Fusion Power, and Why I'm Skeptical of It

JoshNH4H · 2017-11-22 14:32:56

We were talking about energy sources for transportation in this thread when Elderflower brought up fusion power. I said that I'm skeptical of it, and I want to explain why.

But before I do, I want to put some guardrails on this conversation. Specifically, Cold fusion, "LENR", and all that other nonsense is outside the scope of this discussion. If you think these are real then you're being fooled. But more importantly, please do not discuss them in this thread. As far as I am concerned, the only kind of fusion is "Hot Fusion" (Plus muon-catalyzed fusion). If you want to talk about energy sources that most scientists would say are impossible, please do it elsewhere.

Now, here's what I'm skeptical about as far as fusion power is concerned, and why:

I am not skeptical that it is possible to make fusion reactors work. We do it pretty frequently, and someone in the other thread also pointed out that fusion weapons generate way more energy than they consume. What we have trouble doing is controlling the reaction and directing it towards peaceful purposes. I don't believe that we are at the point, near the point, or even really approaching the point where artificial, controlled fusion is the right source of energy in any application.

The reason why I think this is that fusion is hard to make happen, much harder than fission or pretty much any other kind of energy-producing reaction. It requires all sorts of expensive and precise magnets and lasers. Current designs haven't managed to sustain fusion for long enough to generate more than the energy required to heat them up to fusion temperature, let alone to generate enough energy to make them self-heating. The basic reason is that it's extremely difficult to do so.

It's not just that current reactors are inadequate to the task (pretty much any task, to be honest). It's that no conceivable reactor using these technologies ever could be.

Let's look at a few proposed applications for fusion reactors:

Small-scale power, for example individual houses or blocks
Large-scale mobile power, for example on cargo ships
Large-scale stationary power, for the grid
Large-scale mobile power, on rockets

Basically, the important variables here are power density (W/kg) and cost ($/W and $/J).

Cost is of primary importance for the first three and secondary importance for the last one, while power density is mostly unimportant for the first and third, of secondary importance on the second, and of primary, overwhelming importance on the fourth.

If power and energy density are important, fusion is probably not the power source for you. Fusion plasmas are by their nature not very dense and compared to (for example) nuclear fission do not produce that much power per kilo. On top of that, they require a much, much heavier apparatus to control and direct the reaction. Consider: Chemical fuels sometimes ignite upon contact and once ignited a well-designed rocket engine need not move a single part to continue firing. Likewise, a rocket powered by nuclear fission needs only a minimal amount of machinery to sustain and control the fission reaction; sometimes (for a well-designed reactor) the reaction controls itself.

For similar reasons, fusion is unlikely to be cost-competitive with other technologies any time soon. Consider that a nuclear fission reaction is similar in all respects but the need for superconducting magnets or high-powered lasers to sustain the reaction. Consider that any fusion reaction will still tend to produce some amount of nuclear waste as a result of neutron release and the release of other high-energy particles, and also that a better-designed fission reactor could produce much less waste than reactors currently do, plus that solar power produces none. Consider that the intermittent but high-power nature of the reaction will require a substantial amount of energy storage. Consider that, on top of all the advanced machinery to control and sustain the fusion reaction, you will also need a lot of machinery to convert the product energy to power just like in any other system.

Perhaps fusion will eventually make sense. But not today and not any time soon.

Terraformer · 2017-11-22 15:44:50

What if we build the reactors really, really big? Very difficult on Terra, given the vacuum required, in space that's not a problem. If we can get fusion working for space habitats, that opens up a lot more options for colonisation (basically, anywhere that has enough of the right type of matter to build the habitats and power the reactor). It also provides up with plentiful power for beamed propulsion.

I'm interested in the potential for using fusors as cheap neutron sources. If we can push it close to break even, and get neutrons for a low enough energy cost, then that opens up other options for energy production, using Lithium or Calcium-40.

