Documentation of the extreme aridity (approx. 3% relative humidity) of the lower stratosphere and the rapid decrease of mixing ratio with height just above the polar tropopause (20-fold in the 1st km) was begun by Dobson et al., (1946) in 1943. They recognized that this extreme and persistent aridity must be dynamically maintained else it would have been wiped out by turbulent diffusion. This led Brewer (1949) to hypothesize a stratospheric circulation in which all air enters through the tropical tropopause where it is freeze dried to a mass mixing ratio of 2 to 3 ppM. This dry air then spreads poleward and descends through the polar tropopauses overpowering upward transport of water vapor by diffusion which would otherwise be permitted by the much warmer temperatures of the polar tropopauses. Questions can indeed be raised as to the absolute magnitudes of stratospheric mixing ratios, the effective temperature of the tropical tropopause cold trap, the reality of winter pole freeze-dry sinks and the representativeness of the available observations suggesting an H/sub 2/O mixing ratio maximum just above the tropical tropopause and a constant mixing ratio from the tropopause to 30 to 35 km. However, no model that better fits all of …

Space charge as it affects ion beam transport is reviewed. The approach here will be to derive beam-current criteria for divergence from space charge, review recent theoretical models for fractional space-charge neutralization, discuss space-charge-related observations on ion-beam transport in a specific experimental system, and briefly note several applications using space charge. Experimental measurements of effective space charge are discussed for a dc ion-source test stand using a 90/sup 0/ double-focusing magnet for species separation and for a solenoidal lens magnet for trim focus of the ion beam preparatory to entrance into a 400-kV accelerator column.

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