What is happening here?

This forum is for specialized infomation important to the construction and safe operation of the high voltage electrical supplies and related circuitry needed for fusor operation.
Post Reply
Sarvesh Sadana
Posts: 48
Joined: Mon Aug 22, 2016 9:55 pm
Real name: Sarvesh Sadana
Location: Fremont, CA

What is happening here?

Post by Sarvesh Sadana »

I was doing running a test earler at higher voltages than I normally go. Striking the plasma required > 15 kV, but when I did so several very bright spots started to occur, and a small section of my grid disintegrated.

I don't believe my grid melted, as the overall wattage draw wasn't significantly higher than I have done before, so I am unsure as to what exactly occurred in the chamber.
Screen Shot 2017-02-21 at 6.01.26 PM.png
The bright spot where the grid disintegrated. It might be a strong electric field which attracted some of the plasma.
Screen Shot 2017-02-21 at 6.02.40 PM.png
The grid is disintegrating.
Screen Shot 2017-02-21 at 6.02.58 PM.png
The end result, as the pressure has shot up and the voltage has fallen due to the increased temperature.

What did I do, and how do I ensure it doesn't happen again?

The full video of operation is here, if you are interested. https://drive.google.com/file/d/0By1UXG ... lEVEk/view
User avatar
Dennis P Brown
Posts: 3159
Joined: Sun May 20, 2012 10:46 am
Real name: Dennis Brown

Re: What is happening here?

Post by Dennis P Brown »

It appears to me that your pressure is a bit high (also, I see the characteristic color of nitrogen gas) in first series of pics and also, the chamber is likely contaminated; the hot spots on the cathode are often debris/contaminates 'burning' off.

I see in your series of pics that the color changes to a more correct shade (last pic) as the system appears to 'burn' away contaminates and also appears to take on the a more characteristic-like color similar to deuterium. Oxygen and water vapor (or even pump oils) are banes and one needs to go to high vacuum first; also, follow good 'house cleaning' procedures when the system is open. This sometimes mean's cleaning parts/surfaces with (high grade) alcohol using a lint free cloth and wearing gloves before installing parts.

I always (after opening to air or if the system has been down for a day) to take it to down 10^ -5 torr before nearly closing the gate valve and adding D2. This provides both a clean system and a good starting point. Further, I purge the D2 feed line during the 10^-5 torr run, too. I also note that my first run at current will require a higher starting pressure (12 - 20 microns/milli-torr) before I burn my system in with the plasma; thus allowing me to slowly lower/reach 6 milli-torr operating pressure with a strong plasma glow (raising my voltage without current runaway.)
User avatar
Richard Hull
Moderator
Posts: 14991
Joined: Fri Jun 15, 2001 9:44 am
Real name: Richard Hull

Re: What is happening here?

Post by Richard Hull »

Regardless of causes, an open grid wire is a point of high field and will continue to cause issues and melting. You need a new grid. What is yours made of?

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
Sarvesh Sadana
Posts: 48
Joined: Mon Aug 22, 2016 9:55 pm
Real name: Sarvesh Sadana
Location: Fremont, CA

Re: What is happening here?

Post by Sarvesh Sadana »

I don't believe it is the contaminants burning off, as a part of the grid itself disintegrates, leaving a small gap. I believe the weak spot was caused by an imperfection in the welding process; when I welded it, there were a few places where the weld broke off.

I was using 99.95% tungsten wire with a diameter of 1 mm to form the grid, which was then spot welded together. Although, as it came from China through ebay, it may have had some impurities and imperfections. Should I consider using a thicker wire (e.g. 2 mm) next time?
User avatar
Dennis P Brown
Posts: 3159
Joined: Sun May 20, 2012 10:46 am
Real name: Dennis Brown

Re: What is happening here?

Post by Dennis P Brown »

If your welded tungsten burned away then yes, thicker is needed!

My cathode is steel with no welding (3.5 mm thick, and I hand bent it (far easier than it sounds. Used a steel pipe in a vice as a "form" to get four 'circles (using leverage!) With the aid of vice grips I then "twisted" these coils into a 'rough' cage - one roughly horizontal and three vertical at roughly 120 degree angles to the vertical.)) My cathode runs to near white heat at highest power levels and has held up well (for now) for over twenty minutes straight and has enabled me to get fusion (I am reaching power levels of nearly 1500 watts, so it is being tested!)

Hot spots are where the field and/or current is being forced to concentrate and that often implies a thinner area. Welding can do that.
Post Reply

Return to “High Voltage - Fusor Input Power (& FAQs)”