Catching Element Formation In The Act ; The Case for a New MeV Gamma-Ray Mission: Radionuclide Astronomy in the 2020s

High Energy Astrophysic

High power electron diode for linear induction accelerator at a flash radiographic facility

International audienceWe investigated a new cathode design and beam transport with the EPURE axis-1 injector in order to increase the beam characteristics at 3.8 MeV, 80 ns FWHM from the 2.0 kA nominal current to 2.6 kA corresponding to an average current density of 82  A/cm2. Such current increase is highly desirable for improving the x-ray dose and hence radiographic performances. To achieve this, a time-dependent model based on the particle-in-cell method was developed in order to simulate the injector. Using results from calculations based on this model, a 17.2 cm AK gap diode with a larger radius cathode (3.175 cm) was designed, manufactured and tested. Experimental and calculated currents and emittances are qualitatively compared. The study provides a detailed understanding of the beam dynamics inside this type of high current, high energy injector

Comparison of six simulation codes for positive streamers in air

We present and compare six simulation codes for positive streamer discharges from six different research groups. Four groups use a fully self-implemented code and two make use of COMSOL Multiphysics®. Three test cases are considered, in which axisymmetric positive streamers are simulated in dry air at 1 bar and 300 K in an undervolted gap. All groups use the same fluid model with the same transport coefficients. The first test case includes a relatively high background density of electrons and ions without photoionization. When each group uses their standard grid resolution, results show considerable variation, particularly in the prediction of streamer velocities and maximal electric fields. However, for sufficiently fine grids good agreement is reached between several codes. The second test includes a lower background ionization density, and oscillations in the streamer properties, branching and numerical instabilities are observed. By using a finer grid spacing some groups were able to reach reasonable agreement in their results, without oscillations. The third test case includes photoionization, using both Luque's and Bourdon's Helmholtz approximation. The results agree reasonably well, and the numerical differences appear to be more significant than the type of Helmholtz approximation. Computing times, used hardware and numerical parameters are described for each code and test case. We provide detailed output in the supplementary data, so that other streamer codes can be compared to the results presented here

Comparsion study of different simulation codes for positive streamers

For streamer simulations a range of computational models have been developed by various groups for various purposes. These models differ in dimensionality (2D, 3D), model type (particle, fluid or hybrid approach and further differentiation), their numerical implementation, type of grid and its refinement, and their spatial/temporal discretization. The aim of the present study is to benchmark the results of different simulation codes for axisymmetric single positive streamers in air at 1 bar and 300 K using a fluid model