Does anyone build 100k RPM alternators?
For a bit of additional context: there exists a particular design of alternator suitable for high speeds, such as those obtained by direct turbine coupling. Those were developed primarily for use in ICBM power supplies, where weight is of value while excess overheated gases are essentially bundled for free.

http://en.wikipedia.org/wiki/Flux_switching_alternator

It must also be noted, that first ever turbo-generator, developed and demonstrated by C. A. Parsons in 1884, was of direct drive type running at 18k rpm (with rated power of 7.5 kW).