Annular shaped laser-accelerated proton beams

Since their discovery in 1999 [1] the topic of laseraccelerated ions has attracted high attention both in the field of plasma-physics and in the accelerator community. In a typical laser-accelerated ion scheme a thin metal foil is irradiated by an ultra-intense chirped pulse amplified laser beam which leads to the generation of relativistic electrons. These electrons are accelerated through the cold target material in laser beam direction, escaping at the vacuum boundary and forming a Gaussian-like electron-sheath at the target rear-side. A field-strength on the order of several TV/m leads to an instantaneous field-ionisation of the atoms at the rear surface and an acceleration of these ions perpendicular to the target surface by the Target-NormalSheath-Acceleration mechanism (TNSA, [2]). In the scope of this report we present a technique to alter the transverse proton beam-profile from the ”source” of its creation. With this technique we could already demonstrate the generation of an annular shaped proton beam at the PHELIX laser system with the use of an optimised coneguided target geometry [5]. Experimental Results Figure 1 shows the transverse profile of the proton beam measured with radio-chromic films (RCF, [3]) in a stacked configuration shown in the optical density (OD). The laser parameters for this shot were 118 J before compression with a pulse length of about 500 fs (FWHM) at best focus.