Talk-Polywell.org

Without comment:

https://www.youtube.com/watch?v=JurplDf ... ableMatter

Blech...
I just skipped through the video. Pretty much the usual arguments.
I don't know that guy, but he sounds like Dan Jassby (I think it might be him). He is a joke in the fusion community, particularly the Tokamak community.
Apparently he’s been pissed since TFTR was shut down. Retired back in the 90’s...
Plus, he is focused on Toks without HTSCs and so on and so forth.

Skipjack wrote: ↑

Sun Sep 12, 2021 1:37 am

Plus, he is focused on Toks without HTSCs and so on and so forth.

Makes me wonder what type of scientist he was if he thinks that HTSC are not an essential step to obtain a viable fusion machine.....

With the advent of new commercially available 20T magnets brings me hope. But that huge barrier of no funding is still the 400 pound gorilla in the room.
"Performance of this magnet is similar to a non-superconducting one that was used in an MIT experiment that concluded its experiments five years ago," said MIT's Whyte. "The difference in terms of energy consumption is rather stunning. That magnet, because it was a normal copper conducting magnet, consumed approximately 200 million watts of energy to produce the confining magnetic field. This magnet was around 30 watts, so a factor of around 10 million decrease in the amount of energy that was needed to provide the confining magnetic field."- Commonwealth Fusion Systems

paperburn1 wrote: ↑

Sun Sep 12, 2021 1:46 pm

With the advent of new commercially available 20T magnets brings me hope. But that huge barrier of no funding is still the 400 pound gorilla in the room.
"Performance of this magnet is similar to a non-superconducting one that was used in an MIT experiment that concluded its experiments five years ago," said MIT's Whyte. "The difference in terms of energy consumption is rather stunning. That magnet, because it was a normal copper conducting magnet, consumed approximately 200 million watts of energy to produce the confining magnetic field. This magnet was around 30 watts, so a factor of around 10 million decrease in the amount of energy that was needed to provide the confining magnetic field."- Commonwealth Fusion Systems

True, though the cryo system will consume some energy too (not sure how much). I am not sure that that was included in the 30 watts.

Skipjack wrote: ↑

Sun Sep 12, 2021 2:07 pm

True, though the cryo system will consume some energy too (not sure how much). I am not sure that that was included in the 30 watts.

I believe that the 30W refer to the power consumed by the cryo system to keep the HTSC at superconducting temperature during the working operation.
For sure it does not include the power consumption to bring the HTSC at working temperature, but in a steady state machine that initial power consumption will quickly become marginal.
From 200 MW to 30 W gives indeed the idea of how much the technology advanced in this field in the last 10 years.

I made myself watch this a few days ago. Ego and pessimism aside, the schematic on the need for Q>20 for a Tokamak was interesting. That was from generation efficiency 50% (not unkind) and power conversion to plasma injected power 10% (no idea if that makes sense). So Q=20 just drives the machine with no grid delivered power.

So, what is QMax in a Tokamak? Is there a theoretical bound which might be illuminating in this context?

RERT wrote: ↑

Thu Sep 23, 2021 9:01 am

I made myself watch this a few days ago. Ego and pessimism aside, the schematic on the need for Q>20 for a Tokamak was interesting. That was from generation efficiency 50% (not unkind) and power conversion to plasma injected power 10% (no idea if that makes sense). So Q=20 just drives the machine with no grid delivered power.

So, what is QMax in a Tokamak? Is there a theoretical bound which might be illuminating in this context?

The Q for DT-fusion at the maximum cross section can be estimated as follows. The energy liberated by DT-fusion is 17.6 MeV. The maximum of the DT-fusion reaction cross section occurs at a centre of mass energy of about 65 keV, so with these two numbers we get a maximum Q value of 17.6 MeV / 65 keV which is about 270. But you can of course have fusion with a lower probability with a lower or higher centre of mass energy. Now in a tokamak you will have a thermal plasma at some lower temperature so you would need to do an integral over the temperature distribution as well as taking into account the goodness of the confinement and all losses to estimate the QMax for a tokamak. Probably this number has been estimated, but it will vary from tokamak to tokamak and depend on the plasma mode the tokamak is running in.

RERT wrote: ↑

Thu Sep 23, 2021 9:01 am

I made myself watch this a few days ago. Ego and pessimism aside, the schematic on the need for Q>20 for a Tokamak was interesting. That was from generation efficiency 50% (not unkind) and power conversion to plasma injected power 10% (no idea if that makes sense). So Q=20 just drives the machine with no grid delivered power.

So, what is QMax in a Tokamak? Is there a theoretical bound which might be illuminating in this context?

From what I hear, a electricity producing Tokamak is generally considered to need a Q ~ 30 to be economically viable.
I believe that some are aiming to do it with a lower Q of about 20 (forget which team it was, might have been CFS).

After I posted above I decided that the answer for QMax was probably ‘infinite’. Focussing on a DT plasma, 20% of fusion power is charged particles, which one would think would heat the plasma. It’s tempting to think that Brehmstralung losses need to be replaced, but that’s bunk: the device is thermal, and the x-rays are just another source of heat to raise steam. The simple assumption is that once ignited, the containment needs to be powered, but plasma heating becomes unneccessary.

But… if smart people suggest running with Q=20 or 30, I’m missing something….

RERT wrote: ↑

Wed Oct 06, 2021 8:07 am

After I posted above I decided that the answer for QMax was probably ‘infinite’. Focussing on a DT plasma, 20% of fusion power is charged particles, which one would think would heat the plasma. It’s tempting to think that Brehmstralung losses need to be replaced, but that’s bunk: the device is thermal, and the x-rays are just another source of heat to raise steam. The simple assumption is that once ignited, the containment needs to be powered, but plasma heating becomes unneccessary.

But… if smart people suggest running with Q=20 or 30, I’m missing something….

Yes, they need to achieve ignition, but that IIRC requires a higher Q(plasma) than 10.
I might be wrong here, but that is the number I remember. That is why ARC is so much bigger than SPARC. The Q~10 of SPARC won't be enough.

The only way I can make this thread makes sense is that most of the 4He as well as the n energy us lost to the plasma.

Anybody got a book reference that explains this stuff?

A relatively good starting point is Scott Hsu's paper:
https://arxiv.org/pdf/2105.10954.pdf

Thanks, will look…

...and it was very helpful!

Winding back to the question above, from the point of view of operating conditions Qsci goes from 1 to Infinity in a reasonably narrow range of nt. Qsci = 20 is very close to Q=Infinity in terms of operating conditions. So based on the paper my answer to my question would be no, there isn't a Qmax for a Tokamak. Further, it doesn't seem to me that the critique in the root post 'Q=20 is miles away' makes a lot of sense when Qsci explodes at a certain point.