Astrophys Space Sci (2016) 361:42 DOI 10.1007/s10509-015-2620-0 ORIGINAL ARTICLE Inhomogeneity in laboratory plasma discharges and Stark shift measurement B.M. Obradovi´ c 1 · M. Ivkovi´ c 2 · S.S. Ivkovi´ c 1 · N. Cvetanovi´ c 3 · G.B. Sretenovi´ c 1 · V.V. Kovaˇ cevi´ c 1 · I.B. Krsti´ c 1 · M.M. Kuraica 1 Received: 25 November 2015 / Accepted: 7 December 2015 © Springer Science+Business Media Dordrecht 2015 Abstract Several examples of Stark spectral line shifts from different laboratory plasma experiments are presented. The difference is outlined between the Stark shift caused by the micro field of charged particles on one side and the Stark shift caused by macroscopic electric field occurring in plasma sheaths on the other. In the first case the shift is used for testing of the Stark shifting theory and/or measurement of density of electrons while in the second it is used for mea- surement of macroscopic electric field strength. It is shown that inhomogeneity of the plasma can be used to provide the reference nonshifted lines for exact evaluation of the shift magnitude. Keywords Spectroscopy · Stark shift 1 Introduction The electric field, as a source of Stark shift of spectral lines, in laboratory produced plasmas has, usually, its origin in spatial distribution of electrically charged particles which are constituents of the plasma. In self-sustained electrical This article belongs to the Topical Collection: Life Shifts in Astrophysics and Laboratory Plasma. Guest Editors: Jack Sulentic, Luka C. Popovic. B B.M. Obradovi´ c obrat@ff.bg.ac.rs 1 Faculty of Physics, University of Belgrade, Studentski trg 12, 11000 Belgrade, Serbia 2 Institute of Physics, University of Belgrade, P.O. Box 68, 11081 Belgrade, Serbia 3 Faculty of Transport and Traffic Engineering, University of Belgrade, V. Stepe 305, 11000 Belgrade, Serbia discharges, which are often used to produce laboratory plas- mas, there are two distinct regions with very different char- acteristics. The term commonly used for the first is “plasma” while the second is called “sheath” (Lieberman and Lichten- berg 2005). For instance in a DC discharge, the “plasma” region is referred to as “the positive column” while the “sheath” is named “cathode fall region”. The defining char- acteristics of the “plasma” region is its macroscopic electro- neutrality (n e ≈ n i ), well defined energy distributions which can be close to Maxwell’s, and low strength of macroscopic electric field. But even if the macroscopic electric field is zero, any single atom is subjected to the micro electric field from the neighbouring charged particles in the plasma. This results in a broadening and shifting of spectral lines. Ac- cording to Stark broadening theory (Griem 1974) the shapes and shifts of plasma-broadened isolated lines are mainly de- termined by electron impacts (actually “impact” with elec- tron’s electric field) with the radiating atom or ion. Addi- tionally a smaller contribution arises from the electric mi- crofields generated by essentially static plasma ions. Commonly, the term Stark shift of a spectral line refers to the Stark shift in plasma caused by the micro field. However, the bulk plasma is only one of the regions in an electrical dis- charge. As it is mentioned earlier there is also the “sheath”, a discharge region whose main characteristic is the existence of directed significant macroscopic electric field produced by separation of charged particles. The sheath occurs com- monly in the boundary layers where plasma is in contact with the confiding chamber or the electrodes. Another ex- ample of a high field region is the formation of streamer head. The knowledge of Stark shift data in plasma is essential for the testing of the Stark broadening theory, which can be further used to supply reliable shifts for any spectral line of interest. Apart from theory testing, Stark shifts in plasma are