Jon_Beveryman t1_je7oxqk wrote

No, they're not the same. The (111) plane in FCC and the (1000) plane in HCP are equivalent but if you look down the [111] and [1000] directions you will see that the stacking sequence is different. This is usually described as ABCABC (FCC) vs ABAB (HCP). This is, for instance, why you can have FCC <--> HCP phase transformations produced solely by stacking faults.


Jon_Beveryman t1_je7kkey wrote

(A) there is no thermally induced HCP phase in iron at atmospheric pressure, in pure iron the HCP epsilon phase is solely a high pressure phase, (B) I don't see what the temperature vs pressure effect size has to do with any of this - the assertion was that in the core you'd have the same crystal structure as you would on the surface and it is observably not true.


Jon_Beveryman t1_j0u9ayg wrote

I would certainly believe that either is possible, but good ol' Winterberg seems to think that radiation pressure dominates, since it scales so strongly with temperature - to the order of 5000 TPa on the surface of the tamper! Meanwhile the ablation pressure should scale "only" with the P-T EOS of the tamper material.


Jon_Beveryman t1_j0u3bff wrote

In this application the radiation pressure is pretty minimal yeah. I haven't seen numbers for it myself, but in some other settings where you care about direct radiation pressure & ablation pressure, you usually discard the radiation pressure term entirely unless you are very close to the source or it's an incredibly potent source. For instance, in Teller-Ulam type thermonuclear bombs, the radiation pressure from the fission stage is assumed to provide virtually all of the implosion pressure for the fusion stage [going by unclassified sources only ofc, e.g Winterberg "The Physical Principles of Thermonuclear Explosive Devices"].


Jon_Beveryman t1_j0u2zgl wrote

To be blunt: Helion smells like grift to me. Their recent media blitz on youtube and reddit adds to this impression, for me at least. They have a really unorthodox method, and their claims about radiation safety in their design are at best incredibly optimistic, if not outright misleading. For instance, in a past life I did some work on plasma facing materials for ITER. Anything you expose to a burning fusion plasma is going to suffer a lot of neutron damage, including neutron activation -- i.e the neutrons turn your nice non-radioactive wall material into something quite radioactive. Helion's claims about "low activation" materials for this setting don't really pass my sniff test, professionally.


Jon_Beveryman t1_j0th4sk wrote

So...yes, there is a big role for government labs and government-funded academic groups to do that kind of work. and the Department of Energy supports a lot of that work! But there's a wrinkle here, which is that NIF is "owned" by Lawrence Livermore National Laboratory. LLNL is one of the Department of Energy's 3 "weapons" labs. See, for historical reasons [which you probably know already] the DOE owns the nuclear weapons design mission instead of Defense being in charge. 3 of the DOE national labs [Livermore, Los Alamos, and Sandia] are considered the weapons labs. Livermore and Los Alamos are each responsible for nuclear weapons science and design, while Sandia is responsible for the engineering side. The US also does not test live nuclear weapons since the end of the Cold War, so the weapons labs acquired a new mission - "stockpile stewardship and management". Essentially, "go do a bunch of science to make sure that the nuclear arsenal will still work every time we need it to". A big part of this was figuring out how to experimentally replicate the conditions of a thermonuclear explosion, aka fusion. NIF is first and foremost in support of that effort, and not the energy job.


Jon_Beveryman t1_j0tggqw wrote

Technically it's not! The most effective known way of compressing fusion fuel is to generate an insanely high x-ray pressure using a fission primary. We've been doing that since the 50s! But these have, ahem, other issues for energy purposes.

To achieve ignition temperatures in the fuel pellet, you need really large pressures. Stagnation pressures in previous NIF shots are on the order of 1-10 gigabar, or 800 terapascal; similar attempts at OMEGA, a similar laser facility, have achieved lower but still very high pressures of about 200 gigapascal using direct laser ablation rather than using the laser to produce x-rays. Laser and x-ray ablation are well suited to producing such high pressures, because they can dump a lot of energy into the target very very fast; this allows the ablated layer to reach high energy densities before anything can really start moving.

There are other ways to do it, maybe! For instance there's a startup called First Light which is trying to use light gas guns to produce the requisite pressures using physical impacts. They may have gotten this idea from a somewhat infamous nuclear physicist named Friedwardt Winterberg, who proposed a number of interesting mechanisms for compressing fusion fuel. Like this idea to use a hypervelocity projectile to adiabatically compress a high-atomic-weight gas, which will then get hot enough to ignite the fuel pellet!


Jon_Beveryman t1_j0suwj2 wrote

It is actually not, or at least is mostly not, the radiation pressure. The radiation pressure from the x-rays in ICF is not nearly sufficient to ignite the fuel. Rather, it is the ablation pressure from the outer "shell" of the target, which is very rapidly heated by the x-rays. The rapid heating, of course, produces a very large change in internal energy and therefore a large pressure; the outer layer of the target rapidly expands outwards, in the direction of least resistance, producing "recoil" motion in the form of a shockwave directed towards the center of the target.


Jon_Beveryman t1_j0sthbd wrote

X-ray compression is indeed a physical compression process, just like if you submerged the fuel pellet into a tank of (very high pressure!) water. It is not immediately obvious why X-rays should do this to a solid object, though, and I don't think any of the major news articles on the recent NIF shot explain it very well.

The pressure responsible for the fuel compression is called the X-ray ablation pressure. When X-rays interact with matter, they deposit their energy into the material. Most of this energy goes into heating the material. X-rays do not penetrate especially deep into the material, which means that they dump all of their energy into a very thin (several microns, or less than 1/100th of a millimeter) surface layer. The x-ray pulse is also very short, usually shorter than 10 nanoseconds. The energy density in this surface layer rises very, very fast as a result. This produces a two step compression in the target.

  1. The rise in internal energy corresponds to a rise in pressure in this surface layer. This is a thermodynamic relationship usually expressed through what we call an equation of state. There are a number of commonly used equations of state for high pressure physics; if you are curious to learn more about the underlying math, the Mie-Gruneisen equation of state is a good starting place.
  2. The high pressure in the surface layer pushes surface material out and away from the center of the pellet, in the direction of least resistance. This causes a "recoil" force towards the center of the pellet, in the form of a compression shock wave. This is the primary source of the pressure required for fusion, not the radiation pressure. The radiation pressure from the X-rays is not nearly high enough, but the ablation shock is both high enough pressure and moves fast enough to bring the pellet to ignition.

For more detail on the physics, A.T. Anderson's PhD thesis "X-Ray Ablation Measurements and
Modeling for ICF Applications" is a pretty good and non-paywalled option.


Jon_Beveryman t1_ivrgs3f wrote

Radar is "just" microwave-frequency electromagnetic radiation. The radar antenna emits a wave and then measures the return wave to "see" objects. This return wave can come from other planes, terrain features, and depending on radar design, rainclouds. In order to "see" through the nose of the plane, these electromagnetic waves must pass through the nose without producing any signal back towards the antenna or suffering much energy loss.

The nose covering over a radar is called the radome. Usually the radome is made of something transparent to the wavelengths used by the radar (aka microwave radiation). This is usually a lightweight plastic or foam. The key property is that the material has what we call a low permittivity. You can think of this as the material not interacting strongly with electromagnetic radiation.