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Conversion of Magnetic Energy in the Magnetic Reconnection Layer of a Laboratory Plasma

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Conversion of Magnetic Energy in the Magnetic Reconnection Layer of a Laboratory Plasma ARTICLE Received 5 May 2014 | Accepted 23 Jul 2014 | Published 10 Sep 2014 DOI: 10.1038/ncomms5774 Conversion of magnetic energy in the magnetic reconnection layer of a laboratory plasma Masaaki Yamada1, Jongsoo Yoo1, Jonathan Jara-Almonte1, Hantao Ji1, Russell M. Kulsrud1 & Clayton E. Myers1 Magnetic reconnection, in which magnetic field lines break and reconnect to change their topology, occurs throughout the universe. The essential feature of reconnection is that it energizes plasma particles by converting magnetic energy. Despite the long history of reconnection research, how this energy conversion occurs remains a major unresolved problem in plasma physics. Here we report that the energy conversion in a laboratory reconnection layer occurs in a much larger region than previously considered. The mechanisms for energizing plasma particles in the reconnection layer are identified, and a quantitative inventory of the converted energy is presented for the first time in a well-defined reconnection layer; 50% of the magnetic energy is converted to particle energy, 2/3 of which transferred to ions and 1/3 to electrons. Our results are compared with simulations and space measurements, for a key step towards resolving one of the most important problems in plasma physics. 1 Center for Magnetic Self-organization, Princeton Plasma Physics Laboratory, Princeton University, Princeton, New Jersey 08544, USA. Correspondence and requests for materials should be addressed to M.Y. (email: [email protected]). NATURE COMMUNICATIONS | 5:4774 | DOI: 10.1038/ncomms5774 | www.nature.com/naturecommunications 1 & 2014 Macmillan Publishers Limited. All rights reserved. ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms5774 agnetic reconnection, the breaking and topological Here we report that the energy conversion in a laboratory rearrangement of magnetic field lines in plasma, occurs reconnection layer occurs in a much larger region than previously Meverywhere in the universe, in solar flares, the Earth’s considered. We observe that electron heating occurs outside the magnetosphere, star formation and laboratory fusion plasmas1–4. electron diffusion region and that ion acceleration and heating The most important feature of magnetic reconnection is that dominate in a wide region of the exhaust of the reconnection significant acceleration and heating of plasma particles occurs at layer, which ranges beyond several ion skin depths. The the expense of magnetic energy. An example of this efficient mechanisms for energizing plasma particles in the magnetic energy conversion is the observation of large amounts of high- reconnection layer are identified, and a quantitative inventory of energy electrons associated with the reconnection of magnetic the converted energy is presented for the first time in a well- field lines in solar flares5. In the reconnection region of the defined reconnection layer. The study concludes that B50% of Earth’s magnetosphere and the solar wind, convective outflows the magnetic energy is converted to particle energy, two-thirds have been documented by in situ satellite measurements. Despite of which transferred to ions and one-third to electrons. Our these advances, the exact physical mechanisms for bulk plasma results are compared with numerical simulations and space heating, particle acceleration and energy flow channels remain measurements. unresolved. This paper addresses the key unresolved question: how is magnetic energy converted to plasma kinetic energy during reconnection? Furthermore, the conversion of magnetic Results energy is quantitatively evaluated for the first time in a laboratory Experimental apparatus. We use the MRX facility16 to reconnection layer by assessing the overall energy inventory in a experimentally study the conversion of magnetic energy to well-defined boundary. particle energy in a nearly collision-free reconnection layer. In the classical Sweet–Parker model1, which is based on the Figure 1a shows a schematic of the MRX apparatus, wherein two collision-dominated resistive magneto-hydrodynamics (MHD), oppositely directed field lines merge and reconnect. Experiments the energy dissipation rate during reconnection is small are carried out in a setup in which two toroidal plasmas with B 2 (1/2) ( (B /2m0)VA/LS ) due to the slow reconnection rate; VA is annular cross-section are formed around two flux cores as shown the Alfve´n velocity and S (441) is the Lundquist number2–4. in Fig. 1a. As we induce magnetic reconnection by driving Observations in nearly collision-free space and laboratory oppositely directed field lines towards the X-point (B ¼ 0 at the plasmas show, however, that this prediction is not realized3,4. centre of the layer) using pulsed flux core currents, ions and In the collisionless magnetic reconnection layer, electrons and electrons also flow into the reconnection layer. The ions become