JoshNH4H · 2017-11-22 16:07:43

I'm not sure how big is "big enough" but it depends on the technology I guess. I've seen an estimate that Jupiter would be a star if it were made from Deuterium so we can start there.

More seriously it's my understanding that fusors are really bad as far as q is concerned, and that the polywell is a refinement that's supposed to make q better. I wish I understood the plasma physics involved well enough to pass judgment on whether it's a good approach or a bad one, but I just don't and I'm not sure anyone out there does either.

There's definitely some scaling effect with fusion reactors (they catch more and more of the outgoing energy the bigger they get, if you can maintain the confinement).

It feels like a field that's ripe for disruption (although in this case by a genuinely new idea rather than a new business model), and it's really interesting to talk about, but at a macro level it's hard to believe it will pan out. (Solar power, for example, will also be quite economical in vacuum locations in the inner system)

JoshNH4H · 2017-11-22 16:11:44

By the way, what's the idea with Ca-40?

Ca-40+n->Ca-41, but that has a half-life of 100,000 years. I suppose you get roughly 8 MeV by adding the neutron, but surely there's a better reaction than that one?

louis · 2017-11-22 16:20:18

Hot fusion is a chimera. Cold fusion seems much more likely as a technology. Talking of which, Mr Rossi is on the road again...a new demo...

http://ecatworld.org/

Rogue, rebel or revolutionary genius...you decide.

But there have been quite a few patent applications relating to cold fusion technology or LENR - centred on nickel-hydrogen systems.

JoshNH4H · 2017-11-22 16:21:58

It would behoove you to read the posts you are commenting on. For example, this was the second paragraph of my post:

I wrote:

I want to put some guardrails on this conversation. Specifically, Cold fusion, "LENR", and all that other nonsense is outside the scope of this discussion. If you think these are real then you're being fooled. But more importantly, please do not discuss them in this thread. As far as I am concerned, the only kind of fusion is "Hot Fusion" (Plus muon-catalyzed fusion). If you want to talk about energy sources that most scientists would say are impossible, please do it elsewhere.

louis · 2017-11-22 16:31:37

The discussion point is why you are sceptical of fusion power. Unless you are omniscient, others may through debate be able to clarify your scepticism. If you wanted to discuss only hot fustion power, then you should have made that clear in the heading. The fact is that many reputable scientists believe cold fusion or LENR is possible or that they have achieved it. The numerous patent applications/approved patents in this area demonstrate that to be true.

JoshNH4H wrote:

It would behoove you to read the posts you are commenting on. For example, this was the second paragraph of my post:
I wrote:
I want to put some guardrails on this conversation. Specifically, Cold fusion, "LENR", and all that other nonsense is outside the scope of this discussion. If you think these are real then you're being fooled. But more importantly, please do not discuss them in this thread. As far as I am concerned, the only kind of fusion is "Hot Fusion" (Plus muon-catalyzed fusion). If you want to talk about energy sources that most scientists would say are impossible, please do it elsewhere.

Terraformer · 2017-11-22 16:44:29

Haven't people patented anti-gravity machines before? You don't need to prove your invention works in order to be granted a patent for it.

Solar power is more effective in space, but it also degrades faster, and is limited to the inner solar system. What would it's EROEI be at Jupiter? Granted, we could use mirrors to focus the light, rather than using a lot more expensive solar cells.

JoshNH4H · 2017-11-22 16:55:40

louis wrote:

The discussion point is why you are sceptical of fusion power. Unless you are omniscient, others may through debate be able to clarify your scepticism. If you wanted to discuss only hot fustion power, then you should have made that clear in the heading. The fact is that many reputable scientists believe cold fusion or LENR is possible or that they have achieved it. The numerous patent applications/approved patents in this area demonstrate that to be true.

There are lots of threads where you can post about your nonsense fusion. Do it there and leave this one alone. Don't try to make excuses for your failure to read the bolded and italicized text in the second paragraph of my post.

