archermarks

Appreciate the clarification. I am familiar with RANS modeling to a reasonable extent but not with a lot of the modern developments. We're still kind of at the stage of trying to find something that works as well as Prantdl's mixing length model, or maybe a k-epsilon model. Additionally, while our processes are turbulent, the turbulence differs significantly from classical turbulence.

For instance, turbulence in Hall thrusters is likely to be governed by an inverse energy cascade, where small disturbances magnify to larger and larger scales and the dissipation occurs via convection at device-scale instead of by diffusion at kolmogorov scales.

Ok, so looking at that phase diagram, we operate in a drastically different regime than you might be thinking. The pressures we get to are maybe 10^-3 Torr at best. The propellant is extremely rarified and not particularly hot.

Unless I misunderstand your meaning, I guess in a certain sense you could frame it in that way, where you have a rotational drift which provides energy to an axially-propagating wave. I'm not well-versed in black hole plasma physics. It is worth noting that anomalous cross-field transport is a problem that exists in lots of plasmas, including fusion plasmas like tokamaks and probably also in astrophysical plasmas.

Both of those, but also additional mass due to extra plumbing and such. It's definitely a thing we could do but it's not without it's problems. Also, Hall thrusters don't tile well as they're annular/ring-shaped devices. There are linear thrusters which would tile better but they tend to work fairly poorly.

That's what we do! But that's also what we've been doing for 40 years and it's nice to have some theoretical grounding to why stuff works.

I'm really not following what you're talking about. What is the phase diagram you're referring to?

I mean density in terms of amount of plasma through the device. You can cluster these together too but that gets complex from a systems design perspective. It's something that's being researched though

MHD is a specific set of a assumptions you can make to reduce the plasma fluid equations to something more tractable. This includes things like assuming infinite conductivity and combining all of your species into a single "fluid". Those are reasonable assumptions for astrophysical plasmas but not at all for Hall thrusters, so we do not use MHD.

In my work, we use the Euler equations for ions (assuming they lack viscosity), with electromagnetic and ionization source terms. Electrons are treated as massless and generally steady with respect to the ions. Both of their equations of motion can be derived starting with the Boltzmann equation and applying simplifying assumptions.

Yes, thank you. It's a closure problem in the former sense.

They could help with improving magnetic confinement on high power density thrusters.

Yes, it is. There's a lot of small thruster startups out there. See Orbion, Thrustme, and Applied Ion Systems for examples.

VASIMR is a specific type of magnetic nozzle thruster built by Ad Astra and which has never flown. Hall thrusters have been flown since the 70s and 80s and have been rapidly developed since the mid 2000s to fill a variety of niches. Magnetic nozzles are cool and will likely be very important for low power thrusters and future high power thrusters, but at this point VASIMR doesn't have many concrete advantages over Hall thrusters except perhaps in flexibility of propellant choice.

No. Hall thrusters are low temperature plasma devices, and operate in a completely different plasma regime than fusion devices. The ions dont get that fast, only a few tens of thousands of meters/second. The electrons only reach 10-100 eV, a far cry from the hundreds of thousands of eV in fusion devices.

We build them all the time. The difficulty is measuring the right regions of the plasma with the right precision. The plasma is pretty sensitive to perturbations so you can't always just stick a probe in them. This often restricts us to using noninvasive measurements like laser diagnostics, but because the plasma is quite rarified, it can be difficult to get a good signal to noise ratio. Additionally, the laser diagnostics can be restricted in what frequencies (spatially or temporally) they can measure, depending on the technique employed. Lastly, even if we fully understood the process, modeling it would remain a challenge (for all the same reasons that turbulence modeling is, despite the fact that turbulence is much better understood)

They're really cool to see running. Often not so dim either.

The main constraint is power usage. In principle you want to accelerate your fuel particles as fast as possible. At a fixed electric power though, the faster you accelerate each individual particle, the less particles you can accelerate. This conserves propellant but reduces thrust. Consider that you could use a photon rocket, basically a flashlight, where your propellant is moving at the maximum possible speed. This will have great propellant efficiency but you won't move much unless you have TW of power. Since you want to get somewhere in a finite time and don't have unlimited power, in practice you want something with lower propellant efficiency.

So yes, you could use a particle accelerator to accelerate ions, and it might be quite efficient, but you would need a ton of power if you wanted a reasonable amount of thrust.

The "material" in this case is vacuum and plasma. Yes, something small and electromagnetic is affecting the conductivity of the plasma, and we have an idea what it is, but that doesn't mean we know how to simulate it or predict its effects on the thruster.

The fuel actually has quite high mass compared to a typical rocket engine. We use Xenon (131.293 g/mol) or Krypton (~83 g/mol), compared to air which is (~29 g/mol).

