Is baking worth it?

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Michael Bretti
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Re: Is baking worth it?

Post by Michael Bretti » Sun Mar 18, 2018 1:47 am

John Futter,

Thank you with providing the above example, which is a good initial comparison of the practical mechanisms use-able during vacuum conditioning. For systems like a fusor, glow discharge cleaning can certainly provide the fastest and easiest way of conditioning, and for these purposes is probably the most practical, also given the vacuum levels encountered. For other systems, such as e-beam or ion based beam-on-target systems, proper glow discharge cleaning may not be as viable as bake-out. Unfortunately, glow discharge cleaning is not always available based on system topology, though as you illustrated, can be very practical depending on the system.

One thing however, which I detail more below, is that thermal desorbtion and ionization desorbtion work from two very different mechanisms, so they cannot be directly compared in terms of effectiveness based purely on ionization energy. Each has strengths and weaknesses associated with them, and it depends on a wide variety of conditions.

Several things should be considered beyond just immediate cleaning speed only due to ionization energy. Glow discharge cleaning is based heavily on system topology and surface exposure to the plasma, and heated bakeout and UV can be used to cover more internal surface area in chambers with complex geometries with many extensions that are not directly bombarded with ions. For ionization based disassociation, glow discharge is superior to UV for direct exposures. Glow discharge cleaning (as well as UV) however is also generally only effective for water vapor adsorbed near the surface or immediate monolayers, where baking liberates gases adsorbed deeper in the metal itself as well as surface adsorbed monolayers (hence why it is primarily used for the bulk of preparation of systems in UHV where hydrogen is deeply adsorbed in the metal.) Especially for o-ring based systems, heating will much more effectively condition and desorb gases buried in the o-ring, where the glow discharge will only affect the immediate exposed surface, and could be more damaging to the o-rings in the long run.

For baking, effectiveness is measured not by dissociation by ionization energy, but with activation energy in kcal/mol - they work by two different molecular mechanisms of desorption, so speeds are not directly comparable by ionization energy alone between the two methods.

Therefore, for deeper level vacuums, baking becomes the more practical solution since it desorbs deeper gases that glow discharge cleaning could not actually reach. Especially for o-ring systems, glow discharge only minimally helps for total water vapor loads for long-time scale conditioned systems compared to thermal conditioning since the bulk of water vapor is deep within the material. Glow discharge is immediately faster for water vapor surface monolayers, but deeper gases are not affected, so pumping times will be faster in the long run for deeper vacuum levels for thermal conditioning compared with purely glow-discharge. However, both can be used together to increase the overall speed and effectiveness as opposed to just one method. For fusors though, such extreme vacuum levels or worry about deeply adsorbed gas loads are not much of a concern, especially for the short runs times encountered in general, so for practical purposes for most fusor users glow discharge and/or some light baking is plenty to condition the system.

Again, it all goes back to the system configuration and what exactly is being run with it. It may be necessary to desorb water first with baking prior to further glow discharge cleaning to remove a majority of the gas load and contaminants before running the plasma. Often times in high energy physics applications, a system is baked first then followed with a glow-discharge cleaning. I found this interesting paper on an example that employs both methods, could be an interesting read:


Michael Bretti
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Re: Is baking worth it?

Post by Michael Bretti » Tue Mar 20, 2018 3:28 am

I was going back through some of the vacuum texts I have since I have been more interested in the mechanisms of adsorbtion, desorbtion, and the various ways of affecting this in the system (thermal, ion bombardment, photon emission, etc.) Some interesting things to take note which would probably have direct impact on the performance of the fusor, particularly relating to glow discharge cleaning. I will say that the derivations and associated experimental data on these various effects is very fascinating and strongly relevant to fusor operation and potential optimization.

It turns out that higher energy is not actually better for glow discharge cleaning depending on a variety of factors. Since glow discharge cleaning leads to ion bombardment of the surface, several things happen: ion dissociation of molecules such as water or volatiles on the surface, sputtering, and trapping. The effects of cleaning immediate monolayers are apparent. At higher energies however, which has been mentioned previously, sputtering occurs. This mechanism does help the release of gas, but is not a free ride. Gas is liberated from sputtering either by direct sputtering away of the surface material which liberates trapped gas, or through gas sputtering, where the trapped gas is replaced by the impinging ions. This will cause saturation in the metal sites from ion bombardment, as higher energies cause more ion penetration and thus higher ion concentrations in the metal. Sticking coefficients and trapping probabilities greatly increase with increasing energy. Even at voltages of only 1kv, this will be enough to start sputtering and saturating the surface with impinging ions much more so than at hundreds of volts potential. Now this can be good or bad for a fusor depending on the gas. If glow discharge cleaning is carried out at higher potentials with deuterium, this has the benefit of releasing trapped residual atmospheric gases in the metal while also saturating the metal with deuterium. This will be released spontaneously over time during further pumping, heating, and bombardment of the surface. This could help increase neutron yields of the system. However, if glow discharge cleaning is carried out at higher potentials with the residual air, or other gases such as nitrogen, argon, etc, this will have a detrimental effect in that while you may be removing water vapor from the system, you are effectively replacing it and saturating the metal with the gas that glow discharge cleaning is being run with. This would have detrimental effects on neutron performance.

For lower energies, trapping probabilities, or sticking coefficients, greatly diminish. Therefore, glow discharge cleaning can be carried out more safely with minimal trapping at lower energies, in the hundreds of volts range, maybe 100-300V. This could help reduce water vapor and other gas monolayers without sputtering damage or saturation of the metal with unwanted gases if not using deuterium for glow-discharge cleaning.

In essence, if you are running glow discharge cleaning at several thousands or tens of thousands of volts, near normal fusor operating voltages, this should be done with the system fully purged and filled with deuterium. As Andrew Seltzman mentioned, after baking he runs plasma cleaning with deuterium, which noticeably improves yields. This can be directly attributed to a.) effectively removing moisture and other gases with baking without the introduction of further gases trapped in the system, followed by b.) further replacing residual trapped gases with deuterium and helping to further saturate the surfaces with deuterium prior to a run. If you are using air or other gases to glow discharge clean prior to running the fusor, this should be done at greatly reduced voltages (hundreds of volts, not thousands) to minimize penetration and saturation of unwanted residual gases. These gases that become trapped from the ion bombardment will be released as the chamber is heated up through normal operation, as well as bombardment from ions during full operation. If you can afford to do it, glow discharge cleaning with deuterium at higher potentials after initial baking would be the best way to prep a fusor system for improved neutron yields. However, too high voltages will also see a possible negative effect as well, since at certain levels sputtering yields will exceed trapping, especially as the surface becomes saturated, further trapping of the gas will be decreased over time. There should be an optimal balance. There is also a maximum theoretical saturation per unit surface area, which can be calculated. If you want an extra boost to saturate the surfaces, further cooling the chamber after it is baked and filled with deuterium while glow discharging will greatly increase sticking potentials, and allow for much higher quantities of gas to be adsorbed.

This illustrates another potential benefit of baking vs. glow discharge cleaning, depending on your system and how you are operating. Baking will liberate trapped gas molecules without introducing trapped gases into the system. This ultimately results in an effectively much "cleaner" system with total adsorbed gases in the system reduced. For fusors, actually "contaminating" the system by loading and saturating it with deuterium would actually be beneficial. For other vacuum work, where the goal is to remove all residual gases, baking is the preferable solution, utilizing very low potential plasma cleaning if further needed.

I would say that it should be very possible to take advantage of the mechanisms of adsorbtion and desorbtion, and utilizing proper baking and glow discharge cleaning to prep the chamber you could achieve greater yields per run. Proper extended baking and pumping to as high vacuum level as possible, followed by glow discharge cleaning with only an environment of deuterium, at the appropriate voltage levels (not to high or too low, balance between sputtering and saturation), and further cooling of the chamber during glow-discharge cleaning with deuterium would be the most sound method of preparation for the fusor for increased yields. It may take more time and work, at the cost of a bit more deuterium gas used, but in theory run performance should be improved.

Jerry Biehler
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Re: Is baking worth it?

Post by Jerry Biehler » Wed Mar 21, 2018 7:54 am

I threw a 250w ushio mercury uv lamp in my chamber once to see what happened. Lit it up with my tig welder, worked great.

In the pic below you can see the laptop connected to the RGA. The huge spike is mass 18 for water when the lamp fired up and started running. I didn't leave it running long since those lamps were never intended to run without air cooling, worse I have it sitting in a 10^-5 vacuum.

ImageUshio Mercury Lamp in vacuum chamber by Jerry Biehler, on Flickr

ImageIMG_1237 by Jerry Biehler, on Flickr

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Richard Hull
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Re: Is baking worth it?

Post by Richard Hull » Wed Mar 21, 2018 3:13 pm

Nice photos! Yeah, you sure don't want to over heat the lamp. I assume it is a quartz envelope.
How did it do on getting the water vapor out?

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
Retired now...Doing only what I want and not what I should...every day is a saturday.

Michael Bretti
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Re: Is baking worth it?

