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Abstract
Ultrashort, 25-fs pulses of x-ray free-electron laser (XFEL) radiation (9.3-keV photons; a pulse energy of 2 mJ) have been micro-focused on the surface of 3-micrometer Cu foil. The interaction experiment has been performed at the European XFEL facility in Schenefeld (Germany). In this contribution emphasis will be placed on the diagnosis and qualitative analysis of emitted highly charged Cu ions using an ion expansion model based on a shifted Maxwell-Boltzmann distribution of ion velocities. This phenomenological approach makes possible to evaluate the charge states of ions belonging to copper stable isotopes 63Cu and 65Cu and to determine the voltages by which ion jets are accelerated. This work, describing the properties of ions produced from a mid-Z elemental target exposed to ultrashort x-ray FEL pulses at an irradiance of 10 exa-watts per square centimetre, sheds light on processes occurring in volumetrically heated dense matter on several timescales.