Comment by yummypaint | Hacker News Reader

Comment by yummypaint | Hacker News Reader

I'm a practicing nuclear physicist and do work on gamma ray interactions with actinides. It's possible i'm missing something important, but i think this article is an utter mess and was clearly written by someone with absolutely no understanding of what they are saying. Attempting to learn anything from this article will be counterproductive.

Here is an explanation of chirped pulse amplification: https://www.rp-photonics.com/chirped_pulse_amplification.htm... This technique is for producing optical photons, which will have less than 10 ev of energy. In the same way that you can't focus the sun's light to a point that gets hotter than the surface of the sun (violates second law of thermodynamics), it isn't obvious how low energy laser pulses can be useful for this. The article offers no explanation whatsoever. Maybe the electric field across the nucleus can be made strong enough to induce scission?

In general, if you want to interact with the nucleus you need photons on the order of 1 Mev or more, whose wavelengths are comparable to the size of the nucleus. These are gamma rays, which are not optical photons. There are ways to boost optical photons to those energies (like inverse compton scattering), but the article says nothing about that either. I would think inverse compton scattering of a chirped pulse from an electron packet in an accelerator will completely destroy the sharp timing and reflect the distribution of the electrons instead.

While I was a graduate student at UT Austin, I attended a talk By Dr. C.P.J. Barty, Chief Technology Officer for the National Ignition Facility and Photon Science Directorate at the Lawrence Livermore National Laboratory, titled “Extreme Gamma-ray Sources and the Dawn of Nuclear Photonics”. It was quite fascinating. The basic idea is to use a linear accelerator to accelerate packets of electrons to relativistic speeds, and then to interact those packets with an infrared laser pulse (inside the laser cavity), to generate tunable extreme Gamma-ray laser pulses. They were able to demonstrate burning of nuclear waste products, as well as distinguishing between different elemental isotopes based on their absorption.

I haven't heard anything about the project since, but it sounded like a promising approach for burning nuclear waste, particularly when using a superconducting accelerator to do energy recycling.

Mourou's Nobel lecture [1] has brief explanation of the envisioned process. The idea is to use intense laser pulses to accelerate deuterium ions into a tritium target. The deuterium-tritium fusion then generates high-energy neutrons that can transmutate nuclei.

This is probably not impossible, but I have doubts if this could be implemented efficiently at scale.

[1] https://journals.aps.org/rmp/abstract/10.1103/RevModPhys.91....

You can do that with fusor. It does direct acceleration without laser generation losses.

What are the energy requirements? Is there residual energy to capture?

Doing dt fusion to make neutrons for other purposes is a pretty common technique in general. It sounds like the twist is using lasers to provide the energy instead of a traditional accelerator. Commercial neutron generators that exist now can approach fluxes comparable to that in small reactors, but the main difference is the energy distribution. Reactors are much more low-energy dominated so it's true that other reactions can become accessible with a high energy quasi monoenergetic source. However, i think we are still missing orders of magnitude of capability before it could become feasible. On top of that, laser based facilities like NIF are probably the most expensive and slowest-repeating way to generate neutrons right now. Plus when you make that many neutrons alot of other stuff will end up getting activated and pottentially become waste itself.

Personally i think there also need to be some nuclear chemistry advances to be able to efficiently and cleanly separate isotopes before they can be transmuted. There is some neat work being done on this by people at FRIB trying to extract medical isotopes etc from the beam dump.

I don't think it's correct to think of a laser as a source in some thermal equilibrium. "Concentrating temperature" passively from sources in thermal equilibrium is forbidden, but there's nothing preventing "concentrating power".

Pulsed lasers bring material interactions into a highly non-linear regime - photon intensity is so high that multiple photon absorption is common. In the typical nuclear decay regime you are concerned with single photon absorption, and the gamma ray intuition is correct. There are also a number of approaches where various targets hit with ultrafast lasers produce controllable flux of gamma rays which are used in downstream experimentation.

