Is Polywell better than a fusor?

[pstudier]

From http://en.wikipedia.org/wiki/Polywell

Despite initial difficulties in spherical electron confinement, at the time of the 2005 research project's termination, Bussard had reported a neutron rate of 10e9 per second (based on detection of roughly three neutrons per test[6], giving a wide confidence interval). He claimed that the fusion rate achieved by WB-6 is roughly 100,000 times greater than that Farnsworth managed to achieve at similar well depth and drive conditions.[7][8] However, researchers at the University of Wisconsin-Madison claim a neutron rate of up to 5x10e9 per second. [9]

So the neutron production rate and Q's are about the same order of magnitude.

[Helius]

Madison is probably the Pinnicle of the Fusor; whereas the Polywell wb6 was just a very early attempt. If that early attempt achieved the same results as the pinnacle of the Fusors, then we are well on our way for some amazing progress. Let's see what results wb7 will bring. This is still an early attempt. I really feel that a result of Q>1% would be amazing, but I don't expect such positive results....yet.

The Polywell opens up a whole new set of optons for tuning not avalable to fusors.

[JohnP]

I thought the whole point of polywell was to solve the electron losses inherent in fusors... the polywell & magnetic shielded grid were a radical departure. If UWM found other ways to achieve the same results then why bother with the complications of polywell?

[scareduck]

Good grief, I see this same mistake all over the place. It also showed up in the Slashdot thread on focus fusion when Bussard's work came up. What Bussard claimed to have done was to get 10^9 fusions/s at very low drive voltages (5 kV-14 kV, with two runs at 12.5 kV). This is a vitally important qualification to grasp, and one of the main reasons I'm optimistic at all about this design. You can get plenty of fusions at high well voltages, but Bussard's point was that the machine efficiency was magnified many times over earlier designs to the point that he was getting fusion at what had been absurdly low voltages for traditional Farnsworth-Hirsch fusors.

Edit: And WRT the UWM fusor, it still suffers from the same problems all such devices would. Say it actually were capable of Q>1. At that point the negative grid melts and you lose your plasma and everything else. It's just not feasible as a steady-state machine. That might be why they were looking into a pulse-mode device.

Edit 2: The UW Madison team was driving their fusor at 120 kV at 6A in the run they claimed was 5e9 n/s. By contrast, Bussard's best run was one of his two 12.5 kV runs at 800A. Looking at power in, that's 720 kW at Madison vs. Bussard's 10 MW. Judged by power in and power out, the Madison guys have Bussard lapped five times over, but then Bussard barely had time to turn on WB-6, too. (I may not be doing the power calculations correctly, so somebody say something if I'm doing something wrong here.)

[JohnP]

The UW Madison team was driving their fusor at 120 kV at 6A in the run they claimed was 5e9 n/s. By contrast, Bussard's best run was one of his two 12.5 kV runs at 800A. Looking at power in, that's 720 kW at Madison vs. Bussard's 10 MW. Judged by power in and power out, the Madison guys have Bussard lapped five times over, but then Bussard barely had time to turn on WB-6, too. (I may not be doing the power calculations correctly, so somebody say something if I'm doing something wrong here.)

If UWM had 5x the neutrons using 7% of the power that Bussard was using then UWM got 70x the power/neutron efficiency vs WB-6.

[MSimon]

I'd have to look it up but I think you are confusing coil current with grid current.

[TallDave]

Now that I think about it, I'm not sure you can multiply the B field amps times the well depth voltage and get a meaningful number.

If you compare the well depths (ten times greater for the UW-Mad experiment) and use the ^4 B field scaling (the B field strength is the limit on the well depth for a Polywell iirc viewtopic.php?t=230), you would expect to get 10e9/sec x 10^4 = 10e13/sec from a Polywell of WB-6 radius at the UW-Mad well depth.

(I'm not sure the B field to well depth equivalency I'm doing here is right. I'm assuming they scale roughly proportionally.)

Of course, if you're running a gridded system, you're getting much larger electron losses. So the gridded device isn't even trying to be a prototype for a device that could scale into net power.

