  Electric propulsion requires exhaustive ground test campaigns to obtain an
accurate characterization of the propulsion devices, or thrusters, used by the
spacecraft. Among the many plasma parameters, accurately measured during the
tests, that of the ion velocity is key, and can be measured using non-intrusive
tools such as Laser-Induced Fluorescence (LIF) diagnostics. The ion velocity is
inferred by Doppler shift measurements that presupposes a precise and accurate
knowledge of the wavelength of excitation of the ions at rest. Today electric
propulsion is moving towards the use of Krypton as a propellant, due to the
dramatic increase in cost of the more advantageous Xenon gas propellant,
commonly used until now. In this work the
4d$^4D_{7/2}\rightarrow$5p$^4P^\circ_{5/2}$ transition of Kr II is used for LIF
diagnostic in hollow cathode discharge.
