3D particle-in-cell simulations of small-diameter self-magnetic-pinch diodes

摘要

Flash x-radiography usually involves the creation of a pulse of intense bremsstrahlung whose duration is short compared to the motion of the object being radiographed. In many applications it is also desirable to have a very bright source that produces high-spatial-resolution images. For these applications a figure-of-merit (FOM) that provides a measure of the source brightness can be defined as, FOM = D/R², where D is the dose in CaF2 at a distance of 1 m from the source and R (usually referred to as the radiographic spot size) is a number that characterizes the spatial extent of the radiation source. One method of creating a high FOM source uses a self-magnetic-pinch (SMP) electron-beam diode. [1] In relevant experiments on RITS-6 [2] and Mercury [3] inductive voltage adders IVA's, magnetically-insulated electron-flow in the adder can be a significant fraction of the current available to drive the SMP. It is believed that the magnetically-insulated flow produces an undesirably large radiographic spot if it is allowed to strike the x-ray target. Therefore, the approach adopted on RITS-6 and Mercury has been to dump this electron flow prior to the x-ray target. This usually involves the use of very large hardware and a loss of up to 50% of the available current. In this talk, we will present results from 3D LSP [4] particle-in-cell simulations of the coupling of magnetically-insulated electron-flow to an SMP diode. These simulations show that asymmetries in the electrical power-flow can cause the radiographic spot to wander. These simulations further show that nearly all the electrical current can be coupled to the SMP diode. However, in this situation the radiographic spot size is larger than cases without electron-flow in the SMP diode. In this paper we use LSP to explore methods of stabilizing the position and reducing the size of the radiographic spot.