When lightning strikes an aircraft, the structure undergoes significant thermomechanical stresses for about a hundred microseconds due to the formation a high-temperature and high-pressure plasma. To improve our knowledge of these plasmas, ONERA is currently testing an innovative X-ray imaging technique, based on the X-ray phase contrast (XPCI), which is sensitive to refractive index gradients in matter. Currently, these developments are mainly carried out at synchrotrons. However, access is limited to a few days per year, and lightning current generators are difficult to transport over there.
In this context, having an intense, compact and pulsed laboratory X-ray source makes more and more sense. A way to get there is to develop a compact pulsed electron source, delivering a focused electron beam. ONERA, but also the international scene, consider an approach based on pseudo-spark switches. Pseudospark discharge is a low-pressure gas discharge (~10-2 mbar), initiated by hollowcathode mechanisms. When electrical breakdown occurs, a diffuse plasma forms allowing the pseudo-spark to conduct currents of several kA with minimal electrode wear. Simultaneously, the pseudo-spark emits a focused electron beam of several tens of keV for a few tens of nanoseconds, reaching kiloAmpere currents. ONERA has developed an expertise in the 2000s on pseudo-sparks (see figure opposite), and wishes to extend its expertise. On the one hand, we aim at increasing the repetition rate of pseudo-sparks, needed for plasma dynamics studies. On the other hand, we aim at making these sources more compact to consider other applications than lightning, for example for in-flight electron beam diagnostics.
The goal of the post-doc is to develop and characterise an electron source based on a pseudo-spark switch which is compact, pulsed with a high repetition rate (kHz) and intense (kA). Your first task will be to get familiar with an existing pseudo-spark operating at low repetition rate (100 Hz). After optimizing the triggering electrical circuit, you will be able to develop a high repetition rate model. You will characterise the electron beam and compare with PIC simulations (code developed in the hosting group). Finally, depending on the working progress, a commissioning of the electron source as plasma diagnostics and/or X-ray source can be considered. This will imply to compare the electron beam performance with existing electron guns or to evaluate the quality of the X-ray source in the context of phase contrast imaging.