Theoretical magnetic field strength

[Moontanman]

Is there any reason to think that magnetic fields we are currently able to generate are as strong as magnetic fields can be or is it possible to generate magnetic fields of any strength we desire given enough power?

 

I ask this in relation to the nuclear light bulb reactor and possible confinement of a fission reaction by a magnetic field instead of a quartz glass bubble...

 

http://en.wikipedia.org/wiki/Nuclear_lightbulb

[EdEarl]

Magnetars have magnetic fields stronger than any on earth.

[studiot]

Here was an early attempt at this

 

http://en.wikipedia.org/wiki/ZETA_(fusion_reactor)

 

Here is a later one

 

http://en.wikipedia.org/wiki/Tokamak

[Moontanman]

Magnetars have magnetic fields stronger than any on earth.

 

 

I'm not sure how this relates to us generating a magnetic field strong enough to confine a fission reaction.

 

Here was an early attempt at this

 

http://en.wikipedia.org/wiki/ZETA_(fusion_reactor)

 

Here is a later one

 

http://en.wikipedia.org/wiki/Tokamak

 

 

studiot I am thinking of a fission reaction not fusion..

[EdEarl]

 

I'm not sure how this relates to us generating a magnetic field strong enough to confine a fission reaction.

 

OP: Is there any reason to think that magnetic fields we are currently able to generate are as strong as magnetic fields can be or is it possible to generate magnetic fields of any strength we desire given enough power?

 

Magnetars are characterized by their extremely powerful magnetic fields of 10⁸-10¹¹tesla.^[5] These magnetic fields are hundreds of millions of times stronger than any man-made magnet,

We are limited practically by materials strength, available power, and money to spend, any one of which prevents us from making a magnet as strong as a magnetar.

Edited July 6, 2013 by EdEarl

[studiot]

 

studiot I am thinking of a fission reaction not fusion..

 

 

 

Sorry I missed that

 

 

Well, fusion reactions involve much simpler particles than fission and these have the advantage of being charged so we can use the relationship between electricity and magnetism to exert real containment forces on the participating particles. Further we have a relatively few particles involved, undiluted by non participating particles so the confinement space is much smaller.

 

Fission, on the other hand has an abundance of uncharged neutrons to control. These, of course, are not amenable to magnetic fields. Further the fraction of participating or reactive particles in even a refined mass of fissile material, is quite low.

 

 

I would suggest that with his background, Swansont is your man for this.

Edited July 6, 2013 by studiot

[Moontanman]

 

 

Sorry I missed that

 

 

Well, fusion reactions involve much simpler particles than fission and these have the advantage of being charged so we can use the relationship between electricity and magnetism to exert real containment forces on the participating particles. Further we have a relatively few particles involved, undiluted by non participating particles so the confinement space is much smaller.

 

Fission, on the other hand has an abundance of uncharged neutrons to control. These, of course, are not amenable to magnetic fields. Further the fraction of participating or reactive particles in even a refined mass of fissile material, is quite low.

 

 

I would suggest that with his background, Swansont is your man for this.

 

 

At the temps of a nuclear light bulb the ions are all charged and most of the energy is emitted as hard UV, neutrons are not used to heat water like a solid core reactor, but the ions are too heavy to be contained by any magnetic field we can currently produce... Hard UV is either absorbed by hydrogen which is expelled to produce thrust or in a stationary reactor the hard UV is used to produce electricity directly (my take at least) through something similar to photovoltaics...

[studiot]

Fission produces a great many neutrons.

 

The fissile material in your linked reactor is uranium hexflouride, which is subject to all the factors I outlined earlier.

 

The point is that fission also produces a great deal of heat.

In your reactor this passes through a quartz wall and heats a propulsion gas.

 

Are you suggesting magnetic fields to direct this ionised gas?

That is sensible.

 

However you also need a generator to create the electric current to generate the magnetic fields.

 

Would this fit comfortably on board the rocket?

 

Further, at the end of the day, you also have to control your fission reactor in the normal manner.

