We have measured the brightness of several helium free jet sources. Five converging nozzles with diameters between 0.6 and 5 m, and three tube nozzles with diameters between 2 and 10 m were studied at stagnation temperatures of 77 and 300 K and at stagnation pressures depending on nozzle size from 350 to 17000 kPa. Smaller nozzles produced higher brightness beams with values approaching 10 28 ͑s sr m 2 ͒ −1. At low-temperature quantum effects on the helium collision cross section significantly decreased the source brightness. We explore the possibility of producing even higher brightness sources with smaller diameter.

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[1] We present a novel explanation of the physical processes behind one type of cloud and ground-level tornadogenesis within a supercell. We point out that the charge separation naturally found in these large thunderstorms can potentially serve to contract the preexisting angular momentum through the additional process of the electric force. On the basis of this, we present a plausible geometry that explains why many tornado vortices begin at storm midlevel and build downward into ground-level tornadoes. A simple model based on this geometry is used to demonstrate the strength of the electric force compared to the required centripetal acceleration to maintain cloud midlevel tornado vortices measurable as tornado vortex signatures (TVSs). Furthermore, a model based on this geometry is used to get a time estimate for tornado vortex formation. From this we are able to identify a plausible value for the threshold charge density that would lead to tornadogenesis and tornado maintenance on the timescale of a few minutes. We show that the proposed geometry can explain the observations that ground-level tornadoes thrive in regions with high shear and large convective available potential energy (CAPE) and are able to make some predictions of specific measurable quantities. Motivation[2] Many of the physical conditions required to initiate tornadoes are well understood on a broad qualitative level but many quantitative details remain unknown. On the qualitative level, it is apparent that the deep rotation of a supercell thunderstorm along with a buoyant updraft and a rain-driven downdraft create a probable environment for tornadogenesis. Yet, we still do not quantitatively understand the full range of energy inputs which allow particular storms to spawn tornadoes while other storms with similar macroscopic properties do not. Furthermore, we cannot currently predict the length (minutes to hours) or intensity of the tornadoes that do form [Davies-Jones et al., 2001]. This suggests the presence of one or more physical thresholds in the system that, once exceeded, can spawn a tornado. Systems in which the relevant threshold is not achieved, therefore fail to form a tornado. As an example, evidence indicates that a certain amount of boundary layer shear in conjunction with a certain level of convective available potential energy (CAPE) is required for a tornado to occur [Rasmussen and Blanchard, 1998].[3] One clear attribute is that most tornadoes spawn from supercells and associated convective activity. In particular, Trapp et al. [2005] showed that 79% of all measured tornadoes for the years 1998 -2000 came from supercells. We also know that around two thirds of this 79% of supercellspawned tornadoes begin their rotation aloft (2 -7 km) and then descend to form ground-level tornadoes in what is measured as a descending TVS [Trapp et al., 1999]. This means that roughly half of all the sampled tornadoes began their intense rotation high in a supercell cloud and then built downward to make ground-level tornadoes. This suggest...

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