Experimental investigations of the vortex-chain generated in the two-component plasma with the sheared flow of magnetized electrons
We present further experimental results on the observation of electron vortices in an electrostatic plasma lens at the strong radial gradient of electron density. Anharmonic low frequency large-amplitude potential waves propagating in azimuth direction are observed. The recently observed results...
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| Zitieren: | Experimental investigations of the vortex-chain generated in the two-component plasma with the sheared flow of magnetized electrons / Yu. Chekh, A. Goncharov, A. Evsyukov // Вопросы атомной науки и техники. — 2006. — № 6. — С. 115-117. — Бібліогр.: 14 назв. — англ. |
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Chekh, Yu. Goncharov, A. Evsyukov, A. 2015-05-20T17:20:26Z 2015-05-20T17:20:26Z 2006 Experimental investigations of the vortex-chain generated in the two-component plasma with the sheared flow of magnetized electrons / Yu. Chekh, A. Goncharov, A. Evsyukov // Вопросы атомной науки и техники. — 2006. — № 6. — С. 115-117. — Бібліогр.: 14 назв. — англ. 1562-6016 PACS: 52.25.Xz, 52.27.Jt, 52.35.-g, 52.35.Fp, 52.35.Kt, 52.35.We, 52.35.Mw https://nasplib.isofts.kiev.ua/handle/123456789/81797 We present further experimental results on the observation of electron vortices in an electrostatic plasma lens at the strong radial gradient of electron density. Anharmonic low frequency large-amplitude potential waves propagating in azimuth direction are observed. The recently observed results give additional confirmation that the observed waves are the manifestation of the vortex-chain generated at the azimuthal sheared flow of magnetized electrons compensating ion beam. This work was supported in part by the Ukrainian Academy of Sciences, Contract No. 0105U005319 en Національний науковий центр «Харківський фізико-технічний інститут» НАН України Вопросы атомной науки и техники Basic plasma physics Experimental investigations of the vortex-chain generated in the two-component plasma with the sheared flow of magnetized electrons Article published earlier |
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Experimental investigations of the vortex-chain generated in the two-component plasma with the sheared flow of magnetized electrons |
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Experimental investigations of the vortex-chain generated in the two-component plasma with the sheared flow of magnetized electrons Chekh, Yu. Goncharov, A. Evsyukov, A. Basic plasma physics |
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Experimental investigations of the vortex-chain generated in the two-component plasma with the sheared flow of magnetized electrons |
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Experimental investigations of the vortex-chain generated in the two-component plasma with the sheared flow of magnetized electrons |
| title_fullStr |
Experimental investigations of the vortex-chain generated in the two-component plasma with the sheared flow of magnetized electrons |
| title_full_unstemmed |
Experimental investigations of the vortex-chain generated in the two-component plasma with the sheared flow of magnetized electrons |
| title_sort |
experimental investigations of the vortex-chain generated in the two-component plasma with the sheared flow of magnetized electrons |
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Chekh, Yu. Goncharov, A. Evsyukov, A. |
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Chekh, Yu. Goncharov, A. Evsyukov, A. |
| topic |
Basic plasma physics |
| topic_facet |
Basic plasma physics |
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2006 |
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English |
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Вопросы атомной науки и техники |
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Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
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Article |
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We present further experimental results on the observation of electron vortices in an electrostatic plasma lens at the
strong radial gradient of electron density. Anharmonic low frequency large-amplitude potential waves propagating in azimuth direction are observed. The recently observed results give additional confirmation that the observed waves are the manifestation of the vortex-chain generated at the azimuthal sheared flow of magnetized electrons compensating ion beam.
|
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1562-6016 |
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https://nasplib.isofts.kiev.ua/handle/123456789/81797 |
| citation_txt |
Experimental investigations of the vortex-chain generated in the two-component plasma with the sheared flow of magnetized electrons / Yu. Chekh, A. Goncharov, A. Evsyukov // Вопросы атомной науки и техники. — 2006. — № 6. — С. 115-117. — Бібліогр.: 14 назв. — англ. |
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| first_indexed |
2025-11-25T22:31:40Z |
| last_indexed |
2025-11-25T22:31:40Z |
| _version_ |
1850566012794044416 |
| fulltext |
Problems of Atomic Science and Technology. 2006, 6. Series: Plasma Physics (12), p. 115-117 115
EXPERIMENTAL INVESTIGATIONS OF THE VORTEX-CHAIN
GENERATED IN THE TWO-COMPONENT PLASMA
WITH THE SHEARED FLOW OF MAGNETIZED ELECTRONS
Yu. Chekh, A. Goncharov, A. Evsyukov
Institute of Physics, National Academy of Sciences of Ukraine,
03680, pr. Nauky 46, Kyiv, Ukraine, e-mail: chekh@iop.kiev.ua
We present further experimental results on the observation of electron vortices in an electrostatic plasma lens at the
strong radial gradient of electron density. Anharmonic low frequency (νci << ν ∼ νpi << νpe, νce) large-amplitude
(ϕosc >> kBolTe/e, kBolTi/e) potential waves propagating in azimuth direction are observed. The recently observed results
give additional confirmation that the observed waves are the manifestation of the vortex-chain generated at the
azimuthal sheared flow of magnetized electrons compensating ion beam.
