is ammonia the cavity 35 changes the thermal coupling between the heat source 11 and the evaporator 21 and works in two modes nominal and degraded modes depending on the temperature of the heat source 11 i the nominal mode occurs while the temperature of the heat source 11 is below the critical point c2 of the second working fluid as illustrated in fig2 a the heat input h iev1 at the evaporator 21 is proportional to the heat input h i1 at the external surface of the saddle 31 also the heat output h o1 at the condenser 23 is proportional to the heat input h iev1 at the evaporator 21 for illustration purposes the arrows indicating h i1 h iev1 and h o1 are represented with similar dimensions ii the degraded mode occurs while the temperature of the heat source 11 is above the critical point t c2 of the second working fluid the degraded mode relies on the second working fluid having a low thermal conductance in a gaseous state as illustrated in fig2 b the heat input h iev2 at the evaporator 21 is less than the heat input h i2 at the external surface of the saddle 31 during degraded mode the heat transfer from the heat source to the evaporator is reduced due to the saddle.