High Temperature Polyimide Tape - 20mm x about 30M.
I am not sure on the thickness but I am guessing 1 mil film with 1.5 mil adhesive for a total of 2.5 mil.
Polyimide tape was developed by DuPont in the late 1960s under the brand name Kapton. It remains stable across a wide range of temperatures, from −269 to +400 C. It is good for electronic repair due to its toughness and ability to withstand high temperatures.
The tape above is not made by DuPont so it is not the brand Kapton, but it is the same product. In this case it is branded Koptan.