Neutron Generation in a Plasma Diode with Electrons Insulated by a Constant Magnetic Field

Abstract

Neutron generation in a vacuum plasma diode with accelerating voltage to 280 kV and laser source of deuterons in the anode was investigated. The maximum neutron yield in a regime with magnetic insulation of the electrons in the reaction D(d, n) 3 He is Q dd = 5·10 7 per count, which is 10 times higher than the value without a magnetic fi eld. A hollow cylindrical NdFeB magnet, which is placed inside the vacuum volume of the diode and is also the diode’s cathode, is used to create a magnetic fi eld. The conversion on the basis of numerical modeling showed that the neutron yield in the reaction T(d, n) 4 He can reach 5·10 9 per count. Small pulsed neutron generators based on sealed vacuum and gas-fi lled accelerator tubes are widely used for neutron logging of oil-gas wells and ore holes, fast elemental analysis of the composition of matter, and detection and identifi cation of dangerous hidden substances [1]. In vacuum accelerator tubes, different methods of suppressing secondary electron emission with the cathode are used to increase the effi ciency of neutron generation [2]. At high accelerating voltage, magnetic insulation in the diode gap of an accelerating tube between the cathode and plasma anode is effective for suppressing the electron current [3, 4]. So, assemblies made of permanent ring magnets with longitudinal magnetization, which are arranged outside the vacuum space of the diode, are used for this purpose; but, this increases the radial size of the neutron source [4]. The present work examines the possibility of implementing a technical solution where a permanent magnet, made of a material based on the compound NdFeB, is arranged inside the working vacuum space of the diode. In such a design, the magnet is a hollow cylinder and simultaneously fulfi lls the function of the cathode of the diode system of the tube. In the region of secondary-electron emission (inner surface of the cathode) the induction of the insulating magnetic fi eld is two or three times greater than the induction of the fi eld created by magnetic elements arranged outside the vacuum space. Experimental studies of neutron generation in a plasma diode with magnetic insulation of the electrons by the fi eld of a permanent magnet were performed on a model equipped with a laser source of deuterons and an Arkad’ev–Marx generator [5]. A sectional schematic view of the setup is shown in Fig. 1. The pressure in the working volume was ~5·10 –2 Pa. Direct acceleration of deuterons, extracted from the laser plasma in an electric fi eld formed by feeding a positive high-voltage pulse to the anode, toward the cathode was accomplished in the diode. A plasma-forming target in the form of a TiD pellet was arranged on the top of the anode; the radiation from a solid-state laser with an active element based on yttrium-aluminum garnet activated by neodymium and wavelength �™ = 1.06 �´ m was focused onto the pellet. The power density of the laser radiation in the region of plasma formation was q = 5·10 10 W/cm 2 ; the laser pulse duration was �≤ las = 7 nsec. The cathode, made of a material based on NdFeB compounds, comprised a hollow cylinder with outer diameter 8 cm, inner diameter 4.5 cm, and height 4 cm. A neutron-forming target made of TiD or (CD 2 ) n