rf antenna
made of two straight rods and a connecting ring. It
generated a transverse rf magnetic field and thus excited
the first (m = 1) azimuthal mode of helicon waves. A
13.56 MHz, 1 kW generator supplied the antenna in a
continuous regime. The right part of the device in Fig. 1 is
an auxiliary plasma source based on an ECR discharge
driven by a pulsed 2.45 GHz magnetron. Two schemes of
experiment were used: (1) ECR plasma at low rf power
input from the antenna, and (2) helicon discharge at
normal rf power. Experiments were done with Ar gas at
pressures 0.5−5 mTorr.
Fig. 1. A schematic of the device. 1 – rf antenna; 2 – ECR
chamber; 3 – separating grid; 4 and 5 – probes
In the first scheme, the plasma density in the quartz
chamber raised up to some maximum and then fell down
to zero during the magnetron pulse. The absorption of the
low-power rf signal in the antenna circuit, which was
measured by a directional coupler, shows a few
resonances at some specific, resonance densities. These
resonances arise from the excitation of standing modes, as
was revealed by measurements with magnetic probes. The
resistance of a plasma load is shown in Fig. 2 as a
function of density.
Fig.2. Plasma resistance vs density, for a straight m=1
antenna; pAr = 0.7 mTorr, and B0 = 115 G
Fig. 3. Schematic diagram of the absorbed power and
power loss vs density
The influence of absorption resonances on the
discharge behavior is illustrated in Fig. 3. There the
absorbed rf power is schematically shown along with the
power loss from plasma, which is proportional to density.
The power balance constrains the discharge to
intersection points. However, at the middle point the
equilibrium is unstable. Maximum absorbed power
(dashed line) is the total power delivered by generator. If
the power decreases, the upper intersection disappears and
the discharge must jump to lower density. On the
contrary, with increasing power the lower intersection
vanishes and the discharge jumps to higher density [3].
56 Problems of Atomic Science and Technology. 2003. № 1. Series: Plasma Physics (9). P. 56-61
Experiments with a stimulated helicon discharge
seem to argue for this speculation. Figure 4 shows
oscillograms of the rf power absorbed in the ECR plasma
when the magnetron pulse is ending and density is
decreasing. At low rf power, one can see two absorption
maxima at some resonance densities. With increasing rf
power, an additional ionization appears and absorption
resonances become saw-shaped. If the rf power is high
enough, after the end of the ECR pulse a self-sustained
helicon discharge arises. Notable is that it can exist in
several discrete states with densities that are close to
resonance densities. Depending on power the jumps
between these states can occur. This result is consistent
with observations of the cavity modes in other experiment
[4].
Fig. 4. Dependences of absorbed power on time, at
increasing forward rf power
A similar effect occurs when the external magnetic
field varies at a fixed rf power. From dispersion relation it
follows that the helicon wavelength is proportional to
B0/n. As boundary conditions fix the wavelength of
standing mode, the resonance density is proportional to
the magnetic field. Thus, with increasing magnetic field
the absorption curve in Fig. 3 moves to the right. At some
critical magnetic field, it slips out from under the loss line
and the discharge jumps to lower density. From Fig. 3, it
is also clear that the reflected power (the distance between
the discharge point and the incident power line, Pinc)
becomes zero just before the break off. This effect was
really observed experimentally: with increasing B0 the
density grows while reflected power goes to zero, and
then the discharge disruption occurs [5].
The values of critical magnetic field and stable
densities are determined by the shape of absorption curve.
The latter strongly depends, among other factors, on the
antenna dimensions and position. An obvious and
experimentally confirmed result is that if the antenna length
is equal to the wavelength, or if the antenna mid-plane
coincides with a node of the standing wave, the
corresponding resonance cannot be excited because actions
of different antenna parts compensate each other [2,6].
Matching network, which transforms the antenna
impedance to the generator output, can strongly deform
the absorption curve. It is clear that if matching
conditions are adjusted to low, non-resonance plasma
resistance, then at the resonance, when the resistance is
maximum, bad matching results in that the absorbed
power is minimum rather than maximum.
Using the above technique, we compared relative
efficiencies of various antennas. Twisted antennas [7]
gave identical absorption characteristics because standing
helicon wave rotates only in time and has not spatial
helicity. Fields from a phased antenna with right-hand
rotation (m=+1) penetrate deeply into plasma and
maximize in resonance conditions. At left rotation (m=−
1), the field do not penetrate to the plasma center. The
excitation with a single-loop, azimuthally symmetric m=0
antenna differs from the m=1 excitation. It shows neither
distinct absorption resonances nor standing helicon
waves, contrary to theoretical prediction [3] and for still
unclear reason.
2. ION ACOUSTIC TURBULENCE
Another important problem is how the helicons transfer
their energy to plasma. The matter is that the helicons
themselves are weakly damped waves. They are low
sensitive to collisions because their energy is concentrated
in magnetic fields rather than in the particle motion. The
real damping is found to be orders of magnitude more
than calculated from the binary collisions.
For explanation of the anomalous helicon absorption
several linear mechanisms were proposed, such as Landau
damping [8] and linear conversion of the helicons in
short-scale Trievelpiece-Gould modes [9]. Unfortunately,
none of them was directly confirmed or rejected in
experiments. Formerly, anomalous helicon absorption
was observed in experimental conditions that are strongly
different from those considered here [10,11]. It was
attributed to the excitation of short ion-sound waves.
Recently, it was theoretically assumed that parametrically
excited ion-acoustic turbulence might be responsible for
the helicon absorption in a helicon discharge as well [12].
Indeed, ion- acoustic waves were detected in the helicon
discharge by a microwave scattering technique [13]. We
employed probe technique to examine the low frequency
oscillations in our device.
Two shielded probes shown in Fig. 1 were used for
the interferometry. One of them (marked 4 in Fig. 1) was
movable azimuthally at a distance of 5.5 cm from the axis
while another (5) in the radial direction across the plasma
column. Signals from the probes were added by a mixing
circuit and forwarded to the spectrum analyzer.
In the helicon discharge the rf probe reveals a wide
noise spectrum of plasma potential oscillations peaking in
the low frequency range as well as around harmonics of
the pumping frequency (Fig. 5). The amplitude of side-
band oscillations near the fundamental frequency is
comparable with that at low frequencies (LF) and can
even exceed the potential oscillations at the pumping
frequency itself.
Fig. 5. The spectrum of potential oscillations
57
The noise arises as a result of combined action of the
rf field and the external magnetic field only. It is
extremely low in the ECR plasma at zero rf power as well
as in inductive discharge driven by the same antenna, but
without magnetic field. In this latter case, the spectrum is
shown by the dashed lines in Fig.6.
With increasing magnetic field the noise is excited in
a threshold manner. The threshold on the rf power, was
measured with use of the ECR plasma as seen from Fig. 6.
Fig. 6. Noise amplitude vs antenna rf current
Fig. 7. Interferometric signal from two probes
Fig. 8. Dispersion of the LF oscillations
At lower magnet ic f ie lds , the no ise
demonstrates a fair correlation in the azimuthal direction.
In Fig. 7, the frequency spectrum is shown of a joint
signal from two probes separated by d=2 cm in the
azimuthal direction. Considering that the phase velocity is
vph≈ d∆f where ∆f is the frequency distance between
neighboring maxima, one can determine the dispersion
(Fig. 8). The phase velocity is found to be constant over
the frequency range and to coincide with the ion-acouistic
speed, at electron temperature of 4 eV. Varying the gas
pressure we compared electron temperatures measured by
the Langmuir probe with those calculated from the phase
velocities. A good agreement proved that the observed
oscillations are really the ion-acoustic waves. They were
found to propagate azimuthally in the direction of
electron gyration, which coincides with the right-hand
helicon rotation (m=+1). With increasing pressure the
noise amplitude rapidly decreases.
In contrast with the azimuthal correlation, the radial
correlation of oscillations is quite short. In Figs. 10a and
10b the azimuthal and radial spatial correlation functions
are plotted at the same frequency and low magnetic field.
It is with increasing magnetic field the azimuthal
correlation rapidly vanishes.
Fig. 9. Azimuthal (a) and radial (b) correlation functions
for the LF oscillations.
Two excitation mechanisms most conform with the
azimuthal propagation of ion acoustic waves. The first is
the parametric instability driven by the rf field.
Measurements have shown that along the axis the maxima
of the noise amplitude correlate with maxima of the Bz-
field of standing m=1 helicon wave. This fact says in
favor of the parametric excitation in the inner plasma
regions, as long as the Bz field is produced only by
transverse rf currents.
The origin for parametric instability; i.e. electron
drift relative to ions is characterized by oscillatory
electron velocity, u, or by electron excursion, ξ = u/ω.
These values were estimated for specific experimental
conditions with use of the computation code based on
linear cold-plasma theory [2]. Figure 10 shows the
distribution of the azimuthal velocity in cross-section of
the plasma column computed for the parabolic density
profile at z = 25 cm and for 1 A current in antenna shown
in Fig. 1. The peaks seen in peripheral regions underneath
Fig. 10. Contour plot of uθ in plasma cross-section ( flank
circles show the positions of antenna legs)
58
the antenna legs are due to intense electrostatic fields
excited via mode conversion of helicon waves into
Trivelpiece-Gould waves at strong density gradient. Peak
positions correlate with measured maxima of the LF
oscillations. Even at 1 A antenna current, the peaks in Fig.
10 are high enough, maxuθ ≈ Tev.0350 . The appropriate
amplitude of electron azimuthal excursion is of the order
of Larmour radius. At maximum antenna currents
available in our experiments, 20−30 A, the oscillatory
velocity can amount to thermal electron velocity, vTe.
To compute the dispersion of the LF oscillations in
helicon plasma, several simplifying assumptions can be
made. First, considering very small scales of the LF
waves, one can neglect the plasma and pumping field
nonuniformities. Next, one can use the potential
approximation. Indeed, estimations show that the main
part of the side-band spectrum has to be electrostatic
rather than helicon waves. Then one can use the
dispersion relation for LF oscillations [14], in which
parametric effects are determined by the parameter
ξk ⋅=Ea where k is the LF wave number. In
calculations, the excitation of LF oscillation, Ω, as well as
of two side-band satellites near the fundamental
frequency, 0ωΩ ± , was included whereas the effect of
higher harmonics was neglected. In Fig. 11, an example is
shown of the dispersion computed for some typical
experimental parameters; cosϕ = 0.042 where ϕ is an
angle between k and 0B ; and Tev.u 050= . With
increasing k; i.e. with increasing Ea the nonlinearity first
appears as the shift of ReΩ above the ion-acoustic
frequency. When Ea ranges approximately 0.7 to 1.2, the
instability arises with maximum growth rate about the
lower hybrid frequency. Note this value to considerably
exceed that predicted by the kinetic ion-acoustic
instability [12]. The computations for various values of
cosϕ, 0B , and Ar pressures showed that the instability
always occurs when aE lies approximately in the above
mentioned range; however, it turns out to be low sensitive
to other parameters. Thus, the parametric effect of the
pumping field is found to be strong: it substantially
modifies the acoustic dispersion and gives rise to the
instability with high growth rate.
Fig. 11. The LF dispersion in uniform pumping field. pAr =
5 mtorr; B0 = 115 G; and n0 = 4×1011 cm−3
One more possible mechanism for excitation of
the LF oscillations is a two-stream instability driven by a
dc electron drift current (e.g., [15]). The latter can be
induced, for instance, by a ponderomotive force
))(()( Buuuf ×+∇−= c/emep , (9)
where u and B are, respectively, the ac electron velocity
and magnetic field, and the angular brackets denote time
averaging. This force can result in stationary drift flow of
electrons across B0 with velocity ))(( 0 pd ˆeB/c fzu ×= .
Though we failed to detect the azimuthal electron drift,
computations show that it can be so substantial as to drive
the instability. We estimated ud with u and B computed in
linear approximation with use of the code [2]. Figure 12
shows the vector field of du in the central part of plasma
cross-section, at z = 20 cm. One can see two off-center,
slowly rotating vortices, and a rapid jet that moves
through the axis and arises mainly due to centrifugal drift
of electrons in the field of ponderomotive force. The jet
velocity is 5×104 cm/s at AI = 1 A and thus gets
supersonic value at antenna current above a few A. This
jet can be efficient driver of ion-acoustic waves.