You could, you know, read a little more, especially for physics that is outside your scope of study. Just the fact that the person in the article has a Nobel Prize should be enough for you to take a moment to google a little and learn about physics that you might simply not be aware of.

CPA with Ti-Sapphire produces photons that are optical (near-IR specifically, usually 800nm) but it is their interaction with matter, usually solid density matter that is how you achieve that causes such violent reactions, it is in the so-called "laser-plasma interaction," specifically that the material you shoot with ultra-short laser pulses is immediately ionized into a plasma and that light matter interaction leads to all sorts of energetic by-products, including (easily) MeV scale radiation. Now the radiation does not have necessarily good beam qualities, which is part of the current focus of research, but it does happen and it is easy to reach MeV scale.

A good example of how different this regime is that if you're sharp yet ignorant you might have balked at the idea of it being a plasma given near-IR photons should not be able to ionize matter to a decent degree. The reality though is these interactions are not baby's first "single-photon" tree-level diagram quantum interactions, instead the intensity is high enough you model the ionization interaction as a so-called "many photon ionization" process, where you either say many photons pile on-top of each other (which has issues in accuracy honestly) or better, the external field is strong enough it's on scale with the coulombic field at the bound electron's radius that it distorts the field seen by the electron leading to ionization[0], similar to the Stark effect but with an AC field instead of a DC field.

EDIT:re-reading the "hotter than the sun" comment, it looks like I misread it and apologize, it's not as bad as I read it. Basically, think about the natural timescale of this system and realize that the timescale needed to produce gamma rays is pretty short.

Ok, I won't bore you with figuring it out yourself, basically the whole intense interaction before material recombines is at longest a few hundred picoseconds, in fact, the electron acceleration that is MeV+ that then you can convert into x-rays happens on the optical wavelength timescale (that, yes 3fs is the natural timescale! at least for the electrons). So, you can in fact achieve KeV+ plasma "temperatures" for very short periods by focusing the energy of a plasma, ~joule or so, into small spaces (again, natural space scale is laser wavelength, although it's really the laser spot here, so say 100 wavelengths so about 10s of microns) in a short time (picoseconds) you easily achieve yes astrophysical scale temperatures for very short times and small scales--but enough to produce high energy particles and even model astrophysical phenomena! In fact it's a grant writing/masturbatory phrase we use but these systems have even been used to do so-called "laboratory astrophysics[1]" that is model astrophysical phenomena in the lab by creating plasma conditions that you can actually probe experimentally. Again, since we're all laser physicists, the fact the plasmas last picoseconds or nanoseconds isn't a problem because we use the same optical systems that create the matter to probe them that can probe the system on the picosecond scale.

One last note, this is the entire premise of inertial conefinement fusion, you create stellar core pressures and temperatures in a ~mm hohlraum for a very short time, nanoseconds, but that is enough to generate fusion conditions for the deuterium which is what matters. Ditto for creating plasma conditions for radiation generation although that's even shorter, again femto to picoseconds.

[0] https://en.wikipedia.org/wiki/Tunnel_ionization

might as well throw it in here but the usual way it is modeled is by old Russian work of ADK

http://jetp.ras.ru/cgi-bin/dn/e_064_06_1191.pdf

which is a somewhat flawed approximation of PPT

http://jetp.ras.ru/cgi-bin/dn/e_023_05_0924.pdf

[1] https://iopscience.iop.org/article/10.1088/1741-4326/aae598

Okay well I am not a particle physicist, but I did design and build laser plasma ablation devices, and I will echo the skepticism of the particle physicist.

> easy to reach MeV scale

Thermal photons in MeV!? Whoa. Going to need a citation on that one.

Yikes indeed! with the headline, I was thinking that this was some variant of the National Ignition facility, trying to heat up samples enough to break the nucleii... but yeah, without massive intensities like at the NIF I don't wee how optical photons would even be noticed by the heavy nucleii.

I'd hope there's more to it that the journalist just failed to catch...

Well I did the research and what I googled indicates you are completely wrong.

/s

Reply on news.ycombinator.com