how to restart a fusion reactor in space?

[the.punk]

but like if you were to go to the outer planets and you jettison your depleted fuel. that would keep going until it gets caught in a planets gravity and crashes. the chances of someone finding and having bad intentions with it is pretty much nil

on a side note that made me LOL. my mother thinks that satalites sit in orbit for a while, then they get caught by gravity after a long time so the fall back down. i couldnt for the life of me explain to her that satalites fall down because they are still in the atmosphere and so over time they slow down to the point where their velocity cant hold them in orbit anymore.

The problem with fission reactors in space is not the waste itself. The problem is more that the reactors create waste. A fusion reactor that don't produce this waste is more efficient, because it can use the energy of its fuel better. This waste of fission reactors has still energy that the reactor can't use.
And for this reason it is better to have a reactor with no waste.

 

[Mindblast]

Actually, it is completely opposite. Tokamaks are closer to continuous operation, but have the problem that they can currently only reach got plasma parameters at large dimensions.

Well from what read they can never be operated continuously as they have to continuously increase the current in the primary coil to keep the plasma moving.

The Polywell design on the other hand is only able to reach it's reported great power output during short pulses. And between pulses, it uses even more power than it produces during the pulses, though the Polywell design seems to be better in effectivity as earlier inertial containment reactors.

AFAIK Bussard had to run his tests in short pulses because he was working on a low budget and couldn't afford building a setup with superconducting magnets for the electron confinement. But i think the general idea for a net reactor was indeed continuous operation.

The problem for both is reactor designs is Bremsstrahlung, a kind of radiation you get, when you accelerate charged particles. Tokamaks produce a constant energy loss that way, Polywell and other fusor designs currently produce extremely strong radiation bursts between the pulses, when the ions have to change their direction of travel and get pushed into the core again.

Yes.. Bremsstrahlung losses are one of the major concerns with the Polywell design. I'm loosely following the discussions over at http://talk-polywell.org/bb/index.php It seems Bussard had some arguments as to why Bremsstrahlung losses would be smaller than fusion power gain. There is also a paper about this from 1991 i think. Critics say that he argumented based on his conception of how the polywell works, which is not verified yet.
I guess we'll have to wait what the research of Dr. Nebel brings up who picked up the research on the Polywell after Bussards death. From what i read he is under a publishing ban by the Navy again so it might be a while.

But well, Bussard said his own invention is better in a Internet Video, so it must be the truth right?

Well you can't blame the man for believing in his idea. And obviously there must be enough hard science in there to justify 10 years of funding by the Navy and publishing bans. I saw the video and well yes.. Bussard might be a bit on the positive thinking side concerning the Polywell. From what i read so far Dr. Nebel seems to be a much more down to earth solid scientist which will do the project good to either prove or disprove the feasibility of the design.

I sure do hope its going to work as i'm currently working on a new addon using Polywell Fusion reactors.

 

[RisingFury]

Well from what read they can never be operated continuously as they have to continuously increase the current in the primary coil to keep the plasma moving.

Actually, it has to do with heating. Plasma is a conductor, so it's possible to heat it by passing a current through it. Now... you can't really put two wires into the plasma and expect a current to pass all around it, so they produce a current using induction. Ever increasing current in the heating coil will make sure that there is a constant change of magnetic field... and that will induce a current in the plasma. The problem is that your current can't be infinitely large. Superconductors lose their superconducting properties once a certain current is reached.


Ideally, the energy gained from fusion would heat the plasma, but in Tokamak reactors, insufficient fusion power is reached...

 

[Urwumpe]

Well you can't blame the man for believing in his idea. And obviously there must be enough hard science in there to justify 10 years of funding by the Navy and publishing bans.

I won't blame anybody for selling his product, who would fight for his idea, if not Bussard himself? :lol:

But you overestimate the science behind the Navy, they have not really high standards on scientific realism for theoretical projects, especially when it is about reactor technology. They afford financing 6 failures, if they get one serious breakthrough that way. (Is not much different to other basic research programs)

And also, the money which they dropped into the Polywell projects was not that large AFAIR, they spend about 25 times more for the early phases of projects with lower technological risk.