Edited July 6, 2013 by studiot

[Moontanman]

Fission produces a great many neutrons.

I know this, solid core reactors use the neutrons to heat water but the gaseous core reactor does not.

 

The fissile material in your linked reactor is uranium hexflouride, which is subject to all the factors I outlined earlier.

Did you bother to read my link?

 

The point is that fission also produces a great deal of heat.

In your reactor this passes through a quartz wall and heats a propulsion gas.

In this case most of the energy is released as hard UV.

 

Are you suggesting magnetic fields to direct this ionised gas?

That is sensible.

I am suggesting a magnetic field to contain the uranium hexifloride plasma, at 25,000 degrees the gas would be a plasma. Possibly static electricity fields could help as well.

 

However you also need a generator to create the electric current to generate the magnetic fields.

Yes, the hard UV is converted directly to electricity much like solar panels do visible light.

 

Would this fit comfortably on board the rocket?

 

Further, at the end of the day, you also have to control your fission reactor in the normal manner.

Please elaborate...

http://en.wikipedia.org/wiki/Gaseous_fission_reactor

My inspiration comes from this article, I have been told it makes some unwarranted assumptions but is relatively accurate in it's broad outlines.

 

http://members.shaw.ca/bru_b/Liberty_ship_menupg.html

[studiot]

 

Did you bother to read my link?

 

 

Yes I did read it and I may have missed something vital. If so I'm sorry and would be pleased for you to point this out.

 

However I should correct one or two points where you may have misunderstood the article, it was rather short on detail.

 

Firstly the article states "the vast majority of the electromagnetic emissions would be in the hard ultraviolet range"

 

You seem to have translated that into the 'majority of the energy', which the article did not say.

Further neutron emissions are not electromagnetic emissions. further they are not, in themselves radioactive.

 

Yes using the energy of the nuclear fire to megaheat exhaust propulsion gas is a desireable aim.

 

The article has no details of the fission reactor itself or its control or shielding.

Further to develop and maintain the intense magnetic fields you would require a significant electrical generator (presumably powered by nuclear energy)

 

All of this add significant weight to the beast.

 

It's sort of like the fact the the internal combustion engine need to devert some of its power to the oil pump, the water pump, the fuel pump and so on.

 

Nevertheless the it is worth pursuing and re-examining the idea from time to time as new materials and techniques become available.

 

go well

Edited July 7, 2013 by studiot

[Moontanman]

 

Yes I did read it and I may have missed something vital. If so I'm sorry and would be pleased for you to point this out.

 

However I should correct one or two points where you may have misunderstood the article, it was rather short on detail.

 

Firstly the article states "the vast majority of the electromagnetic emissions would be in the hard ultraviolet range"

 

You seem to have translated that into the 'majority of the energy', which the article did not say.

Further neutron emissions are not electromagnetic emissions. further they are not, in themselves radioactive.

 

Yes using the energy of the nuclear fire to megaheat exhaust propulsion gas is a desireable aim.

 

The article has no details of the fission reactor itself or its control or shielding.

Further to develop and maintain the intense magnetic fields you would require a significant electrical generator (presumably powered by nuclear energy)

 

All of this add significant weight to the beast.

 

It's sort of like the fact the the internal combustion engine need to devert some of its power to the oil pump, the water pump, the fuel pump and so on.

 

Nevertheless the it is worth pursuing and re-examining the idea from time to time as new materials and techniques become available.

 

go well

 

Here is the design I am talking about, isp of 3000 seconds, our best chemical rockets are about 450 seconds.

 

http://members.shaw.ca/bru_b/Liberty_ship_pg10.html

 

Third, a gas cored reactor has several potential "scram" modes, both fast and slow, and the speed of the reaction is easily "throttled" by adding and removing fuel or by manipulating the vortex. A 'scram' is an emergency shutdown, usually done in a very fast way. For example: a gas cored reactor can be fast scrammed by using a pressurized "shotgun" behind a weak window. If the core exceeds the design parameters of the window, which are to be slightly weaker than the silica "lightbulb," then the "shotgun" blasts 150 or so kilos of boron/cadmium pellets into the uranium gas, quenching the reaction immediately. A slightly slower scram which is implemented totally differently is to vary the gas jets in the core to instill a massive disturbance into the fuel vortex. This disturbance would drastically reduce criticality in the fission gas. A third scram mode, slightly slower still, is to implement a high-speed vacuum removal of the fuel mass into the storage system. Having three separate scram modes, one of which is passively triggered, should instill plenty of safety margin in the nuclear core of each thruster.

