A lot depends on the design of the feedthroughs/coil supports.
It may have been a COTS vs custom problem i.e. time and money.
Plus if they went with the WB-6 type design - 1 coil supported and the other 5 tied to the supported coil you are not going to get much cooling.
Once you go for full custom you might as well design for LN2 and continuous operation. It will have to be done sooner or later.
Another KOS Diary On IEC/Bussard
Dr. Nebel - thank you for taking up the polywell project. It means the world to humanity if it pans out. There are a lot of people wishing nothing but the best for your team.
As for the ITER/fusion politics, a successful polywell does not mean massive job losses - the ITER personnel will be redirected to other projects; Polywell spacecraft propulsion implementation, luna/mars colonization & mining, asteroid mining, etc... so many reasons to actually hope this pans out rather than despise it.
I don`t think there will be massive layoffs with ITER, rather the project itself will get scrapped and the personnel re-assigned.
Regardless of the final outcome of WB-7 results, we know the project is in the best possible hands.
cheers!
As for the ITER/fusion politics, a successful polywell does not mean massive job losses - the ITER personnel will be redirected to other projects; Polywell spacecraft propulsion implementation, luna/mars colonization & mining, asteroid mining, etc... so many reasons to actually hope this pans out rather than despise it.
I don`t think there will be massive layoffs with ITER, rather the project itself will get scrapped and the personnel re-assigned.
Regardless of the final outcome of WB-7 results, we know the project is in the best possible hands.
cheers!
You make excellent points. Who knows, maybe "the ITER people" will end up using IEC devices to help power tokamaks in order to squeeze out power from D and T. Success of the IEC method means another tool in the scientific community's arsenal. I mean arsenal in a peaceful term of course. A hammer does what the hand directs it to.zbarlici wrote:Dr. Nebel - thank you for taking up the polywell project. It means the world to humanity if it pans out. There are a lot of people wishing nothing but the best for your team.
As for the ITER/fusion politics, a successful polywell does not mean massive job losses - the ITER personnel will be redirected to other projects; Polywell spacecraft propulsion implementation, luna/mars colonization & mining, asteroid mining, etc... so many reasons to actually hope this pans out rather than despise it.
I don`t think there will be massive layoffs with ITER, rather the project itself will get scrapped and the personnel re-assigned.
Regardless of the final outcome of WB-7 results, we know the project is in the best possible hands.
cheers!
e
This is just a general answer about pulsed vs. steady-state devices:
Both the WB-6 and the WB-7 are inherently pulsed machines. Neither is actively cooled. Take a look at the numbers. These machines are injecting 1- 10 Mw of power. That would get a little pricey for a true steady-state machine. But it's cheap on a pulsed machine.
However, as far as the plasma is concerned these are steady-state discharges. The 100 microsecond to 1 millisecond discharges are longer than any of the plasma timescales. Consequently, the physics learned should have application to steady-state devices.
Both the WB-6 and the WB-7 are inherently pulsed machines. Neither is actively cooled. Take a look at the numbers. These machines are injecting 1- 10 Mw of power. That would get a little pricey for a true steady-state machine. But it's cheap on a pulsed machine.
However, as far as the plasma is concerned these are steady-state discharges. The 100 microsecond to 1 millisecond discharges are longer than any of the plasma timescales. Consequently, the physics learned should have application to steady-state devices.
rnebel,
I had my milliohm meter at the lab until one of the guys fried it by failing to follow my audio-tape checklist. I used to routinely put it across the coils on WB-3 to measure the resistance after tests, so I could determine when it had cooled off sufficiently for another test. We definitely knew about the thermos bottle effect, but I don't think they ever bought a replacement meter.
I sometimes deliberately heated the unit and kept it at a fairly steady temperature to help degas it.
Aren't RGAs handy? It didn't take me long to fall in love with them.
All,
Regarding the damping of the coils, on all the machines I worked on, I installed damper diodes across the coils, just as you would for a relay coil driven by DC. You do have all the normal inductance issues for turn-on, but for turn-off the coils essentially shorted across the diode. There is a time constant at play, but this configuration is highly damped.
I had my milliohm meter at the lab until one of the guys fried it by failing to follow my audio-tape checklist. I used to routinely put it across the coils on WB-3 to measure the resistance after tests, so I could determine when it had cooled off sufficiently for another test. We definitely knew about the thermos bottle effect, but I don't think they ever bought a replacement meter.
I sometimes deliberately heated the unit and kept it at a fairly steady temperature to help degas it.
Aren't RGAs handy? It didn't take me long to fall in love with them.
All,
Regarding the damping of the coils, on all the machines I worked on, I installed damper diodes across the coils, just as you would for a relay coil driven by DC. You do have all the normal inductance issues for turn-on, but for turn-off the coils essentially shorted across the diode. There is a time constant at play, but this configuration is highly damped.
Tom,Tom Ligon wrote:
Regarding the damping of the coils, on all the machines I worked on, I installed damper diodes across the coils, just as you would for a relay coil driven by DC. You do have all the normal inductance issues for turn-on, but for turn-off the coils essentially shorted across the diode. There is a time constant at play, but this configuration is highly damped.
