Transient Electrostatic Fields and Related Energetic Proton Generation with a Plasma Fiber

We observe a hollow structure and a fine ring in the proton images from a petawatt scale laser interaction with a ``cone-fiber'' target. The protons related to the hollow structure are accelerated from the cone-tip surface and deflected later by a radial electric field surrounding the fiber. Those associated with the fine ring are accelerated from the fiber surface by this radial electric field. This field is found to decay exponentially within 3 ps from about $5\ifmmode\times\else\texttimes\fi{}{10}^{12}\text{ }\text{ }\mathrm{V}/\mathrm{m}$. Two-dimensional particle-in-cell simulations produce similar proton angular distributions.