What it is, eventually, is just one alternative approach to fusion.

 

[Mindblast]

Actually, it has to do with heating. Plasma is a conductor, so it's possible to heat it by passing a current through it. Now... you can't really put two wires into the plasma and expect a current to pass all around it, so they produce a current using induction. Ever increasing current in the heating coil will make sure that there is a constant change of magnetic field... and that will induce a current in the plasma. The problem is that your current can't be infinitely large. Superconductors lose their superconducting properties once a certain current is reached.


Ideally, the energy gained from fusion would heat the plasma, but in Tokamak reactors, insufficient fusion power is reached...

Yes the heating part is true.. but this only works up to a certain plasma temperature beyond which other means of heating are needed.
But afaik you also need the current in the plasma to confine it properly.
I just read up a bit on the topic and obviously other means of creating the current continuously are being researched too so probably a non pulsed Tokamak is possible as well.

 

[Urwumpe]

Yes the heating part is true.. but this only works up to a certain plasma temperature beyond which other means of heating are needed.

Actually not, plasma remains plasma at any temperature, but the heating effect is not linear.

But afaik you also need the current in the plasma to confine it properly.
I just read up a bit on the topic and obviously other means of creating the current continuously are being researched too so probably a non pulsed Tokamak is possible as well.

It makes no sense to have a pulsed tokamak. You don't build a massive toroidal magnetic field for doing pulses which could otherwise be done more effective with a linear accelerator.

The pulsed operation of Tokamaks is more a result of scale (Twice the size = 1/4th of the magnetic field strength needed for having a special plasma configuration) and experiment operations.

 

[Mindblast]

But you overestimate the science behind the Navy, they have not really high standards on scientific realism for theoretical projects, especially when it is about reactor technology. They afford financing 6 failures, if they get one serious breakthrough that way. (Is not much different to other basic research programs)

Yes maybe i'm overestimating the significance of the Navy funding this. But i'm glad they do as i think the Polywell and probably other designs using other approaches to fusion than magnetic or inertial confinement deserve being looked into as long as there is a chance it can be made to work. Especially considering the huge amounts of funding flowing into the established fusion research. (like ITER being estimated at 5-10 Billion €)

And also, the money which they dropped into the Polywell projects was not that large AFAIR, they spend about 25 times more for the early phases of projects with lower technological risk.

Yep, i think its somewhere around 1-2 Million $/year over the 10 years or so that Bussard worked on it.

What it is, eventually, is just one alternative approach to fusion.

Yea i guess we can agree on that.

---------- Post added at 02:18 PM ---------- Previous post was at 01:50 PM ----------

Actually not, plasma remains plasma at any temperature, but the heating effect is not linear.

Yea because the ohmic resistance of the plasma goes down with higher temperature. So beyond a certain point it makes no sense to use the induced current for heating.

It makes no sense to have a pulsed tokamak. You don't build a massive toroidal magnetic field for doing pulses which could otherwise be done more effective with a linear accelerator.

I'm not sure there might be a misunderstanding here. I was refering to the Tokamak as being pulsed because the current through the coils used to induce the plasma current can't be ramped up forever as RisingFury pointed out above. So you need to restart the thing periodically, thats what i meant with "pulsed". (several minutes per pulse)

I'm not sure what you are trying to say with the linear accelerator ? Pulsing the toroidal coils for accelerating the plasma ? Would this even work ?

 

[RisingFury]

Hmmm... I wonder if it would be possible to create an alternating magnetic field, using an AC current. That would still induce a current in the plasma, but it would be alternating as well. It would heat the plasma, but the magnetic field produced by the induced current in the plasma would be alternating as well... so I don't know if that would present a problem.


I'm sure someone's thought of that before... I just don't know the reason why they don't use it.

 

[the.punk]

That with the induction using the magnetic field is a good point. I read somewhere that we also could heat the plasma with microwaves.

 

[Urwumpe]

I'm not sure there might be a misunderstanding here. I was refering to the Tokamak as being pulsed because the current through the coils used to induce the plasma current can't be ramped up forever as RisingFury pointed out above. So you need to restart the thing periodically, thats what i meant with "pulsed". (several minutes per pulse)

You don't need to increase the power infinitely, you actually can remove heating power once fusion starts and the plasma heats itself. What you can't remove is the power needed for containment. The more heat you get, the more containment power you need.

