An FF engine does not generate any significant amount of heat. The pulsed magnetic field constricts pinpoints of gas into plasmoids & those pinpoints is where the fusion occurs. The tokamak/ITER is the method that requires a sustained high temperature plasma.
https://focusfusion.org/index.php/site/article/heat/
The plasmoids have a top radius of 0.0018 cm.
https://focusfusion.org/index.php/site/article/re_analysis_of_texas_data/
Decaborane density is 950 kg/m^3.
http://www.webelements.com/compounds/boron/decaborane_14.html
plasmoid volume: 2.4 x 10^-8 cm^3
decaborane in plasmoid: 2.3 x 10^-8 g
I cannot find the heat capacity of decaborane in a freely available source, so I’ll use an absurdly high estimate of 100 J/g-K. Boron has a heat capacity of ~2 J/g-K at 600° K.
http://www.efunda.com/materials/elements/HC_Table.cfm?Element_ID=B
Based on that assumption, heating the decaborane in the plasmoid to 1 billion K imparts 2000 J (rounded down) of heat energy into it.
heat capacity calculator: http://www.ausetute.com.au/heatcapa.html
At a 1 KHz pulse rate that is 2 x 10^6 J/s or 50 kWh/day. Over the same time period, the engine generates 120,000 kWh of electricity.
The heat generated is less than 0.05% of the electricity recovered from the particle beam & x-rays.
The amount of heat produced can raise the temperature of 1 liter of water 30 K per minute.