Idea: X-Ray reflection

[mattman]

Hello,

This is just a hypothetical idea.


What would happen if we reflected the Bremsstrahlung radiation off the walls and back into the center plasma?



1. The walls would definitely absorb some energy and heat up over time. This would depend on the surface area and the reflectance of the walls. This heated wall would certainly affect the rate of energy recovery in either a direct conversion scheme or a steam cycle.


2. Would the reflected X-rays reheat the plasma? How much reheating could this method actually do? Would it reheat the ions and the electrons differently?


Has anyone posted something like this before? If not, this would be an interesting topic to research.

[rjaypeters]

I recommend you look at how hard it is to efficiently reflect x-rays.

[D Tibbets]

As mentioned, it is difficult to reflect x-rays at acute angles. Read about Chandra X-ray telescope.
It is not so hard to reflect x-rays at very shallow angles (say 1 degree) To reflect the x-rays back into the center of a plasma container would require dozens to hundreds of carefully arranged mirrors. Each mirror would absorb some of the x-rays. You might be lucky to obtain ~ 1 % of the x-rays being finally reflected.

With visible light, glass lenses reflect ~ 5% of the light hitting them at right angles. With appropriate coatings, this can go down to ~ 1 %. At shallower angles of impact this reflection increases, and at shallow enough angles, even dirt can reflect the visible light enough to create mirages (mirages). With reflective coatings, the reflection of light goes up- ~ 90-95% with aluminum, gold, silver, etc. Again, with appropriate dielectric coatings, this can be increased to ~ 99% reflectivity .
I don't know how much reflection of x-rays occurs with smooth metal surfaces with 90 degree incidences, but I suspect it is ~ 1% or less. With the shallow angles of incidence, this reflective component increases as with any other wavelength of light, till finally , with many small angle reflections, you can bring x-rays to a focus. In this , the net total angle of reflection may only be eg: 10 degrees (effectively, it is more like a refractory lens, than a reflective mirror). To achieve 180 degrees of net reflection, you might need to use 18 times as many mirrors, with 18 times as much absorption losses compared to the x-ray telescope.

I suspect the scheme for absorbing X-rays and directly converting them to electrical energy at perhaps 80% efficiency (as proposed by E. Learner at LPP (Focus Fusion)) is at least several orders of magnitude more efficient. Even thermal conversion at ~ 30% efficiency would be much better than trying to directly reflect the x-rays back into the plasma.

The entire field of light reflection/ absorption/ transmission/ refraction is fascinating with many applications in optics, solar cell efficiency, shielding, stealth, etc. The new meta materials that have a negative index of refraction may be tremendously important. I don't know how these materials might effect x-ray (or gamma ray) manipulation.

Dan Tibbets

[KitemanSA]

My concern is whether we want to "heat" the plasma at all. Mono-energetic is the goal. Heating by X ray..., is it good?

[ltgbrown]

you might need to use 18 times as many mirrors

What about nano structures? It seems like every week we hear about some new capability because of nanostructures. Nanostructures are playing a larger and larger role in the increases in efficiency being achieved in photovoltaics, perhaps increasing reflection (vice absorption or decreasing reflection) is ripe for further research.

[D Tibbets]

Good point. If the x-rays are initially monoenergetic, the idealistic reflected x-rays would also be monoenergetic. But, I suspect the energy distribution of the x-rays is complex. I assume the dominate flux of x-rays would ideally be lower energy than the average temperature of the plasma would suggest. Central convergence of the electrons and ions would result in increased densities, and thus Bremsstrulung collisionality in the core where the electron speeds are low, and the ion densities are greatest. That is one of the central arguments for making P-B11 fusion practical. These lower energy x-rays could partially reheat the plasma, with carefull electron voltage adjustments making up the rest. I don't know if an elliptical potential well could be maintained under these conditions, or if the spread in the x- ray energy ( there would be a gradient in the energy and flux, depending how far from the center the x-ray was created) would be tolerable. A complex problem, even if x-ray reflection is possible.

PS: Some might think that there is x-ray reflection in ICF (a bomb), but I think this is a misconception. The walls of the confining material will absorb the x-rays and be heated to nearly the same temperature of the X-rays and thus emit similar wavelength X-rays almost as fast as it is absorbing them. In this case the confinement, during the brief time it holds together, is acting more like a high efficiency insulator rather than a reflector.

Dan Tibbets

[bwana]

why is it so hard to reflect xrays? Think of light, if you were not allowed to use mirrors, how could you bring light back to its source? use a mass of fiberoptic bundles and curl them back to the source. I know that xrays can be conducted by a waveguide. It's just wire. In fact, back in the '80s, Teller had the bright idea to generate an xray laser. Just set off a thermonuclear device in space contained within a porcupine of metal rods. Each metal rod is individually aimable. What comes out of the metal rod is an xray laser that is target to the ICBM you are trying to shoot down.

[D Tibbets]

I do not understand the atomic level interactions of reflection/ refration of light to answer your question with confidence. The reflection of light generally involves interaction with electrons.

As far as converting energy ( x-rays/ UV, Visible light, IR) into a coherent laser beam, it has been done. The problem as I understand it is that the efficiencies of the conversion is very low. Much less than the efficiency of thermal conversion of the energy to electricity which could then be fed back into the machine.

