Electron Demagnetization in a Magnetically Expanding Plasma Justin M. Little Space Propulsion and Advanced Concepts Engineering (SPACE) Laboratory, University of Washington, Seattle, Washington 98015, USA Edgar Y. Choueiri Electric Propulsion and Plasma Dynamics Laboratory (EPPDyL), Princeton University, Princeton, New Jersey 08544, USA (Received 25 April 2019; revised manuscript received 13 August 2019; published 2 October 2019) Electron demagnetization in a magnetically expanding plasma, a fundamental process for plasma flow and detachment in magnetic nozzles, is experimentally investigated using a rf plasma source and magnetic nozzle (MN). Measurements of the plasma potential spatial profile reveal an ion-confining potential surface, indicative of the edge of a magnetized plasma, that extends along the outermost magnetic flux surface. The downstream extent of the potential surface scales inversely with a characteristic electron Larmor radius, which agrees with an existing theory [E. Ahedo and M. Merino, Phys. Plasmas 19, 083501 (2012)] for electron demagnetization via finite electron Larmor radius (FELR) effects. These results represent the first experimental evidence of FELR demagnetization, and provide an empirical metric for the significance of FELR effects based on the degree of separation between electron and magnetic flux surfaces. With this metric, a critical magnetic field strength is found that ensures electrons remain magnetized through the MN turning point, thus avoiding the rapid plume divergence associated with premature demagnetization. DOI: 10.1103/PhysRevLett.123.145001 Plasma flow along an expanding magnetic field, in addition to being a naturally occurring phenomenon in the solar wind [1] and astrophysical jets [2], also plays a significant role in plasma processing [3] and propul- sion technologies [4–6]. The nature by which the plasma flow decouples (i.e., detaches) from the magnetic field is fundamentally important as it marks the transition between magnetized and unmagnetized flow regimes, thus impac- ting a variety of critical physics such as mass and energy transport [7], instability growth [8], and wave propagation [9]. Detachment is especially important for space electric propulsion concepts that utilize magnetic nozzles (MNs) [10] because plasma propellant returning along the mag- netic field degrades performance and represents a hazard to sensitive spacecraft components. Theories for MN plasma detachment have been proposed based on a variety of physical processes, including colli- sions [11,12], instabilities [13,14], finite Larmor radius effects [15,16], and magnetic field perturbations [17,18]. Experiments have shown that the ion streamlines and plasma density profile diverge less than the downstream magnetic field [13,19–21], in agreement with theoretical models [22]. Despite its importance, experimental inves- tigation into electron demagnetization in the downstream region is notably absent from the literature. In this Letter, we experimentally examine electron dynamics in a MN using measurements of the plasma potential profile, and use the resulting insight to answer the question: what is the cause and consequence of electron demagnetization in a magnetically expanding plasma? Experiments were performed using an 18.5 cm long, 7.5 cm diameter rf plasma source (PS) mounted inside a 7.6 m long, 2.4 m diameter dielectric vacuum chamber (Fig. 1). The MN consisted of two electromagnetic coils (r c ¼ 7.51 cm) positioned near the exit of the PS. The field strength at the center of the MN, B 0 , was controlled with the coil current, I B , such that B 0 ½G ≈ 21I B ½A. Data were obtained at a fixed rf frequency, delivered power, argon z0 ts (a) (b) FIG. 1. (a) Schematic of the plasma source, electromagnets, magnetic field lines (dashed), intersecting magnetic field line (dashed bold), probe range (shaded), and measurement arcs (solid arcs). (b) Photograph of the operating plasma source (P ¼ 500 W, _ m ¼ 2 mg=s) and magnetic nozzle (I B ¼ 20 A). PHYSICAL REVIEW LETTERS 123, 145001 (2019) 0031-9007=19=123(14)=145001(5) 145001-1 © 2019 American Physical Society