Optimization of initial gal distribution in plasma focus discharges
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Національний науковий центр «Харківський фізико-технічний інститут» НАН України
1999
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| Назва видання: | Вопросы атомной науки и техники |
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| Цитувати: | Optimization of initial gal distribution in plasma focus discharges / N.I. Ayzatsky, A.N. Dovbnya, N. G. Reshetnyak, V.V. Zakutin, E.Yu. Khautiev, V.I. Krauz, M.A. Krasnogolovets, Ju.Ya. Volcolupov // Вопросы атомной науки и техники. — 1999. — № 3. — С. 88-90. — Бібліогр.: 5 назв. — англ. |
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Digital Library of Periodicals of National Academy of Sciences of Ukraine| id |
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irk-123456789-813692016-04-15T13:29:40Z Optimization of initial gal distribution in plasma focus discharges Ayzatsky, N.I. Dovbnya, A.N. Reshetnyak, N.G. Zakutin, V.V. Khautiev, E.Yu. Krauz, V.I. Krasnogolovets, M.A. Volcolupov, Ju.Ya. 1999 Article Optimization of initial gal distribution in plasma focus discharges / N.I. Ayzatsky, A.N. Dovbnya, N. G. Reshetnyak, V.V. Zakutin, E.Yu. Khautiev, V.I. Krauz, M.A. Krasnogolovets, Ju.Ya. Volcolupov // Вопросы атомной науки и техники. — 1999. — № 3. — С. 88-90. — Бібліогр.: 5 назв. — англ. 1562-6016 http://dspace.nbuv.gov.ua/handle/123456789/81369 en Вопросы атомной науки и техники Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
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Digital Library of Periodicals of National Academy of Sciences of Ukraine |
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DSpace DC |
| language |
English |
| format |
Article |
| author |
Ayzatsky, N.I. Dovbnya, A.N. Reshetnyak, N.G. Zakutin, V.V. Khautiev, E.Yu. Krauz, V.I. Krasnogolovets, M.A. Volcolupov, Ju.Ya. |
| spellingShingle |
Ayzatsky, N.I. Dovbnya, A.N. Reshetnyak, N.G. Zakutin, V.V. Khautiev, E.Yu. Krauz, V.I. Krasnogolovets, M.A. Volcolupov, Ju.Ya. Optimization of initial gal distribution in plasma focus discharges Вопросы атомной науки и техники |
| author_facet |
Ayzatsky, N.I. Dovbnya, A.N. Reshetnyak, N.G. Zakutin, V.V. Khautiev, E.Yu. Krauz, V.I. Krasnogolovets, M.A. Volcolupov, Ju.Ya. |
| author_sort |
Ayzatsky, N.I. |
| title |
Optimization of initial gal distribution in plasma focus discharges |
| title_short |
Optimization of initial gal distribution in plasma focus discharges |
| title_full |
Optimization of initial gal distribution in plasma focus discharges |
| title_fullStr |
Optimization of initial gal distribution in plasma focus discharges |
| title_full_unstemmed |
Optimization of initial gal distribution in plasma focus discharges |
| title_sort |
optimization of initial gal distribution in plasma focus discharges |
| publisher |
Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
| publishDate |
1999 |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/81369 |
| citation_txt |
Optimization of initial gal distribution in plasma focus discharges / N.I. Ayzatsky, A.N. Dovbnya, N. G. Reshetnyak, V.V. Zakutin, E.Yu. Khautiev, V.I. Krauz, M.A. Krasnogolovets, Ju.Ya. Volcolupov // Вопросы атомной науки и техники. — 1999. — № 3. — С. 88-90. — Бібліогр.: 5 назв. — англ. |
| series |
Вопросы атомной науки и техники |
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| fulltext |
OPTIMIZATION OF INITIAL GAS DISTRIBUTION IN PLASMA FOCUS
DISCHARGES
N.I. Ayzatsky, A.N. Dovbnya, N.G. Reshetnyak, V.V. Zakutin, E.Yu. Khautiev*, V.I. Krauz*,
M.A. Krasnogolovets**, Ju.Ya .Volkolupov**
NSC KIPT, Kharkov, Ukraine; *RSC, Kurchatov Institute, Moscow, Russia; **KGTURE,
Kharkov, Ukraine
INTRODUCTION
As the increasing number of plasma focus (PF)
installations turns to be operated at power supply
energies(W) above 100 kJ, new factors become evident
to come into play that prevent obtaining the neutron
yields (N) predicted by the well-known scaling N ~ W2.
This is mainly due to the fact that with an increasing
energy there is no increase in the current flowing in
plasma filament compressed on the axis of a discharge
system proportionally to W, and a part of discharge
current starts to flow at the periphery [1]. It is also
known that the main condition of matching the moment
when the discharge current reaches its peak value and
the moment when the plasma (current) sheath coincides
with the axis leads to a perfectly certain ratio of PF-
system parameters which can vary with optimization of
discharge conditions: V2C2/r2l2P = const, where V and C
are, respectively, the initial voltage and capacity of a
capacitive energy accumulator, l is the accelerating
electrode length, r is the radius of internal electrode
(anode), and P is the initial working-gas pressure. This
ratio shows that with changing power supply parameters
it is necessary that either the discharge system sizes or
the working gas pressure should be appropriately
changed. It is more preferable to carry out matching by
varying the working gas pressure P, for in this case
there is no need to perform complicated work associated
with changes in the accelerator sizes.
There are two optimum points on initial pressure
in the PF discharges. If pressure is relatively low, then
the formation of uniform plasma sheath takes place. The
latter effectively carries away the gas from the initial
region of discharge volume, that essentially increases
the electrical strength of internal electrode gap required
for peaking the discharge power at a final stage [2]. As
the pressure rises, the discharge dynamics at the final
stage of PF formation as well as the mechanisms of
neutron generation become optimized [3]. However, in
practice, the pressure can be increased only up to a
certain limit, following which a noticeable deterioration
in operating conditions of the installation occurs.
1. EXPERIMENTAL INSTALLATION OF
RESEARCH TECHNIQUE
One of the ways to optimize the PF system in the
neutron yield is to create profile gas distributions, where
the working gas density in the vicinity of insulator is
lower than the one in the region of PF formation. For
this purpose, PF discharges have been investigated on
the installation CPF-1M (W ~ 40 kJ, V ~ 25kV,
T/2 ~ 7 µs) at pulsed neutral-gas supply conditions with
the use of an electrodynamics valve. The studies were
performed by two methods shown schematically in
Fig. 1.
Fig.1 The scheme of the installation CPF –1M
1 - insulator; 2 - electrodes; 3 - gas radial filling;
4 - axial filling; 5 - a tip; 6 - vacuum volume
In the first case, the gas (deuterium) was let in
radially in the middle part of a coaxial-type discharge
system, through 30 holes, each of 4 mm in diameter, in
a cylindrical external electrode with a diameter of 130
mm. The internal electrode was 80 mm in diameter; the
electrode length was 240 mm. The insulator was made
of alundum. Before being filled with gas, the vacuum
chamber was pumped out to a pressure of < 10-5 torr.
Voltage and discharge current, time and integral
parameters of X-rays and neutron radiation, energy
spectra of ion beams generated in the PF have been
measured in the experiments. The discharge was
photographed in visible and X-ray regions of the
spectrum.
The quantity of gas in the discharge volume and
its distribution along the axis were controlled within
certain limits by varying the delay (τ) between the
moment of valve opening and the onset of discharge.
2.EXPERIMENTAL RESULTS AND DISCUSSION
PF-discharge characteristics have been
investigated in a wide range of delays. At given
experimental conditions the PF discharge was observed
in the delay τ range between 1.0 and 2.5 ms. At τ = 1.0
to 1.4 ms the discharge behavior is similar to that at a
low pressure regime with a steady-state gas filling of the
vacuum chamber [2, 4], the only difference being that
the singularity here exists for a longer time of ∆τ
= 0.6… 0.8 µs.
As it can be seen from the voltage and current
oscillograms in Fig. 2, a succession of discharge current
disruptions and the corresponding voltage spikes up to
100 … 120 kV take place.
ВОПРОСЫ АТОМНОЙ НАУКИ И ТЕХНИКИ. 1999. №3.
Серия: Ядерно-физические исследования. (34), с. 88-90.
88
Fig. 2.Typical waveforms of voltage (upper) and current
(lower) of the plasma focus discharge.
At this stage, intense bursts of hard X-rays are
registered. The integral pinhole-camera pictures taken in
hard X-rays from the end of the discharge system
(Fig. 3) display that the hard X-rays are emitted not only
in the near-axis region of the anode.
Fig. 3 (top) Pinhole-camera pictures in hard X-rays
(viewed from the end) obtained at various delays τ.
From left to rate: an open hole with a diameter of 1 mm;
aluminium foil closed hole (foil thickness δ =0.4mm; δ
=0.8mm) (bottom) Framing pictures of the discharge
(viewed from the end).The interval between the frames
-0,5 µs.
Well-localized or diffuse sources of hard x-rays
bremsstrahlung can be seen on different parts of the end
of the internal electrode. This points to the fact that
kinetic instabilities are excited as early as at the stage of
radial compression of the current sheet which emerges
at the end of the internal electrode to the region with a
decreasing neutral-gas density. In this case, the current
sheet is split into separate filaments. The instabilities at
the nonlinear stage transfer the plasma to a turbulent
state. This results in an abnormally high (of about 10-1
Ohm) resistance of the current channel. Plasma
electrons and ions get accelerated to high energies in the
arising strong electric fields. The pinhole-camera
measurements (3 input holes, each being 1 mm in
diameter) using various absorbing filters give the
average energy of the main portion of beam electrons to
range between 30 and 100 keV.
A similar picture of inhomogeneous
luminescence of the end of the internal electrode is also
observed in the framing pictures of the discharge (Fig.3
bottom). It is seen that there is no axial localization of
plasma and it consists of separate filaments.
