Well, there are a bunch of such XFELs around (not sure if China has any yet), but they're research, not for production applications. That's how we can know they

Well, there are a bunch of such XFELs around (not sure if China has any yet), but they're research, not for production applications. That's how we can know they're such bright sources, how big they tend to be, how expensive they are to build, etc.