plasma confinement with radiation pressure

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idobox
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plasma confinement with radiation pressure

Postby idobox » Sun Jan 04, 2009 9:49 pm UTC

As much as I understand, the main problem with fusion is plasma confinement. Magnetic confinement is complex, expensive, and leads to building things like ITER, and until now haven't achieved positive yield. Electrostatic confinement (fusor) has a lot of problems, gravitational confinement requires a star, and inertial confinement works well for H bombs, but no so well with sonofusion.

What we need is a repulsive force to forbid the nuclei to hit the walls, so I thought, why not radiation pressure?
radiation confinement.jpg
radiation confinement.jpg (20.05 KiB) Viewed 1677 times

The plasma would be rather conductive, so the radiation will bounce back and off, with a small part of the power converted to plasma heat. The reflected RF wave should have a little wider spectrum, because of doppler effect on the moving nuclei, but mostly the plasma will emit a fuckton of blackbody radiation, which should be rather well reflected by the superconductive shell. Finally, we should get nucleus-nucleus bremsstrahlung and fusion byproducts. I have still no idea how to protect the shell from the gamma/Xray radiation, nor how to convert the power generated to a usable form.

So, sure it would need a few petawatts of RF waves to get enough radiation pressure, but this power should be rather well conserved, so the once boot up, the device shouldn't require much power input.
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Re: plasma confinement with radiation pressure

Postby SmashtheVan » Mon Jan 05, 2009 7:36 am UTC

something tells me that massive amount of radiation needed to provide this pressure would totally destroy any attempts at coming to a zero exchange point(energy in=energy out, forget the actual term they use) for the reactor. The inertial confinement is essentially the same concept, to my (limited) knowledge anyway, except the radiation is created externally and not as a product of the plasma. Since the inertial confinement would be much more efficient, I doubt anyone would be rushing to build an RC reactor if they can make an IC reactor for much cheaper operating costs

edit:
Just realized a flaw in this radiation confinement idea. Inertial confinement is successful because it supplies a constant and even pressure on all sides of the fuel. Relying on a radiation-based confinement would mean that all the photons that are the source of this pressure would have to originate from the plasma at the same time, in all directions, and at the same wavelengths. However, photon emission is very random(as far as time of emission goes), and so I don't think radiation pressure would be an effective confinement technique for plasmas

take these opinions with a grain of salt as I am an undergrad with just a very basic understanding of plasmas and confinement techniques through my own personal readings. if you have any suggestions on reading materials for plasmas, id be glad to take them.

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Re: plasma confinement with radiation pressure

Postby dic_penderyn » Mon Jan 05, 2009 9:53 am UTC

The ITER project will use the most powerful magnetic magnets we have ever built. But ............
There may be another way

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Re: plasma confinement with radiation pressure

Postby SmashtheVan » Mon Jan 05, 2009 11:38 am UTC

dic_penderyn wrote:The ITER project will use the most powerful magnetic magnets we have ever built. But ............
There may be another way


this is inertial confinement concept, slightly elaborated further. With the inertial confinement technique it is easy to put the fuel under high pressures necessary to create plasmas, but there is still the problem of igniting the fuel to achieve the thermonuclear reactions. The HiPer process uses an extra single short pulse(on the order of 10-100 femtoseconds) laser at one spot to basically start the thermonuclear reaction one spot on the inner fuel core. Normally the lasers are stopped by the plasma region, but this short pulse of shorter wavelength(and higher energy, should go without saying) can penetrate the plasma region and ignite the solid compressed fuel. The thermonuclear reaction then spreads quickly around the rest of the fuel, and it burns outward into the plasma. This is the "fast ignite" system and is a process that should increase the gain of the reactor when compared with the standard inertial confinement method.

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Re: plasma confinement with radiation pressure

Postby idobox » Mon Jan 05, 2009 12:28 pm UTC

This things is exciting, but I highly doubt laser inertial confinement will ever be used for power generation, because you have to change the fuel pellet between every shot, unless they find a way to use gaseous fuel.

An other interesting approach is the polywell. They use 6 superconductive loops to confine an electron cloud and use it as an electrode to do electrostatic confinement. The main advantage is that the nuclei are accelerated by an electrical field, so they all have the same energy when entering the system, even though hits provoke thermalisation. So the proportion of hits than actually lead to fusion is much higher. The main inconvenient is that a lot of energy is lost to bremsstrahlung when the nuclei deflect the electrons.
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Re: plasma confinement with radiation pressure

Postby Minerva » Mon Jan 05, 2009 2:59 pm UTC

Isn't "radiation pressure" - whether that's either in a laser system such as NIF or a Teller-Ulam bomb or whatever - basically synonymous with "inertial confinement" anyway?
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Re: plasma confinement with radiation pressure

Postby idobox » Mon Jan 05, 2009 3:23 pm UTC

Not exactly, in inertia confinement, you first implode a fuel pellet at very high speed, that achieves conditions suitable for fusion, and because of the fuel's inertia, the pellet isn't instantaneously vaporized. By definition, inertia confinement is unstable. And inertia confinement is also used in sonofusion, in which it's fluid pressure rather than laser or x rays that implode the fuel. By the way, sonofusion hasn't proved to be working.