As τ increases, the gas distribution, set at short
delays, with the density maximum in the middle part of
electrodes (place of gas filling) and with the density
fall-off in the vicinity of the insulator and at the end of
electrodes, gradually changes. As a result of gas
accumulation in the closed initial region of the
discharge volume, the gas density grows close by the
insulator. In this case, the «singularity» duration on
volt-ampere curves and the amplitude of voltage spike
decrease. Almost in the whole range of delays (τ = 1.1
… 2.2 µs), the neutron yield remains at a level of < 109
neutrons/discharge at W ~ 40 kJ, and it was lower than
at conditions with a constant gas filling of chamber at
the same energy.
The results show that the distribution of neutral
gas with a reduced density in the initial part of the
discharge volume is optimum for creating conditions
when the interelectrode space behind the accelerated
plasma sheath acquires and retains an electrical strength
high enough to maintain a rather long existence of the
«singularity» (< 1 µs). During this time, the discharge
power reaches a value of > 1011 W. On the other hand,
low density in the region of radial current-sheet
compression does not allow to form an axisymmetrical
PF with the highest possible current density in it. As a
result, it appears impossible to form a dense cumulative
plasma jet required for the effective beam-target
mechanism of neutron production [3].
In the following series of experiments the
method of gas inlet into the discharge volume was
modified so that the gas density should gradually
increase from the insulator to the end of electrodes.
With this purpose, a hollow copper cylindrical tip 5 with
a diameter of 140 mm was added to the former design
of the discharge system. The bottom of this tip was set
at a distance of ~ 70 mm from the internal electrode
end. The gas came through the ∅ 12-mm axial channel
in the internal electrode and, being reflected from the tip
bottom, it uniformly filled the internal electrode
volume.
The gas pressure variation with time in the initial
and final regions of interelectrode volume has been
investigated previously [5].
Fig.4 shows the time behavior of pressure curve
for the pressure P0 = 3 atm in the subvalve volume of
the electrodynamics valve. It is seen that the pressure at
both points of initial (1) and final (2) regions of
acceleration space grows linearly up to 3.5 torr,
approximately.
89
Fig. 4
Fig.5 gives the plot of neutron yield as a function
of delay in the same time scale as the pressure curves in
Fig.4.
Fig. 5.
1 - neutron yield with axial pulsed gas filling. Each
point is averaged over 10 discharges.
2 - neutron yield at steady-state gas filling of vacuum
chamber P=3,5 torr (deuterium).
From comparison between the curves in Figs.4
and 5, it can be seen that the neutron yield grows
together with an increasing amount of gas coming into
the discharge volume. Discharge regimes have been
defined at optimum delays. These regimes are
characterized by a high reproducibility of neutron yield
as compared with a radial gas fielling. The maximum
neutron yield has increased up to 1.5⋅1010
neutrons/discharge, this being comparable with the
values corresponding to the scaling (N ~ W2) for a
power of 40 kJ.
At τ~ 2.5 ms, when the gas distribution along the
electrode length becomes almost uniform, the neutron
yield begins to decrease. When pressure nearby the
insulator exceeds pressure at the end of the internal
electrode, the neutron yield value becomes lower than at
steady-state gas filling.
Fig. 6 shows the signals from the type SNFT-3
photomultiplier with a scintillator, recording neutron
radiation and hard X-rays, the time-of-flight lag being R
= 6. 5 m. The energy spectrum of ions accelerated in the
PF was investigated using a magnetic analyzer [3]. This
analyzer can detect ions in a wide energy range (from
10 keV up to 2 MeV) with a high time resolution.
Generation of intense ion beams of energy E ~ 0.8 MeV
is always synchronous with sharp bursts of hard X-rays
high amplitude.
Fig.6.Pulses from hard X-rays (X1 and X2) neutron
radiation’s (n1 and n2).
CONCLUSION
Thus, the axial method of working gas filling
allows one to optimize both the initial and the final
phases of the discharge with the result that a rather high
and well reproducible neutron yield is obtained. This
method of gas filling also made it possible to investigate
discharge regimes (steady-state gas filling of chamber)
at conditions of additional introduction of various
impurity components (Xe, Ar), without deteriorating
dynamic properties of the discharge, which have an
appreciable effect on radiation energy release in the PF.
REFERENCES
1. Openlander N., Measurement of magnetic field and
current dencity distribution in the Frascati 1 MJ Plasma
Focus Device, " Com. Naz. Energ. Nucl. Center Frascati
| Pap. | ", 1978, N 6.
2. Kolesnikov Yu.A., Filippov N.V., Proc. 7th Int.
Conf.on Ionic. Phenomena in Gases, Belgrad, 1965, 2,
p.833-837.
3. Krauz V.I., Salukvadze R.G., Khautnev E.Yu.
Energetic spectra of ion bunches formed in a plasma
focus, Phisica plasmy, V.11 ,N3, 1985, p. 281-287.
4. Krauz V.I. An experimental research of structures in
plasma-focus discharge. A thesis abstract submitted to
the degree of a candidate of physical-mathematical
sciences, Kharkov 1988, p. 14.
5. Goncharenko V.P. Slavnuy A.S. The pulse ionization
pressure manometer. PTE, 1985, N 5, p 242.
89
CONCLUSION
REFERENCES
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