The IR and visible light negative refraction meta materials depend on structures scaled near the wavelength of the light. For the visible light you are talking about perhaps ~ 500 nanometers. Easy to etch, and plenty of molecules involved so they can withstand the relatively weak visible light energies. For X rays you are talking of wavelengths perhaps in the 1 to 50 nanometer range. Much more difficult to produce. The smallest Intel micro processors are now at about 25 nanometers. Even if they could be produced (and has some benificial effect) the thermal loads (nothing can be 100 % efficient) and destructive ionizing X-rays would probably demolish such small structures very quickly. And this ignors other destructive particles that may be present- such as neutrons, alphas, even hotter gamma rays, etc.

Fiber optics work by bouncing light off the walls of the fiber at shallow angles where reflectivity is near 100 %. Just like bouncing light off the outer surface of a piece of glass. At right angles glass reflects ~ 5% of the light, and absorbs/ transmit the remainder. The same reflection happens as the light leaves the glass on the other side. At a shallow angle the reflection may actually reflect light internally at high enough efficiency that the absorption/ transparency of the glass (substance) is the limiting factor for how far the light will travel. The percent of light that is reflected at the surface (outside or inside surface) of a glass fiber is dependent on the wavelength and incident angle of the light.
Just like an X-ray telescope, the X-rays need to hit the surface at very shallow angles for efficient reflection.
For visible light a loop of glass fiber with a radius of perhaps 1 inch would reverse the light. With an energetic X-ray, the needed radius may be hundreds of feet (a pure guess). Over such lengths of fiber, even if it has very low x-ray absorption (very high transparency) most of the X-rays would probably be absorbed- so you are back to the thermal conversion cycle, or the proposed Focus Fusion approach

Dan Tibbets

[bwana]

thank you for your considered reply. i guess the devil is in the details. a little research before opening my mouth (or loosing my typing fingers on the world) would have revealed that the index of refraction for xrays for all materials is ~1.(usually slightly less than 1!) So now we are looking at making a tube, not a fiber. so the reflected xrays can travel in it. the tube however becomes an xray absorber, so you've got to make it really thin.High atomic number materials reflect xrays more efficiently and commonly used materials are gold and nickel. With these, the critical angle is 1 degree at 1 KeV. If your tube has a 1 mm inner diameter, you would need to travel ~3.2 meters to make a ~1 degree deflection. For a 180 degree u turn, you'd need ~half a kilometer of tube in a semicircle ~380 meters across. If your device is 1 meter in diameter and you dont want to use a thousand little tubes, but just 10 big ones (each with a diameter of 100mm), then we are talking about 38 km diameter semicircle. But not in a circle like CERN, but a sphere. And CERN is 27 km. And that's a lot of gold.

no the xrays that come out cannot be recirculated like the electrons. even the sun cant keep its xrays to itself. we have to develop a polymer that absorbs xrays and gives off electrons. a fancy kind of solar panel.

[Giorgio]

I remember reading an article few month ago about an experimental metamaterial design that was actually supposed to reflect x-ray generated inside a cavity.

I don't remember the exact efficiency (I think to remember that it was very low) but considering it was the first time to see such an application I think that there is still lot of room to improve the whole system.
I'll dig in my history to see if I can get it back and post a link to it here.

The point anyhow is exactly what bwana stated in his last lines:
What do we do of the reflected x-ray anyhow?

[bwana]

the reflected xrays would be pointed back into the core to return energy to the soup

[Giorgio]

the reflected xrays would be pointed back into the core to return energy to the soup

By the time you are generating x-ray the soup is already at its optimum energy level.

[D Tibbets]

Also, if the plasma is transparent enough to allow the original x-rays to escape (the plasma is a gas at ~ one thousandths of an atmosphere in a Polywell, and even less in a Tokamak), the recirculated x-rays would also pass through mostly without interacting. The x-ray might have to make thousands or millions, or more recirculations to significantly heat the plasma, Even if the x-rays were recycled at 99% efficiency, the losses would would multiply, till no net gain would be possible. Again, much better to concentrate on converting the energy of the original x-rays to electricity .

X-rays do heat the plasma in a bomb (possibly also in the small plasmoid in the DPF?) but that is because the density is perhaps ~ 1,000,000 times greater. In such a comparison an x-ray passing through a volume in the bomb would need 1 million as many passes in a Polywell- and that assumes a linear relationship. Actually, I think this number would need to be squared (10^12 passes) as the MFP of a particle varies as the square of the density.

One thousand eV x-rays are pretty cool. In a Tokamak the average x-ray energy may be ~5-15 KeV. In a P-B11 Polywell, FRC, or DPF the Bremmstrulung x-rays may be in this range or up to several hundred KeV. How would that effect the X-ray reflection turning radius?

Dan Tibbets

[Giorgio]

I fully agree with your post.
If the Polywelll will generate x-ray in a consistent percentage than the only way to use them will be to find a suitable solution to efficiently convert them in electricity.

[ladajo]

At the least we can use the Xrays to heat water or something. Energy recovery would be the better move. Or even along the lines of some of the nano enabled solar cells, maybe we could get one sensitive to Xrays at the energy levels required.