Determination of the plasma velocity in an imploding wire array from magnetic field measurements by a gradient probe
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MA DIAGNOSTICS
Determination of the Plasma Velocity in an Imploding Wire Array from Magnetic Field Measurements by a Gradient Probe E. V. Grabovski, K. N. Mitrofanov, A. Yu. Koshelev, A. A. Samokhin, V. V. Aleksandrov, A. N. Gritsuk, Ya. N. Laukhin, G. M. Oleinik, and I. N. Frolov Troitsk Institute for Innovation and Fusion Research, Troitsk, Moscow oblast, 142190 Russia Received November 1, 2010; in final form, December 15, 2010
Abstract—A method for measuring the gradient of the magnetic field in the plasma of an imploding wire array is described. Results from measurements of the magnitude and gradient of the magnetic field in a tung sten wire array on the Angara51 facility at currents of ~3 MA are presented. A novel method for calculating the velocity of the currentcarrying plasma in the framework of MHD equations from data on the magnitude and gradient of the magnetic field at a certain point inside the array is proposed. It is demonstrated that a gra dient magnetic probe can be used to investigate the plasma current sheath in plasma focus facilities. DOI: 10.1134/S1063780X11060092
1. INTRODUCTION One of the main problems in investigating the implosion and acceleration of ICF drivers is the deter mination on the parameters of the plasma current sheath (PCS) [1–4]. The required information can be obtained by measuring the magnetic fields in the imploding plasma. Along with optical and laser diag nostics, probe measurements of the magnetic field are an efficient tool for investigating fast flows of the hot magnetized plasma created using highpower current generators. Application of magnetic probes made it possible to prove and quantitatively study the model of extended plasma production in imploding wire arrays—one of the promising types of ICF drivers [1]. According to this model, the plasma is continuously produced from the motionless wire cores and acceler ated toward the array axis by the Ampère force, thereby forming a radial plasma flow directed toward the array axis. At this point, the model of extended plasma production differs from the zerodimensional sheath model [2], which assumes that a compact PCS accelerates as a whole. The spatial distribution of the mass and current in the stage of plasma acceleration determines the effi ciency with which energy is concentrated on the axis and, eventually, the power of soft Xray (SXR) emis sion required to compress a fusion target. Experimen tal determination of the magnitude and gradient of the magnetic field in the imploding PCS is of fundamental importance for the development of theoretical mod els. According to the model of extended plasma pro duction in wire arrays [1], the plasma is generated and then accelerated to velocities on the order of the Alfvén velocity ( M А ≈ 1) in a thin resistive boundary
layer (RBL) on the inner surface of the array. The RBL thickness is much smaller than the initial array radius. It follows from general considerations that it does not exceed the skin depth or the typical scale length of the heterogeneous stru
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