Liquid-nitrogen temperatures (77 K) to provide a very tight pump-beam focus for cw laser operation. Dewar maintains a sample CdS crystal at a stable temperature of ~ 85 K at 20 mTorr. Laser M4 5 + Ar pump 476m Input polarizing beamsplitter Laser M3 Laser M2 Laser " L output (4950m MI microscope objective microscope objective Lcas CdS at ~ 85K FIG. 1. Ring cavity design with optional retroreflecting mirror M4 . 28 Goal: Liquid-nitrogen reservoir of 100-ml capacity is used to hold the CdS crystal temperature for 4 h without refilling. Ahfe of liquid nitrogen = 200 kJ/kg LN2 density=808 kg/m Dewar uses external microscope objectives to achieve the spot size of 5 m on the CdS crystal. A large spot size would likely destroy the crystal because of the 200 kW/cm (pump) threshold. The small spot size is also required to eliminate amplified spontaneous emission. Total heat transfer rate to the crystal (with a spot size 5 m) = power* A = 39 mW Dewar design Evaporation heat loss qe of LN2 qe = c + qr upper part vacuum lower part housing restraint" crystal- liquid- nitrogen reservoir 2" diameter Parasitic heat gain from the surroundings (radiation only) qr liquid- nitrogen reservoir/ 77K (Initial LN2 volume = 100 ml) Heat transfer via conduction q "O" ring restraint crystals 85K (Target) 80 oil sapphire window window copper frame sapphire copper frame (a) (b) FIG. 2. Dewar construction. (a) Cutaway side view. (b) Cutaway front view. 30 Drex Thermal circuits 19 (Exporation) ~40 mW Surroundings 95 LN Reservary (771) Copper Base 9c2 Indium Layer 193 Capper Plate acu Indive Sheet 9cs Sapphire 196 cds Platelet (SSK) Simplified thermal circuit Objective: to determine the mass of LN2 that is needed to continue 4-hr operation Given: The heat transfer via conduction is ~40 mW. Design Task #1: to determine the dimensions of components to ensure 40 mW of heat transfer from the CdS crystal to LN2 Dewar. Design Task #2: Using the energy balance at LN2 Dewar including evaporation heat loss and radiation heat gain, determine the mass of LN2 needed to operate 4 hours