In the near thirty years since the BEAR experiment, many technologies directly or indirectly applicable to an advanced neutral particle beam (NPB) have emerged due to other DOD programs, commercial investments, or other government programs. Key elements of a space-based NPB system include mass, size, and efficiency. The work proposed herein results from investments in the commercial arena, primarily advancing negative ion source, RFQ technologies and the Radio-Frequency-Focused-Interdigital (RFI) linac technology. While all three aforementioned technologies are being used in the commercial world for medical procedures (Boron Neutron Capture Therapy), the beam current levels are low ~ 15 mA compared to that required for a NPB system, the technology is scalable to higher beam currents. Its primary features include: 3X-4X improvement in end-to-end efficiency, and a 3X reduction in mass. These are key elements as the accelerator represents the largest power consumption and the largest structural mass in the system. This SBIR will incorporate recent advances in the linac design. Specifically, we will: 1. Establish RFI linac requirements 2. Perform a preliminary design at requisite beam currents 3. Assess advances within a NPB weapon concept. For the Option, our team will assess new weight saving technologies incorporated in the linac. Approved for Public Release | 18-MDA-9817 (23 Oct 18)