SNAP typically deals with 5 plasma species: electrons, two hydrogenic ion species, and two impurity species. (Currently, only species with charge = 1 are considered hydrogenic, but in the future SNAP will be modified to treat helium as a hydrogenic species. Currently helium is treated as an impurity, see Section .) To calculate the densities of all these species, we obviously need 5 measurements or equations. One typical set of specifications is:
The hydrogenic species ratio is manually specified by the user,
usually on the basis of measurements of the edge light from H
versus D emission. The impurity ratio is derived from a
measurement (by soft x-ray pulse height analysis [PHA]) of the
contribution to by metallic impurities,
where the sum extends over all highly charged impurities. The PHA measurement also provides an estimate of the total .
Divide both sides by ne:
Replace the terms involving nI2 with the measured :
These equations can be solved simultaneously for the ratio without specifying the electron, beam, or hydrogenic densities which are not known prior to the SNAP analysis:
SNAPIN solves Eq. 9 to determine a ratio of light to heavy impurities using data for the metallic and total \ (usually both are derived from PHA, but sometimes hand-entered, i.e., guessed, numbers are used). The value of used in this calculation may not be exactly equal to the value determined by SNAP using chordal VB data. Therefore, if the impurity information is specified using metallic fractions (historically, this was the standard method), SNAP needs to be run twice: for the second iteration, the value of used by SNAPIN to estimate the metallic fraction should be set to the value determined by SNAP from VB on the first iteration.