Ion acceleration and beam quality preservation of structured targets using PetaWatt-class lasers via Hole- Boring Radiation Pressure Acceleration*

Structured solid-density or foam Ion Source Targets (ISTs) driven by PW-class lasers can generate ion beams with desirable characteristics via Hole-Boring Radiation Pressure Acceleration (HB-RPA) mechanism. Since HB-RPA accelerates ions perpendicular to the front surface of the IST, judicious target front surface fabrication translates to engineered velocity distribution. Ion beams with exotic and useful phase-space distribution, such as a convergent ion flow leading to extremely high energy flux at a well-defined focal length, can be generated. These beams have broad applications including nuclear fusion, cancer therapy, warm dense matter generation, among others. In particular, the IST-generated low-emittance monoenergetic ion beam provides an efficient way to generate an ignition spark for a pre-assembled massive nuclear fusion, leading to higher-gain fusion energy. We discuss the limiting factors for the IST acceleration and beam quality preservation including laser profile, polarization, and target density. Specifically, the temperature of the electrons nearly neutralizing the ion beam poses a limiting factor on beam emittance and spectra. Using additional structures placed at the beam path, the electron population coupled with ion beam can be modified, providing a way to improve the ion beam quality.