Sure thing: the ion saturation is the current that you draw while the probe is biased sufficiently negative. If you check out Figure 3, you’ll see that the current is nearly fixed as the probe bias moves further negative. The ion current has “saturated,” which is the value you seek. If you bias too far negative, then other effects come into play and the current will rise further, so it helps to review the curve to identify the true saturation region.

Hope this helps,
David