JoshNH4H · 2017-11-22 17:47:39

Terraformer wrote:

Solar power is more effective in space, but it also degrades faster, and is limited to the inner solar system. What would it's EROEI be at Jupiter? Granted, we could use mirrors to focus the light, rather than using a lot more expensive solar cells.

This is one of the circumstances where I'm less skeptical that the technology will ever be useful. I can definitely see some circumstances (depending on how our energy technologies develop over the next century) where fusion could potentially make some sense. For example, if you want to illuminate the moons of Jupiter or Saturn to Terraform them I could be on-board with the need and use of fusion.

On the other hand, fusion technology isn't the only one that will change in the next hundred years. For example, it's not inconceivable that it will someday be possible to "build" highly efficient solar panels from electric and magnetic fields with electrons trapped in various potential wells that act as semiconductors. A similar technology might be used to make reflectors and artificial magnetic fields (and maybe even an intangible greenhouse to warm the bodies?).

Between those two technologies I honestly have no idea would will make more sense because neither really exists. There will eventually come a time when we're trying to cross interstellar distances when we will need the high energy per kilo of fuel that fusion gives us, but maybe by then we will have mastered antimatter or built giant mirrors that accelerate crafts using light.

SpaceNut · 2017-11-22 19:43:49

I agree that we need to stay focused on the topic of Hot Fusion...

Much like fision the chances for fusion "controlling the reaction and directing it towards peaceful purposes" is always going to be in the back of all efforts as its so easy to corrupt for other purposes.

In order to control the reaction is to be able to feed it with more fuel through the containment field which is why its limited.
The list for practicle uses is just where we will have the issues for some one to be able to pay for a reactor unit as I hate paying for power/ energy in any form as it just cost to much.

The large planets of the outer part of our solar system would be the perfect building blocks to start a fusion reaction with and in time I would think that they would pull together into a single point of light for the outer solar system.

kbd512 · 2017-11-22 22:37:16

Josh,

First, few new technologies were ever cost-competitive at the exact time they came into existence and more generally, decades after they worked well enough to begin to supplant whatever came before them. Jet engines supplanting piston engines in aircraft comes immediately to mind. That is one of the points I tried to make to Louis about the use of solar power at a time when using solar or wind means consuming more fossil fuels, substantially increased cost, and/or substantially decreased usage. I'm completely unopposed to using anything that works in a cost effective manner that emits less pollution than burning coal or gas, but thus far our nuclear, solar, and wind options all have engineering issues to increase their output and reduce cost. Admittedly, those issues are far better understood than some of the major impediments to working fusion reactors. I think some technological advancements were discounted, too.

1. The types of superconducting electromagnets in current generation fusion reactors, which includes ITER, are not REBCO variants. NbTi and Nb3Sn are old technology. On January 18th of this year, National High Magnetic Field Laboratory achieved a continuous field strength of 42.5T using a no-insulation REBCO type coil with a current density of 1,100A/mm^2. It was constructed of .042mm REBCO tape. NbTi tops out at about 15T or thereabouts and overall not nearly so efficient as REBCO's as a function of magnetic field strength produced from the current applied prior to break down of superconductivity. The plasma instability issues pretty much disappear using the higher magnetic field strengths capable of being generated by REBCO. That is not conjecture on my part since all testing thus far has proven that that is the case.

From National Mag Lab News:

Mini magnet packs world-record, one-two punch

The graphs here illustrate why this is so important to the application of a fusion reactor containing thousands of kilometers of HTS coils:

Superconducting Wires and Cables: Materials and Processes by Peter J. Lee

ITER's Superconducting Electromagnet Design:

The ITER Magnets: Design and Construction Status

Alcator C-Mod achieved 2.05ATM in its last run. ITER is expected to achieve 2.6ATM. As that figure goes up, plasma stability goes up, losses and the damage they cause to the containment vessel goes down, and suddenly it becomes possible to run the device for periods of time relevant to determining how well the technology will or won't work.

Since the superconductors are major part of the materials purchase and construction expenses for fusion reactors, better technology means less cost.

