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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.
Stanley K. Borowski, Y-K. Martin Peng, Terry Kammash
Fusion Science and Technology | Volume 6 | Number 1 | July 1984 | Pages 7-29
Technical Paper | Plasma Heating System | doi.org/10.13182/FST84-A23116
Articles are hosted by Taylor and Francis Online.
Auxiliary radio-frequency (rf) heating of electrons before and during the current rise phase of a large tokamak, such as the Fusion Engineering Device (FED) [R0 = 4.8 m, a = 1.3 m, σ = 1.6, B(R0) = 3.62 T], is examined as a means of reducing both the initiation loop voltage and resistive flux expenditure during startup. Prior to current initiation, 1 to 2 MW of electron cyclotron resonance heating power at ∼90 GHz is used to create a small volume of high conductivity plasma (Te ≃ 100 eV, ne ≃ 1019 m−3) near the upper hybrid resonance (UHR) region. This plasma conditioning, referred to as preheating, permits a small radius (a0 ≃ 0.2 to 0.4 m) current channel to be established with a relatively low initial loop voltage (≤25 V as opposed to ∼100 V without rf assist). During the subsequent plasma expansion and current rise phase, a combination of rf heating (up to 5 MW) and linear current ramping leads to a substantial savings in volt-seconds by (a) minimizing the resistive flux consumption and (b) producing broad current density profiles. (With such broad profiles, the internal flux requirements are maintained at or near the flat profile limit.) To study the “preheating” phase, a near classical particle and energy transport model is developed to estimate the electron heating efficiency in a currentless toroidal plasma. The model assumes that preferential electron heating at the UHR leads to the formation of an ambipolar sheath potential between the neutral plasma and the conducting vacuum vessel and limiter. The ambipolar electron field (EAMB) enables the plasma to neutralize itself via poloidal EAMB × B drift. This form of effective rotational transform “short circuits” the vertical charge separation and improves particle confinement. The benefits of this effective electrostatic confinement are tempered, however, by the possibility of significant secondary electron emission from the limiters and vessel wall. A comparison of theoretical estimates and experimental preheating data from the Impurity Study Experiment-B tokamak shows reasonably good agreement and provides some confidence in the preheating power estimates obtained for the FED. Using FED preheating parameters as initial conditions, a single fluid zero-dimensional tokamak model is then used to study the time evolution of the plasma temperature, voltage, and flux requirements during the expanding radius current startup phase. The sensitivity of these parameters to variations in the initial minor radius, oxygen impurity content, and the electron preheating level is also analyzed and discussed.