THE EDGE OF SCIENCE — BROWN 405 



the theory of the plasma state but have led to engineering applications 

 which have been very numerous indeed. 



If we continue to pour more and more energy into an ordinary glow 

 discharge, it turns into what we call an "arc." In an arc, the electron 

 density can rise to 10® times the charge density that is found in the 

 sun at perhaps one-thousandth of the temperature. These kinds of 

 arc studies in the laboratory provide us with powerful tools for study- 

 ing the behavior of the plasma of the sun. Incidentally, at this kind 

 of temperature and pressure a great deal of work is now being done 

 to produce what are called "ion jet" engines. Ion engines may well 

 be the kind of engine that will move spaceships through the inter- 

 planetary space for long sustained flight after chemical rockets have 

 achieved the high initial force necessary to escape the earth's gravi- 

 tional field. Plasma jet engines are capable of providing a driving 

 force over the thousand years you need to reach out into interstellar 

 space. Obviously a great deal of practical engineering must be done 

 before this method of ion propulsion is perfected. 



The high-pressure arcs are the densest form of plasma that we 

 know. Here, all the material that is in the arc is ionized ; everything 

 is in the charged state. Here the theoretical studies are the most 

 characteristic of a plasma because the plasma is pure, undiluted by 

 un-ionized gas. Here also some very practical devices are being 

 worked on, particularly the "magnetohydrodynamic energy con- 

 verter." In a conventional turbine, gas energy is converted into the 

 kinetic energy of a moving conductor which then generates the elec- 

 tricity by cutting lines of magnetic flux, but if a gas conductor, a mov- 

 ing plasma, is used, the intermediate step is completely elimmated. 

 The plasma moving in a magnetic field produces a flowing current 

 which will allow us to produce generators without any moving parts. 

 There are plans for building very large generating stations by this 

 scheme in which the plasma is produced either by nuclear power, fis- 

 sion heat, or from a chemical reaction. 



Making very dense plasmas and going a little farther up into the 

 temperature region of 100,000 degrees K, we find the "shock tube" 

 as a plasma production device. When a mechanical shock wave is 

 driven down a tube faster than the velocity of sound, the shock wave 

 acts as a piston. Just as with a bicycle pump, you get heat because 

 the piston is pushing against the gas and doing work on it, so in the 

 shock tube you can produce very high temperatures. Some of the 

 highest temperatures we have achieved in the laboratory are produced 

 by shock waves. Another phenomenon which has been known to 

 physicists for a long time, but has only recently received attention in 

 the popular press, appears when we try to pull astronauts back out 

 of interplanetary space through our own atmosphere. When a 

 capsule comes down through our own atmosphere, it produces a shock 