2. The blanket design of ITER and similar reactors is not sufficiently efficient for the intended purpose. There are also ways around this problem, but the knowledge gained occurred after ITER was designed.

3. Typically, the way to make things more cost-competitive is to make them smaller, lighter, and cheaper. That is exactly what better superconductor and blanket technologies or designs would do for fusion reactors. Making fusion reactors to power entire continents would be a colossal waste of time and money, even if it was technically feasible to do it. Distributing the power plants makes the grid much more resilient to damage from natural and man made disasters. That's the best reason to have solar panels adorning the rooftops of buildings and houses, but it's the worst way to lower total cost because more of everything involved in power conversion is required. It's not cost competitive at the moment to try to supply all the power required 24/7, nor is building generating stations that compete with nuclear fission in terms of construction costs / operating costs / cost to the consumer. Hopefully one day it will be, but today is not that day. If we spent the same money on fission reactors as we did on the gulf wars, then there would be no energy shortage either now or in the foreseeable future.

People need to understand that it is not necessary to produce a single watt of electricity from fusion to power a spacecraft, nor is continuous fusion power required. Any attempt at generating electricity using continuous fusion power massively complicates such a propulsion scheme and moves it squarely into the realm of things not achievable using current technology. The fusion propulsion research Dr. Slough and associates are undertaking using grant money from NASA will not ever generate a single watt of electrical power. His team uses pulsed fusion power generated by electromagnets that create an exponentially increasing magnetic field trapped between a D-T pellet and a metal foil liner that implodes at supersonic velocities. The exponentially increasing magnetic field causes the D-T pellet fuse, the energy generated from the brief pulse of fusion power converts the metal foil to a super-heated plasma, and then an electromagnetic nozzle expels the plasma out the back of the rocket to create jet power, same as any conventional rocket even though a totally different mechanism generates the reaction mass. It's not a replacement for conventional chemical rockets, as a function of its thrust-to-weight ratio, and is only meant for in-space propulsion.

Most of the heat and radiation are absorbed by the plasma and carried away from the rocket, so no purpose built radiation shielding is required. The propulsion module's "fuel tanks" full of Lithium metal foil shield the humans from the rest of the radiation. It's the only practical way to use fusion in something resembling a conventional rocket engine that I know of. All proposals that attempt to generate continuous fusion power or convert heat to electricity are pipe dreams. We don't even have paper rockets capable of carrying those other proposals into space. Two Falcon 9 Heavy rockets are sufficient to carry a pulse fusion powered Mars Transfer Vehicle to ISS, one for the propulsion module and the other a habitat module. The transit duration is 30 days to 90 days and short stay opposition class missions are feasible.

elderflower · 2017-11-23 04:00:40

Most of the foregoing posts refer to D-D fusion, but I was originally considering D-T fusion. D-D is much more difficult. D-T can be done, and has been done experimentally in tokamak type devices. Problem is that the excess neutrons from D-T fusion irradiate your tokamak making its parts pretty radioactive. For this reason it has only been done, as far as I know, in reactors which have reached the end of their D-D research programme and are about to be decommissioned.
Another drawback is that the half life of Tritium is quite short, the stuff is quite rare and difficult to get hold of and dangerous to handle. Heavy water reactors make quite a lot of it, though.

Antius · 2017-11-23 08:12:21

JoshNH4H wrote:

