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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 523 records · Page 29
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

H-mode plasmas at very low aspect ratio on the Pegasus Toroidal Experiment

H-mode is obtained at $A\sim 1.2$ in the Pegasus Toroidal Experiment via Ohmic heating, high-field-side fueling, and low edge recycling in both limited and diverted magnetic topologies. These H-mode plasmas show the formation of edge current and pressure pedestals and a doubling of the energy confinement time to ${{H}_{98y,2}}\sim 1$ . The L–H power threshold ${{P}_{\text{LH}}}$ increases with density, and there is no ${{P}_{\text{LH}}}$ minimum observed in the attainable density space. The power threshold is equivalent in limited and diverted plasmas, consistent with the FM3 model. However, the measured ${{P}_{\text{LH}}}$ is $\sim 15\,\,\times $ higher than that predicted by conventional International Tokamak Physics Activity (ITPA) scalings, and ${{P}_{\text{LH}}}/{{P}_{\text{ITPA}08}}$ increases as $A\to 1$ . Small ELMs are present at low input power ${{P}_{\text{IN}}}\sim {{P}_{\text{LH}}}$ , with toroidal mode number $n\leqslant 4$ . At ${{P}_{\text{IN}}}\gg {{P}_{\text{LH}}}$ , they transition to large ELMs with intermediate 5 < n < 15. The dominant-n component of a large ELM grows exponentially, while other components evolve nonlinearly and can damp prior to the crash. Direct measurements of the current profile in the pedestal region show that both ELM types exhibit a generation of a current-hole, followed by a pedestal recovery. Large ELMs are shown to further expel a current-carrying filament. Small ELM suppression via injection of low levels of helical current into the edge plasma region is also indicated.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2017

Continuous, edge localized ion heating during non-solenoidal plasma startup and sustainment in a low aspect ratio tokamak

Plasmas in the Pegasus spherical tokamak are initiated and grown by the non-solenoidal local helicity injection (LHI) current drive technique. The LHI system consists of three adjacent electron current sources that inject multiple helical current filaments that can reconnect with each other. Anomalously high impurity ion temperatures are observed during LHI with T i,OV ≤ 650 eV, which is in contrast to T i,OV ≤ 70 eV from Ohmic heating alone. Spatial profiles of T i,OV indicate an edge localized heating source, with T i,OV ~ 650 eV near the outboard major radius of the injectors and dropping to ~150 eV near the plasma magnetic axis. Experiments without a background tokamak plasma indicate the ion heating results from magnetic reconnection between adjacent injected current filaments. In these experiments, the HeII T i perpendicular to the magnetic field is found to scale with the reconnecting field strength, local density, and guide field, while ${{T}_{\text{i},\parallel}}$ experiences little change, in agreement with two-fluid reconnection theory. In conclusion, this ion heating is not expected to significantly impact the LHI plasma performance in Pegasus, as it does not contribute significantly to the electron heating. However, estimates of the power transfer to the bulk ion are quite large, and thus LHI current drive provides an auxiliary ion heating mechanism to the tokamak plasma.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