Plasma Diagnostics

Plasma diagnostics is a pretty interesting area.  The work horse is the Langmuir Probe.  Since this device requires a physical conductor actually inside the plasma, this can get rather interesting as the plasma is easily at 25 KeV, which works out to an effective temperature of 206 million degrees Kelvin.  But you can vary the potential in the probe, and the current in the probe is kept to a minimum.   But this means that you need to supply potential to the probe, and that means a very high voltage power feed through if you think your plasma is at 100 KeV.

But diagnostics of the plasma is essential for understanding the structure.  And if you're experimenting on changing the structure of the plasma, you have to find some way of measuring what you're predicting will happen.  Currently, the amateur work has centered around neutron measurements.  So there's a lot of good ideas and general engineering knowledge available in that area.

A paper pointed out by Garrett Young claims actual measurement of a double potential well formed in the IEC focus.  The inner potential well profile was measured to have a dip of approximately 200 V.  The cathode voltage for this measurement was -7.5 KV, cathode current was regulated at 30 - 40 mA, He pressure was at 20-30 mTorr.  This measurement was made with an amazingly cool LIF measurement apparatus.  What they did was measure the laser induced florescence at 667.8 nm wavelength and then deduce the potential in the core.  Very, very cool system, and as Garrett says, expen$ive.

The cool thing is that they seem to have actually verified a multiple potential well structure in the plasma core.

My post on sci.plasma finally showed up.  Boy does that list have a lag time.  But it's moderated, so I can't complain.  Did get one answer from a Scott Stephens.  He seems to be doing some cool stuff as well, and lamented about the lack of anything of interest to find on the web regarding IEC in this area (i.e. Bussard's ICC effect).

Garrett Young suggested that I look into the research going on with respect to standing waves in the sun after I lamented about the dearth of information on Bussard's standing waves on the fusor forum.  I must have had a sip too much of wine that night and I railed on about the lack of support for five fold symmetry in current modeling and simulation tools.  Must have sounded like a wacko, and I suppose I am in fact one...  Oh well.

I am pretty convinced though, that the standing waves Bussard describes are not waves generated by spherical harmonics.

As an experiment, I went and tried the Google Answers service.  Basically, I re-used my question I asked on the sci.plasma and sci.fusion newsgroups.  You can see the current progress and eventual result here.  The entire process has been very interesting.  The researcher pulled up a number of interesting sites, papers, etc that I had not been able to find in my scouring of the net.

Part of what I'm doing here is a big experiment with the net.  I'm trying to test the limits of this amazing web of knowledge and human interaction to see what it can do.  I'm ignorant about plasma physics and it's been quite some time since I've been in a physics lab.  So this is all like starting over again in school for me.  That means I make a lot of "green horn" mistakes, come up with completely bogus ideas and have to regurgitate a lot before I can keep some ideas down.  This web log is just a record of what I'm doing - the good, the bad and the extremely stupid.

Speaking of stupid, on the fusor.net forum, we were discussing losses in the IEC fusor.  I was trying to understand how the glowing discharge channels, so prominent in the IEC process (i.e. the star-mode discharge) was such a huge source of losses in the system.  I was arguing that the ions would maybe make it out of the electrostatic cusps formed by the grid, but would in all likely hood reenter the system, rather than hit the chamber walls and become neutralized.  Richard Hull wrote in his reply that the electron loss in the fusor was the single biggest loss in the system.  I think he was pretty frustrated with me, as he seemed to be raising his voice in the posting... 

<sigh>  I had forgotten that the vast majority of fusors operate in the so-called glow discharge mode, where the grids themselves ionize the deuterium gas and accelerate the ions towards the center of the structure.   This results in a shit-load of electrons in the system and the system simply leaks like a sieve as far as the electrons are concerned.  D'oh!  Oh well, I'm not trying to show how smart I am with all this stuff...  I'm just a rank amateur, after all, so that's like trying to prove a negative.  There are some really smart people out there, and I don't learn unless I actually ask stupid questions, or have my silly assumptions pointed out for what they are.  <heh>  I do learn, though.  Perhaps too slowly, but what the heck.

In any event, it's clear why Bussard's suggests suppressor grids in his second patent.  It's to keep this massive loss of power due to electrons to a minimal level.  But then again, that's why the magnetic confinement scheme of his first patent is so fascinating to me.  The magnetic field configuration does a great job of keeping the electrons where you want them, instead of just heating the fusor chamber through the power loss.

Just got the book Principles of Plasma Diagnostics by I. H. Hutchinson.  Man, is this a terrific book.  It's the second edition, and it is simply chock full of plasma diagnostic technique and theory.  Wow.  Going to take me several years to figure it all out, but what the heck.  What else am I going to spend my time on.  :)

I obtained a Bertan 30KV 33 mA power supply yesterday reasonably cheap.  Has remote control and is in very good shape.  I need to get a rack for this.  I also cleaned out the workshop.  My wife has a tendency to dump just about everything down there, and it had built up to a density that was getting dangerous.  So we cleaned it out and hauled out to good will a lot of stuff.  Amazing.  I'm rearranging the corner where I'm setting up and it's going fine.

Oh.  At the last minute, I called Huntington and begged them to change my port specifications on the vacuum chamber.  The mini ports were just too small, and the 2.75" port I had allocated for the high voltage feed through was just too tiny.  Luckily, I had just contacted them as they were putting the chamber on the machines to start drilling the holes.  Ye gods, am I lucky or what?  So I've updated the chamber picture, which shows the ports upgraded to 2.75", and the electrical feed through port upgraded to a 6" port.  That size will accommodate a 100KV power feed through with no problem.  Not that I'm going to get anywhere near that much voltage in the near future...  But it's a heck of a retrofit to the chamber once it's completed.  I'll be posting the updated chamber Inventor files shortly.

I also had a minor heart attack when I remembered that Alumina has a dielectric strength of only 230 Volts/mil.  Supports for my chamber would then only handle a potential of around 15 KV.  Wimpy.  It's weird, though.  Some other sources on the web show a dielectric strength of Alumina of 20 KV / mm.  I guess I don't understand the "mil" unit.  Have to investigate.  In any event fused quartz rods should easily insulate the cage from the chamber.  Looking up the definition of "mil" on the web, I find that this is a "thousandth".  So this is an exceedingly strange unit.  This must refer to a 1,000 of a centimeter, or a hundredth of a millimeter, if the 230 V/mil <-> 23 KV /mm relationship is to hold.  God, I simply love unit madness.  <heh>