Fig. 12. Vector field of stationary electron flow across B0
stimulated by ponderomotive force near the axis
The energy density of the LF pulsations was
determined by integrating over the low frequency
spectrum; its ratio to the plasma thermal energy was
found to be of the order of 2×10-4. The maximum density
fluctuations were about 10%. It is not yet clear whether
this level of turbulence is high enough to provide an
effective collision frequency sufficient for the helicon
absorption, or it is a sort of by-effect.
3. DISCHARGE IN NONUNIFORM
MAGNETIC FIELD
The plasma density in the helicon discharge can increase
when the magnetic field is reduced at the antenna location
[16-18]. This effect cannot be completely due to better
confinement as the total plasma production is strongly
increased at the same absorbed power. In our experiment
we used, instead of antenna shown in Fig. 1, a double-turn
m=0 antenna placed at 6 cm from the left end flange. A
naked magnetic probe movable along the axis measured
the Bz wave fields and ion saturation current. It was
calibrated by using the 8-mm interferometer. The helicon
discharge operated at Ar pressure of 5 mTorr and
absorbed power of 1 kW.
Figure 13 shows axial distributions of the magnetic
field and plasma density. Dashed curves were obtained
for the uniform field when all the coils in Fig. 1 were in
action. Solid curves correspond to the case when the left
59
- 2 - 1 0 1 2
- 2
- 1
0
1
2
X (cm)
Y
(c
m
)
set of coils was turned off and thus the antenna was in the
region of decreasing magnetic field. In the latter case,
downstream density at z=25 cm turns out to be 6-fold
higher at the same absorbed power. Also, the wave
amplitude highly exceeds that in the uniform field case.
Fig. 13. Dc magnetic field (a) and density (b) profiles
Proposed explanation of density increase is trapping
and acceleration of electrons by helicon waves, i.e. again
a sort of Landau damping [17]. Our phase measurements
showed that in uniform field the helicon phase velocity
corresponds to electron energy of 100 eV. In the
nonuniform field, it changes from 4 to 15 eV, which is
much more favorable for the electron trapping. In this
case, additional ion spectral lines with excitation energies
∼15 eV were observed. However, our measurements with
a flat probe did not detect any fast electrons, and thus the
nature of enhanced ionization is not clear. Since the effect
is rather strong, some still unknown, basic property of the
helicon discharge is thought to be involved.
4. CONCLUSIONS
Several wave effects important for the helicon
discharge operation were examined. Modeling
experiments with independently prepared plasma allowed
us to measure the total antenna impedance as a function
of density and magnetic field, for various antenna designs
and positions. This is needed for understanding of a short
discharge behavior, e.g., the density jumps.
The ion-acoustic turbulence was observed in probe
measurements under the helicon antenna. It can be excited
as a result of parametric instability or due to azimuthal
electron drift induced by a ponderomotive force.
Measured total energy of the acoustic turbulent pulsations
seems to be insufficient for the helicon absorption.
Plasma production was shown to strongy increase in
nonuniform magnetic field. Further experiments are
needed for revealing the physical reason of enhanced
plasma production.
REFERENCES
1. F.F.Chen and R.W.Boswell. IEEE Trans. Plasma
Sci. 25 (1997) 1245.
2. V.F.Virko, G.S.Kirichenko and K.P.Shamrai.
Plasma Sources Sci. Technol. 11 (2002) 10.
3. K.P.Shamrai. Plasma Sources Sci. Technol. 7 (1998)
499.
4. K.K.Chi, T.E.Sheridan and R.W.Boswell. Plasma
Sources Sci. Technol. 8 (1999) 421.
5. K.P.Shamrai, V.F.Virko, H.-O.Blom et al. J. Vac. Sci
Technol. A 15 (1997) 421.
6. J.P.Rayner and A.D.Cheetham. Plasma Sources Sci.
Technol. 8 (1999) 79.
7. T.Shoji, Y.Sakawa, S.Nakazawa et al. Plasma
Sources Sci. Technol. 2 (1993) 5.
8. F.F.Chen and G.Chevalier. J. Vac. Sci. Technol. A 10
(1992) 1389.
9. K.P.Shamrai and V.B.Taranov. Plasma Sources Sci.
Technol. 5 (1996) 474.
10.L.I.Grigor'eva, V.L.Sizonenko, B.I.Smerdov, et al.
Zh. Eksp. Teor. Fiz. 60 (1971)605.
11.M.Porcolab, V.Arunasalam and R.A.Ellis. Phys.
