Active Stark Atomic Spectroscopy
  Active Stark Atomic Spectroscopy (ASAS) method can be used to determine a high electric field in the diode of an ion or electron accelerator as a function of position and time, including the positions of anode and cathode plasma emission surfaces (in order to obtain the effective accelerating gap). As possible probe beams, we suggest the use of lithium and sodium atoms. The diagnostic provides a means to measure diode quantities spectroscopically with excellent spatial resolution. 
P.I.Melnikov, J.B.Greenly, D.A.Hammer