MadSci Network: Physics

Re: after something changes into a plasma, can it change back into a gas?

Date: Mon Jan 19 10:53:40 2004
Posted By: Richard Bersin, Other (pls. specify below), Senior Technical Staff Member, Emergent Technologies
Area of science: Physics
ID: 1073613396.Ph

Dear Matt:To answer your question I will first tell you about making the 
plasma.  The simplest way to start is to talk about a neon sign.  This is 
a glass tubing sealed with neon gas inside at a low pressure.  There are 
two electrodes inside the tube, so that a voltage can be applied across 
tha gap between the electrodes.  This voltage causes an electrical 
breakdown in the gas, just like a lightning bolt is an electrical 
breakdown in the air at atmospheric pressure.   In the sign, the gas 
begins to glow and the sign is lit up.   Inside the gas there are negative 
electrons and positive neon ions created by the breakdown.   As long as 
the voltage remains on the electrodes these positive and negative 
particles in the gas will be colliding with each other and the neon atoms 
become electrically excited and emit the light light the sign.   As long 
as the voltage remains on the electrodes a current will flow, through the 
electrically charged gas between the electrodes and consume power.  As 
soon as you take away the volage on the electrodes the neon plasma will 
disappear because there is no longer an electrical field between the 
electrodes to keep the gas ionized and conducting. The gas then returns to 
its original state, no glow, and just back to being a gas like it was 
before the plasma was ignited!  For neon gas this is a simple situation 
because neon is chemically inert, so there is no change in the properties 
of the gas.   However, if you put a gas in the tube which becomes 
chemically reactive when in the plasma state, like methane (CH4) it is 
quite possible that chemical reactions will take place within the plasma 
and the chemical structure of the gas mixture may be changed, so when you 
turn off the plasma you may have a differnt chemical gas in the chamber. 
However it will still be a gas!

Dick Bersin......

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