Microwave Confinement

[bwsparxz]

Here is a site I found by a NCSU grad student on his microwave confinement project.

http://billrobinsonmusic.com/Physics.htm

[Wilfried Heil]

Looks interesting, but I don't see any evidence of confinement here. A localized gas discharge, maybe.

[Hector]

Right idea wrong approach.

Even if he gets it to confine; as power input rises so does the skin depth losses.


Hector

[Richard Hester]

RF confinement was considered with many of the other madcap approaches to fusion in the '60s, and is documented in Glasstone's book on thermonuclear fusion (a fascinating book documenting many of the early attempts at fusion before the tokamaks appeared and ate up all the funds like Godzilla). Skin effect losses are mentioned in Glasstone, as well as the need to tune the RF generators to preserve resonance in the larger space resulting from plasma compression.

[Chris Bradley]

Why was/is 'RF confinement' considered? What is the resolving force that causes ions to stay in one place?? You can quasi-confine ions in a Paul trap with an oscillating saddle field between multiple RF electrodes, but what on earth is the mechanism that permits a propagating wave to confine? Surely a wave is 'symmetrical' in that it would be as likely to 'de-confine' as 'confine'.

It has always seemed to me a fundamental error that many wishful hopefuls make that if they see something glowing and bright, spark-in-a-bottle, flash-in-a-pan type stuff, that somehow, by *mechanisms-mysterious*, fusion will be just around the corner.

This is why I commented on this observation before;

viewtopic.php?f=15&t=7173#p49045

[Hector]

Answer: A Ponderamotive force.

In a spherical resonant cavity like the one envisioned in the 60's, the RF energy can confine the ions via the fact that the RF energy has a decreasing radius towards the center of the chamber, thus creating a relative energy difference between the outer and inner most point of the cavity, this causes all charged particles, not only one or the other, but both electrons and ions to be confined, simultaneously. This has been my idea as to how to overcome the limitations faced by a pure electrostatic IEC system since the late 90's.

All charged particles would be confined and no bombardment of the chamber wall would occur, because the particle kinetic energy is equal to the imparted RF energy, therefore only particles that go through inelastic collisions would acquire enough kinetic energy to escape the potential well.

This is a very complex system, with issues like having to consider the dielectric constant of the RF plasma, it's eventual reflectivity since the RF energy cannot penetrate deep into the plasma core and other issues not faced by pure Electrostatic systems. Others include plasma drift, skin losses, etc.


Hector

[Chris Bradley]

I thought the ponderomotive force only really applied to high power pulse laser fields?

According to a quick punching of my calculator;

the ponderomotive force on a deuteron in an RF field of 2.45GHz and (let's say, my rough estimate of best possible field gradient) 10kV/m/m is 3.3E-23N, equiv to acc of 10km/s/s.

So the maximum velocity that a deuteron could have, to be stopped within 1m (let's say this is the chamber radius), would be 20km/s = 4.3eV.

Not enough to even confine a deuteron that has just made it to ionisation!?

To confine a 10keV deuteron by a 2.45GHz ponderomotive force would require an RF E-field amplitude differential of ~5 billion V/m, centre-to-edge.

Is that [calculation] right?

Heaven only knows what actual peak field you'd need there, as the chamber looks like it would fairly well generate a uniform field within it, after reflections.

Am I misunderstanding something elementary here?

[Hector]

No, you have it correct.

The original work on RF confinement was to use it to heat the plasma. However the only modern example of RF confinement of any kind related to fusion left is the FRC devices.

You are also correct that it would take a tremendous amount of power to drive a machine in this way, which brings us to the problem of skin depth losses, which is why they eventually abandoned this concept.

However, with this method you could do away with the need for any kind of physical grids, a tempting carrot.


Hector

[Hector]

Chris,

Now Calculate what the Ponderomotive force for the electrons should be. Now calculate the depth of the potential well that will form as the electrons rush to the center of the well way ahead of the ions.

Get the picture?

It’s not about using the RF energy to directly drive the ions, it’s about using the RF energy to form a deep electrons cloud and negative potential well, which in turn attracts the ions via Inertial Electrostatic Confinement.

Look mom no grids.


Hector

[Chris Bradley]

OK. I'll go along with that (for a short way!). Ponderomotive force is proportional to 1/mass (or acc is 1/mass^2) so I guess there is an essential merit in considering it. In fact, this seems a much much more convincing solution to getting a negative charge at the centre than the Polywell concept, despite the large RF fields you'd need to generate.

I don't think basic equations will help after that as we're then talking about the fields generate by a differential charge (but would DC fields be screened by this ionised medium anyway?). How much spherically-radial polarisation you'd get I do not know. I am still unsure about why you'd necessarily get this in a small vessel.

[Hector]

I don't have all the answers either, but it's an option that has not been explored and no one but me is crazy enough to explore.

Ah look fresh meat.


Hector

[Carl Willis]

You know what I like most about this guy? He's actually DONE something with his idea, rather than just blather about it (the usual end-state of discussions about RF in fusors on this forum). I don't see how it accomplishes the task fundamentally necessary for IEC fusion of establishing an electrostatic potential well, but it is an interesting exploration of microwave discharges.

Brian, he's close to where you live...maybe you could get a lab tour?

The arrangement of microwave-oven magnetrons on the same waveguide, and the simultaneous pulsing of these magnetrons to achieve high peak power, are interesting. I wonder how well this scheme works and how scalable it is. My impression would be that the peak power from these maggies is limited pretty close to their CW ratings by their permanent magnet field and their cathode emissivity. Also, I wonder what mode spectrum results from coupling many of these things together on the same waveguide. Each is itself a collection of several coupled resonators.

Interesting link, Brian.

-Carl

[Wilfried Heil]

Bill Robinson is trying to recreate ball lightning. He believes that BL emits microwaves, so he attempted to find out if the same will work in reverse.

This model was popularized by Kapitza in the 1950s, but no such microwave fields have been found in nature. I agree with him that all theories on ball lightning today have little foundation in reality, so that the only useful data comes from the reports of many observers.

At least he did try some experiments and continues to do so.

[Dustinit]

It is an interesting concept.
One of the problems I can see is that the more RF power you put into the plasma the more conductive it becomes and the more power it reflects due to its conductivity.
ie: plasma impedance moves further from source impedance so SWR increases.
So you have diminishing returns.
This in itself is not really a concept killer if you could perhaps with a fine probe apply a voltage to the conductive plasma and combine IEC and microwave confinement.
Does this seem feasible ?
Dustin.