Rev. Lett. 29 (1972) 1438.
12.A.I.Akhiezer, V.S.Mikhailenko and K.N.Stepanov.
Phys. Lett. A 245 (1998) 117.
13.N.M.Kaganskaya,.M.Krämer and V.L.Selenin. Phys.
Plasmas 8 (2001) 4694.
14.Yu.M. Aliev, V.P. Silin and C. Watson. Zh. Eksp.
Teor. Fiz. 50 (1966) 943
15.R.L.Stenzel. Phys. Fluids B 3 (1991) 2568.
16.G.Chevalier and F.F.Chen. J. Vac. Sci. Technol. A
11 (1993)1165.
17.X.M.Guo, J.Sharer, Y.Mouzouris and L.Louis. Phys.
Plasmas 6 (1999) 3400.
18.O.V.Braginskij, A.N.Vasil’eva and A.S.Kovaljov.
Fiz. Plazmy 27 (2001) 741.
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ХВИЛЬОВІ ЯВИЩА В ГЕЛІКОННІЙ ПЛАЗМІ
В.Ф. Вірко, Г.С. Кириченко, К.П. Шамрай
Представлено результати експериментальних та теоретичних досліджень хвильових процесів у густій плазмі,
які збуджуються ВЧ антенами різних конструкцій в діапазоні геліконних частот. Експерименти проведено в
комбінованому ЕЦР-геліконному джерелі плазми. Виявлено, що виміряна багатопікова структура залежності
поглинання від густини плазми задовільно узгоджується з результатами обчислень і пов’язана зі збудженням
стоячих поздовжніх мод. Зі збільшенням ВЧ потужності досліджено перехід до самостійного геліконного
розряду. В чисто геліконному розряді виміряні спектри шумових коливань, які включають НЧ смугу з
шириною порядку 1 МГц та ряд ВЧ смуг поблизу основної частоти та її гармонік. Збудження НЧ коливань має
поріг по потужності, тобто відбувається параметричним чином, а також по магнітному полю. Ці коливання
ідентифіковано з іонно-звуковими хвилями, що розповсюджуються по азимуту у напрямку обертання
електронів. Їхні кореляційні довжини зменшуються зі зростанням магнітного поля, а інтенсивність різко
збільшується безпосередньо під антеною. Обчислення показують, що в умовах експерименту генерація звуку
можлива як за рахунок параметричної нестійкості, так і внаслідок дрейфової течії електронів в полі
пондеромоторної сили. Досліджено також геліконний розряд в неоднорідному магнітному полі та виявлено
різке зростання генерації плазми порівняно з випадком однорідного поля.
ВОЛНОВЫЕ ЯВЛЕНИЯ В ГЕЛИКОННОЙ ПЛАЗМЕ
В.Ф. Вирко, Г.С. Кириченко, К.П. Шамрай
Представлены результаты экспериментальных и теоретических исследований волновых процессов в плотной
плазме, возбуждаемых ВЧ антеннами различных конструкций в диапазоне геликонных частот. Эксперименты
проведены в комбинированном ЭЦР-геликонном источнике плазмы. Обнаружено, что наблюдаемая
многопиковая структура зависимости поглощения от плотности плазмы удовлетворительно согласуется с
результатами вычислений и связана с возбуждением стоячих продольных мод. При увеличении ВЧ мощности
исследован переход к самостоятельному геликонному разряду. В чисто геликонном разряде измерены спектры
шумовых колебаний, которые включают НЧ полосу с шириной порядка 1 МГц и набор ВЧ полос вблизи
основной частоты и ее гармоник. Возбуждение НЧ колебаний имеет порог по мощности, т.е. происходит
параметрическим образом, а также по магнитному полю. Эти колебания идентифицированы с ионнозвуковыми
волнами, распространяющимися по азимуту в направлении вращения электронов. Их корреляционная длина
уменьшается с ростом магнитного поля, а интенсивность резко возрастает непосредственно под антенной..
Вычисления показывают, что в условиях эксперимента генерация звука возможна как за счет параметрической
неустойчивости, так и вследствие дрейфового течения электронов в поле пондеромоторной силы. Исследован
также геликонный разряд в неоднородном магнитном поле и обнаружено резкое возрастание генерации плазмы
в сравнении со случаем однородного поля.
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