What I call radiation pressure confinement is a stable confinement, as in fusors or tokamaks, where the plasma is kept under fusion conditions until a significant part of the fuel has been consumed.
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Re: plasma confinement with radiation pressure

Postby Minerva » Mon Jan 05, 2009 4:05 pm UTC

Well, a Fusor and a Tokamak have this stable, established field, electrostatic field in the fusor and magnetic field in the Tokamak respectively, that is responsible for confinement.

I suppose a Fusor or a Tokamak are kind of relatively "steady-state" systems, operating in a stable, continuous kind of way, in contrast to laser ICF (or a thermonuclear bomb) where everything happens in a once-off fast, transient pulse - is that what you're getting at?
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Re: plasma confinement with radiation pressure

Postby idobox » Tue Jan 06, 2009 10:51 am UTC

That's what I meant.

The thing is you need a force to fight against the pressure of the plasma, to keep it at high density and temperature.

In stars, this force is gravity.
In fusors and polywells, it's the electrical field
in tokamaks, stellarators and such, it's the magnetic field
In H Bombs, ICF and sonofusion, it's the inertia of the fuel (I know inertia isn't really a force, sue me)

In the ICF, the radiation pressure does not confine the fuel. Well it does, but for a very short period of time, and most of the confinement is done by the inertia of the fuel. A true radiation confinement device would not rely on the inertia, and work about the same way as a fusor or tokamak.
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Re: plasma confinement with radiation pressure

Postby Soralin » Tue Jan 06, 2009 12:51 pm UTC

idobox wrote:This things is exciting, but I highly doubt laser inertial confinement will ever be used for power generation, because you have to change the fuel pellet between every shot, unless they find a way to use gaseous fuel.


If you can figure out a way of producing fusion with IEC, then you can figure out a way of having a big tank of fuel pellets and dropping in a new one after every blast. :)

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Re: plasma confinement with radiation pressure

Postby idobox » Tue Jan 06, 2009 1:46 pm UTC

Even though the idea of blasting deuterium pellets one after the other with giant lasers is kinda cool, I'm not sure this would be industrially applicable. And IEC means inertial electrostatic confinement, it does not use any laser.

I also had an other stupid idea. Since the fusor is limited by hits on the inner electrode, why don't we build a fusor with a Boron-11 electrode, and fuel it with hydrogen, or use any other mix of solid/gaseous fuel ? That way, we wouldn't care about nuclei lost in collisions with the electrode, the small cross section would be less of a problem since the nuclei could hit a large number of targets before losing too much energy, and the energy lost in coulomb collisions would only heat the electrode, so a part of this energy would still be usable. Boron has a melting point of 2349K according to wikipedia, with such temperatures, theorical Carnot efficiency can be quite high, so a good proportion of the energy lost to heat can be reused. And because P-Be fusion produces fast alpha, (almost 9MeV for 3 alpha particles), you can directly convert the output to a 9/3=3MeV, 3/2=1.5 MV DC voltage between the inner electrode and the outer electrode (3/2 because alpha particles have q=+2e). Bremsstrahlung would be minimal, and synchrotron radiation would be negligible.
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Re: plasma confinement with radiation pressure

Postby Tass » Tue Jan 06, 2009 3:23 pm UTC

idobox wrote:Even though the idea of blasting deuterium pellets one after the other with giant lasers is kinda cool, I'm not sure this would be industrially applicable. And IEC means inertial electrostatic confinement, it does not use any laser.

I also had an other stupid idea. Since the fusor is limited by hits on the inner electrode, why don't we build a fusor with a Boron-11 electrode, and fuel it with hydrogen, or use any other mix of solid/gaseous fuel ? That way, we wouldn't care about nuclei lost in collisions with the electrode, the small cross section would be less of a problem since the nuclei could hit a large number of targets before losing too much energy, and the energy lost in coulomb collisions would only heat the electrode, so a part of this energy would still be usable. Boron has a melting point of 2349K according to wikipedia, with such temperatures, theorical Carnot efficiency can be quite high, so a good proportion of the energy lost to heat can be reused. And because P-Be fusion produces fast alpha, (almost 9MeV for 3 alpha particles), you can directly convert the output to a 9/3=3MeV, 3/2=1.5 MV DC voltage between the inner electrode and the outer electrode (3/2 because alpha particles have q=+2e). Bremsstrahlung would be minimal, and synchrotron radiation would be negligible.