We were talking about energy sources for transportation in this thread when Elderflower brought up fusion power. I said that I'm skeptical of it, and I want to explain why.
But before I do, I want to put some guardrails on this conversation. Specifically, Cold fusion, "LENR", and all that other nonsense is outside the scope of this discussion. If you think these are real then you're being fooled. But more importantly, please do not discuss them in this thread. As far as I am concerned, the only kind of fusion is "Hot Fusion" (Plus muon-catalyzed fusion). If you want to talk about energy sources that most scientists would say are impossible, please do it elsewhere.
Now, here's what I'm skeptical about as far as fusion power is concerned, and why:
I am not skeptical that it is possible to make fusion reactors work. We do it pretty frequently, and someone in the other thread also pointed out that fusion weapons generate way more energy than they consume. What we have trouble doing is controlling the reaction and directing it towards peaceful purposes. I don't believe that we are at the point, near the point, or even really approaching the point where artificial, controlled fusion is the right source of energy in any application.
The reason why I think this is that fusion is hard to make happen, much harder than fission or pretty much any other kind of energy-producing reaction. It requires all sorts of expensive and precise magnets and lasers. Current designs haven't managed to sustain fusion for long enough to generate more than the energy required to heat them up to fusion temperature, let alone to generate enough energy to make them self-heating. The basic reason is that it's extremely difficult to do so.
It's not just that current reactors are inadequate to the task (pretty much any task, to be honest). It's that no conceivable reactor using these technologies ever could be.
Let's look at a few proposed applications for fusion reactors:
Small-scale power, for example individual houses or blocks
Large-scale mobile power, for example on cargo ships
Large-scale stationary power, for the grid
Large-scale mobile power, on rockets
Basically, the important variables here are power density (W/kg) and cost ($/W and $/J).
Cost is of primary importance for the first three and secondary importance for the last one, while power density is mostly unimportant for the first and third, of secondary importance on the second, and of primary, overwhelming importance on the fourth.
If power and energy density are important, fusion is probably not the power source for you. Fusion plasmas are by their nature not very dense and compared to (for example) nuclear fission do not produce that much power per kilo. On top of that, they require a much, much heavier apparatus to control and direct the reaction. Consider: Chemical fuels sometimes ignite upon contact and once ignited a well-designed rocket engine need not move a single part to continue firing. Likewise, a rocket powered by nuclear fission needs only a minimal amount of machinery to sustain and control the fission reaction; sometimes (for a well-designed reactor) the reaction controls itself.
For similar reasons, fusion is unlikely to be cost-competitive with other technologies any time soon. Consider that a nuclear fission reaction is similar in all respects but the need for superconducting magnets or high-powered lasers to sustain the reaction. Consider that any fusion reaction will still tend to produce some amount of nuclear waste as a result of neutron release and the release of other high-energy particles, and also that a better-designed fission reactor could produce much less waste than reactors currently do, plus that solar power produces none. Consider that the intermittent but high-power nature of the reaction will require a substantial amount of energy storage. Consider that, on top of all the advanced machinery to control and sustain the fusion reaction, you will also need a lot of machinery to convert the product energy to power just like in any other system.
Perhaps fusion will eventually make sense. But not today and not any time soon.

I agree with all of the above, with one (possible) caveat. Low power density plus high capital cost would appear to make tokamak fusion an economic dead loser. That's assuming it can be made to work from a practical viewpoint. It would then need to begin the slow uphill task of trying to compete with fission from an economic viewpoint. It is a tall order, because the power density of fission reactors is 1-2 orders of magnitude greater and as Josh pointed out, they are technologically much simpler devices.

It only gets worse for fusion. Some 80% of the energy release from De-T fusion is released in the form of fast neutrons, with average energy 14.1MeV. These impact the reactor walls making them brittle and radioactive. Tokamak concepts generally envisage using steel baffle plates to absorb neutron energy which are changed every five years or so. However, the intense gamma activity of the irradiated steel will make this operationally difficult.

There is perhaps one caveat to the (probably) disastrous economics of the Tokamak. Those 14.1MeV neutrons are extremely efficient at causing secondary fission in thorium or depleted uranium blankets. Remember, the average neutron energy in a thermal or even fast reactor is too low to generate appreciable power by direct fission of 238U or 232Th. We talk about breeding nuclear fuel from these isotopes, but they do not directly work as fuel in a normal fission reactor. But with the super high energy neutrons from a fusion reactor they will directly fission, yielding energy and more (but slower) neutrons. And those super-fast fusion neutrons will fission just about any other actinide. A single fission will yield 200MeV of net energy, versus 18MeV for fusion. A fusion reaction that reaches breakeven can therefore drive a fission reactor that has net energy gain of 10. As virtually any actinide will fission in 14MeV neutrons, such a reactor would not generate long lived nuclear wastes and 1kg of U/Th put into the reactor can be expected to completely fission into short-lived fission products. What is more, at such high neutron energy the fission would yield copious secondary neutrons, which would allow very high breeding ratios in the bombarded actinide material. So a single fusion-fission hybrid could (if desired) provide enough fissile fuel for 2-3 fission reactors of the same thermal power.

