Publications

2016

• Double and Single Planar Wire Arrays on University-Scale Low-Impedance LTD Generator
  
  • Safronova Alla S.
  • Kantsyrev Viktor L.
  • Weller Michael E.
  • Shlyaptseva V. V.
  • Shrestha I. K.
  • Lorance M. Y.
  • Schmidt-Petersen M. T.
  • Stafford A.
  • Cooper M. C.
  • Steiner A. M.
  • Yager-Elorriaga D. A.
  • Patel S. G.
  • Jordan N. M.
  • Gilgenbach R. M.
  • Chuvatin Alexandre S.
  IEEE Transactions on Plasma Science, Institute of Electrical and Electronics Engineers, 2016, 44 (4), pp.432-440. Planar wire array (PWA) experiments were performed on Michigan Accelerator for Inductive Z-pinch Experiments, the University of Michigan's low-impedance linear transformer driver (LTD)-driven generator (0.1 Ω, 0.5-1 MA, and 100-200 ns), for the first time. It was demonstrated that Al wire arrays [both double PWA (DPWA) and single PWA (SPWA)] can be successfully imploded at LTD generator even at the relatively low current of 0.3-0.5 MA. In particular, implosion characteristics and radiative properties of PWAs of different load configurations [for DPWA from Al and stainless steel wires with different wire diameters, interwire gaps, and interplanar gaps (IPGs) and for Al SPWA of different array widths and number of wires] were studied. The major difference from the DPWA experiments on high-impedance Zebra accelerator was in the current rise time that was influenced by the load inductance and was increased up to about 150 ns during the first campaign (and was even longer in the second campaign). The implosion dynamics of DPWAs strongly depends on the critical load parameter, the aspect ratio (the ratio of the array width to IPG) as for Al DPWAs on high-impedance Zebra, but some differences were observed, for low-aspect ratio loads in particular. Analysis of X-ray images and spectroscopy indicates that K-shell Al plasmas from Al PWAs reached the electron temperatures up to more than 450 eV and densities up to 2 x 10²⁰ cm⁻³. Despite the low mass of the loads, opacity effects were observed in the most prominent K-shell Al lines almost in every shot. (10.1109/TPS.2016.2538291)
  DOI : 10.1109/TPS.2016.2538291
• Orientation of the X-line in asymmetric magnetic reconnection
  
  • Aunai Nicolas
  • Hesse Michael
  • Lavraud B.
  • Dargent Jérémy
  • Smets Roch
  Journal of Plasma Physics, Cambridge University Press (CUP), 2016, 82 (4), pp.535820401. Magnetic reconnection can occur in current sheets separating magnetic fields sheared by any angle and of arbitrarily different amplitudes. In such asymmetric and non-coplanar systems, it is not yet understood what the orientation of the X-line will be. Studying how this orientation is determined locally by the reconnection process is important to understand systems such as the Earth magnetopause, where reconnection occurs in regions with large differences in upstream plasma and field properties. This study aims at determining what the local X-line orientation is for different upstream magnetic shear angles in an asymmetric set-up relevant to the Earth's magnetopause. We use two-dimensional hybrid simulations and vary the simulation plane orientation with regard to the fixed magnetic field profile and search for the plane maximizing the reconnection rate. We find that the plane defined by the bisector of upstream fields maximizes the reconnection rate and this appears not to depend on the magnetic shear angle, domain size or upstream plasma and asymmetries. (10.1017/S0022377816000647)
  DOI : 10.1017/S0022377816000647
• Emission spectroscopy of partial discharges in air-filled voids in Unfilled epoxy
  
  • Shcherbanev S.A.
  • Nadinov I.U.
  • Auvray Philippe
  • Starikovskaia Svetlana
  • Pancheshnyi Sergey V.
  • Herrmann L.G.
  IEEE Transactions on Plasma Science, Institute of Electrical and Electronics Engineers, 2016, 44 (7), pp.1219-1227. Emission optical spectroscopy is used as a technique to study partial discharges (PDs) in four unfilled epoxy samples encompassing an artificial air-filled cavity. It is shown that emission spectroscopy can be used to estimate the density and the chemical composition of a gas from the spectrally resolved emission and time-resolved pulse shape of the PDs at any time during the aging process. Two scenarios are observed: either PD continues until the sample breaks down (observed in 1 out of 4 samples) or PD stops at a certain point without sample breakdown (3 out of 4 samples). For both the scenarios, a stable initial phase with a gradual decrease of emission intensity from the discharge is typical for a few hundreds of hours of continuous discharge operation. At this stage, the spectrum of the second positive system of molecular nitrogen dominates in the entire spectral range of 350500 nm studied in this work. Furthermore, time-resolved measurements indicate two types of discharges of very different frequencies and magnitudes as well as a decrease of the pressure in the voids as a function of aging time. Then, a sharp 500% increase of the N2 emission is observed 2 days before the breakdown; during the last day a spectrum of CO and some other C-N-O-H containing molecules is observed instead of the spectrum of molecular nitrogen. This allows predicting a breakdown at least a few hours before it happens by analyzing the broad emission spectra behavior. Additionally, the possible role of surface conductivity increase during aging on PD inhibition is discussed. (10.1109/TPS.2016.2576560)
  DOI : 10.1109/TPS.2016.2576560
• Influence of the angular scattering of electrons on the runaway threshold in air
  
