Plasma which are generally trapped into orbits around

Plasma is sometimes
called the fourth state of matter. The other three being solid, liquid, and
gas. The gas become ionized when an electric current pass through it.
Electrons, which are generally trapped into orbits around their atomic nucleus,
like planets around the Sun, can be energetically kicked out of their orbits
and drift freely. These free electrons make such ionized gases very good
conductors of electricity. Furthermore, both the free electron and the stripped
nucleus are electrically charged relative to a neutral atom, and the ionized
gas responds to electric and magnetic fields. As in magnetic fusion, ionized
gases(plasmas) can be trapped into magnetic or even electrostatic bottles. In this
way these plasmas can be insulated and heated to temperatures needed for
nuclear fusion. One method of producing such plasmas is the plasma pinch (neon
light). The neon gas becomes a plasma and glows with radiation output and generates
a magnetic field which traps and pinches the plasma when an electric current
passed through it. Plasmas are macroscopically electrical neutral like the
other three states of matter. Plasmas sometimes act like a solid, sometimes
like an incompressible fluid, and sometimes like a compressible gas. In fact,
it is better to think of solids, liquids, and gases as being three special
varieties of plasma. The atoms and molecules which comprise the relatively
electromagnetically insulated elementary constituents of a solid, liquid, or
gas are broken up.

    In the years
from 1960 to 1964, Fillipov in the Soviet Union and Mather in the United States
independently developed devices to generate, by simple means, hot dense plasmas
of rather short lifetimes. These devices have attracted considerable attention.
They are presently under investigation for a variety of reasons in about 20
laboratories all over the world. Plasma-Focus Device (PFD) is relatively inexpensive,
non-radioactive, low voltage, compact, and very efficient source of plasma and
radiation. With experienced guidance, it can be reproduced in experimental
laboratories around the world for use as a non-radioactive source of X-Rays,
plasma streams, electrons and ion beams, and neutrons for both
applications and basic research.

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    The
operation principle of plasma focus devices lies in the use of energy stored in
a capacitor bank to create a low-density plasma that is electromagnetically
accelerated and subsequently compressed by the Lorentz force(combination of
electric and magnetic force on a point charge due
to electromagnetic fields) associated with
the discharge current (pinch).      There were two mostly used arrangements of a
plasma focus device, commonly called “Fillipov-Type” which has short and large
diameter of discharge arrangement and the “Mather Type” which shows a long
structure with small diameter of discharge arrangement, resembling a plasma
accelerator. In both cases the sequence of events is as follows: After
switching the condenser bank (with a voltage of 20 to 50 kV), breakdown occurs
along the insulator, a plasma is created in the filling gas (mostly deuterium with
a filling pressure of some torr), the plasma moves as indicated due to the
pressure of the rising magnetic field and finally some small fraction of the
plasma is compressed in front of the centre electrode. This is the plasma focus
proper. This focussing of the plasma is normally arranged to happen when the
discharge current is at its maximum; consequently, a very effective compression
is possible.

High voltage applied to
the electrodes immersed in a low pressure (of the order of few Torrs) gas
causes an electrical break-down along the insulator. Fast rise of the current
(with ~1-5 us time scale) leads to the formation of a plasma sheath, which,
driven by jxB force, moves along the electrodes towards their
open end. During this process, the plasma sheath accelerates to a velocity of
107 cm/s and the current rises up to ~0.7-3 MA (depending on a
condenser battery power). After reaching the central electrode edge the sheath
collapses toward the axis forming a dense (up to 1020cm-3),
hot (~1 Kev), elongated plasma structure called “pinch”. Rapid development of
MHD(magnetohydrodynamic which is the
magnetic
properties of electrically conducting fluids) instabilities
causing disruption of the pinch is accompanied by an intense burst of ionizing
radiation: soft and hard X-rays, electrons, ions and neutrons. The neutrons are
from D-D reaction when deuterium is used as a filling gas. Fast (> 100km/s)
plasma stream is also generated along the axis. While the general features of
phenomena occurring in the plasma-focus discharges have been known for
several tens of years
there is still no detailed understanding of mechanisms ruling the structure,
dynamics and destruction of the current sheath which is responsible for
anomalous impedance (caused by micro-instabilities) and subsequent generation
of ion and electron beams which create then large yields of X-rays and
neutrons. So, the plasma generated in the PF is a very interesting object for
basic studies of current carrying plasmas.