Cube’s $$ Titanium Coated Copper Endcaps Destroyed

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Jon Rosenstiel
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Cube’s $$ Titanium Coated Copper Endcaps Destroyed

Post by Jon Rosenstiel »

I’d noticed my cube fusor’s TIER had been gradually dropping off so I opened her up to see if I could find the problem. First to come out was the HV feedthrough with its 304 stainless steel cylindrical cathode. Upon first inspection I didn’t see anything obviously wrong other than a small amount of what appeared to be carbonized oil or grease inside the bottom of the cathode. (In the below image bottom is facing the camera) The material was easily removed using an IPA-soaked Q-tip. A quick test with a NIB magnet showed the material to be ferromagnetic. (Iron being baked-out of the stainless steel?)

A short time later, as I was reviewing the photos I’d taken, I noticed that the insulator’s snout and the cathode’s mounting lug appeared a little copperish in color. Ooooh, that’s not what I wanted to see, not at all. (The black stain on the feedthrough's snout is my clumsiness showing)

Something else that was initially missed but turned up in the same photo was a “whisker” that apparently “grew” out of the stainless-steel micro-tube covering the feedthrough’s Cu conductor. The whisker, if that’s what is actually is, is about 45-degrees off axis, roughly pointing towards an edge created where the viewport bore meets the cube’s central bore. (For reference, the micro-tube’s OD is 3mm)

Lastly is an image showing what happens when one cranks up the power a little too much for a little too long… vaporized titanium.

Jon Rosenstiel
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Richard Hull
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Re: Cube’s $$ Titanium Coated Copper Endcaps Destroyed

Post by Richard Hull »

Wow! Thanks for this report and your findings. It is stuff like this that instructs and informs others doing cube work The cube is a tiny environment and components take it on the chin. I find it amazing that these can take the punishment they do. (Energy concentrated in a small volume.) You show that such systems can be over driven and how a small metal hair can throw things awry in a high field environment within a plasma. Material science is one of those things we have to watch out for. Most of the old boys understand this and your report shines a light on how you pushed the limits.

Let us know about your repair efforts and how they turn out.

Interesting stuff. Thanks again for the report

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
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Liam David
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Re: Cube’s $$ Titanium Coated Copper Endcaps Destroyed

Post by Liam David »

Sorry for your loss... I had the same thing happen, zirconium and titanium blasted off a copper substrate. There are a number of reasons for these kinds of material failures, but I don't think that it's intrinsically the higher power. Beam systems outputting anywhere from 1e8 to >>1e13 n/s dumping megawatts of power into thin titanium layers survive much longer than your total runtime. I think the hole started forming not long after it entered service for other reasons.

What's your base pressure? Throttled pressure before you admit D2?

Your deposit of carbonized hydrocarbons is troubling. Hydrocarbons + water will absolutely trash your coatings. Hydrocarbons cause rapid sputtering. Any water >>1e-7 torr combined with the heat will quickly oxidize and slough off layers. This can also happen during a bakeout.

Another thing that can reduce the neutron rate is the evaporation of the cathode onto the titanium.

Copper has one of the highest sputtering rates of vacuum-compatible materials. I'd definitely suggest replacing the endcaps.

Some time ago I found a paper describing the growth of such whiskers. I'll try and find it. I'm surprised that it didn't cause arcing, although electron current from that point could have contributed to your decreasing efficiency.
Jon Rosenstiel
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Re: Cube’s $$ Titanium Coated Copper Endcaps Destroyed

Post by Jon Rosenstiel »

Liam,

Base pressure: I have a Baratron that, according to the manual, is good down to 10E-05 Torr. It’s usually bottomed out or close to it before I admit D2. One of these days I need to plumb in my hot cathode ionization gauge to get a better idea of where I’m at.

Cathode deposit: XRF shows the black, ferromagnetic material gathered on the Q-tip swab to be 304 stainless steel with a hint of copper. (See attached)

Years back a bunch of us drove up to Pasadena to attend a JPL open house event. I remember seeing a display they had put together on tin whiskers… was quite interesting. (Along with just about everything else they had on display)

Jon Rosenstiel
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Richard Hull
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Re: Cube’s $$ Titanium Coated Copper Endcaps Destroyed

Post by Richard Hull »

What can I say other than....Wow!

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
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Jim Kovalchick
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Re: Cube’s $$ Titanium Coated Copper Endcaps Destroyed

Post by Jim Kovalchick »

Jon,
I don't really doubt your assessment of the content of the black material, but regarding Liam's question about hydrocarbon deposits, I do doubt your XRF's ability to see carbon if it were there. The first K alpha xray for carbon is only 0.277 keV and below what looks to be your instruments cut off. Further, your cotton swab is made of it.

Jim K
Jon Rosenstiel
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Re: Cube’s $$ Titanium Coated Copper Endcaps Destroyed

Post by Jon Rosenstiel »

Jim,

I agree, my XRF (Spectrace 6000 from the 1980’s) struggles to ID magnesium’s K-alpha at 1.25 keV so there is no way it’s going to pick up carbon, but what I was attempting to show is that the black material contains iron. (To back up my original observation that the black stuff was attracted to a magnet) I believe that should rule out hydrocarbon deposits.

As an aside, the XRF did pick up bromine at 11.9 and 13.3 keV in both used and new Q-tips. (I understand it’s used as a fire retardant)

JonR
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Jim Kovalchick
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Re: Cube’s $$ Titanium Coated Copper Endcaps Destroyed

Post by Jim Kovalchick »

Jon,
I have XRF envy for your device!

Jim K
Jon Rosenstiel
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Re: Cube’s $$ Titanium Coated Copper Endcaps Destroyed

Post by Jon Rosenstiel »

Jim,

At one time or another I think I’ve run just about everything in and around our house on the Spectrace. Found that ground coffee contains a small amount of rubidium.

JonR
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Re: Cube’s $$ Titanium Coated Copper Endcaps Destroyed

Post by Frank Sanns »

Hi John.

Most people know that I like asymmetry included in experiments. It can reveal important fundamentals. My question is about your end cap spacing. Have you considered putting and extra inch or two or stainless spacer on ONE of your end caps?

It would be interesting to see if both patterns would be affected and how it would effect the neutron counts and neutron isotropy.

Frank
Achiever's madness; when enough is still not enough. ---FS
We have to stop looking at the world through our physical eyes. The universe is NOT what we see. It is the quantum world that is real. The rest is just an electron illusion. ---FS
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Richard Hull
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Re: Cube’s $$ Titanium Coated Copper Endcaps Destroyed

Post by Richard Hull »

Nice suggestion Frank. If I ever get the cube that I now own set up, I will try this suggestion first off, I think. I worry that the asymmetry would give preference to the easy nearby target and a dispersion of ions/deuterons would hit the long walls of the more distant arm target. Interesting.

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
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