What is a 'current filament' in a Gate Turn-Off Thyristor?
A current filament is the conductive current path in a GTO device.  

Most power GTO's have a large number of smaller thyristors in parallel on the actual device, and as a negative pulse is delivered to the gate to switch them off, they don't all switch off at the same time.  As individual parts of the device switch off, higher and higher currents exist through the parts that are still on.  This is what normally causes a GTO to fail, burning up the individual thyristors that make up the overall die.

Some serious negative currents are normally injected into the gate to force the conduction regions to shrink as quickly as possible.  For high current devices, it is common to have turn off pulses be at least 5 to 10% of the forward current, and not unusual to have the turn off pulses be up to 25 to 30% of the forward current to switch them off as quickly as possible.