Actually, the TGV at commercial speed is already pretty sparky: the highway between Paris and Lyon runs parallel to the railway for many miles, and I drove it several times at night: when the TGV passes you (you can't be mistaken, he's 150 km/h faster than you !) it has burst of sparks flying all around the pantos every second. And that was all of them (they're barely two minutes apart on that section).

There is a large ballast rock strip on either side of the railway, dunno if it's specially larger for high speed railways, but there must be burning metal shards falling on it all the time.

Actually, if the panto was contact-less and the spark was just like lightning, a canal of ionized air, it wouldn't expand past the area between catenary and panto and nothing would get chipped away to the ground.

Pierre