The impurity section of SNAPIN is the least straightforward, partly because it controls a variety of different variables, some of which are related to one another, some of which are not. This menu reads the PHA database, the visible Bremsstrahlung chordal signal, and the H and gas feedback signals.
\* Options are: 1) Standard impurity density model 2) Old impurity density model 3) Zeff Total ..............................<VB : 0.000 > 4) I1: first impurity (z= 6.00 a=12.00) < ---SOLVE--- > 5) I2: second impurity (z=26.00 a=56.00) <Hand : 0.300 > 6) PHA zeff, zeff metals (Read) ...... <0.000E+00,0.000E+00> .... SNAPDAT 7) VB chordal intensity (Read) ....... < 2.631E+11 +/- 14.5%> .. SH-VB-SL 8) VT chordal tangency radius ........ <1.90> 9) Estimate zeff (linear), zeff (center) <0.000E+00,0.000E+00> 10) PLasma species (z,a) .............. <(1,2)(1,1)(6,12)(26,56)> 11) Mix of H/H+D+T .................... <0.15> 12) HElium plasma (special) 13) Taup global ion confinement (s) ... <-0.100> 14) RS Recycle source (Read) .......... < 8.15E+21> ...... SH-HA+,GS-FB-AK 15) RM Recycle Menu
The first and most important function of the impurity section is to specify how SNAP will determine the density of each plasma species. Typically there are 6 plasma species:
Alternately, the can be determined from the measured loop voltage assuming Spitzer or neoclassical resistivity. This procedure works only in ohmic plasmas for which beam-driven and bootstrap currents are small.
A third source of total plasma is from pulse-height analysis of the soft x-ray emission (PHA). This technique works acceptably well in ohmic plasmas, but in beam-heated plasmas the presence of beam neutrals changes the charge-state distribution of the impurity ion distribution, and gives questionable total values. The values of the metallic contribution to however, are still considered accurate during beam heating.
A final source of total is to make one up; this is entered as a hand value.
The only diagnostic source for the metallic contribution to is from PHA. If you request old impurity density model, the information regarding the metallic contribution to is passed to SNAP by specifying the ratio of light to heavy impurities. This requires, unfortunately, also the total which the PHA measured from the PHA spectrum. Choosing the standard impurity density model avoids these complications--it simply defines the contributed by heavy impurities directly. See Section 6.5.
SNAP does not yet allow helium as a hydrogenic species, but it can be handled as an impuritity. Section describes how to treat helium plasmas.