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Name: Fred
Status: educator
Grade: 9-12
Location: OR
Date: December 2007


Question:
How can a plasma form of matter be changed into a gas and what would be an example of this change?



Replies:
Fred,

As you know, plasma is nothing more than ionized gas, a state in which some or all of the electrons of the gaseous atoms have been stripped from the atom leaving a positively charged ion. One can then imagine that plasma may be converted to gas when the high energy state is no longer maintained and the ions combine either with other ions or with electrons to form stable molecules.

Greg (Roberto Gregorius)


Fred-

When does a gas change to a liquid? Or a liquid to a solid? When it cools enough. Same common sense applies to plasma and gas. Being at high temperature is what keeps the electrons from settling onto the nuclei. If that temperature declines (say, from 10,000 degrees Kelvin to "only" 1,000 degK) or the electron-exciting energy bombardment stops (say, stopping the electric current that is being forced through the gas in a Neon lamp ), then the plasma decays and there is only a gas.

Examples: (not too many in common experience):

- Neon lamp On/Off. also Fluorescent lamp, Mercury lamp, Sodium lamp, any "arc" lamp.

- many plasma-based laboratory devices such as sputter-depositers, Reactive-Ion-Etchers (RIE), etc.

- any of our scientific attempts at fusion reactors: tokamaks, laser-implosion, etc.

- The gas in a medium-hot star is pretty well ionized, even in the cooler outer layers. Eventually the star "explodes", (normal end-of-life nova), sends out ionized shock-waves of gas. This gas radiates light, looses energy, cools down, and some of it might eventually collect by gravity to make the cool gas atmosphere of a Jupiter-like planet around a new star.

- The exhaust of an electric plasma torch.

- the powerful glow-ball of an atomic bomb explosion cooling down into an aging mushroom cloud.

- an electric spark, or a lightning bolt.

- a high-power laser pulse laser focused on a spot in mid-air makes a sharp cracking sound and spark-like flash as the plasma-patch suddenly forms then fades.

- in a "Jacob's Ladder" arc between two thick vertical wires, the air that is in the plasma bridge one second, a second later is elsewhere in the air around it. Probably above the arc, given convection.

The solid-liquid transition and the liquid-gas transition are often very sharp with respect to temperature. I don't think the gas-to-plasma transition is like that. It's gradual, and partial-percentages of ionization are the norm. Gasses at 2000K might have less than 1 atom in a billion ionized, but at 20,000K be approaching 100% ionization. Both can be called plasmas in various discussions.

However, plasmas in thick cool gas do keep moderately sharp boundaries. These are the plasmas sustained by non-equilibrium excitations such as electric arcs (as opposed to being surrounded by uniform high temperature). In many plasma machines, and in some Neon lamps (old "flicker-flame" candle-like bulbs that make yellow-orange light between two metal spade-shapes, and the plasma zone moves around erratically) you can see the glowing zone of plasma has edges where the plasma has stopped spreading out, and there's just dark non-ionized gas outside it. Higher gas pressure (> 10 torr) encourages plasma boundaries, lower gas pressure (< 10 torr) tends to allow more spreading. At a boundary you can imagine gas atoms diffusing freely across the border in both directions. But their time-averaged ionization percentage changes depending on which side they're on.

Jim Swenson



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