During a harvester session, the harvester will save all urls in realtime into a file with the name "harvester_tmp.txt".
This file is just created in case you have any issue with the harvester, be it a crash, power loss or similar, to prevent the loss of your harvested urls.
In the event of a power loss, or crash, you can just rename this file to anything you like and will find all harvested urls in it.
It will be deleted the moment you start a new harvester session.