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1. INTRODUCTION
During the interaction of high-intensity laser pulses with matter highly-transient, large-amplitude electric and magnetic fields are generated either by space-charge separation or by the flow of large currents of relativistic electrons. These fields play a role of fundamental importance in many laser-plasma processes. Electric fields due to charge separation can drive the expansion of the ions of the plasmas, leading to production of ion beams in interaction with thin foils (Borghesi et al. , 2006; Roth et al. , 2005), or to Coulomb explosion of plasma channels in interaction with underdense plasmas (Borghesi et al. , 1998). A major step forward in the detection of such fields has been marked by the development of the proton imaging and deflectometry techniques (Borghesi et al. , 2002a , 2002c ; Mackinnon et al. , 2004), which employing laser-driven protons as a particle probe (Borghesi et al. , 2004), have proven to be an exceptionally useful tool for the investigation of ultrafast plasma dynamics. In this paper, after a brief discussion of the principles of the technique, we will present results from some recent experiments in which proton probes have been used to investigate the electron and ion dynamics initiated by high-intensity interaction. An application of transient, laser-initiated fields for control of the angular, and spectral properties of a proton beams will also be discussed.
2. PROTON PROBING TECHNIQUES
The unique properties of protons from high intensity laser-matter interactions, particularly in terms of spatial quality and temporal duration (Borghesi et al. , 2006), have opened up a totally new area of application of proton probing/proton radiography. Several experiments have been carried out in which laser-driven proton beams have been employed as a backlighter for static and dynamic target assemblies, in some cases, a secondary target irradiated by a separate laser pulse. The proton beams emitted from a laser-irradiated foil are highly laminar (Cowan et al. , 2004), and for projection purposes, can be described as emitted from a virtual, point-like source located in front of the target (Borghesi et al. , 2004; Brambrink et al. , 2006; Ruhl et al. , 2006). A point-projection imaging scheme is therefore automatically achieved. The unique capability of this technique to detect electrostatic fields in plasmas has...