Cold-Cathod Field-Emission Vacuum Tube - Page 1
Forum section: electronics / projects

author: d-smes
author_rank: Regular Contributor
timestamp: February 24, 2016, 08:59:29 pm
content: A rather miraculous tube technology is described in a paper athttp://www.cce.umn.edu/documents/CPE-Conferences/MIPSYCON-Papers/2012/PowerElectronTubesAsAnEnablingTechnologyForTheGrid.pdfQuoteQuoteImpressive ratings! In terms of on-state voltage drop, I don't quite understand this one except that it's quite low.QuoteSwitching times are sub nanosecond enabling DC to GHz+ operating frequencies.QuoteThe tubes can also be operated Class A, although I don't know how one would get rid of the heat. Essentially, what's described is an ideal switch! The paper then gives examples of how specific products can be used in Fault-Current Limiters, Geomagnetically Induced Current Protection, HVDC to 3-Phase AC Inverters, Static Compensators, Circuit Breakers, and EMP/IEMI Protection.Sounding too good to be true, I did some reading of the patents athttp://appft.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&p=1&u=%2Fnetahtml%2FPTO%2Fsearch-bool.html&r=0&f=S&l=50&TERM1=Birnbach%2C+Curtis&FIELD1=&co1=AND&TERM2=&FIELD2=&d=PG01It uses graphite foam or graphite felt as the cathode field emitter but beyond that, the patents don't really describe how the tubes work. I can find technical papers describing field emission with Spindt cathodes, but nothing on high power tubes such as this.Is this technology for real? Does anyone besides this company make such tubes? I welcome your thoughts.

author: d-smes
author_rank: Regular Contributor
timestamp: March 01, 2016, 08:20:13 pm
content: I found a presentation that has pictures of the devices:http://mydocs.epri.com/docs/publicmeetingmaterials/1108/6XNSUMJE9MT/John_Kappenman_-_Electron_Tubes_as_an_Enabling_Technology_for_the_Grid.pdfThey show triodes and tetrodes and say they operate at high vacuum (i.e. these aren't gas-filled tubes). Does anyone know the physics of how these work?

author: HAL-42b
author_rank: Frequent Contributor
timestamp: March 01, 2016, 08:55:00 pm
content: Most exotic tube technologies like this are highly classified.Recently I was looking at the Wasenaar Agreement list and it is chock full of ultra high frequency high power tubes.Seems like they are mainly used in radars and satellite transceivers and so on. 100+ GHz frequencies are mentioned in a few places. Only general itemized lists though, nothing specific.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 01, 2016, 11:58:59 pm
content: I don't see anything consistent or coherent...- I've never seen substantiated claims of long life, field emission cathodes. (If they've solved it to the tune of "25 years life", that should be big news. And important patents. I don't see anything about this, even searching with their keywords.)- What the hell is a "vacuum plasma"? It's one or the other... (I think such a term is used informally to refer to a plasma at low pressure (a vacuum, as in, anything less than atmospheric), but it's still an ugly and ambiguous term.)- As far as I know, most or all of their numbers are mutually exclusive. I don't see how it could be possible to switch in "fractional nanoseconds" (which implies the device itself is less than ~30cm across), at high voltages (= large clearances required); or to utilize plasma (ions neutralize the electron flow, allowing higher current density -- the similarity to semiconductors they mention) with vanishingly small voltage drops ("10eV") also at high voltages (long plasma column = lots of steps of 10eV to pass through).- Nevermind that, if they are using plasma (from some kind of gas fill), then ion bombardment of the cathode will rapidly wear it. (This might be acceptable anyway, but that requires discovering a field-emission cathode surface that isn't damaged by erosion.)- If they are using plasma, at a low voltage drop (at high current densities), there's no way they're achieving smooth and continuous, non-latching amplification at the same time.And as an engineer, I also have to ding them for:- They don't have any specs listed for anything, not even as an example, or proof of existence of a test device.- They don't have any sales listings. Must be vaporware...I'd love to get a small demonstration model as a sample (assuming such even exists), but I suspect it's not worth even trying to contact them.Tim

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: March 02, 2016, 12:10:22 am
content: A megavolt in a vacuum tube - could be some issues with X-rays perhaps?

author: calexanian
author_rank: Super Contributor
timestamp: March 02, 2016, 03:10:44 am
content: I did not see anything about cold cathode there. I believe this is one of the many tube factory want to be nuts out there I have to fend off trying to raise some hell. There are some real DARPA bucks out there for people investigating super high power tubes for very very, very high frequencies. There are some specialty doped emitter materials out there that can emit many tens of amps per square centimeter under the right conditions and they could be dealing with emission surfaces of thousands of centimeters. There have been some mega massive tubes that have been made that would fall into this category but like all fringe bleeding edge designs they cost the GDP of a small nation to make. As mentioned the super high voltages mentioned are of a real concern for X rays. I know in the past giant thyratrons have been used in grid power distribution, but that is a gas/plasma discharge type of conduction and does not produce as much harmful stuff due to the low electron velocities and low impact energy.I smell a little B.S. on this one, especially when i saw the patent pending on everything. but there is an application for high power tubes out there.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: March 02, 2016, 03:23:38 am
content: Bah, bring back Hewittic rectifers. Those things look spooky!The self-renewing cathode! "This side UP" though.

