Why operate Turbo - or Vapour Jet Pump during Fusion attempt

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Finn Hammer
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Why operate Turbo - or Vapour Jet Pump during Fusion attempt

Post by Finn Hammer » Thu May 25, 2017 1:46 pm

When the forepump has leveled out at a one digit microns level, the Turbo or VapourJet pumps are started, to bring the pressure down two decades deeper, to x^-2 Millitorr, before the valve to the Deuterium tank is opened.
By backfilling the tank with D2 from this low pressure, there is a good chance that there is mainly Deuterium in the tank, for a fusion attempt.
This backfilling process should level out at around 5-15 millitorr, and it is frequently described as being quite difficult, and to require great skill on the side of the operater, to wiggle the valves involved,( the Deuterium throttle valve and the gate valve between Fusor and Turbo or VapourJet pumps ) to bring this pressure to a stable level. The trade off here is to keep the tank pressure up, while at the same time avoiding to flush too much Deuterium out the drain.
This Fusion pressure ( 5-15 millitorr ) is well within the capability of the forepump alone.
That leads me to the question: Why operate the "deep" pumps at this point at all. Would it not be easier to create a steady pressure using only the forepump (I am thinking: VFD to control the pumping rate) instead of fighting against a pump that is designed to go a lot deeper.
In a way, It looks to me like driving a car with the throttle wide open, while adjusting the speed with the brakes at the same time.

I realize I am thinking way ahead of my experience level here ( experience level = 0 ), and perhaps I should just shut up and build.
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Dennis P Brown
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Re: Why operate Turbo - or Vapour Jet Pump during Fusion att

Post by Dennis P Brown » Thu May 25, 2017 5:40 pm

Your assumptions are based on specific equipment and conditions that don't often hold. However, in general, a turbo pump can be started at a much higher fore pressure than a hot oil diffusion pump because of the issue of oxygen. A molecular drag pump can be started at a still higher pressure.

For an oil based diffusion pump (DP), generally, I start the heating process of the oil when the fore-line pressure drops below 100 microns. By the time the oil really starts to get hot, my system is in the under 5 micron range. That said, a gate valve can always be opened to an operating DP when the chamber pressure is below 50 microns and many oils can handle it if a pressure of 100 microns is admitted.

However, I never admit deuterium into my chamber until I have allowed my fusor chamber to drop into the low 10^-5 torr (Then, of course, I almost close my main gate valve just before I start to slowly admit deuterium.)

But deuterium can be added when the chamber is just into the very low micron range - it will slowly flush the chamber clean but I find that wasteful and contaminates may - like water on the chamber walls - hang around at those rather high pressures and cause major issues when you strike a plasma.

Getting the pressure at the desired value to start a fusor (i.e. when you turn on the high voltage; of course, at a very low value) is not a difficult task nor should it be (if it is, you are going to have major issues trying to run a fusor.) The issue for most people and I find this more true if one doesn't get the chamber to the low end of 10^-5 torr is when the voltage is first raised and the gas first starts to conduct. That can cause issues big time - wild current swings and pressure changes. Also, after one strikes a plasma, holding the current and pressure steady can be a challenge until the chamber and operator are both seasoned ... .

As for loss of deuterium, that depends on your required flow rate and that is determined solely by the gate valve system (that separates the chamber from the high vac pump.) My gate valve is manual and allows very precise closure control so I can leak very little deuterium (via a very good leak valve) into my chamber to hold any exact pressure (usually about 5 or 6 microns; by the way, use microns not milli-torr when talking about those values here in the forum. Yes, both are correct but usage here is microns, not milli-torr.)

We do not use the term "deep" pumps. They are either high vacuum pumps or call it by its name: turbo or DP. You will confuse people if you don't use commonly accepted terms here.

One MUST run a turbo or DP because there is no easy way to hold a few microns total pressure when admitting D2 and having a plasma running that itself can cause large volumes of other gas contaminates to be released. As such, the high vac pump, controlled by a gate valve is generally used.

You really need to read both the FAQ's on these very subjects and posts by users who run fusors routinely like me, and a few others. I am posting these answers mainly for other general readers but you really need to read the FAQ's if you want to avoid such newbie questions that have been answered so many times in the past.

Next, no one here believes any vacuum gauge to read 1 micron (or 2 or 3 ...) because they aren't calibrated to those levels. A digital gauge shows that but one should hardly believe such a number. I have checked my micron gauge against other units and feel it is fairly accurate and my operating pressure (5/6 microns) agree's with other fusor people's known valves for the chamber size I use. Your electronic gauge I seriously doubt is really 1 micron when it says that.

Please, don't "shut up" since your questions (if not routinely answered in FAQ's) are generally welcome and no one starts out "all knowing." As such, your level of experience is more like 0.2 or so rather than 0. You at least know to be on this forum and hopefully, learning.

Do, however, build! I learned most of my knowledge by building, testing and operation. However, the people here prevented me from stupid mistakes. By the way, real fusors are extremely dangerous and will kill if one makes careless mistakes using these very high voltage/current power supplies. So, do be smart about it and ask for help when you have searched and can't find an answer.

Also, get a decent vacuum gauge that reads in the 1-20 micron range within +/-2 microns and goes up to at least 100 microns. The digital gauge you have is useless for any serious vacuum work. There are a lot of fairly inexpensive units available that are rather accurate and great for fusor work.

Aside and useful for most: after talking to a chemist, element names are not capitalized when used in sentences ... never gave it much thought but that is what I was told recently.

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Finn Hammer
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Re: Why operate Turbo - or Vapour Jet Pump during Fusion att

Post by Finn Hammer » Thu May 25, 2017 9:13 pm

Dennis,

Thanks for your comprehensive answer.
I have spent a decade over at 4hv.org, and this forum is indeed a very disciplined arena, but I will try my best to fit in here as well, both when it comes to capitalization, as well as trying to use the right terminology with regard to pressures and pumps -and everything else, i guess!.
I have 2 Baratrons already, and a Quattro 999 full range gauge on the way, so I hope to be able to report accurate pressures when time comes.
And by the way, I also think that the digital readout from the thermocouple is faulty, I get random readings from 1-15 microns and I cannot yet decide whether it is due to leaks, the pump or the gauge.
I have built a multitude of high voltage devices, including one of the largest Tesla Coils in the world, Van de Graaff generators, wound my own high voltage transformers and a variac (oh yes), so when it comes to the high voltage aspect of this hobby, I am not worried, and neither should anyone else be.
https://www.youtube.com/watch?v=l-FAdM_5E-s

And yes, build!.

It is the fact that vacuum is such a hands on art which is so attracting to me, and here is the status of my chamber: A 4 inch pipe leading right into the diff. pump with a 4" gate valve in between, there may also come a cold trap or at least a baffle.

Cheers, Finn Hammer
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Richard Hull
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Re: Why operate Turbo - or Vapour Jet Pump during Fusion att

Post by Richard Hull » Thu May 25, 2017 10:15 pm

I ran my first fusor with no deep, secondary pump attached and did about 100,000n/s back in 1998. It is easily done if you can actually get below 5 microns with your forepump. The draw back is you waste precious deuterium as you must flow deuterium into the system against the running forepump. Once you displace the remaining air with deuterium, you can do fusion.
This is not recommended, but does work provided your forepump is exquisite and your pressure gauge is known to be flawless.

Richard Hull
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Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

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Dennis P Brown
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Re: Why operate Turbo - or Vapour Jet Pump during Fusion att

Post by Dennis P Brown » Thu May 25, 2017 10:27 pm

Since we get all knowledge levels we cannot assume anyone is aware of various dangers; as such, I post accordingly until proven otherwise.

I am very interested in Van de Graaff's since I have built one that stands slightly over five feet tall and has a nearly three foot diameter dome. Would like to find out what you know about those - specifically designs of a high voltage spray (design, and details on the needle array) at the base of the belt; also, pick up needles for the top - location, number, design for both types and belt materials. For instance, return feed polarity spray in the top and best arrangement for wiring in the top relative to the pick up array and return wiring (if any.) Issues of improving performance (excluding a dry box) would be welcome. I have a rather large electro-static deuterium accelerator that I'd like to get working. Built the VdG and a large 60 kV voltage multiplier (for its belt spray - the VdG must have a positive polarity.)

As for punctuation and other issues, not important just passing on some useful usage. Terminology/units matter since people do need to follow what someone posts and be on the same page - jargon can confuse people.

Gate valves have marginal use with a fusor (see the use for a DP but they are generally all or nothing) - having a throttling valve between a DP/turbo and the chamber is far and away the most useful method to control a fusor for real work.

Issues relative to getting a high enough power/voltage transformer should be something you could help a lot of people here! So, feel free to post on that subject especially how you decide to address that issue!

Your chamber will need access ports so I guess that is still in the works for the upper stage and why it isn't there. Your start up stuff is impressive. I guess the deuterium issue will be another on your list besides a neutron detector system. Again, glad to have someone experience with high voltage transformers - without a doubt one of the biggest issues most initial would be fusor builders stumble with and give up (neutron detector appears to be the second one.)

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Steven Sesselmann
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Re: Why operate Turbo - or Vapour Jet Pump during Fusion att

Post by Steven Sesselmann » Fri May 26, 2017 1:02 am

Finn,

Pressure is a function of temperature, and as soon as the plasma ignites, your perfectly adjusted pressure gets all messed up. I am aware of one group here in Australia attempting to make a sealed fusor, and after years of failure they came asking me if I could help, and I declined. An electronic pressure controller is a practical solution that works.

Running the fusor with active pumping and deuterium flow removes contaminants from sputtering and will result in higher fusion numbers.

Steven
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Re: Why operate Turbo - or Vapour Jet Pump during Fusion att

Post by Jerry Biehler » Fri May 26, 2017 1:27 am

gate/pendulum valves work just fine for throttling, you just need to know the max and min conductance which ideally should be in the datasheet for the valve.

I do have a extra ISO100 butterfly with built in motor/pressure controller lying around. I should test it out.

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Re: Why operate Turbo - or Vapour Jet Pump during Fusion att

Post by David Kunkle » Fri May 26, 2017 1:40 am

Steven Sesselmann wrote:Pressure is a function of temperature, and as soon as the plasma ignites, your perfectly adjusted pressure gets all messed up. I am aware of one group here in Australia attempting to make a sealed fusor, and after years of failure they came asking me if I could help, and I declined. Steven
The old PV=nRT. Don't believe I ever heard it put that way on the forum before. I've only thought how more atoms = more pressure inside the fusor. Helps explain why things get so touchy?

I have to ask- what the heck is a sealed fusor? I thought everyone's fusor was sealed. ;)

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Re: Why operate Turbo - or Vapour Jet Pump during Fusion att

Post by Steven Sesselmann » Fri May 26, 2017 1:45 am

David Kunkle wrote:I have to ask- what the heck is a sealed fusor? I thought everyone's fusor was sealed. ;)
Sealed as in a light bulb, evacuate and pinch off. the pressure is then regulated with a getter pump (google getter pump)

Steven
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Re: Why operate Turbo - or Vapour Jet Pump during Fusion att

Post by David Kunkle » Fri May 26, 2017 2:57 am

Completely sealed off was my first thought. But then I thought that can't be it, sounds ridiculous and how could that work? Apparently it doesn't.

What was the purpose of that effort?

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