For small scale tokamaks, the problem is, that containment power requirements jump up quickly once fusion starts. That is why it is not possible yet to run small tokamaks for more than 20 minutes - the needed power running through the superconducting magnets warm the magnets and bring them closer to a quench, which you can fight for some time with cooling systems. But eventually, it is a fight you can't win at such small scales - you pay for each minute you want to sustain fusion.

The bigger your reactor gets, the less electrical power density you need for keeping the plasma contained. The total power input of course increases with size, but not as much as you improve the plasma conditions and the net power output.

I'm not sure what you are trying to say with the linear accelerator ? Pulsing the toroidal coils for accelerating the plasma ? Would this even work?

You have to understand that heating the plasma is on the scale of the particles, the same as accelerating it. How you accelerate it, defines how effective you can heat the plasma. Polywell needs to invert the direction of travel of the fuel often, causing Bremsstrahlung, Tokamaks change the direction of travel constantly by having a ring shaped plasma, but you can also do it with long linear accelerators, firing two stream of fuel particles against each other.

 

[Mindblast]

You don't need to increase the power infinitely, you actually can remove heating power once fusion starts and the plasma heats itself.

Well from what i read you still need a current in the plasma to create a poloidal magnetic field that interacts with the toroidal field so that the field lines effectively spiral around in the torus. This is needed to prevent the plasma from going to the outer wall to much.

What you can't remove is the power needed for containment. The more heat you get, the more containment power you need.

For small scale tokamaks, the problem is, that containment power requirements jump up quickly once fusion starts. That is why it is not possible yet to run small tokamaks for more than 20 minutes - the needed power running through the superconducting magnets warm the magnets and bring them closer to a quench, which you can fight for some time with cooling systems. But eventually, it is a fight you can't win at such small scales - you pay for each minute you want to sustain fusion.

The power (current?) running through the magnets warms them ? This is new to me.. i thought superconducting means zero ohmic resistance hence no resistive heating occurs.
Isn't the temperature problem more from the hot fusing plasma just next to the coils ? In this case it would be a problem of shielding and cooling power.

The bigger your reactor gets, the less electrical power density you need for keeping the plasma contained. The total power input of course increases with size, but not as much as you improve the plasma conditions and the net power output.

Yep that sounds logical.. thats why they have to build such a big reactor as ITER to show net power generation.

You have to understand that heating the plasma is on the scale of the particles, the same as accelerating it. How you accelerate it, defines how effective you can heat the plasma. Polywell needs to invert the direction of travel of the fuel often, causing Bremsstrahlung, Tokamaks change the direction of travel constantly by having a ring shaped plasma, but you can also do it with long linear accelerators, firing two stream of fuel particles against each other.

Ok but the resulting collisions and fusion occuring would probably not be enough to reach net power right ?

 

[Urwumpe]

There is no plasma close to the magnetic coils, which are outside the pressure vessel and not even affected by the radiation.

Super-conduction means extremely little electrical resistance, but not zero resistance, if you pump enough power through a superconductor, it will still get warm.

But I have most my knowledge about how superconducting magnets work, from the LHC pages.

 

[the.punk]

Theoretical a superconducter has nor ohmic resistance, but that is impossible. At 0°k everything is theoreticaly a superconducter.

 

[RisingFury]

Which is why we're researching superconductors that have 0 resistance at higher then 0K. The ultimate goal is to find something that is a superconductor at room temperatures or higher.

By the way, it's not 0°K, it's 0K.

 

[the.punk]

By the way, it's not 0°K, it's 0K.

Thanks for the tip.

 

[jedidia]

Maybe this hasn't any significance for a tokamak, but wasn't there a breakthrough some months ago (I don't no in which experimental reactor) where they managed to breach the pulse gap by shooting a laser at the plasma?

 

[RisingFury]

I think you're talking about NIF - National Ignition Facility, which will use a powerful laser concentrated on a small target to achieve a nuclear fusion pulse...