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Dragonfly, a Pu-fueled drone heading to Titan, gets key NASA approval
Curiosity landed on Mars sporting a radioisotope thermoelectric generator (RTG) in 2012, and a second NASA rover, Perseverance, landed in 2021. Both are still rolling across the red planet in the name of science. Another exploratory craft with a similar plutonium-238–fueled RTG but a very different mission—to fly between multiple test sites on Titan, Saturn’s largest moon—recently got one step closer to deployment.
On April 25, NASA and the Johns Hopkins University Applied Physics Laboratory (APL) announced that the Dragonfly mission to Saturn’s icy moon passed its critical design review. “Passing this mission milestone means that Dragonfly’s mission design, fabrication, integration, and test plans are all approved, and the mission can now turn its attention to the construction of the spacecraft itself,” according to NASA.
M. F. A. Harrison, P. J. Harbour, E. S. Hotston
Fusion Science and Technology | Volume 3 | Number 3 | May 1983 | Pages 432-456
Technical Paper | Divertor System | doi.org/10.13182/FST83-A20866
Articles are hosted by Taylor and Francis Online.
Plasma behavior in the scrapeoff and divertor regions of the single-null configuration of the International Tokamak Reactor (INTOR) has been predicted by means of a one-dimensional model of transport through the plasma sheath at the divertor target. Electron capture during ion/surface collisions is the principal mechanism for the production of neutral gas, which recycles to the target within the divertor chamber. This recycling is analyzed using a model for neutral particle transport through a divertor plasma channel of simple geometry and uniform density that overlays the target; the exhaust of those atoms that can escape through the plasma and enter the pumped region of the chamber is assessed by means of a gas transport model that embraces the characteristics of both chamber and pumping ducts. Estimates are made of the power dissipated by atomic line radiation and by transport of fast atoms to the walls of the chamber; sputtering of both target and walls is assessed. Data are evaluated for the specific case when INTOR is operated under “standard conditions, ” i.e., transport by charged particles of 75 MW to the throats of the divertor through a scrapeoff plasma of average density 5 × 1019 m−3. Under these conditions, the temperature of the scrapeoff plasma is predicted to be ∼90 e V, the flow of ions to each divertor target is ∼1.3 × 1024 s−1, the plasma density adjacent to the target sheath ∼9 × 1019 m−3, and the corresponding plasma temperature ∼25 e V. Fusion reactors in INTOR produce 2 × 1020 alpha particle ⋅ −1 and the exhaust rate of helium gas must be adequate to maintain a low concentration of helium ions in the plasma, i.e., (nHe/nD-T) ≈0.05. The model predicts that this can be achieved with a gas exhaust rate of ∼105 l⋅s−1 (referred to helium atoms and deuterium-tritium molecules at 300 K) and the corresponding burnup fraction is −25%. The operational lifetime of the divertor specified for INTOR will probably be limited by erosion of the stainless steel walls of the chamber, which is estimated to occur at a rate of −1 cm⋅yr−1 of cyclic operation.
