Contrib. Plasma Phys. 53, No. 1, 86 – 91 (2013) / DOI 10.1002/ctpp.201310015 On the Use of a Retarding Field Energy Analyzer for Plasma Flow Analysis ˚ A. Fredriksen 1 ∗ , W. J. Miloch 1,2 , N. Gulbrandsen 1 , and L. N. Mishra 1,3 1 Department of Physics and Technology, University of Tromsø, 9037 Tromsø, Norway 2 Department of Physics, University of Oslo, Oslo, Norway 3 Department of Physics, Tribhuvan University, Lalitpur, Neapel Received 15 December 2011, accepted 07 March 2012 Published online 09 January 2013 Key words Plasma ﬂow, probe diagnostics, retarding ﬁeld energy analyzer, PC simulations. Measurements of plasma ﬂow are of key interest in a number of plasma environments and applications. In laboratory magnetized plasmas, the directional Langmuir or Mach probe is a well-proven ‘in-situ’ diagnostic tool to obtain the ﬂow velocity. However, in non-magnetized or weakly magnetized plasmas, this method does not readily yield reliable velocity measurements, as it has been shown by numerical and experimental studies that the collection of upstream ions to the rearward probe surface can be signiﬁcant. In this study, we have utilized the analysis of data from 3D PIC simulations [W. J. Miloch, Plasma Phys. Contr. Fusion 52, 124004 (2010)] of ion velocity distributions in the vicinity of a negatively biased object embedded in a collision-less, source-free plasma with and without ﬂow. The simulations allow us to study how the grounded probe housing of a retarding ﬁeld ion energy analyzer (RFEA) affects the distribution of ions and their collection at different angles with ﬂowing, electropositive plasma. We ﬁnd that an analysis based on derived plasma potential at different angles with the ﬂow, may provide more consistent results than the Mach probe theory in our weakly magnetized case. Comparisons are carried out with RFEA measurements in an inductively coupled helicon plasma. 1 Introduction Plasmas ﬂow with respect to an inertial system is an important parameter in many plasmas, whether it is a plasma rocket exhaust nozzle, a geographical position, or with respect to a probe or an obstacle in a laboratory device. Flows may provide important information about their driving forces and the dynamics of the particular plasma system, e.g. ExB-drifts, plasma thrust, or plasma expansion. The ﬂow of plasma along expanding magnetic ﬁeld lines occurs in many natural plasmas, like for instance plasma outﬂow from the solar corona [1, 2], or ion outﬂow from the polar ionosphere[3, 4]. Furthermore, it is long established that ﬂows in plasmas help lowering the threshold of instabilities [5], and more recently, it has been shown that cross-ﬁeld turbulent transport in tokamaks can generate parallel ﬂows [6] and play a role in plasma conﬁnement. The study of plasmas and instabilities with ﬂows and ion beams is one of the main objectives behind the construction of the Njord device [7, 8], in which the ﬂow measurements in this article are performed. In this paper, we investigate the use of a RFEA to diagnose ﬂow along expanding magnetic ﬁeld lines in the weakly magnetized, steady-state plasma of the Njord helicon device. The RFEA can be turned around its through- feed axis perpendicular to the ﬂow, so that the RFEA aperture can be pointed at arbitrary angles with respect to the ﬂow. The diagnostics problem may be approached with two different methods. The ﬁrst method is by means of the well-known Mach-probe analysis [9] in which the ratio between ion-saturation currents to a probe surface in the parallel and anti-parallel direction deﬁnes the Mach number, i.e. the ﬂow speed in terms of ion sound speed c s . This method is well proven and widely adopted in magnetized plasmas [10]; while in non-magnetized plasmas it is more disputed [11]. The other method is a straightforward measurement of the plasma potential, ∗ Corresponding author. E-mail: ashild.fredriksen@uit.no c  2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim