RESIDUAL GASES AND RELATED TOPICS IN VACUUM TUBES

by Rene M. Rogers           Sr. Scientist                1995

          It has been said that vacuum tubes, sooner or later, bring those who work with them face to face with every facet of Nature on the grand scale.  Whatever the merits of that, I have had numerous occasions to scour the lore and review the fundamentals in an effort to explain a myriad of strange observations made over the past 40 years in the vacuum tube business.  In 1968, for example, I was working as a general troubleshooter on a backward wave oscillator production line which had just been consolidated with a low power klystron line.  The crisis I had been working on was near resolution so my new boss assigned me to work on a series of chronic problems in small klystrons which might or might not have a common thread associated with "residual gases".  The term "residual gas problem" had come into general usage on the production line to describe some things people were seeing.  My observations following this assignment over the next two years had a profound effect on my concept of the subject and forced me to adopt radically different views than I had previously gathered from the lore of a generation of old time vacuum tube experts.

          Instead of presenting this material in the usual manner with theory first followed by the supporting experimental data, I will try to recall the findings in chronological order and encourage my reader to engage in his own speculations as well as follow the development of the theory favored by myself.  In the end, the curious reader may be left with some tantalizing questions.  Unfortunately, all economic incentives to follow up on this material and do some basic research in the field have long since vanished.  A fuller understanding must await a few inquisitive people who can gather bits of seemingly irrelevant data along the way to other places and eventually put them together in their leisure hours.    

          By way of my initiation, one of the production engineers took me to a bench where a reflex klystron, just returned from a customer, was being tested.  The operation of the tube was normal in all respects except that the repeller voltage was out of specification.  A small percentage of our reflex klystrons had been returned from the field on this account since the beginning of time.  No one knew why and the cost of a serious effort to find out was estimated to be far in excess of the cost of a few replacement tubes.  The interesting thing about this particular tube, however, was the visible glow from within which could be seen through the clear mica window.  The tube was gassy and people were beginning to ask why returned tube failure records were showing high gas levels as the cause of more and more failures.  Of course, failure records are usually prepared by people who have little or no concept of the operation of the device in question, but the records are useful nonetheless because troubleshooters learn to read between the lines.  My guess is that gas levels had not changed as much as the record keeping procedures.

          I brought a microscope over to the test set and soon had the interaction gap in focus with the beam on.  A blue glow was clearly visible where the electron beam crossed the gap.  The engineer told me that he had seen the phenomenon many times and believed it was due to the fact that the tube had a very small leak.  He was of the opinion that the glow was due to the ionization of O2 and N2 from the air that seeped in through this leak.  Leaks of this sort were too small to be detected by the very sensitive helium leak detectors that are used routinely in the vacuum tube business to find larger leaks during production.  The tube we were looking at had been in operation in the field for several hundred hours before the customer noticed that he could no longer adjust the repeller voltage to achieve proper operation.  Other tubes with identical symptoms had recently been returned as well.  These were routinely opened for inspection, but nothing unusual was found.  Some were fitted with new exhaust tubes and subjected to the most rigorous leak testing methods known using the helium leak detector, all with negative results.

          Several years earlier I had been responsible for instrumentation in support of a group of researchers working on gas lasers and had become somewhat familiar with photoemission spectroscopy.  This work had been abandoned and much of the equipment was stored in a warehouse gathering dust.  The company had several warehouses full of such material.  It was too valuable to dump or sell to scrap merchants for pennies on the dollar, but most, if not all of it had zero book value and very few people had any idea what there was or what it might be good for.  I retrieved one of the small spectrometers which covered the band from 3000 to 7000 Angstroms more or less and set up a small dark room on the top of a lab bench.  Using a couple of lenses, I made an image of the klystron gap on the entrance slit of the spectrum analyzer and soon had a chart recording of the emission spectrum of the gases inside the tube.  All of the principle lines were in the H2 spectrum and there was no evidence of O2, N2, CO, or CO2, or any other likely component.  There were some lines not listed in the CRC Handbook, but these were all at levels just above the noise floor. 

          One measurement is easily worth 1000 calculations or, perhaps, 10,000 speculations. 

          In connection with the earlier laser studies, I had prepared a series of glass ampules with two electrodes each and gas samples of H2, N2, O2, CO, CO2, argon, and helium, all at a pressure of roughly 10^-3 mmHg so that a low voltage glow discharge could be established in each.  These were now lost and had to be made anew.  Commercial neon pilot lamps were available from the stockroom.  I used these sources to calibrate the spectrometer and prepare charts of the various spectra for comparison.  The principle lines from each source compared very favorably with the tables in the CRC Handbook although there were always a few weak lines which may have been due to trace impurities in the samples and/or omissions in the tables.  In any case, it was very compelling to compare a spectrum from a klystron gap to one or another of these charts.  I note in passing that there were also unmistakable traces of argon and xenon in the commercial neon lamps.

          Figure 1. is a schematic representation of a reflex klystron.  Such a contraption with a transparent window is an ideal vehicle for the study of residual gases in small sealed-off vacuum tubes.  The operation of the device is described roughly as follows.  The electron beam passes through the interaction gap where it is velocity modulated by an rf field established there by previous electrons.  The velocity modulated electrons leave the gap and enter a drift space with a retarding electric field whose strength is set by a negative voltage on ***** RENE WHAT IS MISSING?**************

                          FIGURE 1.

          All of the electrons are turned around by the retarding field and re-enter the interaction gap.  If the repeller voltage is properly adjusted, the velocity modulation on the electrons entering the drift region will be largely converted to density modulation, or bunches, of the right phase to reinforce the rf voltage at the gap when they return.  The gap is part of a resonant cavity which determines the central operating frequency of the device.  The exact frequency, however, can be pulled slightly by adjusting the repeller voltage and thus the phase of the returning bunches. 

          However good the vacuum is in any device, there are always a large number of gas particles present.  For practical purposes, the vacuum is poor when a significant percentage of the beam electrons collide with one or more of the residual gas particles during the transit time between leaving the cathode and eventual collection at a positive electrode, in this case the cavity resonator.  The vacuum is good when an insignificant fraction of the beam electrons experience a collision.  A collision entails a random change in velocity and transit time which shows up as noise in the rf signal.  There are many sources of random noise in the rf signal and gas levels at which ion noise becomes important are usually high enough to cause other problems.  Very roughly speaking, a residual gas pressure of 10^-4 mmHg is unacceptably high in terms of ion noise as well as ion erosion of the cathode, while 10^-6 mmHg will not lead to ion noise in excess of noise from other causes and the effects of ion erosion will be evident only after operation for several thousand hours.  Residual pressures in the range of 10^-8 mmHg or lower are typically considered ideal. 

          The pressure in a reflex klystron can be measured in the approximate range 10^-3 mmHg to 10^-7 mmHg within a factor of 2 or 3 by measuring the ion current to the repeller.  The electrons in the retarding field region collide with and ionize some of the residual gas particles and the positive ions thus formed flow to the negative repeller.  In [1] we calculate the rate of ion formation for a given beam current and path length in terms of the pressure and the collision cross section of the atoms in question.  This technique is also applicable to almost any vacuum tube with a control grid by operating the grid at a positive potential and the anode at a negative potential relative to the cathode.  No electrons from the cathode will reach the anode under these conditions while the ions formed between the grid and the anode will mostly flow to the anode where they can be collected and measured.  The high pressure limit mentioned above is ill defined, but the low pressure limit is set by the formation of soft X-rays.  The spent electrons are collected at the walls of the resonant cavity at a potential of several hundred volts and the sudden change of velocity of the charged particles give rise to photons (X-rays) with energies up to the beam voltage.  These photons are energetic enough to produce photo-emission at the repeller and we have no way to determine whether an electron flowing to the repeller does so to replace a photo-electron or to neutralize a positive ion.  The ion current becomes small as compared to the photo-emission current at around 10^-7 mmHg in a typical device.

          The pressure in the klystron I looked at first was on the order of 10^-5 mmHg, typically 2 or 3 orders of magnitude greater than new tubes of this type.  During continued observation, however, both the ion current and the intensity of the H2 spectral lines decreased slowly.  There was also a pressure transient at turn on after any extended period of quiescence during which time the cathode was hot but no beam voltage was applied.  Unfortunately, the notebooks, recorder charts, and oscilloscope records of those experiments over the next two years are not available to me, so I must rely on my memory which is much better with regard to the conclusions I drew than to the data on which those conclusions were based.  To a first approximation, the pressure increased in direct proportion to the time that the beam current was turned off as indicated by both the initial ion current at the repeller and the initial intensity of the H2 lines when the beam was suddenly turned on.  After turn-on, the pressure decayed exponentially to a quasi-steady state level with a characteristic time constant of roughly 5 or 10 seconds.  From the rate of ion formation, the volume of the tube and the basic considerations set forth in [2] regarding pressure transients, I estimated that between 1% and .1% of the ions thus formed were somehow driven into the walls or otherwise trapped so that re-entry into the vacuum would take much longer than 5 or 10 seconds.  When beam current was flowing, the device was acting as an ion pump, less inefficient than a conventional sputter ion pump, perhaps, but not a negligible factor either.  The walls of the device, on the other hand, were acting as a relatively steady source of H2 influx, at least so long as the temperature remained constant. 

          After several weeks of studying this phenomenon, the ion current at the repeller became small compared to the X-ray current and the spectral lines became almost undetectable.  When shipped, perhaps a year previously, the klystron required a repeller voltage 270 volts below the cathode voltage to work at the desired frequency.  It now required 290 volts, but the beam current and the output power were substantially the same as when the tube was new. 

          There were several field return reflex klystrons of the same type on hand which had similar symptoms as well as a stock of new ones and I spent a lot of time examining them.  All of the field returns required a more negative repeller voltage, from 2 or 3 volts up to 30 volts, than they did when they were shipped, but not all had measurable gas levels.  The ones that did show gas, however, had H2 as the only detectable species although there were some very weak lines not apparent in every tube.  For the most part, the new tubes had gas pressures below detectable levels, but traces of H2 which vanished entirely after several hours of operation were seen in one or two.  One new tube exhibited a strong spectrum of CO and CO2 which diminished to undetectable levels after several days of continuous operation.  My speculation is that the cathode was not completely broken down before the tube was sealed off the pump.  I never saw a recurrence of this phenomenon.

          The relationship between gas levels and repeller voltage shift was obscure at best, but it soon became clear that some form of semi-insulating layer was attaching itself to the surface of the repeller.  I connected several klystrons as shown in Figure 2. and found that the form of the repeller current vs. cathode voltage function was quite dependent on the age and history of the tube.  In this configuration, the klystron behaved somewhat as if there was a leaky capacitor in series with the repeller after the tube had been operated for several hundred hours or more.  It was well known that barium evaporated from an oxide cathode even under perfect vacuum conditions and that sputtering by ions under less than perfect conditions would increase the rate.  It was also well known that barium is a getter of a variety of residual gases in vacuum tubes.  I could therefore guess that a barium film on the repeller surface was gettering at least some of the gases in the klystron to form a quasi-insulating film.  To test this hypothesis, I put a positive bias on the repeller voltage in the above configuration so as to heat the repeller to several hundred degrees.  The klystron was filled with H2, and only H2, to a pressure in excess of 10^-3 mmHg.  The repeller current display returned to that of a new tube and the frequency vs. repeller voltage characteristic returned to the values recorded when the tube was new. 

          The evolution of barium from an oxide cathode is a very complex and poorly understood process which depends rather critically on the cathode temperature as well as sputtering by ion bombardment.  It is easy to imagine that the rate of formation of a barium film on the repeller in a reflex klystron could vary widely from tube to tube even though all are built by the same people using routine parts and processes.  Barium is very active chemically and forms stable compounds with all but the noble gases, including BaH2 which has a heat of formation of approximately 1.8 ev.  It is doubtful that there was ever enough hydrogen available to form stoichiometric BaH2, but is seems as though a barium film which has taken up some H2 will hold a charge from a few volts up to 30 volts when subjected to a steady ion current from collisions between the residual gas and beam electrons in the retarding field region[3].  It also seems reasonable to suppose that a barium film which has taken up enough hydrogen to become partially insulating will disgorge enough hydrogen to flood a klystron to an uncomfortably high pressure when a small spot of it is involved in a severe arc.  I encourage my reader to do a back-of-the-envelope calculation assuming that a barium film several thousand Angstroms thick has taken up hydrogen atoms on a one-for-one basis, or some fraction thereof, and find the size of a spot which, if vaporized during an arc, would release enough hydrogen to raise the pressure in a typical klystron to a pressure of 10^-4 mmHg.

          During the course of observing these phenomena, I happened to use some liquid nitrogen to cool one of the klystrons and noticed that the repeller current characteristic changed dramatically as indicated in Figure 3.  There was no measurable repeller current until a threshold voltage was reached and then the current changed discontinuously to the value it would have had if there was no film.  There was also some hysterisis observed in this characteristic.  As the repeller temperature returned to normal, the characteristic returned to normal in a continuous manner.  Some friends in a division of the company working with semiconductors came over to see this phenomenon and quickly set forth a variety of scenarios familiar to them to explain it all, but they agreed that an exact understanding would require more study than any of them could fit into their busy schedules.

          A small mystery at this point was my failure to see N2 in any of these tubes because H2 and mass 28, ie N2 and CO, as well as CO2, were routinely observed during and after bakeout in a vacuum station carefully designed specifically to study the evolution of gas species before, during, and after bakeout.  Species identification was done using a mass spectrometer which was sensitive to partial pressures down to 10^-10 mmHg.  The mass spectrometer was not inside the bakeout oven, but it was baked to some degree using electrically heated fiberglass tape wrapped around it.  I suspect that this bakeout was never adequate and that the mass spectrometer, and/or its connecting tubulation, was the source of the N2 and perhaps some of the CO and CO2 as well.  In any case, I never saw spectral lines of N2 or O2 in a sealed off vacuum tube during the next 2 years although I looked very hard for them.

          Perhaps, I reasoned, the leak channels are too small to admit big atoms like N2 and O2 or even helium, but are large enough to admit H2.  I stored my original specimen, now with a hard vacuum, in an H2 atmosphere overnight with no voltages applied and looked for evidence of gas the next morning.  H2 lines and repeller current above the X-ray floor were evident if I looked very hard and used my imagination.  With only the heater power on and no waveguide connection to drain away heat, the temperature of the klystron body would reach roughly 60 DgC, too hot to touch comfortably, and with full voltage and beam current on the temperature would go over 120 DgC.  The next night I applied heater and beam voltage to the tube, set a recorder with a logarithmic response to measure the repeller ion current, and provided an H2 atmosphere for the tube.  When I came to work the next day I could hear the recorder motor grinding against the stop from far down the hall.  The H2 pressure inside the tube was too high to measure.  The heater current was high because heat conducted by the H2 was now a significant factor in the thermal network of the heater-cathode system and the heater was cooler than normal.  This familiar diagnostic indicated a pressure in excess of 10^-3 mmHg, a level consistent with the repeller ion current I observed. 

          The beam current available was roughly half of normal because the cathode was somewhat poisoned and there was a visible film on the mica window, but the spectral lines of H2, and only H2, were still apparent.  It is well known that the ions of residual gas particles formed between the cathode and the anode will fall into the cathode with energies up to the applied voltage and dislodge, or sputter, atoms of the cathode material.  The sputtered cathode material travels in a straight line from the point of origin and tends to stick on whatever surface it strikes.  Atoms striking a hot surface, such as the grid at the gap, may re-evaporate and move on with some going to the mica window.  If all of the cathode coating is sputtered away, the cathode will generally not emit electrons, but if some coating remains the cathode may often be brought back to life by simply keeping it hot and drawing some current from it.  This eventually happened in this case and the gas pressure also decayed to barely detectable levels as well over a period of several weeks.  I assumed that whatever leak channels were responsible for admitting the H2, they became markedly more permeable as the temperature increased.

          One day, while examining new reflex klystrons from the inventory awaiting shipment, I found one with a faint glow in the gap that, to the naked eye, had a slightly different color than the now familiar H2 spectrum.  The ion current at the repeller indicated a pressure on the order of 10^-7 mmHg while spectral analysis proved the gas to be a mixture of argon and neon with no evidence of H2 or any other gas.  Moreover, the principal neon lines were stronger than the principal argon lines, which may or may not indicate anything about the relative abundance of these gases in the tube.  Argon is roughly 1% of the atmosphere (presumably from the radioactive decay of potassium over the past 4.5 billion years) while the partial pressure of neon in the atmosphere is roughly 10^-3 mmHg.  If this tube had a leak to air, where was the N2 and the O2?  A list of collision cross sections for various gas particles is shown in Table I, but the range of values is too small to inspire me to believe that size is an important factor in all of this. 

          After several days of operation with the beam on, the spectral lines and the ion current decayed below detectable levels.  It would appear that the beam was effective as an ion pump for argon and neon as well as H2, CO, and CO2.  This did not surprise me, for I supposed that the pumping mechanism included simple burial of ions impacting solid surfaces with energies of 10's or 100's of electron-volts as well as the formation of exothermic compounds between some gas species and the metal walls of the tube. 

          The next experiment was to put the tube in an enclosure filled with helium while recording overnight the ion current at the repeller with the beam current on.  The repeller current was in the X-ray limit range at the outset and for roughly 8 hours thereafter, but then the pressure began to rise at an ever increasing rate to the level of roughly 10^-6 mmHg after 14 hours.  Helium lines, and only helium lines, were clearly evident in the spectrum.  After beam pumping had been allowed to bring the helium pressure below detectable levels, several days, this experiment was repeated with the tube in an argon atmosphere.  The influx of gas was evident after roughly 4 hours and the recorder motor was grinding against the stop after 14 hours.  The spectrum showed the gas to be argon and nothing else.  It was pretty clear that the size of the particles, as determined by their ionization cross sections, had little or nothing to do with what was going on here. 

          In discussing these findings with colleagues, I learned of a report written by people at a subsidiary of the company.  They had attached a bakeable mass spectrometer to a large klystron with an oxide cathode in order to study residual gas species present before, during and after normal processing, which included bakeout and pinch-off.  They found H2O, H2, CO, CO2, and CH4 before and after bakeout and pinch-off, but no O2, N2, argon, neon, or helium.  The total pressure after pinch-off was 5x10^-7 mmHg.  This decayed to 2x10^-9 mmHg of H2 after 400 hours of operation.  To me, the striking thing about this report, in view of my recent findings, was that after 500 hours of operation the tube developed a leak to air which became evident by the appearance of argon at the mass spectrometer.  O2 and N2 were never observed before the total pressure was too high to continue applying beam voltage.  It would appear that O2 and N2 are quickly captured and tightly held by the inner surfaces of the klystron or by the walls of the leak through which the argon was able to pass relatively unhampered.  The electron beam was on most of the time so ionization and disassociation of any gas molecules in the path of the beam is to be expected.  The ions will, of course, be accelerated by the strong electric fields present in the klystron and will collide with solid surfaces with considerable energy.  Recalling that 1 electron-volt is the energy equivalent of 11,600 degrees Kelvin, we can imagine that the chemistry of ionized and disassociated gases in a vacuum tube is not the same as the chemistry in a laboratory vessel at room temperature. 

          One of the chemists also produced a paper perhaps 10 or more years old reporting the results of a series of experiments in gas diffusion through capillaries of various materials.  The experimental apparatus consisted of two bakeable chambers which could be independently evacuated and back-filled with various gases.  Each chamber included a bakeable mass spectrometer for measuring the partial pressure of individual gas species.  The chambers were connected by a long capillary tube which could be made of glass or metal and coated with various materials like oils or pyrolytic graphite or left plain.  The basic idea was that the expected transit time for a gas particle entering the capillary at one end to emerge from the other end could be calculated exactly in terms of a model in which each encounter with the walls is followed by an instantaneous cosine law rebound.  The actual transit time was always considerably longer than calculated and a new model was offered to explain the experimental results.  In this model, each encounter between a gas particle and the wall is characterized by a sticking probability and, once stuck, by a residence, or sojourn, time.[4] 

Neither the chemist nor I could follow the mathematical theory, a fact which inspired me to take a graduate course in random variables, but we all agreed that the results could be explained only if the diffusing gas particles spent considerable time in a semi-bound state along the way.  This was true for the noble gases, helium, argon, and neon as well as the chemically active gases, O2, N2, H2, CO, and CO2.  The noble gases may not form stable compounds, but they do form tenuous bonds with almost all solid surfaces.  Furthermore, the evidence of beam pumping for the noble gases suggested that their ions were being driven deep within a solid wall and held interstitially for long periods of time.

          There were, of course, books on the subject of high vacuum as well as professional societies interested in the subject, but most people in the dying vacuum tube business were concerned only on a casual basis if at all.  I bought a 1968 publication [5], got a copy of Dushman[6] from the company library, and dug in.  I also reviewed my lab notes regarding some experiments I had done a dozen years earlier with a brighter-than-average co-worker with a PhD in physical chemistry, long since departed to form his own company, when we had occasion to measure the diffusion parameters of sulphur in 18-8 stainless steel.  The details of that experiment are lost, but I used the same technique to make the same measurements in 1970 with regard to Zinc in a Copper-Nickel alloy. [7]

          One doesn't have to read very much in Dushman to learn his view that metals dissolve prodigious amounts of a wide variety of gases and release them back into the vacuum or out into the external environment following the laws of diffusion.  In his model, molecular gases disassociate into their constituent atoms at the surface of the metal and migrate into the solid lattice where they may reside interstitially between the metal atoms or, in some cases, replace one of the metal atoms.  The residence time for a gas atom in one of these configurations depends on the binding energy, called the heat of diffusion, and the temperature.  The relationship is t₀ = e^Qd/kT/f₀ Seconds, where t₀ is the expected residence time in Seconds, Qd is the heat of diffusion in electron volts, k is the Boltzman Constant, and T is the absolute temperature in Degrees Kelvin.  kT is approximately 0.025 ev at room temperature, while f₀ is the frequency of escape attempts per second.  Roughly speaking 10¹² < f₀<10¹³ per second.  Binding energies range from a fraction of an ev to several ev with the result that expected residence times range from nanoSeconds to the age of the universe in our everyday experience.  Dushman also discusses a parameter of scale, a diffusion skin depth somewhat analogous to the more familiar rf skin depth, which describes how far dissolved atoms will migrate in a solid for a given time and temperature.  All this is discussed further in the reference[8].     

          There was very little data available regarding the diffusion parameters for gases in metals and even less agreement in the data that was available, but I was able to estimate on a very rough basis that the 1 hour diffusion skin depth for H1 in iron at 120 DgC was on the order of one millimeter.  Since the bodies of the klystrons I was studying were made of iron several millimeters thick on average, it was not surprising that I was able to achieve a hard vacuum using beam pumping for several days or weeks or that storage in a hydrogen atmosphere overnight would produce the results observed.  These findings were met, however, with howls of outrage from several of the old timers, some in high places, for it went directly against the core lore that all residual gas problems were either internal surface phenomena or leaks to the outside air.  I was familiar with the table napkin calculation to the effect that there are on the order of 10¹⁶ surface traps/Cm^2 on a metal surface and when each of these is occupied by a gas particle the surface is, by definition, covered by a monolayer of adsorbed gas.  Since there are roughly 3.5x10¹⁶ gas particles/Cm^3 in a vessel at 1 mmHg (from Avogadro's Number), the pressure in a 1 liter vessel would be on the order of .17 mmHg if all of the particles in a monolayer were suddenly released.  The purpose of the bakeout procedure, according to my critics, was to dislodge these surface gases and, at the very most, a few gas particles trapped one or two atomic layers below the surface.  When lines such as these are drawn in the sand, the facts of the matter become totally irrelevant and further discourse is not possible.

          It was pretty clear to me how gas atoms and molecules inside the vacuum became ionized by the electron beam and disassociated and accelerated to enormous temperatures by various electric fields and how at least some of these ions would be deeply imbedded in the metal walls, but the reverse process at the outer surface was somewhat obscure.  The heat of formation of 2H1 -> H2 is roughly 4.5 ev while, according to Dushman and others, only atoms of hydrogen can go into solution with a metal.  Even at the melting point of copper, 1083 DgC, over half of the molecules of H2 in a hydrogen furnace have kinetic energies less than 0.1 ev while, according to Boltzman Statistics, only 1 in 10¹⁷ have energy in the neighborhood of 4.5 ev.  Yet, according to experience and the literature, all metals which have been on a trip through a hydrogen furnace at red heat and above come out with hundreds to thousands of parts per million of H1 dissolved throughout.  Where does the energy required to break the molecular bonds come from?  The most plausible scenario seems to be that some molecules on the surface become atoms within the solid lattice by means of quantum mechanical tunneling.  Once dissolved as atoms, H1, at least, diffuses so rapidly at elevated temperature in most, if not all, metals that the distribution becomes uniform relatively soon in objects the size of vacuum tube parts. 

          Volume solubility is, therefore, a surface phenomenon.  At equilibrium, gas molecules arrive at the surface and a few of them are dissolved as atoms at a certain rate while from within atoms diffuse to the surface and re-combine and re-enter the gas phase as molecules at the same rate.  The surface concentration at equilibrium is established throughout the volume at a rate dependent on the temperature, the diffusion parameters, and the size of the object.  The volume solubility of diatomic gases in metals is proportional to the square root of the pressure in the gas phase over the widest possible range of pressures.  This principle is known as Seivert's Law and is a consequence of the fact that dissolved atoms do not re-enter the gas phase as such, but must pair up at the surface to form molecules. 

          The equilibrium pressure inside a void below the surface is enormous, typically millions of atmospheres, so that once a blister has begun to form in any metal in which gas atoms are dissolved, it will grow without limit until the metal is eventually ruptured.  Vacuum tube lore tends to attribute the observation of blisters on copper coming out of a brazing furnace to the presence of oxygen in the copper and the formation of H2O, or steam, but I believe that oxygen is not necessary while a nucleation site, like a void on the atomic level or a grain boundary, is.  When Cold Fusion was a hot topic, it occurred to me that Palladium would be about the last metal I would choose to try and confine dissolved atoms of D1 at high pressure within internal voids.  I would think some form of tool steel or Tungsten-Carbide or some other very hard metal would be a better choice.  Palladium was apparently chosen because of widespread lore to the effect that hydrogen and deuterium diffuse rapidly in it.  In my experience, hydrogen diffuses more or less rapidly in all metals.

          Most metal vacuum tubes baked in air at 400 DgC or higher tend to oxidize and scale rather badly and look as if they had been mistreated, so some form of cosmetic surface finish is typically applied.  The small klystrons I was working with were grit blasted after pinchoff to remove the scale and then painted red with the window and other insulating surfaces masked.  One of the chemists at the plant who routinely operated the residual gas mass spectrometer mentioned previously took a keen interest in what I was doing and told me of an occasion in the recent past when someone thought it would be a good idea to chemically strip and electroplate some small klystrons after bakeout for cosmetic reasons.  These tubes were so universally and profoundly gassy that a connection between the gas and the electroplating was unavoidable and the project was abandoned.  The chemist read the literature extensively and learned that a particularly efficient way to dissolve H1 in a metal is to make it the cathode in an electrolytic bath containing H1⁺ ions.  Hydrogen embrittlement in association with electroplating is widely known and abundantly reported in the literature.  I read in one report that nickel could be so infused with H1⁺ ions this way as to produce a whitish powder on the surface, probably a form of nickel hydride. 

          The chemist went to the warehouses and recovered some hardware once used to study the permeation of gases through thin metal plates.  This apparatus consisted of two stainless steel vacuum chambers separated by a removable membrane along with some valves and vacuum flanges.  A mass spectrometer in one chamber was used to measure the influx of gas through the membrane from the other chamber which had been evacuated and back-filled with a particular gas species.  A number of experiments were planned with the intent of gaining some insight regarding surface factors which might facilitate or inhibit gas permeation, particularly with regard to hydrogen.  I have no notes and little recollection of the course of events, but my impression is that time constraints prevented all but a few experiments.  We did learn, however, that in the case of both cold rolled steel and 18-8 stainless steel plate 0.040 inch thick, the rate of hydrogen permeation increased roughly an order of magnitude after the surface exposed to hydrogen was sand blasted with garnet grit.  Our speculation was that the grit blasting produced a great deal of local stress at the surface and a corresponding increase in local free energy which might be available to accelerate the reaction, H2->2H1.  There was also anecdotal evidence at this time to the effect that cold rolled steel tended to rust faster after grit blasting than it otherwise would.  I have never had occasion to verify either proposition.

          The question arose as to what degree the grit blasting used to prepare small klystrons for painting also contributed to the decomposition of hydrogen-bearing molecules on the surface from the atmosphere and thus hydrogen influx into the new tube.  After a coffee break discussion of the matter, one of the technicians said he had long since made a practice of painting tubes as soon after sandblasting as possible.  He said that the surface tended to discolor after an hour or two and would be downright ugly with rust stains if left unpainted over a weekend.  Furthermore, he said that he could recall more than one batch of tubes left unpainted over a weekend that wound up as scrap, but he did not know the details.  I arranged to get some new tubes immediately after grit blasting to see how hydrogen influx after turn-on varied as a function of elapsed time between grit blasting and turn-on.  Figure 4. shows the gross results of that experiment as I recall them.  My speculation after this experiment was that the outer surface began to accumulate hydrogen immediately after grit blasting and stopped accumulating hydrogen as soon as the outer surface was pacified with a coat of paint.  After turn-on, the temperature would rise to roughly 120 DgC, the hydrogen dissolved at the outer surface would migrate both inward and outward as the temperature increased, finally leaving altogether by way of the outer surface.  One very old timer told me that he had always opposed painting the tubes and preferred to dip them in light oil after pinch-off and ship them as ugly but rugged devices that worked well. 

          I reckoned that a coat of paint immediately after grit blasting was a reasonable compromise

          At some point during this period I was given a new yield problem regarding a small 2-cavity klystron with mica windows.  This tube had been in production for many years with a very high yield, roughly 90% of starts being shipped with very few returns from the field.  The yield had suddenly dropped to roughly 60%.  The rejects suffered from power degradation over time apparently due to the formation of a copper film over the output window.  Gas was somehow suspected as the root cause. 

          These tubes had no grid and no repeller, so it was not possible to connect the electrodes as a triode to measure ion current, nor were the gaps within direct line of sight through the transparent windows.  It was, however, possible to see two of the headers which supported tungsten vane grids, as indicated in Figure 5.  Looking into either window with the beam on, it was apparent that the grids were incandescent and the entire inside of each cavity was brightly illuminated.  It would have been difficult if not impossible to observe low level spectral lines in this background of white radiation without some kind of phase sensitive tagging and a reduced duty cycle.  Metallographic sections through the grid vanes during autopsy showed clear evidence of melting, de-lamination, and grain growth.  See Appendix I.  The tungsten ribbon from which the grids were made was first wrapped around a molybdenum mandrel of the desired cross section and set in a high temperature furnace.  The mandrel was then removed by etching in an acid bath.  The individual grid vanes were then cut from the set ribbon, by hand under the microscope, using toe nail clippers.  The grid was then assembled into the header, by hand under the microscope, using an alignment jig.  The vanes were then brazed to the header, under the microscope, by rf induction in a hydrogen atmosphere using 35-65 Gold-Copper alloy.

            A substantial degree of operator skill was required at every step and essentially all of the new grids looked perfect to both the skilled and unskilled eye.  Nevertheless, some grids melted and gave off copper vapor while others did not.

          Thermal analysis failed to disclose any plausible scenario by which beam interception or ion effects could account for the temperatures which were obviously being encountered here.  I was forced to speculate that de-lamination of the tungsten vanes was somehow at the bottom of the problem.  Pure drawn tungsten wire, which melts at roughly 3500 DgC, is made by repeated drawing and annealing starting with a billet originally made from tungsten powder scintered under pressure at roughly half the melting temperature.  Ribbon is then made by rolling the hot wire.  The end product is like a bundle of long fibers more or less loosely bonded to each other, more or less anisotropic and prone to de-laminate when subjected to repeated thermal or mechanical stress.  The thermal conductivity in the direction of the fibers is high, but goes to near zero normal to the grain when the bonds between adjacent fibers is ruptured.  Even at melting temperatures, radiation cannot account for more than a fraction of the heat dissipation in this case so most of the power due to beam interception had to be conducted to the header or carried off as the heat of vaporization.  My guess was that de-lamination created a barrier to normal heat flow and resulted in the high temperatures observed.

          The puzzle was why some tubes had super hot grids and some did not and why the sudden onset of the problem in a large fraction of the production.  Metallographic sections of new grids did not show de-lamination while all of the grids from failed tubes did.  Moreover, one normal working tube on life test was sacrificed and significant de-lamination was evident in its grids as well.  The stock of tungsten ribbon became suspect and was subjected to random sampling while the gas connection was still considered a possible factor.  Nothing was ever found to indicate a detectable difference in the raw ribbon stock and we were left with the general impression that all of it was subject to de-lamination under severe repetitive thermal and/or mechanical stress.  Some years later, however, a colleague at another company devised a high volume inspection scheme using induced eddy currents which seemed to detect precursors to de-lamination in tungsten ribbon.  De-lamination can be prevented by the addition of 1% to 3% of rhenium to the original billet, but the resultant properties, specifically the electrical conductivity, are not identical to those of pure tungsten.  De-lamination can also be aggravated by the mechanical shock of cutting with wire cutters or toe nail clippers.  This can be avoided by EDM or laser cutting. 

          Although it was not possible to focus directly into either gap, I went about assembling equipment to try and detect scattered spectral lines under pulsed conditions and reduced power to minimize the noise from hot body radiation.   One day, while looking through a microscope hoping to see some color due to ions, I saw a bright green flash at the instant of turn-on which could not be replicated.  Having no idea what might be responsible for this, I went ahead and assembled a phase locked detection system to separate spectral lines from the black body radiation and was able to detect some very weak hydrogen lines, but I never thought residual gas levels could explain or contribute significantly to the basic yield problem. 

          The green flash was not seen again and remained a mystery until it occurred to me that it might have been due to the ionization of barium which had accumulated over time on the grids and been evaporated when the grids were heated.  A dozen years earlier, our cathode guru had described to me an experiment he was doing to measure the rate of evolution of barium from an oxide cathode.  He was using a quartz crystal behind a shutter to collect condensible material coming off the face of an oxide cathode, primarily barium.  Then, using the quartz crystal to control the frequency of an oscillator, he was able to watch the frequency shift and relate that to the increased mass on the crystal.  I increased the heater voltage on one of my test vehicles and left it alone for a week or so with no beam voltage applied.  When I turned the beam voltage on, I saw the green flash again.  Setting the spectrometer to the wavelength of the principle barium line in the green part of the spectrum, I was able to verify my speculation.  Once I knew what to look for, I could monitor the buildup of barium, as well as calcium and strontium which were also active ingredients in the cathode, using much shorter accumulation times.  Of course, all of this was idle indulgence since I had no reason to suspect barium, calcium, or strontium evolution from the cathode to be significant factors in the problem I was studying.

The exact cause of our yield problem with overheated grids remained a mystery for some time and I was diverted to other matters.  One of these involved consultation with a recently acquired subsidiary in the gas chromatography business.  I soon had a portable gas chromatograph on a tea wagon and a need for test samples to study the operation of this instrument.  The atmospheres in brazing furnaces, bell jars, TIG welding hoses, and so on were handy sources and it soon became evident that these atmospheres were not necessarily what people thought they were.  Contamination from leaks in supply lines was widespread, but unless the consequences became catastrophic these were usually ignored.  One exception stands out... a TIG welder who told me he could tell when his hoses needed replacement by the appearance of the weld.  Eventually I got around to sampling the atmospheres in the three stations used to braze tungsten vane grids into headers and found that one of them had a severe leak.  The concentration of O2 in this one was roughly 100 ppm while the O2 concentration in the other two was roughly 10 ppm.  My guess is that this high concentration of O2 was not sufficient to support an explosion, but was sufficient to prevent adequate wetting of the tungsten ribbon by the liquid braze alloy.  I can speculate further that the liquid alloy might very well permeate incipient fractures in the ribbon and provide the heat path which would otherwise be missing if the fractures were left open.  New orders for the products involved were on the decline and rigid configuration controls were in force for the current production to such a degree that these questions were irrelevant and no resources were made available to look into them.