If you install damping resistors in series with the diodes you can reduce the heat load on the coils by dissipating most (more) of the energy in the damping resistors. A lot depends on what kind of diodes you can get to handle the voltages developed.
Cree has some very nice fast high voltage SiC diodes with positive temp coefficients so they can be series/paralleled for the current/voltage required.
http://www.cree.com/products/power_docs2.asp
The 50A 1200V job is only available in chip form (unpackaged) so that is a problem. But there are others on the page.
Another option is to use capacitors to store the energy and bleed it off more slowly. A parallel snubber type configuration. The voltage rise then is just dependent on how much capacitance you can afford. With a deal like that the shots could be much more frequent than with a snubber diode only.
Engineering is the art of making what you want from what you can get at a profit.
Hmm, well, ELMs seem to occur at a much slower pace -- once a second or so. Is the plasma in a polywell really that much more stable? I realize it doesn't have the same problems as thermal nuclear fusion, but do we understand its stability? And can we really say we do, if we don't run it at longer timescales? What's your level of confidence here? Mine's not so high, but mainly that's out of ignorance.....rnebel wrote:However, as far as the plasma is concerned these are steady-state discharges. The 100 microsecond to 1 millisecond discharges are longer than any of the plasma timescales. Consequently, the physics learned should have application to steady-state devices.
I do agree that the experiment will have application, but it looks to me like there's a long road ahead even if you folks are successful.
10Mw would be expensive, I'd guess a good half a mil+, or you get yourself a LOT of lead.... I don't have a good guess at/intuition about the difficulty with heat dissipation though. It would be nice to run it at the second to 10 second timescale without the complexity of going superconducting/liquid He. Nevermind an experiment with pB11....
-Dave
dnavas:
Polywells have good curvature in their magnetic fields everywhere. Tokamaks have average good curvature. Consequently, tokamaks have some regions where localized modes (like elms) can be a problem.
It's unlikely that we will see elms on a Polywell. However, we might see other things. However, we don't expect to see catastropic things. What we may see is modes which increase losses through the cusps or cause cross-field transport across the magneitc field. These can affect the scaling laws. That's why the proposed phase II machine will be a steady-state machine at a considerably larger size. It should resolve these issues.
Polywells have good curvature in their magnetic fields everywhere. Tokamaks have average good curvature. Consequently, tokamaks have some regions where localized modes (like elms) can be a problem.
It's unlikely that we will see elms on a Polywell. However, we might see other things. However, we don't expect to see catastropic things. What we may see is modes which increase losses through the cusps or cause cross-field transport across the magneitc field. These can affect the scaling laws. That's why the proposed phase II machine will be a steady-state machine at a considerably larger size. It should resolve these issues.
Generally the only instabilities that show up on these timescales are resistive MHD. This can be tearing modes (reconnecting MHD modes) or resistive interchange (so-called g modes) or possibly resistive ballooning modes. The latter two won't occur on Polywell's due to the good curvature. Tearing requires sheared magnetic fields, which the Polywell also doesn't have. I'm not real concerned about this timescale.
Dr. Bussard explained this field shape thing to me early on. The terms he tended to use were convex versus concave.
Looking from the perspective of a confined plasma or electrons pushing at a field, a concave field will be stretched when you push on it. Picture air pushing out a balloon. As it stretches, the field or balloon skin becomes weaker. This setup is prone to instability. The ultimate example of a stretched magnetic field would be a loop-shaped solar prominence. When one of those stretches too far, it ruptures with a stupendous release of energy.
The magnetic fields confining electrons in a Polywell are convex from the perspective of the electron cloud inside the magrid. Pushing on the magnetic field compresses it instead of stretching it. The more you push on a convex field, the more it resists. This configuration is intrinsically stable. It does mean you have cusps to deal with.
The "wiffleball" effect is possible because pushing back the magnetic field is so stable. The compression effect tends to squash the cusps closed as an added benefit.
Looking from the perspective of a confined plasma or electrons pushing at a field, a concave field will be stretched when you push on it. Picture air pushing out a balloon. As it stretches, the field or balloon skin becomes weaker. This setup is prone to instability. The ultimate example of a stretched magnetic field would be a loop-shaped solar prominence. When one of those stretches too far, it ruptures with a stupendous release of energy.
The magnetic fields confining electrons in a Polywell are convex from the perspective of the electron cloud inside the magrid. Pushing on the magnetic field compresses it instead of stretching it. The more you push on a convex field, the more it resists. This configuration is intrinsically stable. It does mean you have cusps to deal with.
The "wiffleball" effect is possible because pushing back the magnetic field is so stable. The compression effect tends to squash the cusps closed as an added benefit.
If anyone is interested, there is a technical description of this in "Principles of Plasma Physics" by Krall & Trivelpiece. Section 5.12 has a nice picture of a cusp plasma and the section is called "Stability of a Plane Plasma-Magnetic Field Interface: Energy Principle Analysis".
That's why I like the polywell - it is every where MHD stable plus it has mirrors at every "exit". There's gotta be a way to make it work!
That's why I like the polywell - it is every where MHD stable plus it has mirrors at every "exit". There's gotta be a way to make it work!