Fusor Seasoning and Issues

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Dennis P Brown
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Fusor Seasoning and Issues

Post by Dennis P Brown »

Started up the fusor in a real way; baked it for a few hours and then seasoned it with deuterium and plasma for twenty minutes until it could operate in a stable fashion. I am not convinced the power supply's milli-amp gauge is providing the correct current delivered to the cathode. Also, the micron gauge is a little high but that isn't an issue (drifts some after initial calibration). Was able to control current (at max voltage) very precisely using the deuterium flow valve only (once the chamber high vacuum valve was set). So that is certainly a good sign that the fusor chamber is operating properly.

From the pic's, the current (28 ma) and voltage (31 kV) are at the system continuous max. The pressure ranged from 11 - 17 microns during seasoning ( I'd think that is a bit high for this sized fusor's normal range.) The current/voltage/plasma was extremely stable in the fusor once the chamber finished the seasoning cycle.

The chamber got fairly warm after twenty minutes and the glass view port very warm. After I shut the power off and shorted the HV feed cable, I checked the cathode ceramic insulator on the high voltage feed-thru. It was not noticeably warm nor did the cathode get hot enough to glow at any time during fusor operation cycle - so, this makes me suspect the current delivered to the cathode isn't as high as the power supply gauge indicates. I'd think with those power levels after ten minutes the cathode 'cage' would glow at least red at this power level (see pic.) The power to the cathode should be about 850 watts continuous so that is a lot of power if the current is correct.

I am thinking to install an analog gauge to the top of the HV insulator and test the current against the power supply meter to see if they track accurately. I can do this at a reasonably low voltage (8 - 12 kV) when the chamber is at a higher pressure (lower voltage operation for the plasma.)

I include some pic's for the chamber and gauges during operation of the fusor.

I still feel I'm in the start up phase and a bit of a ways off before I feel comfortable in trying any neutron detection. The main point of this test was to check the new high voltage feed-thru's performance during actual fusor conditions and I felt it did a very good job. Certainly far better in fusor operation performance and current stability at high voltage than my all home-made feed-thru it replaced.
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Normal Max. continuous current
Normal Max. continuous current
System Pressure - reading a few microns higher then real
System Pressure - reading a few microns higher then real
Cathode - not very bright
Cathode - not very bright
Last edited by Dennis P Brown on Tue Mar 01, 2022 4:22 pm, edited 3 times in total.
Matt_Gibson
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Re: Fusor Seasoning and Issues

Post by Matt_Gibson »

I’m able to heat my titanium cathode to glowing red hot in just a few minutes running at 45kV and 13mA. My old 3 wire (.6mm) tungsten cathode would glow red hot in 10 seconds or less.

I have an analog meter suspended between my psu and my fusor feed through. I’ve tried out two different meters and they both read the same, so I’m assuming they are giving accurate current measurements.

Chamber gets too hot to touch after a 10 minute run unless I have the fan blowing.
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Dennis P Brown
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Re: Fusor Seasoning and Issues

Post by Dennis P Brown »

That is exactly what it should be - I am more convinced something is not correct. Glad your analog system worked at the HV cathode - that certainly is a good indication I should do that as well. I should add that larger chambers like mine do tend to operate at lower temps then smaller chambers.

Again, really nice fusor and system you have! Is the voltage multiplier going to remain in air or are you considering oil?
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Re: Fusor Seasoning and Issues

Post by Matt_Gibson »

I had considered going digital for voltage and current measurements, but chose to just keep it simple for now. Analog meter with current shunt inline with the high side seems pretty hard to get wrong seeing as I have multiple return paths to ground and whatnot. Plus, I’ve seen a few others having trouble with their metering so just decided to leave it up there.

Thanks for the compliment, it definitely wound up my pride and joy project!

The multiplier is actually under oil in that enclosure in the back. The other doodad that’s in air is a resistor board for the voltage metering.

-Matt
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Richard Hull
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Re: Fusor Seasoning and Issues

Post by Richard Hull »

that floating current meter is a deadly item. But, then anything like a toroid or exposed line connection in the hot line is deadly.


I think Dennis was looking at your ladder divider voltage meter chain. I always tended to build mine in air as well. As long as inquisitive hands keep away from such chains and the floating ammeter during operation, all is cool.

In many instants digital shunts on digital meters are figured wrong and are in error. All shunted digital meters should be checked at low voltages like 10.00 volts feeding a 200 ohm 1% load with the supposedly calibrated digital meter in series with the load. You should read 50ma. If it does, your meter shunting effort is correct. Now it can still be in error if you place it in a circuit where there are other parallel paths to ground that are subtle and unsuspected. This is why we stress putting it in the fusor's ground path! The only current there is due solely to the cathode, (grid) voltage through the deuterium gas load.