Post by Michael Bretti » Wed Mar 21, 2018 3:50 pm

Jerry Biehler, awesome set up in testing UV desorbtion, it could certainly provide further data and info on this matter.

Below is an article that I have referenced numbers with in my above posts regarding the effects of UV conditioning of a system and its effectiveness. It is an excellent article and very much worth a read for anyone interested in vacuum chamber conditioning:

http://www.normandale.edu/departments/s ... ydown-zone

Also from the same source is an article discussing the differences between baking vs. UV desorption:

http://www.normandale.edu/departments/s ... m-systems-

Finally, again from the same source, an article on the effects of humidity on a vacuum system:

http://www.normandale.edu/departments/s ... um-systems

I would really suggest anyone to take the time and read through all of the articles available on the website. I have gone through every article, and it has provided an excellent resource on high vacuum topics and information without getting highly technical into the derivations and hard physics. I think one of the key takeaways from the information regarding desorption with UV energy is that by utilizing the proper wavelength energy (must by short-wave UV-C to be effective on water), the pumpdown times to reach a specific vacuum level can be decreased by a factor of two or even three as compared with pumping the system down without UV condition, even with very low UV power levels and indirect exposure. UV conditioning also has a direct benefit for systems that are limited in baking temperature, like systems that contain o-ring seals, as many fusors do, which would normally take extended periods of time to bake and condition otherwise, and can be used to help radically reduce these preparation times.

I think another point that should be considered is that beyond just being able to obtain higher ultimate vacuum levels, proper conditioning will benefit run consistency and yields in a fusor in general. It seems as though there is a common notion that because a fusor operates at higher pressures than many other vacuum devices, and can get up and running easily and producing neutrons in even a terribly contaminated, unbaked, and leaky system (by high vacuum standards), that further high vacuum preparation will not produce any noticeable improvements on yields. However, something that is counter-intuitive is just how much water vapor is present in a system, particularly adsorbed in the metal of a chamber, and how much effort is really required to drive all of this excess water vapor out. Even pumping down to high vacuum levels of 10^-6 torr is not enough, and requires further desorption either by thermal, ionization, or photon interaction methods, often times for extended periods of time. For most fusioneers just looking to do fusion and produce neutrons, this level of preparation is not needed at all to see results, but if one wants to squeeze out every possible neutron in a system, then proper preparation of the chamber can only help. If these processes are critical in just about all other high vacuum physics applications, from accelerators, to sputtering and surface coating, to other fusion devices, why would a fusor be any more immune to this decrease in efficiency due to things like water vapor contamination and gas adsorbtion? Any gas other than the process gas will be a detriment to the end process goal, regardless of the system. Other weird molecular effects can occur as well that are not immediately apparent or noticeable (such as the fact that CO will slowly replace hydrogen in tungsten at room temperature and atmospheric pressure.)

A perfect example of these effects for a similarly reproducible neutron producing high pressure vacuum fusion device is the dense plasma focus. DPFs operate at pressure orders of magnitude higher than the fusor, at levels in the several Torr range, operated on deuterium as well for neutron production. Yet in papers and reports working on the optimization of neutron yields and consistency for these types of systems, there is still large emphasis on proper high vacuum conditioning and preparation prior to experimental runs for improving yields. Baking and proper preparation produces noticeable and measurable yield improvements and shot consistency from these systems, and is especially critical for devices operating at lower pulse energy inputs of hundreds of Joules and lower, rather than the typical higher power systems operating at tens of kJ. A fusor follows the same vacuum physics principles as any other plasma physics vacuum device in terms of desorbtion, adsorption, and gas contamination, and proper preparation will see benefits as in all of these other types of systems.

I am currently working on running ion interaction simulations in my free time to explore the effects further of desorbtion, sputtering, ion implantation, and captured ion concentrations per unit area specifically related to glow discharge cleaning for fusor preparation and operation. This will cover the comparison of the above effects between operating glow discharge with hydrogen vs. nitrogen (to simplify simulation input as direct analogs for deuterium and air) at varying ion potentials impinging on stainless steel. Already from playing around with preliminary simulations, the results do point to increased benefits for fusor-specific conditioning using glow discharge with deuterium (after initial baking) at higher potentials, primarily from the standpoint of surface loading of deuterium with reduced sputtering. Replacement of gases in the system, especially with loading the chamber surfaces to saturation with deuterium, will further increase available sites for neutron production from impinging ions on the surfaces during actual production runs, and reduce overall gas loads from other unwanted gases, assuming the chamber is very leak-tight and well pumped to begin with. I will post results of the simulations as I progress.

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