Water-window X-rays allow researchers to visualize biological cells at high contrast without staining agents or other potentially damaging preparations. Multiple challenges, however, have prevented widespread adoption. Previously, the only way to produce water-window X-rays of varying energies was with synchrotrons, expensive facilities that are the size of a large stadium. Most smaller tabletop machines could only produce radiation in fixed energies, and some required high-intensity lasers that need ongoing maintenance.