Proton imaging detection of transient electromagnetic fields in laser-plasma interactions

Due to their particular properties ~small source size, low divergence, short duration, large number density!, the beams of multi-MeV protons generated during the interaction of ultraintense (I .1019 W/cm2) short pulses with thin solid targets are most suited for use as a particle probe in laser–plasma experiments. In particular, the proton beams are a valuable diagnostic tool for the detection of electromagnetic fields. The recently developed proton imaging technique employs the beams, in a point-projection imaging scheme, as an easily synchronizable diagnostic tool in laser– plasma interactions, fields, with high temporal and spatial resolution. The broad energy spectrum of the beams coupled with an appropriate choice of detector ~multiple layers of dosimetric film! allows temporal multiframe capability. By allowing, for the first time, diagnostic access to electric-field distributions in dense plasmas, this novel diagnostic opens up to investigation a whole new range of unexplored phenomena. Results obtained in experiments performed at the Rutherford Appleton Laboratory are discussed here. In particular, the article presents the measurement of highly transient electric fields related to the generation and dynamics of hot electron currents following ultraintense laser irradiation of targets. The experimental capabilities of the technique and the analysis procedure required are well exemplified by the data presented.