Can a light dimmer substitute for a variac when setting up a tesla cilo?

Hi to all,
I have read the posts on changing motor speed with a light dimmer and was
wondering if you could use a dimmer to replace the variac supplying the NST's 
to operate the tesla coil at low power for setting up.
Also I am able to buy a 500va variac and my coil is using 2x15kv 60ma NSTs
Could I use this variac for short runs with a good cool-down after?
If so,  how long should my maximum run time be?
Many Thanks,