All in all, we are far more likely to see fission-fusion hybrid reactors than pure fusion reactors, although it is still debatable whether a hybrid would compete economically against more conventional fast neutron reactors.

One interesting option would be a plasma core reactor, in which both fusion and fission fuels are present as mixed plasma. Fission would yield heavily charged fragments carrying about 180MeV of kinetic energy which would rapidly deposit that energy into the plasma. This heating would result in fusion between the tritium and deuterium ions, which would in turn generate super-fast neutrons, leading to even more fission. A hybrid like this could work on a chain reaction principle, even if the actinide fuel is not fissile (i.e. pure thorium). Such a device could achieve breakeven at relatively small size, because of the much greater density of actinide plasma and the short slowing down length of the fission fragments that provide most of the heating. It is therefore possible for a plasma hybrid to function as a power dense space drive, with both high acceleration and high specific impulse – the ultimate torch ship. Of course, the fission product rich exhaust would be radioactive. But until switched on, all components are low activity – Thorium and tritium.

https://en.m.wikipedia.org/wiki/Nuclear … ion_hybrid

Last edited by Antius (2017-11-23 12:46:18)

Antius · 2017-11-23 13:00:06

Kinetic impact induced fusion:

http://www-pub.iaea.org/MTCD/Meetings/F … _p7-30.pdf

This requires accelerating a projectile into a target of fusion fuel. The kinetic impact generates sufficient thermal energy to trigger fusion. This method of fusion power is relatively energy efficient. However, the projectile velocity must be ~1000km/s, which makes this an inherently large scale machine. If it is possible to accelerate a projectile at 100,000g, then the accelerator must be 500km long.

JoshNH4H · 2017-11-23 13:41:34

kbd512,

My argument is not that fusion power won't get cheaper than it is today (since no fusion reactor has ever contributed net power to the grid, the cost of fusion energy is currently infinite). It's that it's hard to believe that it will ever be cheap enough to be the energy source you would choose to use in any particular circumstance.

It's impossible to say for sure, because the technology doesn't really exist now in a usable form. But speculating based on comparison to other, currently-existing technologies (and the near-term developments thereof) it seems like fusion probably won't be competitive even once we get the technology to a state where it could be "ready" because it requires most of the same components as a comparable-sized fission plant, plus more components to sustain a fusion reaction.

Terraformer · 2017-11-23 14:15:33

Whatever happened to polywell? They're still plugging away at it. I recall they were asking for $100-200 million to build one that could break even, which would make it worthwhile giving them the money if they had even a 1% chance of success - the benefit if it works is far more than $20 billion.

What are the problems in the way of creating a viable fusion reactor? Is it mainly that most collisions don't result in fusion, and the energy is lost to thermalising the plasma?

A big problem with the Fusor is that the electrodes get destroyed, which the Polywell solves by replacing them with a virtual electrode made of electrons. Could we use magnetic fields to direct the ions away from the electrodes and towards each other? Or towards a stationary target?

Terraformer · 2017-11-23 14:34:47

Ah, that's the calcium I was looking for. Ca-46, which makes up 0.004% of all calcium. Or Ca-44, which makes up 2%. Both of the isotopes just above them, with half lives of around 5 days and 163 days respectively. The latter is stable, the former is effectively stable and is prevented from decaying. Both decay through beta minus decay, I think. Given that I want to generate net power, I think it was the first one I had in mind. I remember that the decay energy is far greater the 1 MeV it would theoretically take to generate a neutron - about 5 MeV I think? But if we had a cheap enough source of neutrons to irradiate a block of Ca-46...

