Contrib. Plasma Phys. 55, No. 7, 538 – 544 (2015) / DOI 10.1002/ctpp.201400102 On the Conductance of the Gas Discharge Plasma at Space Anisotropic Excitation D. Zhechev 1∗ , V. Steflekova 1 , J. T. Costello 2 , and N. Bundaleska 3 1 Institute of Solid State Physics, Bulgarian Academy of Sciences, 72, Tzarigradsko Chaussee Blvd., 1784 Sofia, Bulgaria, Fax: 359-2-975-36-32 2 NCPST and School of Physical Sciences, Dublin City University, Dublin 9, Republic of Ireland 3 Instituto de Plasmas e Fus˜ ao Nuclear, Instituto Superior T´ ecnico, Universidade de Lisboa, Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal Received 03 November 2014, revised 09 March 2015, accepted 11 March 2015 Published online 27 May 2015 Key words Gas discharge, magnetic state, (self-) alignment, orientation, optogalvanic effect. The galvanic manifestation of two transformations of degenerate magnetic states of the neon atom, namely self-alignment → non-alignment and alignment ↔ orientation is studied by monitoring the voltage across the discharge in a range of hollow cathode discharge (HCD) lamps. The application of a magnetic field to the discharge disorders the self-alignment. Alignment and orientation are optically induced and their effect on the discharge conductance is compared by measuring the optogalvanic signals. The degenerate states investigated are found to contribute to the gas discharge conductance in a manner which depends on their degree of coher- ence. Various hollow cathode discharge media are studied including Ne/As, Ne/Cu, Ne/Ni, Ne/Cd, Ne/Li and Ne/Ca in the corresponding trademarked HCD spectral lamps. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 Introduction The study of interfering quantum states has grown and enriched our understanding of atomic systems [1]. W. Hanle aligned optically an excited atomic state by using linearly polarized spectral line radiation opening up a broad field of study in coherences in atomic systems [2]. Absorption of linearly polarized light prepares a coherent superposition of degenerate magnetic states m such that Δm =0, ±2. Later the same coherence was found to be also an inherent attribute of gas discharge plasmas. It arises without any external stimulus i.e., it is due to the space anisotropy of some internal process of excitation (self-alignment). In fact, all real gas discharge sources generate some degree of self-alignment. Another anisotropic excitation, i.e. absorption of circularly polarized light leads to coherence of type Δm = ±1 known as orientation [3]. Both (self-) alignment and orientation manifest themselves optically, i.e. as linearly- and circularly polarized spontaneous emission from interfering m-states. In a real gas discharge medium either alignment or orientation means that the corresponding Δm =0, ±2 or Δm = ±1 allowed transitions and resulting states dominate in number over other permitted m-states. Therefore, certain definitive m-states dominate along some arbitrary space axes. On the other hand both ionization and excitation of atoms are anisotropic in low-temperature plasma and its conductance should depend, generally, on the magnetic states m too. This nonenergetic contribution to the conductance manifested itself implicitly in some earlier investigations [4, 5]. In this study the excited ensemble of atoms is investigated galvanically at the two Δm- transformations, i.e. i) self-aligned → non-aligned and ii) aligned ↔ oriented. A hollow cathode discharge (HCD) is used which is known to be a medium where the excited atoms are self-aligned by the characteristic beam like fast electrons [6, 7]. Since self-alignment is an intrinsic property of HCDs the comparison in the conductance between self- aligned and non-aligned atoms is based on the magnetic destruction of this coherence (which we designate case * Corresponding author. E-mail: spectron@issp.bas.bg, Phone: +359 2 9795767, Fax: +359 2 9753632 © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim