Here's a thought which has been running through my head for a long time:

If you had a small, but REALLY powerful synchrotron, you could flood it with some sort of matter (possibly a plasma or particular solid), and get this matter whipping around at a steady 80% c or so. If there were many, as opposed to the customary two or four 'kick' stations (which use a magnetic field to impart kinetic energy to the particles within the field), one of these could be configured to impart more speed to the particles at a single point around the accelerator than at the others. Now, if this station accelerated the particles within its field to, say, 95% of c, the mass of these particles (to an observer) would appear to increase, therefore, their effective gravitational effect on their surroundings would increase. Theoretically, if the relativistic mass of these particles was high enough, they might create a gravitational field at this point large enough to start to collapse the matter stream into a singularity.

Maybe.