[asymmetric_implosion]

Generally, vacuum systems are not designed to be pressurized. Also, vacuum is to create a clean environment. Adding a fluid means you are adding something to the system. You can take it out and wipe all the surfaces down but you still need to pump out any residue. Also, the leak rate is likely so small that it would be hard to see with a liquid. How would you localize it without taking the system back apart.

Helium leak detection by puffing helium outside the chamber under vacuum allows you to localize the leak without contaminating the chamber. It’s a tried and true technique used across the vacuum community.

[asymmetric_implosion]

Using cold gas makes the chamber cold so it will be hard to see the gas leak which tends to be very small quantities.

When vendors qualify components, it is common to place a detector on the vacuum side of the system and puff small amounts of helium on the air side. Those of us that are users tend to use the same protocol because you can localize (~4″) the leak as helium moves quickly, easily penetrates the leaks and does not occur in natural air at a reasonable quantity. It is a bit of an art to get started but the key is very low flow rates or small puffs. Don’t start spraying helium everywhere. If you take your time and do it well, a vacuum system like FoFu-1 can be leak checked in ~1 hr. With some practice you can localize a leak to less than 1″. Normally, you can find the leaking spot if you localize the leak to 4″. The common problems are damage to the sealing surface, debris on an o-ring or damage to a knife edge. Debris can be as small as a human hair or as large as finger nail clipping (yeah, I saw it once). Good vacuum practice will eliminate the debris. Damage can be accidental or due to manufacturing problems. Usually you will feel them before you see them. I hope someone has fingernails. Run them over sealing surfaces. You can’t hurt the metal but you will generally feel some damage before you see it.

A trick we use is to pump the system in pieces by using blank flanges to test the chamber separately from the electrode assembly region. I can’t speak for anyone else but I find solving a few little problems better than solving one huge problem.

[asymmetric_implosion]

Lerner: You don’t need 10 people to design a switch that already exists. (http://www.amazing1.com/sparkgap.htm) Scroll to the bottom. Look at Rail gap. Typically a 16 week lead. Switch N2 to Ar-O2 (10%) as listed by Maxwell pulse power and you get a 75 kV, 1 MA switch. This is not the first time I’ve said it and I’m sure John Thompson has mentioned it to you more than once as John introduced me to the switches. You need a ~100 kV trigger to take them down with ~10ns jitter. North Star High Voltage can build it if they have the time. Richard Adler is one of the best if not the best person for the job. I have used North Star triggers at ~40 kV with a jitter of 20-30 ns for years and we have never had a problem that wasn’t our own doing. We run up to ~100 kA per switch for over 10,000 shots without opening the switches for cleaning. I’ve run over 1000 shots without changing the working gas. You should be able to get at least 250 shots without any maintenance. Maintenance is polishing the rails and wiping away any built up deposit. Takes about 1-2 hr per switch at most. (Brian L. Bures, Mahadevan Krishnan and Robert E. Madden “Relationship between Neutron Yield and Macro-scale Pinch Dynamics of a 1.4 kJ Plasma Focus over Hundreds of Pulses” IEEE Trans. Plasma Sci. Vol. 39 No 12 pp 3351-3357 (2011) doi: 10.1109/TPS.2011.2170588)

If you don’t like the Rail gap, look for the MMCS switch the French use on the Sphinx machine (50-100 kV, 1 MA). Sandia uses laser triggered spark gaps with 1 ns jitter so there is always that option if you want to spend the money for four of those.

You speak of the black art of pulse power and I agree to a point. I am no pulse power expert, more of a dabbler and user, and there are three (really two as Sandia’s switch costs way too much) alternatives that will work without designing a custom switch and are much faster than 20 mon.

Vacuum leaks: Derek is on the right track. Buy an RGA and find the leak with helium. RGA is 4-6 weeks away from SRS if they don’t have one in stock.

Joe: My frustration is that it looks like we will see a 1000 ways to make a PF pulse power system wrong before it is made reliable. There are at least 3 if not more 300-500 kA machines in this country put together on a shoe string budget. One might say that they are not as big but they contain all the key parts to make a 3 MA machine. I know Eric has hardly talked to our group at AASC. I get it; we are a company. I know KSU folks helped build the machine but I don’t know more than that. The UNLV folks used a design by Bruce Freeman, formally of Texas A&M, now with Raytheon (probably designing the new switches). Bruce operated a 2 MA PF at A&M for years and I’m sure he didn’t have these problems. NsTec has a 3 MA PF and I’m pretty sure they don’t have these problems. There are always the folks at NTU/Singapore, DENA or Pavel Kubes. There are a number of folks in the US and around the world that have solved these problems and moved on to study the physics. The PF community is very open to new members and very willing to help whether it is physics, pulse power or components. Great in roads have been made between LPP and physicists around the world (Iran, UK) but why are they not seeking help from an expert in pulse power that would visit and clean up the pulse power problems? This is my concern for LPP in more detail.

I really appreciate the small group as the AASC group is only 4-5 and I know the struggles. The key for us was to bring in folks that were experts as consultants at the right time. We let them solve the problem and tell us to become users and more slowly advanced users. We will never replace these experts in skill but we learned to stand on our own. When we trip, we run back to them asking for more help. We had a good team in place and we still had to talk to experts to keep moving forward. I can tell you that it never took the experts more than a few weeks to identify a problem and propose a fix whether it was a switch, transmission line or machining problem. The parts can take a while but more than a few months seems unreasonable.

[asymmetric_implosion]

I don’t recall saying that fusion is impossible. I did say that the data seems to support that fusion has more downsides than up. I maintain my position of I don’t know if fusion will work as an energy source. NIF is unlikely to meet Q>1. The experiment at Sandia MAGLIF has some interesting potential. I’m rooting for those guys as I am rooting for the PF. Both Z-pinch like technologies will have substantial hurdles to overcome after Q>1. All fusion approaches will struggle with materials problems. Pulse power approaches are going to struggle with switches. Neither problem is trivial and fundamental physics might be a damper.

When I said outsider, I meant someone unfamiliar with the PF or the program. I don’t consider the LPP effort a failure, but I think it is in jeopardy in some respects. I am baffled by the mishaps that keep cropping up. Lerner has confirmed my concern that the right people aren’t in place to use the machine. It is a common problem in small groups; sometimes you need a master and you get are jacks of all trades. It is not an easy problem to contend with. A questionable team can do more damage than disagreeable physics to the reputation of a field.

I think the lack of the right people is the heart of the fusion problem across many platforms. The big projects lack imaginative and effective leaders that can use their technical expertise and people skills to find champions in the funding agency that will support them. They get bogged down in gov’t policy rather than using gov’t policy as a tool to further their programs. Effective gov’t contractors do this masterfully. The approach might have been incorrect but good science does not need a practical end in the near term. Small groups tend to be hampered by a lack of the right people and resources. Great small businesses get creative to bring in people on the cheap. I wish I had a suggestion.

–From the eyes of Charles Seife and folks like him—

If one wants to compare the LPP effort with the tokamek there are some sobering comparisons. Let’s take the tokamek achievements: Q>0.5, T and confinement realized but not density. Realizing two of three of the fusion gain triad of temperature, density and confinement time is failure as ITER and other tokameks are failures or more politely, have not met expectations. Eric states LPP has reached two of the three necessary parameters. The inability to achieve the third is failure. The PF has been touted as a low cost alternative to ITER and NIF but the frequent comment to things falling behind schedule is more money is needed. Sounds like tokameks and NIF to me… All we need is 1 kJ of laser energy and now we need 2 MJ. All we need is 3 MA and tomorrow we need 6 -10 MA which means a new machine and a bigger budget.

—Back to reality—-

There is the potential for dangerous parallels. As Eric said, none of the problems encountered so far are difficult to fix. As someone that has operated PF devices, I am baffled by the length of the delays; a year for arcing and months for a vacuum leak. Twenty months to design a switch???? My on-going concern with LPP is not the physics; it is the ability to demonstrate the physics due to machine problems that never seem to end.

[asymmetric_implosion]

Common practice is to fix the leak rather than try some kind of gas blanket. The gas blanket tends to be more work than fixing the vacuum leak. Even scratched surfaces can be polished or face cut with relative ease. The most serious problem is when a knife edge is damage. Typically a new flange must be welded on or the part replaced.

[asymmetric_implosion]

From my experience, the primary problem with metal seals is thermal cycling rather than temperature alone. If you think about thermal expansion, the seal gets better with temperature as everything expands. Thermal cycling can lead to small leaks but they are typically on the 1E-7 or 1E-8 Torr scale.

Depending upon the seal locations i.e. system design, the seals can be protected from shock and plasma. We seldom had problems with metal seals at even modest repetition rate (~5 Hz) that led to ~100 C chamber temps. At higher temp (>400C) you need to look for special seals as the common sealing materials (copper, organics) no longer work. I think folks tend to favor soft iron but that could introduce new problems as the material is magnetic. Organic seals tend to be more resistant to thermal cycling (<100C) and movement.