  • Chanrion Olivier
  • Bonaventura Z.
  • Bourdon Anne
  • Neubert Torsten
  Plasma Physics and Controlled Fusion, IOP Publishing, 2016, 58, pp.044001. The runaway electron mechanism is of great importance for the understanding of the generation of x- and gamma rays in atmospheric discharges. In 1991, terrestrial gamma-ray flashes (TGFs) were discovered by the Compton Gamma-Ray Observatory. Those emissions are bremsstrahlung from high energy electrons that run away in electric fields associated with thunderstorms. In this paper, we discuss the runaway threshold definition with a particular interest in the influence of the angular scattering for electron energy close to the threshold. In order to understand the mechanism of runaway, we compare the outcome of different FokkerPlanck and Monte Carlo models with increasing complexity in the description of the scattering. The results show that the inclusion of the stochastic nature of collisions smooths the probability to run away around the threshold. Furthermore, we observe that a significant number of electrons diffuse out of the runaway regime when we take into account the diffusion in angle due to the scattering. Those results suggest using a runaway threshold energy based on the FokkerPlanck model assuming the angular equilibrium that is 1.6 to 1.8 times higher than the one proposed by [1, 2], depending on the magnitude of the ambient electric field. The threshold also is found to be 5 to 26 times higher than the one assuming forward scattering. We give a fitted formula for the threshold field valid over a large range of electric fields. Furthermore, we have shown that the assumption of forward scattering is not valid below 1 MeV where the runaway threshold usually is defined. These results are important for the thermal runaway and the runaway electron avalanche discharge mechanisms suggested to participate in the TGF generation. (10.1088/0741-3335/58/4/044001)
  DOI : 10.1088/0741-3335/58/4/044001
• A computational analysis of the vibrational levels of molecular oxygen in low-pressure stationary and transient radio-frequency oxygen plasma
  
  • Kemaneci Efe
  • Booth Jean-Paul
  • Chabert Pascal
  • van Dijk Jan
  • Mussenbrock Thomas
  • Brinkmann Ralf Peter
  Plasma Sources Science and Technology, IOP Publishing, 2016, 25 (2), pp.025025. Vibrational levels of molecular oxygen, O 2 ( v &#8201;&#8201;<&#8201;&#8201;42), are investigated in continuous and pulse-modulated low-pressure radio-frequency oxygen plasma with a global modelling approach. The model is benchmarked against a variety of pressure-, power- and time-resolved measurements of several inductive and asymmetric capacitive discharges available in the literature, and a good agreement is obtained. The sensitivity of the model with respect to the vibrational kinetics, the wall reactions and the spatial inhomogeneity of the charged particles are presented. The simulations without the vibrational levels are also shown for the sake of comparison. (10.1088/0963-0252/25/2/025025)
  DOI : 10.1088/0963-0252/25/2/025025
• Distinct responses of the low-latitude ionosphere to CME and HSSWS: The role of the IMF B^z oscillation frequency
  
  • Rodríguez-Zuluaga J.
  • Radicella S. M.
  • Nava B.
  • Amory-Mazaudier Christine
  • Mora-Páez H.
  • Alazo-Cuartas K.
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2016. In this work an attempt to identify the role of the interplanetary magnetic field (IMF) in the response of the ionosphere to different solar phenomena is presented. For this purpose, the day-today variability of the equatorial ionospheric anomaly (EIA) and the main ionospheric disturbances are analyzed during one coronal mass ejection (CME) and two high-speed solar wind streams (HSSWSs). The EIA parameters considered are the zonal electric field and both the strength and position of its northern crest. The disturbances being the prompt penetration of magnetospheric electric field (PPMEF) and disturbance dynamo electric field (DDEF) are studied using the magnetic response of their equivalent current systems. In accordance, ground-based Global Navigation Satellite Systems receivers and magnetometers at geomagnetic low latitudes in the American sector are used. During both phenomena, patterns of PPMEF related to fluctuations of the IMF are observed. Diurnal and semidiurnal magnetic oscillations are found to be likely related to DDEF. Comparisons among the EIA parameters and the DDEF magnetic response exhibit poor relation during the CME in contrast to good relation during the HSSWSs. It is concluded that the response of the low-latitude ionosphere to solar phenomena is largely determined through the oscillation frequency of the IMF B z by affecting the generation of the PPMEF and DDEF differently. This is seen as an effect of how the energy from the solar wind is transferred into the magnetosphere-ionosphere system. (10.1002/2016JA022539)
  DOI : 10.1002/2016JA022539