To scale up thrusters you can simply make them bigger, but that leads to a lot of wasted space as the thrust is only produced by a ring-shaped region of the device. To improve thrust density, you can nest these inside each other (see this page on my research group's website https://pepl.engin.umich.edu/project/high-power-hall-thruste...). More recently, we have been looking at running smaller thrusters at higher power densities, which was previously impractical due to the way older thrusters shaped their magnetic fields. Nowadays we can design thrusters such that very little power is lost to the walls so high power density operation is looking more feasible.

Per the second question, you would need a very efficient thruster to limit the amount of mass needed as propellant. If you have an entire planet, you might look into using a photon rocket. Alternatively, you can try a massive solar sail or magnetic sail but I'm not an expert on these things.

Depends on what you mean by order i suppose. The simplest are algebraic (zero-order) models where we prescribe the anomalous transport as an explicit function of other plasma parameters. More complex ones are coupled systems of hyperbolic PDEs, which are typically first-order in space and time.

If you are referring to the order of accuracy of the simulations, that's typically 1st to 4th order, depending on method used.

Hall thrusters do not involve any form of detonation or condensation. They're run on inert rarified gases. There are ionization instabilities in Hall thrusters for which I suppose you could make an analogy to detonation. Rotating spoke instabilities in Hall thrusters do resemble RDEs, to an extent. However, these are low frequency modes and as the post discusses, they are insufficient to explain enhanced transport. The instabilities that cause anomalous transport are probably high-frequency modes which occur faster than the characteristic timescales of ionization.

It's pretty unlikely that anomalous transport comes down to quantum fields. The instabilities that probably cause it are complex but are classical in nature.

In this case, it's different than lift. We can run CFD simulations of wings and get accurate estimations of lift/drag etc. We cannot do that for Hall thrusters without already having run the thruster, as we need to calibrate the anomalous transport parameters to make the simulation match experiment. This makes the simulations much less useful in design than they are in other fields, which is a real drawback.

Another point to make is that testing these devices is expensive. They are low thrust propulsion systems that are designed for thousands of hours of continuous operation. They need to be run in very high vacuum, and for higher power devices there are only a handful of vacuum chambers in the world sufficient for the task. Additionally, Xenon is expensive (dramatically moreso since the Russian invasion of Ukraine), so developing better predictive simulations is critical.

I actually work directly on this problem as part of my PhD. I work in designing simple "closure models" for Hall thruster anomalous transport that can be implemented in simulations to improve how well they match with experiment. It's a really interesting problem, and like the OP alludes to, it probably comes down to turbulence and plasma instabilities which are really hard to measure and even harder to include self-consistently in a device-scale simulation.

The really tricky thing is that the simulations show that basically every aspect of Hall thruster performance is extremely sensitive to the choice of anomalous transport parameter in certain parts of the discharge, but completely insensitive to it in other parts of the discharge. this makes validating models difficult, as many different models could agree in the small region of the discharge where it matters, but completely disagree in other parts of the discharge, and we would have little chance of discriminating between them.

This doesn't have basically anything to do with the linked article. This study suggests that the oldest known Homo Sapiens fossils in Africa are even older than previously thought, which gels with other evidence. Here's the abstract:

"Efforts to date the oldest modern human fossils in eastern Africa, from Omo-Kibish and Herto in Ethiopia, have drawn on a variety of chronometric evidence, including 40Ar/39Ar ages of stratigraphically associated tuffs. The ages that are generally reported for these fossils are around 197 thousand years (kyr) for the Kibish Omo I, and around 160–155 kyr for the Herto hominins. However, the stratigraphic relationships and tephra correlations that underpin these estimates have been challenged. Here we report geochemical analyses that link the Kamoya’s Hominid Site (KHS) Tuff, which conclusively overlies the member of the Omo-Kibish Formation that contains Omo I, with a major explosive eruption of Shala volcano in the Main Ethiopian Rift. By dating the proximal deposits of this eruption, we obtain a new minimum age for the Omo fossils of 233 ± 22 kyr. Contrary to previous arguments, we also show that the KHS Tuff does not correlate with another widespread tephra layer, the Waidedo Vitric Tuff, and therefore cannot anchor a minimum age for the Herto fossils. Shifting the age of the oldest known Homo sapiens fossils in eastern Africa to before around 200 thousand years ago is consistent with independent evidence for greater antiquity of the modern human lineage."

The African origin of Homo sapiens is pretty well established these days.

Oh I'm well aware, I work on this stuff, but I'm still curious about their system, and any on-orbit data they might have. We're working on krypton Hall thrusters right now in our lab and trying to characterize the differences in operation. There's some non-obvious scaling factors we're working to understand in order to try and better optimize thrusters for krypton operation. We have a recent paper (free pdf) about this at pepl.engin.umich.edu/pdf/2021_JoAP_Su.pdf.

This is really cool. Electric propulsion for small satellites is a hot field and has been advancing rapidly, and this is no exception. The rising cost of Xenon is a real problem in the field right now so it's great to see the work being done on alternate propellants (it would be great if we knew more about SpaceX's thrusters, which run on Krypton).

Doing some math, their total thrust efficiency seems to be around 29%, which is really good for a thruster of its size being ionized by RF power.