It is required to use a fan to cool the LC31 coupler on power levels of 100 watts or more. This means a fan is recommended for ST plasma tube for more than 75 volts, and required for the BAT tube, regardless of voltage setting.
This fan from Cooler Master is quiet, cheap and moves a lot of air.
This fan needs 12 volts power. For example this power supply from eBay. You may want to use floor protector pads or a foam pad to minimize vibration and sound.