1. Figure 10 shows the equilibrium composition of a hydrogen plasma as a function of temperature and pressure. It is seen that the plasma is approximately 9-10percent ionized at temperatures of 15,000 K. This degree of ionization is more than adequate to place the conductivity in the Coulomb-dominated regime in which the conductivity varies as T312, a desirable range in order to avoid electrothermal instability effects. Further addition of energy to achieve higher levels of ionization would probably not be justified. To overcome these difficulties, there are three poss ible options:
2. mentioned earlier, monatomic gases would avoid frozen flow loses due to dissociation and still provide sufficient conductivity at temperatures of 10,000-15,0000K.
3. 9 and an effective cross section Qeg 110II 2 = 10-19m2, together with z = fl=3x105 m/sec, we calculate e T e - T -e
4. The following monatomic gases might be considered: H, He, Ne, A, Kr, and Xe. Of these, He and Ne should be discarded because of their high ionization potentials and high associated frozen flow losses. The cross section for H is about the same as for H2 at 15,000 K (i.e., 15 X 10-20 mz), so it may be eliminated for this reason, in addition to the fact that the use of H implies frozen losses due to dissociation. Although the ionization potential for Xe is low, (12.13 eV), its cross section is 19.7 x 10-20m2, and so it may be eliminated. The remaining candidates are argon and krypton, whose properties are listed below in Table 4.