Interestingly, UW-Mad mentions Polywell.

http://fti.neep.wisc.edu/iec/inertial_e ... fineme.htm

[scareduck]

TallDave wrote in the recent scaling thread that pstudier calculated the power in at 175 kW because of "drive voltage is 12.5 Kev at a current of 14 amps". Not sure where he got that from, though. The numbers I used were from Bussard's final report, but again, I may be misusing them. It's clear that there are three terms for power input (at least), one for powering the magnetic part of the magrid, one for powering the electron cloud, and one for the electrostatic field on the magrid. (I'm assuming the electrostatic field power is minimal.)

[TallDave]

TallDave wrote in the recent scaling thread that pstudier calculated the power in at 175 kW because of "drive voltage is 12.5 Kev at a current of 14 amps". Not sure where he got that from, though.

I think it's the lab notes. There are some references to the electron current, but the units are hard to find.

http://ecow.engr.wisc.edu/cgi-bin/getbi ... nl0107.pdf

The current seems to be a function of the losses. So the UW-Mad team probably had a much higher current when you adjust for the different radii.

[TallDave]

It's clear that there are three terms for power input (at least), one for powering the magnetic part of the magrid, one for powering the electron cloud, and one for the electrostatic field on the magrid. (I'm assuming the electrostatic field power is minimal.)

Someone correct me if I'm wrong, but I think, in theory, you should be able to ignore the powering of the magnetic field (ideally these would be superconducting so no current required to overcome resistance losses) as it does no net work.

So basically your power in would be approximately the electron current needed to replace your electron losses and keep the well depth where you want it.

[scareduck]

Re-reading Bussard's final report (pp. 12-15) he was using 12.5 kV @ 10-40A to drive the electron gun, so 500 kW for the electron gun. (The plot shows an exponentially increasing curve to 40A, but Bussard or one of his lab people sketched in 40A throughout the run on the graph on page 12 because he apparently thought the "secondaries" [whatever those are] would have boosted the current enough to make the waveform square.)

[TallDave]

So 500kw for WB-6, 120kV x 6A = 720kw for UW-Mad, and from the picture the latter had a much smaller radius.

Anyone want to attempt a conversion factor for the radii?

[MSimon]

I'd really like to sample the high speed parts of the system (neutron counter, light detector, HF currents, ionization detectors, etc) at 16 bits 100 MSPS for at least a minute of operation. That is 12 GB of data per high speed channel.

Low frequency systems could be sampled at lower frequencies. (1 MSPS).

I'd like to sample an antenna placed in the reactor room to get an idea of the RF environment.

There ought to be a lot of information in the cross correlation of the data.

The lab should have a 10 MHz (rubidium clock controlled) distributed pulse with the cables equalized to better than .3 nS. (that is max minus min of 3").
For time synchronization.

[drmike]

Re-reading Bussard's final report (pp. 12-15) he was using 12.5 kV @ 10-40A to drive the electron gun, so 500 kW for the electron gun. (The plot shows an exponentially increasing curve to 40A, but Bussard or one of his lab people sketched in 40A throughout the run on the graph on page 12 because he apparently thought the "secondaries" [whatever those are] would have boosted the current enough to make the waveform square.)

Secondaries are electrons created from collisions with either neutral gas or walls. Since the grid is positive, any secondaries there will be reabsorbed rapidly. But on the ground wall, they may flow towards the grid and count as current. You also get positive ions, but they are mostly at very cold temperatures and do not contribute much to the over all current flow.

The pictures I saw were spherical balls on line with the cusps. These seem like thermionic (or cold cathod) electron sources, not "guns" in the technical sense that I'm used to. The power arguments don't change though - you need a certain current at a certain voltage and that takes I*V power - the name doesn't change that fact!

If it is a straight electron source, then the Child-Langmuir law will apply. There is only so much current you can get across a gap of a given voltage. It seems to me you'd want to turn on the magnetic grid after you pump current for ionization, otherwise the electrons will be trapped outside the MaGrid instead of inside. I assume Bussard left notes on the turn on sequence, but I haven't run across them in the reports yet.

Once it is turned on, if the electron losses are low enough, there should be a good power gain. I think this is the main difference between the FH fusor and the Bussard fusor - the electrons don't hit the grid so often. We'll see with more tests.

[MSimon]

I belive he discusses the turn on sequence in the UWisconsin results paper:

UWisconsin Lab Notes