Extensive work was done on gas core reactors, and 25 years ago several experimental designs were built and run successfully. There were technical challenges, but nothing that seems insurmountable or even especially difficult given our current computer and material skills.

 

 

This engine produces 1,200,000 pounds of thrust, with an exhaust velocity of 30,000 meters per second, from a thermal output of approximately 80 gigawatts. This equates to an Isp of 3060 seconds. Several sources state that a gas core NTR can exceed 5000 seconds Isp, so 3060 is well inside the overall performance envelope. The three turbopumps from the SSME are run at low power levels, and even losing a pump allows the engine to continue running as long as there is no damage to the nuclear core. Lets assume this design is able to achieve a thrust to weight ratio of ten to one, so the engine and all of its safety systems, off-line fuel storage, etc, weighs 120,000 pounds. I think we can build this engine easily for 60 tons.

 

 

Now admittedly this style engine has some problems, the biggest of which would be helped greatly if not completely by a magnetic field supporting and confining the plasma.

 

We have made some pretty good strides in magnets just in my lifetime, permanent type magnets to be sure are quite a bit smaller and more powerful than they were capable of 50 years ago, but things like super conductors, and static charge force fields are also up and coming technologies.

 

http://science.nasa.gov/science-news/science-at-nasa/2005/24jun_electrostatics/

 

 

"Using electric fields to repel radiation was one of the first ideas back in the 1950s, when scientists started to look at the problem of protecting astronauts from radiation," Buhler says. "They quickly dropped the idea, though, because it seemed like the high voltages needed and the awkward designs that they thought would be necessary (for example, putting the astronauts inside two concentric metal spheres) would make such an electric shield impractical."

Buhler and Lane's approach is different. In their concept, a lunar base would have a half dozen or so inflatable, conductive spheres about 5 meters across mounted above the base. The spheres would then be charged up to a very high static-electrical potential: 100 megavolts or more. This voltage is very large but because there would be very little current flowing (the charge would sit statically on the spheres), not much power would be needed to maintain the charge.

 

 

http://www.telegraph.co.uk/technology/news/7487740/Star-Trek-style-force-field-armour-being-developed-by-military-scientists.html

 

Now I know that neutrons are not affected by these fields but in a space craft the entire engine doesn't have to be shielded just enough to make a neutron shadow that protects the crew.

 

Right now the weight of the individual nucleons prohibit them being contained in a magnetic field but what i want to know are we dealing with technology that can conceivably be built or are magnetic fields of that magnitude prohibited by physics.

[swansont]

A couple of points to clarify:

 

Neutrons do have a magnetic moment (~ -1.9 nuclear magnetons). Free neutrons are also radioactive, decaying with a lifetime of about 15 minutes.

 

The majority of the energy released in fission is carried off in the very massive fission products. The amount in the neutrons, photons (and neutrinos) is small.

 

 

AFAIK the only magnetic confinement of neutrons has been in cold traps (energy << 1 eV); it's not the field, as such, it's the field gradient that's important. If you can suffer the loss of the neutrons, though (which is not a small issue, unless the reactor is large), then you just have to ionize and confine the fuel, which is probably similar to fusion issues. But then how do you do the heat exchange with the propellant?

[Moontanman]

A couple of points to clarify:

 

Neutrons do have a magnetic moment (~ -1.9 nuclear magnetons). Free neutrons are also radioactive, decaying with a lifetime of about 15 minutes.

 

The majority of the energy released in fission is carried off in the very massive fission products. The amount in the neutrons, photons (and neutrinos) is small.

