You can use a synchrotron to produse a contimuous pulse train of X-rays.
An X-ray tube can be modulated by modulating it's current, it will modulate
amplitude and spectrum at the same time.
If you need more speed, you can use acousitic exitation to transfer energy
between two x-ray beams is a crystal. (phonon modulation of X-ray
diffraction).
There are LED's which generate UVB, easy to modulate