PACS: 52.25.Xz, 52.27.Jt, 52.35.-g, 52.35.Fp, 52.35.Kt, 52.35.We, 52.35.Mw
1. INTRODUCTION
In the theory of plasma physics, plasma is often
assumed to be quasineutral. The deviation from this rule
is possible for beam plasmas [1], diode plasma systems
[1, 2], various plasma-optical devices [3, 4], plasmas with
high-frequency electromagnetic oscillations, and
obviously in one-component plasmas [5]. Recent
researches of tokamak plasma also reveal the fact that the
assumption of quasineutrality does not allow to analyze
all the variety of tokamak operation regimes correctly [6,
7]. We investigated the possibility for nonneutral vortices
to be formed in the two-component low-temperature low-
density collisionless ion beam plasma consisting of
positive unmagnetized ions of medium-energy beam and
magnetized electrons. Such a medium can be formed in
the electrostatic plasma lens (PL) [3, 4, 8]. This lens is the
axially symmetric plasmaoptical device intended for
positive ion beam manipulation. In fact, it is an electron
trap, where electrons are retained in longitudinal and
radial directions by the electrostatic and magnetostatic
fields, respectively. Electrons are generated by the
peripheral ions through the ion-electron emission from
electrodes of the lens, some part of these electrons
remains in the lens providing equipotentialization of
magnetic field lines [3, 4]. In the used experimental
conditions magnetic field is strong enough to magnetize
electrons, but practically has no effect on ions. Similarly,
the voltage applied to the lens electrodes is sufficiently
high to realize overthermal space-charge electric field but
it is still small for ion trajectories to be effectively
bended. The electric field of the negative space charge in
the lens and externally applied magnetic field results in
the azimuthal drift of electrons with the velocity
V = [E × B]/B2, (1)
where E is the electric field intensity, B is the magnetic
field induction. It is known that the radial gradient of the
drift velocity (or shear) causes the excitation of strong
instability [9-11]. This gradient may arise due to the
presence of radial magnetic field gradient or nonuniform
distribution of electron density. The excitation of
instability results in the bunching of electrons and their
self-consistent electric fields cause additional vorticity of
electron trajectories. As it was shown theoretically
[10,11], these bunches, having approached some density,
can create the chain of electron vortices with closed
trajectories of electrons. In [12] we represented the first
experimental results on the observation of nonlinear stage
of the instability at large radial gradients of electron
density where large-scale electron vortices were observed.
In this paper we present some addition results, which
were not placed in [12] as well as experimental results
obtained recently.
2. EXPERIMENTAL SETUP AND APPROACH
A vacuum arc ion source [13] with a grid anode and
three-electrode, multi-aperture, accel-decel ion extraction
system was used (see Fig. 1). Ion beamlets extracted from
emission holes widen during propagation in the space
between source and lens to form practically uniform ion
beam current density at the lens inlet aperture.
The source operates in a repetitively-pulsed mode and
produces moderate-energy, broad, heavy metal ion beam
with principal parameters for the work described here:
beam pulse duration – 100 µs (0.5 pps), beam extraction
voltage ϕacc – 12 and 24 kV, beam current Ib = 150 and
400 mA, Cu ion species.
Fig. 1. Scheme of experimental setup:
1 – vacuum chamber; 2 – ion source;
3 – capacitive or Langmuir probes; 4 – plasma lens;
5 – ion beam; 6 – collector
The electrostatic plasma lens (Fig. 2) had an input
aperture diameter of 7.4 cm and a length of 16 cm. The
maximum positive potential ϕL = 1 kV was applied to the
central lens electrode and several symmetrically arranged
adjacent pair electrodes, the other electrodes being
grounded. Most experiments were performed at high
voltage applied to the central electrode and one pair of
adjacent electrodes. In [12] we used DC potential ϕL, in
the present work we used repetitively pulsed feeding of
electrodes. In the latter case the lens electrodes where
mailto:chekh@iop.kiev.ua
116
connected to the source of accelerating voltage of the ion
source through a resistive divider. The repetitively pulsed
feeding of electrodes is needed to eliminate the self-
sustaining discharge in the lens [14]. Also using this
regime simplifies measuring the electric potential at low
frequencies by capacitive probes. The magnetic field with
the induction of 40 mT at the PL center was created by
permanent magnets. The vacuum chamber pressure was
≤ 1.5 × 10-5 Torr, allowing plasma formation within the
lens volume by the beam itself via ion-electron emission
from the lens electrodes.
Fig. 2. Plasma lens:
1 – permanent magnets; 2 – magnetic conductor;
3- magnetic field lines; 4 – electrodes; 5 – approximate
localization of electron layer
The azimuthal and radial distributions of electric
potential were measured using a system of capacitive
probes. The scheme of probes used in [12] is represented
in Fig. 3a. The shields of these probes are not fully
enclosed, and we assume that this is the reason for
incorrect probe operation at low frequency range. This
force us to use Langmuir probes to measure long time
variation of electric potential which we call “background”
potential in [12]. Now we use fully enclosed probes
represented in Fig. 3b. This enables us to measure
simultaneously high and low frequency dynamics of
electric potential using capacitive probes only.