If you want to put layers of seals you have another problem, trapped volume. If you use multiple levels of seals typically two layers with organic seals you can achieve a higher base vacuum when you pump the volume between the seals to ~1E-3 Torr. The leak rate across the inner seal is reduced by ~1E5 so you can achieve better base vacuum such as ~1E-9 Torr instead of ~1E-7 Torr. For a PF device, you can get away with 1E-6 to 1E-7 Torr base vacuum without any problems. Some folks operate at base pressures as high as 1E-4 Torr. I don’t know of anyone that would use metal seals in multiple layers. Metal seals are good to 1E-11 Torr at the worst. The typical problem with vacuum is virtual leaks. Imperfections in the surface, physical absorption of things like water and gas out of materials are generally the limiting factor in base vacuum. For high vacuum systems, it is common to bake the vacuum chamber at ~100C to motivate the water to leave the surface of the vacuum chamber. I’ve never done it for a PF but I have done it for high vacuum deposition of metals. There are other tricks with chamber material choices like Ti instead of SS304 or coating and SS304 chamber with Ti or Nb.

[asymmetric_implosion]

Most of the article is pretty accurate talking about NIF and ITER. Fusion has a horrible reputation of meeting expectations and proposed deadlines even after they have been extended more than once. Even DOE is losing faith as they are not fighting to defend budgets in the face of R&D cuts. I think the public, politicians and fellow scientists are tired of empty promises of limitless energy while pouring money down a sink hole.

I can understand the dig at LPP and Tri-alpha being not nice but look at it from the outside. Have dates slipped on FoFu-1? Are unexpected problems slowing progress that should not be a problem (switches, arcing, etc)? It seems to be walking a similar path as the other projects. One might argue that all projects slip on the timeline and unexpected things happen but I would argue that at the end of the day people remember success over slipped dates. When failure is all you have to offer, slipping dates become a real problem. I sympathize with the problems of the plasma focus. I’ve had my share and I was trying to make a neutron source which is easy by comparison.

I agree that fusion is hard but I don’t know if it is possible using known and possible approaches (leave the gravity generator out of it). If possible, is it economically viable? Don’t know. I’ve done some back of the envelope math a couple times that shows it is difficult without big leaps in key components that may or may not be possible.

One can argue that outside forces like big oil are conspiring against fusion. Why would they? Fusion enjoyed a healthy budget and some of the best minds trying a myriad of approaches for over fifty years (yes, the DPF is in that list as Livermore tried to ride that pony back in the 70’s) without success. It comes down to how to measure potential. The upside of fusion his huge, but the downsides are many. Everyone needs to balance the potential for themselves. Charles Seife see more downsides than up. Sadly, the experimental data seems to support his position.

[asymmetric_implosion]

I’ve worked with SRS RGA’s in the past. You can do gas sampling or leak checking down to the 1E-9 Torr range with ease. The key problems we’ve encountered (not on PF devices) are sensitivity to plasma environment as one might expect, electronics problems with pulse plasmas (200A, 100 ms), disruption of the comm link to the computer and the variable maintenance cycle (depends on what deposits where and when).

I think most of the problems are universal and could be dealt with using proper electronic and vacuum isolation. I don’t know if you could run the RGA with the PF on-line but some sort of He monitoring on-line will probably be required down the road so might as well find the problems now. SRS software is straightforward to use. I learned it in about 20 minutes so I could do leak checking and base vacuum analysis. SRS is pretty good about maintenance but it does cost. I recommend their 100 AMU model for FoFu-1. Most of the key gases are less than 50 AMU.

[asymmetric_implosion]

The report is interesting. Glad to see the arcing problem is under control for now.

Does anyone else see the “apple core”? It looks more like wishful thinking than proof. With this image I can’t tell the depth of the filament so it might be light in a more complex pattern that flattens to look like the filament. The image quality is not very good. I’ve seen ~500ps images from lower current PF devices with superior clarity. It’s nice to have the fast framing camera but the set up appears to need some improvement to get better image resolution. Good luck to Fred in pushing beyond 60 um per pixel.

[asymmetric_implosion]

Zapkitty: Don’t you need enough length for a compressor section and heat transfer length to extract the heat from the helium to a less expensive medium? I can’t speak for LPP but I would want enough helium primary loop length to get outside the radiation shield area so the secondary loop could use coolants with better thermal properties that might normally suffer activation.

We cool our anode with water presently. The volume of water to cool the anode is small but the water lines and cooling region are more than 20X what is used in the anode. Is Helium going to cool the vacuum chamber and cathode as well? That is more volume to be considered. Your math is on the extremely low end. If converted to standard cubic meter the total FF demand to get started by your math is 37% of the 1996 demand I listed earlier. Granted that today’s demand is larger, bringing up FF systems could be a huge dent in our helium stores. You also neglect that even a recycled system has losses. Perhaps the FF systems can produce enough to helium to keep up with loss but that adds another complication as you need to either store the helium or process it on site.

Mechanik: I was thinking of indium. My bad. 🙁

[asymmetric_implosion]

I forgot about Li producing tritium. Scrap lithium as it would require significantly more regulation due to flowing radioactive coolant. Can’t have a PF power source on every corner if it has radioactive coolant. The site license would be impossible to get in a town.