author: calexanian
author_rank: Super Contributor
timestamp: March 02, 2016, 06:09:44 am
content: Down at M.U. I found some notes from the owners father who started the company discussing general design points of tubes and the one item that stood out as what the heck is this was his statement that metallic barium chunks floating in liquid mercury is the best emitter. Quite how that would work is beyond me. I am assuming it works as some kind of ignitron. Sounds quite scary to try! Perhaps he intended it to be the recipe for a mercury arc rectifier like pictured above.

author: HAL-42b
author_rank: Frequent Contributor
timestamp: March 02, 2016, 08:12:55 am
content: You learn something new every day. I wonder why Barium.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 02, 2016, 09:22:45 am
content: Barium has a low work function, so the cathode-spot temperature can be lower, and higher in current density.Barium amalgamates with mercury (to the tune of a few percent in solution, forming intermetallics at higher concentrations), but I suppose a solution or amalgam could be used.Normally, operation of a mercury-pool arc rectifier is described as, the arc tends to strike a "spot" on the pool, this being the cathode spot. It heats up locally, causing emission. Of course, mercury won't really get very hot before vaporizing, but perhaps the input of gas affects the density of the plasma (it would certainly act to cool it and increase density, but whether it acts to improve emission, I have no idea). The vaporized mercury condenses elsewhere on the bulb and drains back down, quite efficiently dissipating power (it acts as a heat pipe!).Mercury arcs have a deionization time in the milliseconds, so they are only suitable for line operated equipment. Xenon thyratrons were also quite popular, having similar speed (e.g., C6Js used in machine tools -- early VFDs!). Only light gasses, namely hydrogen, were useful in high speed thyratrons for radar modulators and accelerators. (There were krytrons for nuke ignition, too, but I don't know what they were filled with, other than radioactive krypton or nickel to seed the discharge.)Tim

author: d-smes
author_rank: Regular Contributor
timestamp: March 02, 2016, 04:31:50 pm
content: Quote from: calexanian on March 02, 2016, 03:10:44 amMy thoughts exactly. But then I saw a key patent was granted last year:https://patents.google.com/patent/US9025353B2/enIt's painful reading... incorrect terminology, etc. (like Tim said). The patent describes a fault-current limiting device where four of these cold-cathode tubes are integrated within a common vacuum envelope. It then describes magnetic interactions that take place at high current due to the close proximity of the four tubes. It solves this problem with ferrite shielding within the envelope. Figure 8 is worth a look. A LOT of engineering went into this! Unfortunately, the pdf link doesn't work, but the same figures are in the application here:https://patentimages.storage.googleapis.com/pdfs/US20120081097.pdfThe above patent refers to another patent for a description of how this technology works:https://patents.google.com/patent/US4950962A/enIt is a switching tube for an X-Ray tube based on this technology (So yup, HV = X-Rays). It says "the cathode is formed of a graphite foam sleeve mounted on a support structure" and that the tube "is held under vacuum, preferably by getter pumps" which I think implies this isn't a gas filled tube. And this paragraph from the patent describes the operation:QuoteSo it's electron flow and not ion flow which I think addresses a few of Tim's comments. The patent goes on to say:QuoteBut I still don't understand how they achieve such high current density. Is it just the high voltage and high surface area of the graphite foam cathode?P.S. Nice picture Alex!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 02, 2016, 07:16:43 pm
content: Hmm, seems like "carbon foam" is the special bit? Still don't see that performance can be terribly good (or consistent/reliable or low voltage drop), though.This looks related:http://psec.uchicago.edu/Papers/Reticulated_vitreous_carbon_field_emission_cathodes_for_light_source.pdfWhich goes withhttp://www.google.com/patents/US6054801Tim

author: calexanian
author_rank: Super Contributor
timestamp: March 02, 2016, 11:21:58 pm
content: Carbon is actually used to increase the work function of a surface so it would be safe to assume they are not envisioning this device working on the same principals of a traditional emitter. Or once again it is all just bullshit.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 03, 2016, 12:05:25 am
content: I expect work function doesn't matter very much; if you're dropping a few hundred kV anyway, it might very well be an advantage: lower leakage.Or to put it another way, terminal voltages will be scaled by a geometry factor and a work function factor. Lower operating voltages should be had with lower work functions and smaller geometries (closer electrode spacing), and vice versa.

author: d-smes
author_rank: Regular Contributor
timestamp: March 03, 2016, 12:57:30 am
content: Actually, cathode is described as "graphite foam". Is that the same as carbon foam?Regardless, can someone explain the material work function significance? Somehow, the on-state voltage drop of this technology is related to the work function and it's implied to be quite low. That is, on-state voltage at many kA's is mV or less (first post says 10eV, which is the part I don't understand). How does that work? In gas-filled cold-cathode tubes, it's 10's of volts at far less current.Tim- Interesting reading! Thanks for the links.

author: calexanian
author_rank: Super Contributor
timestamp: March 03, 2016, 04:51:46 am
content: The work function of a material is essentially the energy level required for it to give up electrons. In this case the energy is expressed as heat. It is a trade off between the amount of heat a material needs to to allow its electrons to be drawn away, and subsequently replaced with electrical current and said materials melting point/vapor pressure.