Numerical Simulation of Beam Interaction with Plasma and Gas in Diodes for Flash X-Ray Radiography

Abstract

Summary form only given. A number of questions should be solved in the course of development of new diodes for radiographic applications. (1) In self magnetically pinched diodes plasma near electrodes can significantly affect electron beam focusing and Bremsstrahlung formation. (2) It is necessary to know the influence of angle spread and initial rotation of electron beam on focusing properties. (3) If gas-filled focusing cell is used, the result depends on choice of gas density. To answer the above questions PICNIC hybrid code was developed, which consists of several interrelated parts: 2D PIC code calculating dynamics of relativistic particles (electrons and ions), 3D FDTD solver for Maxwell equations, 2D MHD description for thermal electrons and ions, Monte-Carlo code PPJZMA defining particles momentum and angle after collisions. For the calculation of magnetic field the current through the anode surface is accounted. Generator parameters were the next. Diode with gas-filled focusing cell is analyzed for generator with parameters: 5 MV voltage with 30 kA current. For self-magnetically pinched diode the generator used 10.0 MV, 200 kA, 50 nsec FWHM electric pulse.Calculation results show, that diodes operate in different regimes depending on current value: initially quasi-uniform beam motion in vacuum, then unstable motion of high-current electron beam in plasma. Return current and magnitudes of local current densities inside plasma play an important role in these collective effects of beam-plasma interactions. These processes significantly depend on beam initial parameters, spread of angle velocities and energies. For rod-pinch diode incidence angles of electrons during Bremsstrahlung generation may vary in 50deg.