Hole-Boring Radiation Pressure Acceleration of parabolic front surface target generates tightly focused energetic ion beam compatible with fast ignition

Fast Ignition (FI) provides a promising alternative to conventional Inertial Confinement Fusion (ICF) by reducing sensitivity to Rayleigh Taylor Instability and yielding higher gain. However, FI requires a separate ignitor beam which must be ultrashort(sub-ps) and tightly focused. We show via Particle-In-Cell simulations that a target with parabolically shaped front surface can be accelerated and focused to a very small spot using a readily available laser. A simple theoretical model based on Hole-Boring Radiation Pressure Acceleration mechanism is developed to explain the result. This scheme is scalable and can be used for a wide range of laser power to focus ions to variable focal lengths and can generate fs-scale ultrahigh energy density/flux beams. We show as an example that an ultrahigh power (100PW) ultrashort O(fs) laser can generate FI-compatible beams with almost O(kJ) beam energy focused to a spot of a few microns.