JoshNH4H · 2017-11-23 16:33:42

Based on the signals coming out from the Polywell team I'm somewhat more optimistic than either inertial confinement or magnetic confinement fusion, but on the whole I just don't know enough to say if it's real or if they're just overstating the promise of the results.

Wikipedia has a pretty cool chart that's helpful here. It's pretty important to look at the energy gained from neutron bombardment by the increase in the nuclear binding energy, which can be substantial (the binding energy associated with a neutron will generally rise from 0 to 7 MeV just by being absorbed by a nucleus, for example).

Antius · 2017-11-24 08:20:26

My understanding of plasma physics is limited, but there is one thing I have never understood. The Lawson parameter for self-sustaining fusion within plasma is the product of particle energy, particle density and average confinement time. When the product exceeds a critical value, fusion becomes self-sustaining. Energy determines particle speed; density is a function of temperature and pressure; simplistically (assuming no collisions between ions) average confinement time is average speed divided by vessel diameter.

One simple way of increasing particle confinement time would be increasing the size of the reactor vessel, as at any given speed it will take an ion a greater period of time to cross the vessel and reach the adjoining wall if the vessel is larger. A spherical vessel of sufficient size filled with plasma should therefore have no difficulty reaching ignition, it is only a question of minimum vessel size and plasma density. The vessel walls can be protected from erosion by injecting a boundary layer of cold gaseous fuel through pores in the vessel walls. This would negate the need for a magnetic field. Within a large enough plasma, the majority of particles will tend to collide with each other and only those close to the outer edge would have a high probability of colliding with a cold gas particle. On this basis, a temperature gradient would develop within the plasma and fusion reaction rates would be much greater towards the centre. Neutron collision with the cooler outer gas particles would tend to flatten the temperature profile of the reactor somewhat, but the edges would be cold and would gradually diffuse into the centre. Power could be extracted by bleeding hot plasma from the outer edges of the plasma through a magnetic nozzle into an MHD generator. This would also remove waste helium ions.

Presumably, there is an underlying reason why such a simple device has not been built. Maybe the minimum size or plasma pressure for this arrangement are too large to be practicable? Maybe there are plasma stability issues that I am not aware of. Maybe the power density of the device is too low to be practicable. But I have never heard this discussed. Any ideas?

Last edited by Antius (2017-11-24 08:30:49)

JoshNH4H · 2017-11-24 10:53:00

Depending on what kinds of confinement time you're looking to achieve, the system you're describing is either Inertial Confinement Fusion or a star.

I don't think the assumption that molecules don't collide is a very good one but I guess I do accept it as a very rough approximation

Terraformer · 2017-11-24 12:19:58

As far as I can tell, break-even fusion is actually pretty easy to achieve if you have access to a giant vacuum and build a massive reactor. You only have to go a few hundred kilometres to find somewhere it could be done. Unfortunately, that's a few hundred kilometres straight up.

JoshNH4H · 2017-11-24 13:02:46

It's definitely possible to overstate how easy it is. We have inertial confinement systems already, after all, and they're not really good enough. The size of the ICF vessel isn't really the limiting factor either.

It's my understanding that Q is defined as "The energy produced by fusion reactions divided by the initial heating energy required to heat the plasma", with breakeven being Q>1. Containment energy is excluded from the breakeven calculations.

The vacuum would probably help as far as fusion is concerned because it seems like something that would benefit from economies of scale, but it's not enough to make it get to breakeven. Gases that are rapidly expanding into the vacuum tend not to collide at all which sort of puts a damper on the rate of fusion.

JoshNH4H · 2017-11-24 13:19:43

Also, if you're looking for isotopes to bombard with neutrons, can I recommend Ca-48?

Ca-48+n->Ca-49->Sc-49->Ti-49

Ca-49 has a half-life of 9 minutes and Sc-49 of 57 minutes, so the energy will be released pretty quickly although not immediately.

