A low-pressure cesium vapor diode operating under a no negative space charge condition has emitter and collector with a 2cm2 area. The collector has a work function of 1.81V. The work function of the tungsten collector can be adjusted by varying the pressure, pes, of the cesium vapor. The emission constant is unaltered by the cesium and is 600,000Am2K2. The temperatures of the emitter and of the collector are 2100K and 1100K, respectively. The effective emissivity of the emitter is 0.3 on the side facing the collector and 0 on the opposite side. The only loss mechanisms are the cost of emitting electrons, excess energy of these electrons, and the radiation from the emitter toward the collector. 1. Which emitter work function maximizes the efficiency? 2. What is this efficiency