Fig. 3. Schemes of cylindrical capacitive probes:
a) previous version [12]; b) wholly isolated probe;
1 - metal shield, 2 - glass insulator;
3 - copper conductor; 4 - lacquering
The probes were introduced nearly parallel to the system
axis, their sensitive tips being placed in the central cross
section of the PL.
3. EXPERIMENTAL RESULTS
We used high voltage applied to the neighboring
electrodes for annular electron density distribution to be
created. Electrons emitted from the lens electrodes have
to equipotentilize magnetic field lines, following to step-
like potential distribution over the electrode system
applied externally. In turn, the step-like radial potential
distribution corresponds to the mentioned above annular
electron density distribution.
As it was anticipated, the maximum amplitude of the
waves was observed in the rage of localization of the
potential step. The observed large-amplitude anharmonic
waves (Fig. 4) were found to propagate in the E × B drift
direction with the constant angular velocity (E is the
background electric field).
Fig. 4. Radial potential distributions in the PL central
cross section; 1 – background potential; 2 – lowest
potential achieved during the oscillation period;
the radial dimension of the vortex is denoted
by a double-headed arrow;
Ib = 400 mA, ϕacc = 12 kV, ϕL = 1 kV (DC), mθ = 5
For the conditions observed, electron and ion
Langmuir frequencies are νpe ≈ 300 MHz, νpi ≈ 1.7 MHz
and gyrofrequencies equal to νce ≈ 1.4 GHz, νci ≈ 20 kHz,
respectively, while the observed frequencies of
oscillations ν ≈ (1…2) MHz therefore the conditions
νci << ν ∼ νpi << νpe, νce is satisfied. The frequency of
rotation ν* = ν/mθ of the constant-phase regions around
the PL axis, as well as the number of wavelengths (mθ)
within the 360° azimuthal angle interval, depend on the
distance of a potential step (created by electrodes) from
the axis and on the magnitude of this potential step. For
the same electrode potential ϕL = 1 kV, the frequency ν*
was found to change within 200…500 kHz, while mθ
being within 4…6, depending on the potential distribution
over electrode system. The temperature of electrons
emitted from lens electrodes is near Te ≈ 10 eV while the
effective temperature of ions Ti ≈ 100 eV [8], therefore
the amplitude of potential oscillations ϕosc >> kBolTe/e,
kBolTi/e, where kBol - Boltzmann constant, e – electron
charge.
We imply that a “vortex” is localized structure with
closed trajectories, i.e. the vortex has to have separatrix.
In the investigated conditions electrons are strongly
magnetized, and we can omit the inertial terms in the
equation of motion for electrons. As the electron plasma
in the PL is low-temperature and collisionless, we can use
simple drift equation of motion, which has the solution
(1). Equation (1) defines the one of the main principles of
plasmaoptics: the magnetic field lines have to be
equpotential, also from this equation it follows that
equipotentials of electric field correspond to electron
trajectories [3]. Thus we can clearly recognize formation
of vortices by the presence of local extrema of the
potential distribution in the plane, which is normal to
magnetic field. Local minima correspond to vortex-
bunches with the local excess of electrons, and maxima
117
correspond to vortex-holes with the luck of ones. Fig. 4
indicates the presence of electron vortex-bunches
displaced in the radial direction approximately between
17 and 28 mm, that is the radial size of the separate vortex
is near 1 cm. The small maximum near r = 17 mm on the
distribution 2 in Fig. 4 is just saddle point which does not
represent the vortex. The mode number mθ = 5 represents
the number of vortices in the “chain” displaced along the
azimuthal direction.
The curve 2 in the Fig. 4 was obtained through
calculations from distributions of the “background”
potential and oscillation amplitude measured with
Langmuir probe and capacitive probe, respectively. As it
was mentioned above, using repetitively-pulsed voltage
ϕL and capacitive probe represented in Fig. 3b, we have
measured dynamics of the electric potential without
splitting it on long time (background) variations and high
frequency oscillations (see Fig. 5). From Fig. 5 one can
conclude that there is the local minimum of radial
potential distribution near r = 25 mm revealing the chain
of electron vortex-bunches.
It should be noted that the considered conditions are
far beyond the optimal ones of PL operation at low
magnetic fields.
Fig. 5. Oscillograms of electric potential obtained in the
PL central cross section at various distances from the PL
axis with the capacitive probe represented in Fig. 3b;
vertical sweep – 135 V/div; horizontal sweep – 20 µs/ div;
) r = 0, b) r = 20 mm, c) r = 25 mm, d) r = 30 mm, e) r
= 35 mm; Ib = 150 mA, ϕacc = 24 kV, ϕL = 1 kV (pulsed)
This work was supported in part by the Ukrainian
Academy of Sciences, Contract No. 0105U005319
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