Measurement of the ionic emission generated by an electron–photon conversion target

A time-of-flight diagnostic made of a double sheet sensor has been designed for the search of ions emitted by an electron–photon conversion target under the impact of an intense relativistic electron beam. This device was tested on the PIVAIR prototype accelerator. Signals attributed to ions emitted from the target were observed for different target elements. The ions C+ and H+ were identified by time-of-flight measurements, and their mean velocities were found to be equal to about 1.6×108 and 5×108 cm/s for C and Ta targets, respectively. The appearance, rise time and saturation values of the ionic current are close to those obtained from preliminary calculations based on an ionic emission model (VAPOR) which has been coupled to a Maxwell–Vlasov PIC code (M2V). The ionic current signal is characterised by a plateau, the level of which corresponds to the Child–Langmuir current (25 A for H+ ions). Furthermore, the fall-off of this signal is synchronised with the electron beam one. This behaviour represents the de-confinement of the ion beam under the effect of its own space charge.