Question on quick voltage ramp-up

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Anze A Ursic
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Question on quick voltage ramp-up

Post by Anze A Ursic »

Hello,

I've posted before in the HV section of this forum about the Spellman DF3 power supply I have, but this pertains to something else. The supply in question requires a minimum voltage setting of about 15-16kV to which it ramps up in about 10 seconds. There doesn't seem to be a setting that changes this. I am curious how bad this is in practice for a fusor. I've only ever ran my device to 5kV at about 15mTorr. We have a better HV input now, a much better 18" long ballast resistor, etc, but I am still worried that this quick ramp-up may cause instabilities in the plasma or something else.

Would one way of countering this effect be to pump down the chamber to very low pressures (<1mTorr), have the supply ramp up but not form plasma, and then slowly increase the pressure, letting the plasma form? Then stabilize the pressure at 15mTorr or so and actually continue slowly increasing voltage? We do have a really nice pressure controller so we can just set it to 15mTorr and it'll go there quick and stabilize.

Maybe I'm overthinking this, but again, I've only ever done 5kV and I ramped up to that slowly (probably 1-2 minutes or more), but I may be overly cautious, in which case, let me know! Any info is appreciated!

AAU
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Richard Hull
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Re: Question on quick voltage ramp-up

Post by Richard Hull »

You have the right idea. If you are forced to a minimum HV at 15kv, do not admit any gas at all! bring the gas in very slowly and bring the voltage up from there while jockeying the gas at the same time. It is how the fusor is run regardless of initial voltage. Tune for minimum smoke.

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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Anze A Ursic
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Re: Question on quick voltage ramp-up

Post by Anze A Ursic »

Richard, thank you! When you say "bring the gas in very slowly" are we talking about air or Deuterium?

My understanding is that fusors are operated by going to about 10-15mTorr vacuum, sparking a plasma and then increasing the voltage to the typical 30-35kV ranges. Then the turbopump is turned on, which evacuates all the remaining air and the pressure drops to below mTorr, the plasma extinguishes and then Deuterium is slowly added, which brings up the pressure back to the 10-15mTorr range and you (hopefully) get fusion.

In my case, should I evacuate the chamber to below the mTorr range, let the power supply ramp up to 15kV then reintroduce some air to raise the pressure to 15mTorr wherein I'll already get a plasma and then continue raising the voltage to 30-35kV, then pump down again to below mTorr and introduce deuterium? Or should I evacuate the chamber to below mTorr, get to 15kV and just continue raising the voltage to 30-35kV with no plasma formation (because of the very low pressure) and then directly introduce Deuterium, skipping the air plasma formation?

This may be overly pedantic, but I am trying to hone down every single fine detail for this run to be successful. Again, any feedback is much appreciated!

Anze
Matt_Gibson
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Re: Question on quick voltage ramp-up

Post by Matt_Gibson »

Hi Anze,

You’ll want to get your pressure down via turbo before letting in deuterium. Don’t let any air into the chamber, it’ll mess up operations.

Once you’ve dropped below 1 micron, you should start adjusting your chamber valve in preparation for deuterium flow.

-Matt
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Dennis P Brown
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Re: Question on quick voltage ramp-up

Post by Dennis P Brown »

Richard does mean add deuterium gas. One does it slowly via a leak type valve so the current does not run away as the high voltage starts to really create a plasma. Just as Matt is asking about having a low enough starting vacuum (i.e. 10^-4 torr or lower) the voltage at 14/15 kV is still too low to create any real conductive path under those very low vacuum conditions. This allows one to use deuterium gas to control the fusor; in fact, this is exactly how I control my fusor - it really helps to prevent as many runaways - of course, one has to have a 'leak valve' that releases the high pressure behind it extremely slowly into the chamber. Hence, normal valves are unsuitable for fusor work. Some people use a pin hole cap with a more normal valve to better enable gas flow control.

For instance, once you ignite a plasma at 15 kV (make sure the current is in a safe range), you'd never want to 'ramp' up the voltage at that fixed pressure because otherwise, a massive current surge would occur (and one could damage/destroy the power supply!) So, with chamber at 10^-4 torr and the power supply at 15 kV, slowly start raising the chamber's pressure. At some pressure the plasma will ignite - make sure your current isn't exceeding your systems safe operating conditions. Then under these conditions, one has to decrease the pressure in order to raise the voltage for an active plasma already at 15 kV.

So, the plasma is ignited with deuterium gas at 15 kV; then as you lower the gas inflow rate, you closely watch your current and add more deuterium gas to keep the current up in a safe range. Keep following this procedure until you reach the max voltage you plan on using. Do note a runaway condition with the current is likely to occur and you must lower the voltage to protect your power supply. As the system 'cleans', this will occur less often.

Critical point to note: Always keep the current in the safe known operating range for the transformer and diodes! Burning out a diode is a pain - destroying your transformer will ruin your day (or many weeks till you decide what to do about a new one.)

For a fusor run, using air as the trace gas to create a 'first' light plasma for the run would be rather counter productive - one always uses deuterium for any real fusor run. I only use air when I 1) just checking the power supply system 2) want to burn out the chamber to help reduce known trace contaminates and don't want to waste deuterium gas. Bottom-line: best not to use air to establish a plasma. Start with deuterium.

Also, no where did you mention throttling the chamber to reduce the turbo's performance. Otherwise, you will waste a vast amount of deuterium gas trying to reach the operating pressure.

A few asides FYI: pressure for a fusor isn't absolute so the pressure you are saying may or may not work for your fusor - it can vary depending on many parameters of your system. Also, measurable fusion gets going near/above 20 kV but as one approaches/exceeds 30 kV it is far greater.
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Richard Hull
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Re: Question on quick voltage ramp-up

Post by Richard Hull »

As noted above in the responses.....
You have a vacuum system to take the air out!
Why re-introduce it?
You are struggling to get a pure deuterium atmosphere into the chamber.
Whenever fusion folks are talking about introducing gas into the chamber it is only deuterium.
Operating a fusor is an art and not a science. Like all art, technique is needed and is developed in the hands-on experience from the doing.
I have written extensively on this in the FAQs.

viewtopic.php?t=9174

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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Anze A Ursic
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Re: Question on quick voltage ramp-up

Post by Anze A Ursic »

Ah, ok, so I think I get it. A true fusor run is no air any time and it's pressure controlled, not voltage controlled. So I can go up to 30-35kV at extremely low pressures, then control the pressure via Deuterium flow to get plasma and (hopefully!) fusion. I see.

Two notes: I have an MKS mass flow controller that can be remotely operated, so you can set different flow rates rather quickly to increase pressure. It's in SCCM which is good because I know a lot of people here talk about flow rate in ml/min, so that's a 1:1 conversion factor. The chamber we have is pretty big for fusor applications, so I may need more flow than the typical run.

Further, I have an MKS T3B gate valve controller just before the turbo pump so turbo is easily throttled. We actually use it in pressure control, so you set the pressure and it reads the voltage from the pressure gauge to get to the proper pressure. It is kind of slow in terms of reaction time, so I may not rely on it solely to get to the 10-15mTorr range for fusion but it's very good at stabilizing the pressure at a specific point, so that's excellent. Once fusion starts I'll just tell it to do 14mTorr (at this range, it jumps +/- 1mTorr every few seconds), so the run should be steady.

Thank you!

AAU
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Richard Hull
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Re: Question on quick voltage ramp-up

Post by Richard Hull »

Voltage determines the pure probability of quantum tunneling. (fusion) Low voltage-low fusion probability (cross section)
Pressure determines how much fusion fuel is present per unit volume in the reactor that can be turned into fusion ready deuterons.
Current determines how many deuterons, (ionizations of the gas), that might be present at a maximum to do the fusion probability dice throw.

All three are critical to doing fusion never think one is better than the other, though voltage sets a detectability limit for the amateur.

viewtopic.php?p=86189#p86189

viewtopic.php?p=89829#p89829

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: Question on quick voltage ramp-up

Post by Dennis P Brown »

Maybe I wasn't clear but it is a bad idea to have the power ramped all the way up to full on the variac (setting 35 kV) and then try and add deuterium to 'strike' the plasma. Such a situation would very likely create a runaway current that could damage or even burn out your transformer. Certainly it will be hard on the diodes - of course, that is what the ballast resister is supposed to help prevent but it is best not to depend on that to prevent damage as you frankly lower the voltage to lower the current. Besides, you'll likely then end up below 15 kV and have to start over. So, get a low vacuum, set your system to 15 kv and slowly ramp up the pressure to create a plasma (likely the current will runaway and you'll need to lower voltage/pressure to control the situation.) Then when all is stable, then adjust pressure and voltage to achieve the 35 kV or so. Since your supply automatically starts at 15 kV, that should be your starting point, not 35 kV.

Controlling flow also requires adjusting the throttling valve for the chamber to the turbo - so that can make controlling chamber pressure a bit more difficult.
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Anze A Ursic
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Re: Question on quick voltage ramp-up

Post by Anze A Ursic »

Ok, that make sense (again!). Wish I could try this a few times myself but since even with the initial 15kV voltage we are technically producing X-rays, we cannot do it without an agent present measuring exposure... kind of sucks, but is what it is.

And I do have a nice 125W, 18" long 100kOhm resistor in series with all of this to hopefully prevent any damage in case of a runaway.

Anze
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Re: Question on quick voltage ramp-up

Post by Anze A Ursic »

Also, Dennis, while I now understand balancing the pressure and voltage/current, the final pressure at fusion voltages (~30-35kV) should be around 10-15mTorr right? Or should I not care about being in that range and just focus on maintaining a nice plasma at those voltages?

Anze
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Re: Question on quick voltage ramp-up

Post by Dennis P Brown »

Good question - as I mentioned, the exact pressure your chamber requires will depends on your system design and equipment (to some extent.) Generally, it is, as you suggested, between 5 to 15 microns pressure and most people do operate their fusor in this range (smaller chambers have higher operating pressure.) Of course, it is possible your detector is off in calibration and the reading might then be somewhat different.

However, the issue, as you pointed out, is your power supply is unique: it has to start at 15 kV (mime starts at "0" using a variac.) This makes the process more difficult. So, what you need to do first is get the supply to turn on without a 'runaway' of current. To do this I'd suggest that you get the fusor below 1 micron, and this should enable you to get the power supply to turn on (it reaches 15 kV) without striking a plasma - yet. Then carefully raise the deuterium pressure in the chamber so your fusor becomes conductive - i.e an electrical plasma/glow occurs.

One must watch both the chamber and the current very closely - as the pressure in the chamber rises - using the leak control rate of deuterium entering the fusor - at some point the plasma will ignite (and the current will rise rapidly/spike.) As long as the current doesn't exceed your supplies allowed value, you have an acceptable pressure to start with. If the current jumps too high, lower the pressure and/or voltage to control the current (however, your supply can't be lower than 15 kV so that is then problematic.)

If the current is acceptable and you have a stable plasma at 15 kV your ready to begin raising the system's voltage. To raise the voltage without a current run away you need to lower the chamber pressure first (current will drop.) To compensate for the current drop (i.e. keep the current at an acceptable value) you slowly raise the voltage (say 17 - 18 kV). Keep up this iteration process until you 1) get to the desired operating voltage or 2) get a high enough rate of counts on your neutron detector that enables you to test for real fusion.

As you attempt these steps, the current is likely to 'runaway' requiring you to lower the voltage to get the current back to the desired range. Hopefully, this doesn't extinguish your plasma or shut the supply down.

Your power supply information should enable you to determine the max allowed current. Obviously, try not to exceed that value - that is why fusors need ballast resisters - they prevent damage to the transformer or diodes from a sudden current 'spike/runaway' giving you time to lower the voltage before supply damage occurs.

The process requires experience and depends on a host of parameters so the only way to determine how your system performs is to try it. As your system is run and 'cleaned' you'll find it gets easier to start up and maintain a steady plasma with no runaways.
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Re: Question on quick voltage ramp-up

Post by Anze A Ursic »

Thanks again, Dennis. I guess now having the T3B pressure controller is more useful than ever. I do understand what you are saying and I'll adjust my plan accordingly, but out of pure curiosity, why couldn't one go to the final voltage (~35kV) at 10^-4 Torr and then slowly add Deuterium? It seems to me as though if one had an incredibly precise way of inputting deuterium where he could vary the flowrate pedantically and slowly, the runaway current effect wouldn't take place, or am I wrong?

Again, just curious. The closer I get to a true fusion run, the more fine details I learn. Shame I can't practice with air plasma but the second we expect to get X-rays, we're under the purview of the health and safety office, which is completely understandable, just limits my experimental experience before going into the real thing.
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Re: Question on quick voltage ramp-up

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One certain can do as you suggest; just that the danger of a drastic runaway that damages your supply increases with increased voltage. If it is computer controlled (the power supply) and it can react in fractions of a second, and with your ballast, likely all would be ok when trying that method. However, that might take a long time before the fusor started to behave and create a stable plasma on first light up (load the walls, remove trace contaminates.) Meaning you might struggle to get stable operation started. Still, up to you to try what you want and see what is what. As long as you are confident the supply will survive, then I guess why not? Can learn from the experience no matter the outcome.
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Re: Question on quick voltage ramp-up

Post by Anze A Ursic »

Somewhat unrelated, but I just noticed whenever I opened up the main gas inlet valve the pressure immediately rises. At first I thought my swagelock tubing system was bad, so I redid it and tested it, and noticed the MKS mass flow controller we use (179a11cr38m) is not actually rated for such low pressures and it seems to leak badly. To a point where we can't get past 20mTorr with the turbopump. So now I'm curious how other people control flow of D2 remotely. I know several people who have manual valves next to them, but I'll be about 7m removed from my device in another room, so how would I go about controlling the pressure remotely? Are there any inexpensive ways this could be done or am I gonna have to cut 7 meters worth of tubing to get the bottle next to me?

Anze
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Re: Question on quick voltage ramp-up

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I have been following this and Dennis has given great advice. You have a special limitation that none of us have as we are private, self-funded and private property owners with our fusors in our homes, private labs, garages or outbuildings. We get to police ourselves. We are the purse holders, machinists, electronics builders, instrumentational specialists, the vacuum system experts and technicians. We are the rad-safe officers as well in our situation. As such we, have all gained a lot of experience over these many years in this type of system and, as noted, know what to expect in the way of radiation emissions from the fusor at all levels of voltages via our own measurement taken and recorded

I have held out of this discussion due to your special situation and need to please many masters who seem to have little understanding related to fusion or the amateur fusion effort related to operation of this type of device. Hopefully, once you radiation department actually measures around the operating device, they will come to realize the fusor's rather benign nature. At 7 meters distant, I am sure they will find any radiation, be it neutron or X-ray, to be immeasurable.

If you are forced to operate at this distance the cheapest solution would be the 7meters of gas line to the device. Operating, flowing deuterium, (differential pumping in-effect is what is needed), and controlling it is critical.

All the best of luck on solving this issue.

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: Question on quick voltage ramp-up

Post by Liam David »

I would advise against a 7m gas line for deuterium purity reasons relating to conductance. You should be able to find a suitable MFC on Ebay for <$200, and probably <$100 if you look long enough. Something in the 10-20 SCCM range should work fine.
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Re: Question on quick voltage ramp-up

Post by Anze A Ursic »

Thank you Richard for your response!

Liam, we actually have a bunch of MKS MFCs here but the issue is they're not rated for such high pressure differentials. I'm struggling to find one on Ebay that would suit my needs or maybe we could use the ones we have and I'm missing something?

Not sure how much money we can still spend, but worst case scenario I could attach a gear to a manual valve there and get a stepper motor with a gear to open / close it. If I make a large enough gear size differential it could be pretty sensitive too I suppose, for minute adjustments.

Either way, I'll try and find a solution with something we already have.

Anze
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Re: Question on quick voltage ramp-up

Post by Dennis P Brown »

I assume here the dual gauge step down regulator has been tried to near atmospheric pressure and has already failed.

One could make a work-a-round. Use an empty small gas cylinder and evacuate it (turbo); then back fill with anywhere from a 50 torr
to ? of deuterium gas; then use your flow controller - now the differential pressure is significantly smaller - to control inflow to your chamber. Obviously, it requires testing to determine the max. pressure that the unit can withstand - easy to determine.
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Re: Question on quick voltage ramp-up

Post by Matt_Gibson »

I got a near perfect condition MKS MFC for $71 on eBay. It’s a 10sscm type and works great and has a web interface that makes setup and zeroing a snap. Keep watching eBay and be prepared to snipe! I see this being sold new for over $4k still. Deals are out there if you look.

-Matt
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Anze A Ursic
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Re: Question on quick voltage ramp-up

Post by Anze A Ursic »

Matt,

Any chance you could provide the model number? Mine is a MKS 179a11cr38m. But we have a metric ton of these things in the lab so I'll go check now to see what we have and if we have anything close to what you have.

Anze
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Re: Question on quick voltage ramp-up

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Hi Anze,

It’s a GM50A013101RMM020.

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Anze A Ursic
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Re: Question on quick voltage ramp-up

Post by Anze A Ursic »

Interesting, the maximum pressure differential seems to be 10-40 psid for your model, which is also outside the fusor pressures right? Honestly, I'm not quite familiar with mass flow controllers, so I'm wondering if mine just has a bad seal or something. Also, yours seems to have a maximum inlet pressure of 150 psig. But I assume you're using a lecture bottle or something similar of D2, which is often 1500-2000 psig, so how do you account for that?

Any help appreciated, thanks!
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Re: Question on quick voltage ramp-up

Post by Matt_Gibson »

I’m using a syringe with deuterium in it. To be fair, it gets touchy under 1sccm of flow.
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Re: Question on quick voltage ramp-up

Post by Anze A Ursic »

In my infinite wisdom, I just realized the thing I attached onto my chamber is a mass flow METER, not CONTROLLER.... brilliant.
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