Virtual Source Dynamics

Abstract

As discussed in Sect. 7.1 the model of the virtual source describes the spatial emission of the proton beam starting from a point source in front of the irradiated target. The temporal characteristics of the proton emission have been discussed theoretically and are supported by experimentally gained arguments in several publications [1–5]. The acceleration of the protons takes place on a picosecond timescale. Additionally, it was shown that the proton beam has an intrinsic velocity chirp [2, 3]. This means that energetic protons are accelerated first followed by the slower ones. For pump–probe experiments (e.g. proton imaging) the short acceleration time is more relevant since after a few millimeter the proton bunch is temporally stretched due to the broad energy distribution (cf. Sect. 9.3). Nevertheless, from the change of the emission characteristics for different proton energies, the temporal evolution of the acceleration field can be estimated even if no quantitative time scale can be given. In a previous experiment at the Max-Born-Institute [3] a fluctuation of the proton beam pointing was observed with a magnifying Thomson spectrometer. This phenomenon was also observed in the proton “streak deflectometry” experiments described in Chap. 12. Thus, a further experiment was applied, combining the advantages of both experiments—a continues energy dependent detection of several well separated parts of the proton beam.