How can I reduce the power dissipated across this MOSFET without affecting avalanche?
OK -- Lots of details left out, and I'm assuming that if you can put a cap in series with the coil, you can drive it from the low side.
If you've got enough power being dissipated that your TO-220 FET requires a &quot;comically large heat sink&quot; then no matter what you do to dissipate the heat, that component will get hot.  If you want to reduce the heat (and lower your power consumption, to boot) then you need to recapture the energy and use it.
M1 and L1 you have.  When you turn off M1, D1 conducts and dumps energy into C1.  Size C1 to get the voltage you want.
The key here is that if you just put a resistor in parallel with C1, it'll get hot instead of the FET -- if that's OK, do it and be happy.  Just size it so that C1 is mostly discharged at the end of a cycle.  If it *is&quot; a problem, then design a circuit that'll take the energy you're dumping into C1 and feeds it back into your +12V line.  This'll require a switching supply, probably a lot like an off-line switcher, and it's probably not worth it unless you really need to keep the heat out, or you have more than a couple of watts of power to recover.



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