Characterization and optimization of a flash X-ray source for dense spray radiography

Summary form only given. Liquid sprays analysis, as those generated by fuel injection systems, implies diagnosis procedures able to produce information for matter in different phases from liquid to vapor. The high speed sprays produced by the fuel injection systems, need a fast and reliable diagnosis technology. Previous works on N₂ cryogenic jets in a high pressure chamber had shown that the use of flash X-ray system can provide valuable data over wide range of parameter conditions. The flash X-ray (called DIKEV), is a transportable blumlein table top source build at GREMI laboratory to perform radiography of fuel injection sprays. This device delivers high X-ray doses, a few mGy per pulse at the output window, in pulses of tens of nanoseconds duration and operates from single shot up to 50 Hz repetition rate. DIKEV efficiency optimized for charging voltage ranging from 10 to 25 kV, leads to the production of X-ray photons with energy ranging from 5 to 40 keV. The low part of this photon energy range, where most photon are produced , is matched for the radiography of dense spray of light elements. The X-ray focus size is not controlled by electrostatic grids as in case of conventional X-ray tubes, but is correlated with the pulsed electric field topography in the X-ray diode. A significant reduction of the X-ray focus size was achieved. This have been possible thanks to the high efficiency of the carbon fiber as field emitter cathode; which permitted an important reduction of cathode size (graphite rods of diameter ranging from 0.5 mm to 2 mm and 10 mum carbon fiber) without dose reduction up to 1000 shots. The density calculation from Beer Lambert's law, requires the precise knowledge of X-ray spectrum. For four different metallic anodes, a DIKEV full spectra characterization has been made with a germanium photodetector. The most important K,L characteristics lines below 30 keV for each anode were identified together with Bremsstrahlung spectra. In contrast to Mo case where continuum spectrum is dominant, a very high, characteristic line to Bremsstrahlung ratio was measured for Cu anode. Best anode material can be easily selected to be adapted to given spray characteristics.