 

 

AFAIK the only magnetic confinement of neutrons has been in cold traps (energy << 1 eV); it's not the field, as such, it's the field gradient that's important. If you can suffer the loss of the neutrons, though (which is not a small issue), then you just have to ionize and confine the fuel, which is probably similar to fusion issues. But then how do you do the heat exchange with the propellant?

 

 

Heat exchange with the propellant is via radiative exchange.

 

Also, to repeat, due to the extremely high temperature gradient in the motor, the main cooling of the fissioning mass is not conductive but radiative, a mode which is inherently less susceptible to perturbations. (Having no working fluid for cooling means no material characteristics for the working fluid must be considered.) This radiative cooling mechanism is what allows the "lightbulb" system to work. The silica bulb just has to be transparent enough to let the gigantic power output of the fissioning core flow through, while keeping the radioactive material of the core safely contained inside the thruster. No radioactive materials leak out of the exhaust, it is completely "clean."

 

BTW swansonT I am not talking about controlling neutrons via a magnetic field...

[EdEarl]

What probability of uranium hexafluoride escaping into the environment in the event of a crash? Is not a fusion rocket safer?

[Moontanman]

What probability of uranium hexafluoride escaping into the environment in the event of a crash? Is not a fusion rocket safer?

 

 

Well since we can't make a fusion reactor it's difficult to say. We can make a gaseous core fission reactor. Please see the link for safety measures.

 

http://members.shaw.ca/bru_b/Liberty_ship_pg10.html

[swansont]

Heat exchange with the propellant is via radiative exchange.

 

The heat exchange in the fuel that gets you to 25000K is energy deposited by the fission fragments. Because you are at that temperature, you emit UV light.

 

If the fuel is hydrogen, why would it absorb this radiation?

 

BTW swansonT I am not talking about controlling neutrons via a magnetic field...

 

Good, because you won't. How do you keep the fuel magnetically separated from the propellant?

[studiot]

I said swansont was your man on this subject.

Edited July 7, 2013 by studiot

[Moontanman]

The heat exchange in the fuel that gets you to 25000K is energy deposited by the fission fragments. Because you are at that temperature, you emit UV light.

 

If the fuel is hydrogen, why would it absorb this radiation?

Well according to the article hydrogen absorbs radiation at this frequency quite strongly.

 

 

Good, because you won't. How do you keep the fuel magnetically separated from the propellant?

In the article it suggests a fuses silica wall cooled by the hydrogen propellant. My idea is to use a magnetic field to contain the fuel plasma.

[swansont]

Well according to the article hydrogen absorbs radiation at this frequency quite strongly.

 

There is no "at this frequency" for a blackbody emitter, which is a continuous spectrum. You have absorption lines in hydrogen, but it will be transparent at most of the frequencies. Atomic hydrogen, at least, absorbs at ~121 nm, but nowhere else nearby. Doppler and pressure broadening will only gain you so much width to the line. Molecular hydrogen probably has some more states, but will still likely be inefficient.

 

In the article it suggests a fuses silica wall cooled by the hydrogen propellant. My idea is to use a magnetic field to contain the fuel plasma.

 

The fused silica wall allows for conductive heat transfer, which you are eliminating. Magnetically you keep the fuel in but how do you keep the hydrogen out?

[EdEarl]

 

 

Well since we can't make a fusion reactor it's difficult to say. We can make a gaseous core fission reactor. Please see the link for safety measures.

 

http://members.shaw.ca/bru_b/Liberty_ship_pg10.html

I read all the pages of Liberty_ship. However, in the event of a crash, the hydrogen explosion will be large and hot. What containment for the UF₆ is capable of sustaining that blast? What is the probability it will crack and release UF₆ into the environment? I did not see any such estimate.

[Moontanman]

I read all the pages of Liberty_ship. However, in the event of a crash, the hydrogen explosion will be large and hot. What containment for the UF₆ is capable of sustaining that blast? What is the probability it will crack and release UF₆ into the environment? I did not see any such estimate.

