” North Korea is going to need access to a pretty powerful warhead—probably in the 250-300 kT”
and EMP is faily insensitive to the specific size of the nuclear blast - A 1-10kT warhead will do the trick. It’s the gamma-yield that matters. If NK has a “fizzler” warhead as has been speculated, which is an enhanced gamma-yield warhead they may, indeed have an existing EMP capability.
Right on, RFEngineer! Exactly what I have been saying. Enhancing gamma output is possible independent of yield.