ions move quite differently from each other due to two-fluid demagnetized at a distance of the ion skin depth (di ¼ c/opi, 3,4,6,7 dynamics ; furthermore, differential motion between the where opi is the ion plasma frequency) from the X-point where magnetized electrons and the unmagnetized ions generates strong they enter the so-called ion diffusion region, and they change Hall currents in the reconnection layer. In the two-fluid their trajectories and are diverted into the reconnection exhaust formulation, the Ohm’s law of MHD should be replaced by a as seen Fig. 1b. The electrons, on the other hand, remain generalized Ohm’s law to describe the force balance of an electron magnetized through the ion diffusion region and continue to flow flow, namely, towards the X-point. They become demagnetized only when they reach the much narrower electron diffusion region j ÂB ÀrÁPe me dVe (B10d B1 cm in MRX) as seen in Fig. 1b. Electron currents E ¼ Zj þ e À ð1Þ e ene e dt flow dominantly in the out-of-plane direction (Y) near the centre (X-point) as shown in Fig.1c. For standard conditions, the 13 À 3 Here the conventional notations are used with E being the electron density and temperature are, neB(2–6)  10 cm , electric field and B reconnecting magnetic field, Ve the electron Te ¼ 5–15 eV, for B ¼ 0.1–0.3 kG,S4500; the electrons are well- flow velocity, j the current density, je is the electron current magnetized (gyro-radius, reB1mmooplasma size, L) while the 3 density and Pe the spatially dependent electron pressure tensor . ions are not. The mean free path for electron–ion Coulomb A large out-of-plane electric field caused by the Hall currents at collisions, lmfp, is in the range of 6–20 cm that is larger than the the reconnection layer (jHall  B) causes an increase in the layer thickness, and as a result, the reconnection dynamics are reconnection rate3,4,6,7 by inducing rapid movement of the nearly collision-free and dominated by two fluid and kinetic reconnecting field lines. This explains why the reconnection rate effects3,4. In this plasma, the energy exchange between electrons in collisionless plasmas is much faster than the classical Sweet– and ions is small since the characteristic energy transfer time is Parker rate. longer than the electron confinement time. The ion skin depth is In spite of recent progress, a major question remains 6–8 cm and the electron skin depth is typically 1 mm. We use unresolved: how is energy transferred from the magnetic field (R, Y and Z) coordinates where BZ is the reconnecting field to plasma particles? A simple two-dimensional (2D) numerical component and Y is the out-of-plane direction. Helium plasmas simulation would expect that field line breaking and energy with a fill pressure of 4.5 mTorr are used for this study to facilitate dissipation is localized in the small electron diffusion region ion temperature measurements. No external guide field is applied whose width is on the order of the electron skin depth (de ¼ c/ope, for this study. The plasma beta in the inflow region is about 0.1. where c is the speed of light and ope is the electron plasma Our measurements are carried out in a steady state reconnection frequency). However, significant acceleration and heating of both phase which last 20–30 ms, significantly longer than the Alfve´n ions and electrons have been observed and analyzed in a wide time (B1 ms). region of the actual reconnection layer8–15 of laboratory and We document comprehensively the dynamics of plasma space plasmas. Although quantitative studies of energy flow have particles and determine the mechanisms for energy conversion been recently reported based on multiple satellite data12,15, to our in the reconnection layer using extensive in situ diagnostics. The knowledge, a comprehensive analysis of energy inventory over main diagnostic is a 2D magnetic probe array that measures the a well-defined reconnection layer has not been made. A evolution of all three components of the magnetic field at 4200 quantitative analysis of the energy conversion rate together with locations in the reconnection plane. The array consists of seven the identification of heating mechanisms and location would probes with a separation of 3 cm along Z. Each probe has provide key insights into the energy conversion processes. 35 miniature pickup coils with a maximum radial (R) resolution 2 NATURE COMMUNICATIONS | 5:4774 | DOI: 10.1038/ncomms5774 | www.nature.com/naturecommunications & 2014 Macmillan Publishers Limited. All rights reserved. NATURE COMMUNICATIONS | DOI: 10.1038/ncomms5774 ARTICLE Magnetic probe array Equilibrium field coil Magnetic field lines 37.5 cm Flux core R Y Z R Ion diffusion region Electron diffusion region Ion flow Electron flow Electron Plasma current outflow Y Z Plasma inflow Inflowing magnetic fieldlines Separatrix Out-of-plane magnetic field Figure 1 | MRX apparatus and two-fluid reconnection. (a) MRX apparatus and the reconnection layer. Each flux core (darkened sections) contains both toroidal field (TF) and poloidal field (PF) coils. The distance between the surfaces of the two flux cores is 42 cm.
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