 

 

It did list several ways the reaction could be contained but you must have missed the risk mitigation chapter....

There is no "at this frequency" for a blackbody emitter, which is a continuous spectrum. You have absorption lines in hydrogen, but it will be transparent at most of the frequencies. Atomic hydrogen, at least, absorbs at ~121 nm, but nowhere else nearby. Doppler and pressure broadening will only gain you so much width to the line. Molecular hydrogen probably has some more states, but will still likely be inefficient.

 

I am beginning to believe this little hobby horse idea isn't even good enough for science fiction. For some reason I was under the impression that radiative transfer of the energy from the reactor would be absorbed particularly well by hydrogen.

 

 

 

The fused silica wall allows for conductive heat transfer, which you are eliminating. Magnetically you keep the fuel in but how do you keep the hydrogen out?

I was under the impression that the silica wall was transparent to UV and that the hydrogen would absorb it. helium can be added to make the plasma more electrically conductive, the fused silica would have to stay if the hydrogen would be a problem but the magnetic field could hold the plasma away from the walls and floor under acceleration...

 

http://en.wikipedia.org/wiki/Gas_core_reactor_rocket

 

Gas core reactor rockets are a conceptual type of rocket that is propelled by the exhausted coolant of a gaseous fission reactor. The nuclear fission reactor core may be either a gas or plasma. They may be capable of creating specific impulses of 3,000–5,000 s (30 to 50 kN·s/kg, effective exhaust velocities 30 to 50 km/s) and thrust which is enough for relatively fast interplanetary travel. Heat transfer to the working fluid (propellant) is by thermal radiation, mostly in the ultraviolet, given off by the fission gas at a working temperature of around 25,000 °C.

 

 

 

http://en.wikipedia.org/wiki/Gaseous_fission_reactor

Spacecraft[edit]

The spacecraft variant of the gaseous fission reactor is called the gas core reactor rocket. There are two approaches: the open and closed cycle. In the open cycle, the propellant, most likely hydrogen, is fed to the reactor, heated up by the nuclear reaction in the reactor, and exits out the other end. Unfortunately, the propellant will be contaminated by fuel and fission products, and although the problem can be mitigated by engineering the hydrodynamics within the reactor, it renders the rocket design completely unsuitable for use in atmosphere.

One might attempt to circumvent the problem by confining the fission fuel magnetically, in a manner similar to the fusion fuel in a tokamak. Unfortunately it is not likely that this arrangement will actually work to contain the fuel, since the ratio of ionization to particle momentum is not favourable. Whereas a tokamak would generally work to contain singly ionized deuterium or tritium with a mass of two or three daltons, the uranium vapour would be at most triply ionized with a mass of 235 dalton (unit). Since the force imparted by a magnetic field is proportional to the charge on the particle, and the acceleration is proportional to the force divided by the mass of the particle, the magnets required to contain uranium gas would be impractically large; most such designs have focused on fuel cycles that do not depend upon retaining the fuel in the reactor.

In the closed cycle, the reaction is entirely shielded from the propellant. The reaction is contained in a quartz vessel and the propellant merely flows outside of it, being heated in an indirect fashion. The closed cycle avoids contamination because the propellant can't enter the reactor itself, but the solution carries a significant penalty to the rocket's Isp.

Energy production[edit]

For energy production purposes, one might use a container located inside a solenoid. The container is filled with gaseous uranium hexafluoride, where the uranium is enriched, to a level just short of criticality. Afterward, the uranium hexafluoride is compressed by external means, thus initiating a nuclear chain reaction and a great amount of heat, which in turn causes an expansion of the uranium hexafluoride. Since the UF₆ is contained within the vessel, it can't escape and thus compresses elsewhere. The result is a plasma wave moving in the container, and the solenoid converts some of its energy into electricity at an efficiency level of about 20%. In addition, the container must be cooled, and one can extract energy from the coolant by passing it through a heat exchanger and turbine system as in an ordinary thermal power plant.

However, there are enormous problems with corrosion during this arrangement, as the uranium hexafluoride is chemically very reactive.