Helium was injected into expanded boundary divertor discharges in Doublet III to evaluate the helium exhaust and enrichment capabilities of the divertor. Helium was found to enter the main body of the plasma readily, achieving a concentration similar to that without the divertor. At a helium concentration in the main plasma of 5% of n̄e, helium pressure of 5 × 10−5 torr was observed in the pumping duct with no enrichment of helium relative to atomic hydrogen, while at higher densities helium pressure decreased and significant de-enrichment was observed. Simplified calculations of the relative penetration depths of neutral hydrogen and helium are consistent with these results.
In the attempt to understand the behavior of HfO2-based resistive switching devices at low temperatures, TiN/Ti/HfO2/W metal–insulator–metal devices were fabricated; the atomic layer deposition technique was used to grow the high-k layer. After performing an electroforming process at room temperature, the device was cooled in a cryostat to carry out 100 current–voltage cycles at several temperatures ranging from the “liquid nitrogen temperature” to 350 K. The measurements showed a semiconducting behavior in high and low resistance states. In the low resistance state, a hopping conduction mechanism was obtained. The set and reset voltages increased when temperature decreased because the thermal energies for oxygen vacancies and ions were reduced. However, the temperature did not influence the power absorbed in the reset transition, indicating the local temperature in the filament controls the transition. The set transition turned from gradual to abrupt when decreasing the temperature, due to a positive feedback between the current increase and the Joule heating at low temperatures.
Experiments designed to heat electrons by Landau damping of waves at approximately twice the lower hybrid frequency have been carried out on Doublet IIA. This objective is in contrast to other lower hybrid experiments which are designed to heat ions using frequencies corresponding to the lower hybrid resonance frequency. Up to 500 kW of rf power. was applied to discharge with approximately 100 kW ohmic input using parallel wavelengths chosen to optimize the spatial distribution of the power deposition based on linear or quasi-linear Landau damping.
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