I thought a good deal about this one. It sounds good: protons being accelerated by the field coliding with a target, heating it and releasing high energy alpha particles which more than recharges the field. Energy is generated by the heating of the target electrode and possibly by tapping off excess field, letting the electrons left in the target out to join the alpha particles that left.

Deuterium and lithium migth work as well.

The protons would probably knock electrons lose from the negative electrode, thereby draining the voltage though. They don't need nuclear energies to fly off and follow the alpha's, since the field pushes them out they just need to break free of the metal.

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Re: plasma confinement with radiation pressure

Postby idobox » Tue Jan 06, 2009 4:48 pm UTC

In a p-Be fusion, the output energy is in the form of 3 high energy 4He++, so any net energy output you might get would be by converting this kinetic energy into electricity through magnetic induction or capacitor charging, the latter being the easiest.

The thing would be a spherical vacuum chamber with three electrodes:
- the central electrode, greatly negative, coated or made of Boron. An interesting alternative is to build a shell of positively charged boron shell around the negative electrode, to reduce the probability of e-/p collision, but heat transfer would be more difficult. A heat machine takes the thermal energy of the boron to produce electricity. The power obtained that way will be inferior to the power consumed by proton acceleration.

-a collector electrode, that will collect the fast 4He++, building voltage until they arrive at null speed. Since all He don't have the same energy, this electrode might need to be divided into two concentric electrodes, the inner one catching slower alphas, and the other one catching the faster ones. Tensions should reach more than 1MV. The faster He have an energy of around 4MeV if I remember well, which means 2M DC building up. The outer electrode would be to the ground to avoid arcing between the device and surroundings, meaning the inner target electrode would be at -2MV potential.

-an emitter electrode. If I remind well, the maximal cross-section is around 600KeV. Since the Boron is standing still, it means the p have to be accelerated around 1.2MeV. The emitter electrode is around this voltage, and hydrogen atoms or protons are inserted in the device at this radius. The electrode must be a grid because fast He will have to go through it. Depending on the energy of 'slow' He (I've read the value somewhere), the emitter grid and the 'slow receptor' might be the same electrode.

The device will consume energy for proton acceleration. Upon collision, this energy will be converted for a part into heat, for a part into bremsstrahlung (electrons moved around), kinetic electrons and fusion reactions.
Heat can be harvested with a steam turbine, the blackbody radiation of the electrode should not be wasted if all the other components are good reflectors.
I don't see how to harvest bremsstrahlung.
The kinetic electrons will convert to either DC charge when they hit the inner electrode or heat when they stay in the lattice. If the electron is knocked backwards, it will be accelerated toward the outer electrode and produce X-rays or maybe gamma ray at impact, and energy will be lost.
When fusion occurs, it will produce 3 fast He, in fact 1 fast He and 2 very fast He. This energy will be harvested by charging the capacitor. To avoid loss of kinetic energy in collisions, the He must traverse the least matter possible, so the inner electrode must be as small as possible. With a small electrode, the probability a p goes through it is significant, but that's not a problem because the p is confined and will hit the electrode again until all energy is lost to heat. Same thing happens with protons rebounding on the electrode.

In and out, the two most energy losses are bremsstrahlung (which is not lost if converted to heat) and steam turbine efficiency.
If the electrode is near boron melting point, i.e. 2350K, the theorical Carnot efficiency will be around 90% (1- Tcold/Thigh = 1- 280/2400).
Accelerating a proton takes Einput=1.2MeV
A succesful fusion provides Eoutput=8.7MeV
If C=0.9 is heat recuperation efficiency and L the percentage of energy converted to heat, we get:
Heat production = L.Einput
recycled heat energy = L.C.Ei
Energy produced by fusion = (1-L).Eoutput
Energy consumed for acceleration = Einput

Total energy Et = L.C.Ei + (1-L).Eo - Ei = (C.Ei - Eo).L + Eo - Ei
with numerals
Et = (0.9*1.2 - 8.7)L + 8.7 - 1.2
Et = -7.62 L + 7.5
Et>0 if 7.62L < 7.5
L < 7.5/7.62
L < 98.4%

if the heat recycling efficiency falls to 70%, we get
L < 7.5/7.86
L < 95.4%

I don't know the cross-section of fusion and coulomb collision, but if coulomb is less than 20 times as frequent as fusion, and if we find a way to convert bremsstrahlung to heat, then it looks like energy production is possible, with nothing more complicated than a big gold-plated (for blackbody reflection) cathodic tube and a steam turbine.
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Re: plasma confinement with radiation pressure

Postby idobox » Tue Jan 06, 2009 4:49 pm UTC

Mwahahah!

The nobel is mine.
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Re: plasma confinement with radiation pressure

Postby Soralin » Tue Jan 06, 2009 10:45 pm UTC

idobox wrote:And IEC means inertial electrostatic confinement, it does not use any laser.


Oops, yeah, ICF I guess (Inertial Confinement Fusion). Too much time looking at polywell. :)


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