How is the helium temperature regulated?
The temperature of the helium is determined by the pressure in the tank, which is in turn determined by the heat leak into the tank and the impedance of the vent line. The impedance of the vent line is controllable, and set by all of the helium thrusters. By throttling the vent line we can change the tank pressure and so control the temperature. In normal operation the helium boiloff exceeds the amount of gas needed to cancel drag, and the excess is null-dumped, meaning that equal amounts are vented in opposite directions so as to produce no thrust. The excess can be throttled independently of the thrust produced by the gas, and this is what regulates the temperature.
