Recirculating and energy recovery ion linear accelerators

High-power superconducting ion linear accelerators play a vital role in both scientific research and industrial applications. However, their RF usage efficiency remains relatively low. In this paper, we review two innovative concepts: recirculating ion linear accelerators and energy recovery ion linear accelerators. These approaches significantly improve RF efficiency by enabling the ion beam to traverse the same superconducting cavity multiple times and by recovering the energy of the accelerated ion beam to return it to the RF cavities. We also propose a demonstrator for the recirculating ion linear accelerator using a scaled, low-energy electron accelerator, and discuss the potential application of the energy recovery ion accelerator in a full energy recovery electron-ion collider. These concepts offer promising avenues to substantially reduce both the construction and operational costs of superconducting linear accelerators.