The United States has a rich history in high energy particle accelerators and collidersboth lepton and hadron machines, which have enabled several major discoveries in elementary particle physics. To ensure continued progress in the field, U.S. leadership as a key partner in building next generation collider facilities abroad is essential; also critically important is the exploring of options to host a future collider in the U.S. The "Snowmass" study and the subsequent Particle Physics Project Prioritization Panel (P5) process provide the timely opportunity to develop strategies for both. What we do now will shape the future of our field and whether the U.S. will remain a world leader in these areas. In this white paper, we briefly discuss the US engagement in proposed collider projects abroad and describe future collider options for the U.S. We also call for initiating an integrated R&D program for future colliders.

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Increasing the Main Injector beam power above ∼ 1.2 MW requires replacement of the 8 GeV Booster by a higher intensity alternative. In this paper, we consider an 8 GeV linac Booster replacement that produces 8 GeV H- beam for injection into the Recycler Ring or Main Injector. This upgrade will maximize the beam available for neutrino production for the long baseline DUNE experiment to greater than 2.5 MW and enable a next generation of intensity frontier experiments. The 8 GeV linac takes ∼ 1 GeV beam from the PIP-II Linac and accelerates it to ∼ 2 GeV in a 650 MHz superconducting RF linac, followed by a ∼ 2 to 8 GeV pulsed linac using 1300 MHz cryomodules. The linac components incorporate recent improvements in superconducting RF technology. The linac configuration and beam dynamics requirements are presented. Injection options are discussed, including use of an 8 GeV Accumulator Ring. Foil-based injection is the present standard but R&D toward implementing laser-assisted injection could enable a significant improvement. Research needed to implement the Booster replacement is described.

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