While electronically abysmally simple, metering can be tough for those not use to it.

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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Re: Fusor Seasoning and Issues

Post by Matt_Gibson »

Definitely not a safe solution, but I don’t get anywhere near the exposed bits during operation and periodically check the mechanical connections for “tightness”. I’d go through the chamber return, but the chamber is sitting on an aluminum frame that has a few other devices that are connected to ground. I may try to insulate the chamber at some point…

-Matt
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Dennis P Brown
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Re: Fusor Seasoning and Issues

Post by Dennis P Brown »

Might I suggest a large, reasonably thick plexiglass shield mounted to the aluminum frame (to cover the upper section.) If you accidentally trip towards the device the shield could be a life saver. Also, I have a master ground point and a plexiglass rod that has a metal screw on its other end. I attached this metal point to the ground wire. I use this to confirm anything that was hot is now safe before touching any part of the high voltage system. I assume your high voltage system has a bleed off resistor. If not, a good practice to install one. Still, I'd never assume caps are safe till they are checked against a ground.
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Re: Fusor Seasoning and Issues

Post by Matt_Gibson »

I’d still need to contend with the resistor string, but could probably plexiglass box it up. I’ll probably look into this a littler more, right now I stand way back (out of the picture) and operate a signal generator as control.

I also have a grounding cable that I use to discharge the multiplier through the ballast (save the diodes from peak currents).

Have you tried measuring power consumption using one of those plug-in power meters? I know they aren’t super accurate, but it should give you an idea of whether or not you’re in the ballpark, no?
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Dennis P Brown
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Re: Fusor Seasoning and Issues

Post by Dennis P Brown »

Well, the voltage on my system is accurate so once I measure the current at the cathode, the power has to be that product - so a 'power' meter isn't really necessary. Of course, if you mean the variac that would give me some indication but I still prefer the method you are using - but only for a one time test of the readings.

Once you confirm your meter reading, wouldn't it be prudent to remove that meter attached in the air to your cathode? Besides being a bit dangerous it will cause some corona issues.

Bleed resistors (mega-ohm) are superior to shorting (even through a ballast - 'only' 70 k-ohm) the multiplier. They also are an added safety measure.
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Re: Fusor Seasoning and Issues

Post by Matt_Gibson »

Yup, I meant at your variac. If you’re up for high side measurements, then that would be better.

I’m not having any corona issues (yet) up to 60kV, but plan to go to 75kV with my new psu upgrade.

I’ll probably rig up a meter on the return side to my multiplier at some point so that I can see both voltage and current in the same physical location.

My resistor string does a good job of bleeding the VM multiplier down, but I still use a grounding wire before going near the hv bits.

-Matt
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Richard Hull
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Re: Fusor Seasoning and Issues

Post by Richard Hull »

Yes, the Voltmeter resistor string will bleed down the HV after shutdown over time. However, the theory in exponential decay of the stored voltage under the voltmeter load says the voltage will never reach zero. We know that in the real world it will hit zero at some point. (A point where theory and the real world collide).

Still, if you want to manually access any HV system that contains capacitance. Shorting the output is always a must-do!

In my water arc work where the capacitor discharged explosively into a water load over a few microseconds. Stored inductive "reflex energy" usually left the 30kv capacitor charged to 3-5kv. Upon shorting prior to the next shot, sometimes I got a nice little snap. At other times, nothing noticeable. still it was nice knowing a crowbar was across the water arc gun and the main supply was off. No shortcuts with kilojoule toys is allowed.

A humorous note here... Once I forgot to lift my copper crowbar and fired the gun. This threw the crowbar into the air instead of welding it across the gun's input bolt heads.

The fusor is no explosive discharge device with no reflex energy path, and the voltmeter will bleed it down over time. However any HV system should be shorted when hands are monkeying within the system.

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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Dennis P Brown
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Re: Fusor Seasoning and Issues

Post by Dennis P Brown »

On to my more mundane fusor problems or at least imagined problems. I hooked up a cheap analog milliamp meter to my fusor input high voltage (HV) connector to compare to my analog power supply meter (which is connected to a different side of the diode bridge compared to my fusor 'hot' side input.) Hence I was concerned that the power supply meter was not following the real power delivered to the fusor.

Well, that issue is settled - the two meters tracked very closely. At 30 milliamps under full fusor conditions (deuterium gas, at voltage) the power supply meter agreed within a reasonable accuracy with the HV feed meter (the meter's drifted a bit during the time to take the photo's due to the turbo still winding up.) But basically, the current at the HV input to the fusor agrees with the panel meter.

As a sanity check, I measured the HV cable's resistance (the two ends) and it was 0.2 ohms. I also checked the resistance between the cathode cage and to of the connector to the HV assembly and that unit measured 0.4 ohms. So neither the cable or HV feed-thru are in any way preventing the fusor from generating full current/voltage generated by the power supply; that is, I am getting the full 850 watts delivered to the cathode.

So still lost on why my my cathode cage does not 'glow' super hot.
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Dennis P Brown
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Re: Fusor Seasoning and Issues

Post by Dennis P Brown »

Sorry about the pic's - the camera refuses to focus properly. Also, can't edit the grammar error in the one sentence; have we lost edit privilege's by author's?
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Re: Fusor Seasoning and Issues

Post by Matt_Gibson »

Your cathode wire looks pretty thick, maybe it just needs to run longer?

I read a thread where someone found a bad connection that was robbing them of their power. Maybe something is loose? A thermal camera might help?
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Dennis P Brown
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Re: Fusor Seasoning and Issues

Post by Dennis P Brown »

At high frequency, I'd certainly suspect a cable/feed-thru issue that is not apparent to an ohm meter but this is just 60 Hz. So I'd think the ohm meter is accurate; however, since I am still confused, maybe the cable or feed-thru is the issue on power as you suggest. But the current and voltage at 60 Hz appears very steady so I'd think connection issues would display during a run - maybe others recall that thread and its details.
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Richard Hull
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Re: Fusor Seasoning and Issues

Post by Richard Hull »

That cathode wire cage is certainly beefy. What gauge is it?

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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Dennis P Brown
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Re: Fusor Seasoning and Issues

Post by Dennis P Brown »

Sorry, mis-read the scale (analog scale.) The wire is .12 inch or about 3 mm so that would be 8 gauge from a chart.
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Re: Fusor Seasoning and Issues

Post by Matt_Gibson »

That’s a hefty cathode! I think that you’ll need longer runs.
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Dennis P Brown
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Re: Fusor Seasoning and Issues

Post by Dennis P Brown »

Well, guess the 850 watts isn't going to enable the cathode cage to heat up enough to glow even a dull red - pity; however, I now wonder if that reduces the available free electrons in the chamber and if so, wonder if that would affect my number of free deuterium ions available? If that does occur then the devices' fusion rate might be impacted? Wonder if anyone has examined if this occurs?
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Dennis P Brown
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Re: Fusor Seasoning and Issues

Post by Dennis P Brown »

Thinking more on this, I do recall that in old time radio vacuum tubes, getting the filament to glow bright red to 'boil off' electrons was crucial to its operation - availability of energetic electrons would be extremely important to ionize the deuterium gas.
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Dennis P Brown
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Re: Fusor Seasoning and Issues

Post by Dennis P Brown »

Guess I'll wait till Richard, who certainly knows more on this subject then most any one else, might be able to clarify this matter. Much better than my pure speculation (through, that often doesn't stop me from posting ;) )
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Richard Hull
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Re: Fusor Seasoning and Issues

Post by Richard Hull »

First off, at red orange to bright orange, most grid materials are not electron emitters. (they have not reached their emissive work function) Bare Tungsten at white heat will boil off a lot of electrons. Most early vacuum tubes relied on a white hot tungsten wire. Tube filaments were coated in later years with low work function barium compounds to allow great electron emission at a low red heat. This allowed for a much longer core filament life.

We Do Not want electrons boiling off the grid in a fusor!!! We want shell and chamber wall area emission of electrons! This is where we want the electrons to make deuterons to accelerate to their full energy and arrive at the cathode.

When a grid gets too hot, you will have electron avalanche runaway! The fusor will act like a very efficient gas diode and current will runaway in an attempt to deliver amperes of electrons to the shell. Many here have seen this and many have not.

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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Dennis P Brown
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Re: Fusor Seasoning and Issues

Post by Dennis P Brown »

Thank you; vacuum tube knowledge does payoff - not a topic people who only have used miniature black boxes called microchips would ever know. The issue of electron runaway is certainly useful to know as well nor something I would suspect as an issue to avoid. So, keeping the cathode cool isn't a problem for a fusor. Good to know.
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Re: Fusor Seasoning and Issues

Post by Richard Hull »

I did experience electron runaway in fusor III and early fusor IV with fine wire Tantalum grids getting white hot (1999-2004). Tantalum is a great filament material. I never used tantalum after that and have used only heavier gauge Tungsten wire ever since 2005. I have had rare runaways since then, but I was pushing the power in a bit too hard. My fault.

Runaway is very possible in any fusor and the cathode general only needs heavier material and a decent sink at the stalk. Wire grids are far more susceptible to runaways than the modern cylinder grids in the more recent BOT systems.

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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