Plasma is sometimescalled the fourth state of matter. The other three being solid, liquid, andgas. 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 orbitsand drift freely. These free electrons make such ionized gases very goodconductors of electricity. Furthermore, both the free electron and the strippednucleus are electrically charged relative to a neutral atom, and the ionizedgas responds to electric and magnetic fields.
As in magnetic fusion, ionizedgases(plasmas) can be trapped into magnetic or even electrostatic bottles. In thisway these plasmas can be insulated and heated to temperatures needed fornuclear fusion. One method of producing such plasmas is the plasma pinch (neonlight). The neon gas becomes a plasma and glows with radiation output and generatesa magnetic field which traps and pinches the plasma when an electric currentpassed through it. Plasmas are macroscopically electrical neutral like theother three states of matter. Plasmas sometimes act like a solid, sometimeslike an incompressible fluid, and sometimes like a compressible gas. In fact,it is better to think of solids, liquids, and gases as being three specialvarieties of plasma.
The atoms and molecules which comprise the relativelyelectromagnetically insulated elementary constituents of a solid, liquid, orgas are broken up. In the yearsfrom 1960 to 1964, Fillipov in the Soviet Union and Mather in the United Statesindependently developed devices to generate, by simple means, hot dense plasmasof rather short lifetimes. These devices have attracted considerable attention.They are presently under investigation for a variety of reasons in about 20laboratories all over the world. Plasma-Focus Device (PFD) is relatively inexpensive,non-radioactive, low voltage, compact, and very efficient source of plasma andradiation.
With experienced guidance, it can be reproduced in experimentallaboratories around the world for use as a non-radioactive source of X-Rays,plasma streams, electrons and ion beams, and neutrons for bothapplications and basic research. Theoperation principle of plasma focus devices lies in the use of energy stored ina capacitor bank to create a low-density plasma that is electromagneticallyaccelerated and subsequently compressed by the Lorentz force(combination ofelectric and magnetic force on a point charge dueto electromagnetic fields) associated withthe discharge current (pinch). There were two mostly used arrangements of aplasma focus device, commonly called “Fillipov-Type” which has short and largediameter of discharge arrangement and the “Mather Type” which shows a longstructure with small diameter of discharge arrangement, resembling a plasmaaccelerator. In both cases the sequence of events is as follows: Afterswitching the condenser bank (with a voltage of 20 to 50 kV), breakdown occursalong the insulator, a plasma is created in the filling gas (mostly deuterium witha filling pressure of some torr), the plasma moves as indicated due to thepressure of the rising magnetic field and finally some small fraction of theplasma is compressed in front of the centre electrode. This is the plasma focusproper. This focussing of the plasma is normally arranged to happen when thedischarge current is at its maximum; consequently, a very effective compressionis possible.
High voltage applied tothe electrodes immersed in a low pressure (of the order of few Torrs) gascauses 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 theiropen end. During this process, the plasma sheath accelerates to a velocity of107 cm/s and the current rises up to ~0.7-3 MA (depending on acondenser battery power).
After reaching the central electrode edge the sheathcollapses toward the axis forming a dense (up to 1020cm-3),hot (~1 Kev), elongated plasma structure called “pinch”. Rapid development ofMHD(magnetohydrodynamic which is themagneticproperties of electrically conducting fluids) instabilitiescausing disruption of the pinch is accompanied by an intense burst of ionizingradiation: soft and hard X-rays, electrons, ions and neutrons. The neutrons arefrom 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 ofphenomena occurring in the plasma-focus discharges have been known forseveral tens of yearsthere is still no detailed understanding of mechanisms ruling the structure,dynamics and destruction of the current sheath which is responsible foranomalous impedance (caused by micro-instabilities) and subsequent generationof ion and electron beams which create then large yields of X-rays andneutrons. So, the plasma generated in the PF is a very interesting object forbasic studies of current carrying plasmas.