Neutron absorption results in an energy of 5.14 MeV (most likely in the form of a gamma). The first decay releases 5.26 MeV (by beta emission) and the second releases 2.00 MeV (also by beta emission), for a total of 12.4 MeV per neutron.

Still probably a net loss relative to any known neutron production but it's pretty good all things considered.

Antius · 2017-11-24 13:19:47

JoshNH4H wrote:

It's definitely possible to overstate how easy it is. We have inertial confinement systems already, after all, and they're not really good enough. The size of the ICF vessel isn't really the limiting factor either.
It's my understanding that Q is defined as "The energy produced by fusion reactions divided by the initial heating energy required to heat the plasma", with breakeven being Q>1. Containment energy is excluded from the breakeven calculations.
The vacuum would probably help as far as fusion is concerned because it seems like something that would benefit from economies of scale, but it's not enough to make it get to breakeven. Gases that are rapidly expanding into the vacuum tend not to collide at all which sort of puts a damper on the rate of fusion.

Pressure is another variable. At 100million K, hydrogen gas is ridiculously diffuse, about 1milligram per cubic metre at 1atm. At 1000atm, density increases 1000 fold. A magnetic field cannot sustain those sorts of pressures. A physical vessel could. But the plasma would need to sustain a temperature gradient to prevent the vessel walls from eroding.

Last edited by Antius (2017-11-24 13:26:42)

New Mars Forums

Announcement

#1 2017-11-22 14:32:56

Fusion Power, and Why I'm Skeptical of It

#2 2017-11-22 15:44:50

Re: Fusion Power, and Why I'm Skeptical of It

#3 2017-11-22 16:07:43

Re: Fusion Power, and Why I'm Skeptical of It

#4 2017-11-22 16:11:44

Re: Fusion Power, and Why I'm Skeptical of It

#5 2017-11-22 16:20:18

Re: Fusion Power, and Why I'm Skeptical of It

#6 2017-11-22 16:21:58

Re: Fusion Power, and Why I'm Skeptical of It

#7 2017-11-22 16:31:37

Re: Fusion Power, and Why I'm Skeptical of It

#8 2017-11-22 16:44:29

Re: Fusion Power, and Why I'm Skeptical of It

#9 2017-11-22 16:55:40

Re: Fusion Power, and Why I'm Skeptical of It

#10 2017-11-22 17:47:39

Re: Fusion Power, and Why I'm Skeptical of It

#11 2017-11-22 19:43:49

Re: Fusion Power, and Why I'm Skeptical of It

#12 2017-11-22 22:37:16

Re: Fusion Power, and Why I'm Skeptical of It

#13 2017-11-23 04:00:40

Re: Fusion Power, and Why I'm Skeptical of It

#14 2017-11-23 08:12:21

Re: Fusion Power, and Why I'm Skeptical of It

#15 2017-11-23 13:00:06

Re: Fusion Power, and Why I'm Skeptical of It

#16 2017-11-23 13:41:34

Re: Fusion Power, and Why I'm Skeptical of It

#17 2017-11-23 14:15:33

Re: Fusion Power, and Why I'm Skeptical of It

#18 2017-11-23 14:34:47

Re: Fusion Power, and Why I'm Skeptical of It

#19 2017-11-23 16:33:42

Re: Fusion Power, and Why I'm Skeptical of It

#20 2017-11-24 08:20:26

Re: Fusion Power, and Why I'm Skeptical of It

#21 2017-11-24 10:53:00

Re: Fusion Power, and Why I'm Skeptical of It

#22 2017-11-24 12:19:58

Re: Fusion Power, and Why I'm Skeptical of It

#23 2017-11-24 13:02:46

Re: Fusion Power, and Why I'm Skeptical of It

#24 2017-11-24 13:19:43

Re: Fusion Power, and Why I'm Skeptical of It

#25 2017-11-24 13:19:47

Re: Fusion Power, and Why I'm Skeptical of It

Board footer