The Focus Fusion Society › Forums › Lawrenceville Plasma Physics Experiment (LPPX) › ionized gas cathodes
I recently stumbled upon Focus Fusion and found it to be fascinating and was surprised by the simplicity and beauty of the involved machinery and physical principals.
After reading Dr. Lerner’s latest publication, I have come to understand that the plateauing of the fusion yield is believed to be caused by impurities introduced by vaporization of cathode material. That got me thinking and here are my 10 cents:
What if the cathodes were made out of ionized gas? Ionization could be realized by shining laser light into the reactor at appropriate times. The possible benefits would include: (1) No vaporizing cathode material; (2) Greatly reduced x-ray absorption by cathode; (3) High level of control over the conductivity of the gas-electrodes by varying the laser intensity; (4) Possibly some pre-ionization of other gas volumes in the reactor through mixing.
I’m curious to read what the community thinks!
Greetings,
Tim
There has been some discussion about pre-ionization. That will depend on how the experiments go with the new tungsten electrodes. Look at the January 27 report. You may also be interested in the News and Archives.
Francisl, thank you for the links. I’d like to point out that I’m not suggesting pre-ionizing all of the gas in the reactor. Rather, I suggest shining laser light into the reactor to primarily ionize the reactor volume that is currently occupied by the cathodes. In other words, take out the 8 electrodes, install 8 quartz windows in the same place were the electrodes used to join the head plate and ionize the gas column where the electrodes used to be. When the caps fire, electrical resistance will be the smallest along the ionized gas columns, so the ionized gas columns will become your new electrodes with the benefits listed in my previous posting.
Wouldn’t this idea suffer from the same difficulty as the problem of designing a real-life lightsaber? That is to say, how do you give the electrodes a tip, or end-point?
We always like new ideas–keep them coming. But the electrodes do have to be solid in a Mather-style DPF. If you eliminate the anode, the currents in the center will pinch together as soon as the current starts flowing, so you would need a much larger radius insulator to slow that down. If you put the insulator at the edge of the vacuum chamber, you get a Fillipov-style DPF. You don’t really need any pre-ionization, but you have less flexibility of design and you need a lot more energy for the same current.
Ivy, I don’t think the gas cathodes have to end at the same plane as the center-anode. The current should stop flowing axially away from the back wall once the center electrode (solid) ends.
Dr. Lerner, thank you for your encouragement! I don’t know enough about the differences between Mather/Fillipov-style DPFs to comment on the necessity of solid electrodes. But I’d like to point out that I only want to replace the outer electrodes with gas electrodes and keep the design of the center electrode and insulator unchanged. If the resulting design doesn’t fit with Mather/Fillipov-style DPFs, maybe a new name can be found 😉
Ivy Matt wrote: Wouldn’t this idea suffer from the same difficulty as the problem of designing a real-life lightsaber? That is to say, how do you give the electrodes a tip, or end-point?
Could he use intersecting laser beams to shorten the path of least resistance?
Michio Kaku shows how to build a plasma lightsaber in this youtube video:
Tim_Petrik wrote: Ivy, I don’t think the gas cathodes have to end at the same plane as the center-anode. The current should stop flowing axially away from the back wall once the center electrode (solid) ends.
Dr. Lerner, thank you for your encouragement! I don’t know enough about the differences between Mather/Fillipov-style DPFs to comment on the necessity of solid electrodes. But I’d like to point out that I only want to replace the outer electrodes with gas electrodes and keep the design of the center electrode and insulator unchanged. If the resulting design doesn’t fit with Mather/Fillipov-style DPFs, maybe a new name can be found 😉
That actually sounds like an elegant solution to a lot of the cathode problems the reactor would run in to such as avoiding degradation of a solid cathode, but presents some new engineering challenges, and may require more energy than its worth.
I have a potential idea that seems it should use a lot less power, and should have less heat concerns – how about making the cathodes and maybe the anode of carbon nanotubes? It is a highly conductive and strong material, should be relatively cheap, and is structurally light enough to allow x-rays to pass through, although CNT has a somewhat high attenuation rate. My guess is the xrays may cause too much degradation of a CNT anode, but some long-term tests appear promising. Nevertheless, the relatively high x-ray attenuation of CNT makes it a less than ideal choice for an anode in this reactor. The cathodes might be able to be designed in a way to lessen the loss of X-rays. In a vacuum CNT is estimated to have a higher melting point than beryllium. I know next to nothing about the physics of such an application, but thought I might bring it up.
Is it possible that when the axial coil is used the dimple in the end of the cathode might not be necessary? Elimination of that dimple sure would simplify cathode cooling.
Well, I should have known that LPPF would have known about such a material – after all it has been around for over a decade. Don’t I feel stupid…
Apparently, Lerner is considering CNT as a coating for the electrodes.
http://lawrencevilleplasmaphysics.com/carbon-nanotubes-may-protect-electrodes/
I assume its absorption of X-rays has been deemed too great to use CNT as a monolithic cathode, but it seems the properties of CNT would allow for a smaller cathode…
If a glow discharge works well for starting the shot, would a glow discharge around all of the electrodes reduce the intense reactions along the whole length of the electrodes?
Here is possible method to produce an effect similar to glow discharge along the full length of the cathode arms. Indirectly heated cathodes could be placed along the full length and on both sides of each of the cathode arms. These heated elements would create a thermionic electron emission that would ionize the nearby gas similar to a glow discharge. The heating elements would only have to be operated a short time before each shot. A side effect is that the heater elements could also heat the chamber to a specified temperature so that there is more uniformity in shot to shot performance.
This approach could be tested in a college physics lab. Place a Jacob’s ladder spark gap in a bell jar with the same pressure and fuel as used in a dpf. Put indirectly heated cathodes along the sides of the spark electrodes. Observe the operation of the spark discharge with and without the use of the cathodes.