One approach involves an investigation of alternating current systems for balloon use, Solid-state tech- nology has now produced highly efficient light weight inverters that offer to make practicable the use of alternating current devices on balloon platforms, Devices of this type weighing less than twenty-five pounds can now deliver 3 KVA of 400 cycle power with close frequency control and efficiencies exceeding 85 to 90 percent. With respect to high altitude sampling this will allow the use of low-friction, brushless motors delivering a given horsepower at high efficiencies and lessening the critical heat dissipation problem. There will be little, if any, weight penalty in going to AC systems since the somewhat higher total efficiency should result in lower battery weight. High altitude operation also offers some interesting advantages with respect to the use of cascade impactors. At low densities, efficient impaction can be obtained at relatively low jet velocities. This means that we may be able to abandonthe relatively inefficient constant volume pumps with their low sampling rate (22 cfm) and go instead to multistage turbines which will deliver much higher volumes, giving the radiochemists more adequate samples to analyze. A second, entirely different approach to high altitude sampling is largely an outgrowth of our past work with crystalline zeolites. This involves the idea of using an evacuated zeolite bed as a pump. If sucha bed is maintained at liquid nitrogen temperatures (-196°C), ten grams of Linde Type 13X zeolite will adsorb over one standard liter of dry air at pressure altitudes equivalent to 170, 000 feet (0.5 mm Hg). verified many times in the laboratory. This is a fact we have Furthermore, this process is quite rapid and tests indicate that at 130, 000 feet, 85 pounds of zeolite will be able to adsorb over 100 standard cubic feet of air in less than 100 minutes. Figure 5 shows in principle the basic features of a particle sampler which utilizes a liquid nitrogen cooled zeolite pump. Such a sampler is presently being designed for flight tests in late summer 1961 and this unit is expected to have the following characteristics: WEIGHTS Zeolite Adsorbent 85 lbs Liquid Nitrogen 45 lbs Containers and Hardware 70 lbs TOTAL 200 ibs PERFORMANCE GOALS 156 Volume Sampled 100 + scf Sampling Rate 1.0 scf/min Sampling Altitude To 140, 000 feet