Tin activates as it is used in activation counters. Argon might be an option as it is pretty immune to neutrons but probably has miserable thermal properties otherwise people would use it as a coolant in next gen reactors. Neon is pretty expensive as are other inert gases.

[asymmetric_implosion]

The PF has a bit of an advantage as it doesn’t require liquid helium. One could recycle the helium in a loop because the thermodynamic efficiency is reasonable as you are trying to cool the helium using ambient air. Taking cold helium and turning it into liquid is a very inefficient process because everything his hotter than the helium. Thermodynamics is not in your favor. I don’t have an idea for the loss rate of helium for a PF. I know some next gen fission technology is built around helium coolants so I have to believe the helium release is monitored because Xe and other gases with radioactive isotopes would mix with a gaseous coolant and get released if coolant is released.

I personally favor liquid metal coolants as they tend to remove heat well at modest flow rates and low pressure. If the atomic number of the coolant has the be less than Be, Li is the only option but you have to worry about the neutron activation and all that goes with it. I remain unconvinced that Be is a viable anode material so other options with sodium exist in my mind. Sodium is a proven coolant material that operates at atmospheric pressure. Water is always an option but the flow rates and pressure (which is also true of He) could be an interesting engineering challenge.

[asymmetric_implosion]

vansig wrote: presently as far as i know, Helium is seldom recycled.

it’s just too precious to let go.

can we scoop it from earth’s upper atmosphere?

Helium recycling is really picking up at the national labs due to cost and Department of Energy rules. The large helium using labs like the national high field lab at Florida State was instituting helium recycling for their large superconducting magnet systems. The commercial magnet systems are probably going to follow suit in the next few years as helium costs or lack of supply might damage their business. Higher temp superconducting magnets are also in the works for MRI systems. We could drop a great deal of demand (~20%) if SC magnets worked at 10 K instead of 4 K.

Given the concerns about helium, how can one build a power grid based upon helium cooled reactors (fusion or fission)? Seems a bit problematic.

[asymmetric_implosion]

What are you trying to image? Pinches are bright enough emitters that optical back-lighting is not necessary. Most back-lighting experiments must be in the x-ray regime (1-10 keV) so you can see through the pinch region and get the mass density.

Interferometry in the pinch region requires intense UV lasers at the low energy limit like more like soft x-ray lasers at 50-100 eV. As I recall, LPP is not a fan of interferometry techniques. Even if the wavelengths are correct, the time is so short/power so high you’d likely damage the detector. Most cutting edge plasma diagnostics are running with ~10 ps lasers which is more than enough. Most of us low-rent folks are using ~1 ns lasers for diagnostics before the pinch implodes to measure the plasma density and the local magnetic field.

Associated components like non-linear optics are interesting for pinch research but they need to be evolved substantially to ensure the measured signals are real representation of the data before the non-linear optics are used.

[asymmetric_implosion]

Electron temperature is typically measured using the relative line ratio in emission spectrum of characteristic lines or by measuring the bremsstrahlung continuum shape. The ion temperature is measured from the broadening of spectral lines, typically in the x-ray. Sandia National Lab had a huge affair a few years back about 200 keV ion temperatures in iron plasmas. There is a PRL published by Malcolm Haines of Imperial College about it. The debate about the source rages to this day.

Electron heating and cooling: To get an ion hot, you must undergo many, many collisions with electrons that have more energy but each transfer is small because of momentum conservation so only a fraction of the electron population can supply energy to the ion. In the case of cooling, the electrons collide with the ion and take a fraction of the energy. The electrons radiate the energy away as brems when they slow. Those greedy electrons go back for more. While many electrons take away energy only some can provide it. The electrons also transport energy (heat) very efficiently as they are highly mobile. A typical pinch has a temperature profile with the temperature peaked near the center. If electrons are able to move, they transport that heat out of the core plasma to the edge.

The general rule of plasma is the electrons are the dominate particles for energy transport, radiation and current carrying. The ions are responsible for momentum carrying and mass. These rules are pretty universal. Pinches are a bit funny as the ions have such large kinetic energies before the pinch implodes which allows them to have large ion temperatures when the pinch stagnates on axis. Other plasma systems cannot give the ion much energy as they are slow to respond to electric fields unlike electrons and they are so heavy. The PF allows the ions to move quickly due overwhelming magnetic pressure behind the plasma sheath that pushes both the ions and electrons at the same speed. If an ion moves at the same speed as an electrons but has 2000X the mass, it has 2000X the energy. Easy to see how the ion can have higher temperatures than the electrons in a pinch. The thermalization is complex so don’t read too much in the 2000X number upfront. Those pursuing fusion wish that cold electrons could coexist with hot ions.

Can’t speak to the onion or LPP’s plans for it.

[Author]

Posts