Value chain · CLM-1049
In plain terms
The fifty-year magnetron rests on a cathode that runs cool because the tube heats it only as much as it needs, on tube-life curves from 1950s transmitter valves, and on twelve radio-station valves that ran ten years without failing. No magnetron had been life-tested that way, and the test tubes Raytheon looked into poisoned themselves in under an hour.
The claim as recorded
Raytheon's magnetron life case rests on a measurement and two extrapolations: optical pyrometry through windowed oven magnetrons shows the carburised thoriated tungsten cathode running at 1893 K at 190 mA, dependent on anode current alone and following Richardson-Dushman, so a scaled 20 kV SPS tube at 235 mA and 129 mA/cm2 would run at 1862 K; Ayer's 1952-53 triode data give 75,000 h at 2000 K (from more than 30,000 h observed) and a factor of 3 per 50 K, extrapolated to "230 years" at 1850 K, 77 at 1900 and 26 at 1950; twelve WWV time-signal triodes ran 850,000 tube-hours without failure at about 1900 K; and the SPS design (0.030 inch wire, 40 per cent carburised, 1862 K) is given "in excess of sixty (60) years", Table 7-3 ">50 Years" at 3.2 kW and 0.8 of that at 5.0 kW - while Raytheon's 1980 report states "There is no comparable life data on any 'CW' magnetrons run on a low-ripple power supply at low cathode temperatures" and that its windowed test tubes poisoned and stopped within about 40 minutes from residual gas, and the 1981 study's remaining-issues list makes understanding the noise and the inefficiency prerequisites, saying "it would be premature to start the development of the SPS space tube".
Evidence behind this: T3-tube developer's life case: one measurement and two extrapolations, with the source's own caveats.
Scope and limits
The temperature is measured on 1 kW oven tubes and projected, not measured, for the 20 kV design; the life curves are for negative-grid triodes with heated filaments, extrapolated by a factor of 27; the WWV valves' end of life "was never determined". The 1980 summary gives "5 to 10 years" at 200 mA for the oven tube where the 1981 introduction gives "10 to 20 years" at two-thirds current. Thorium desorption is dismissed on Danforth 1960, not tested.
What it assumes
Decarburisation as the only failure mechanism; an emitting area of 1 cm2 for the oven cathode; Ayer's curves valid 150 K below their data.
Where it comes from
PDF p.77 / 5-14: "With previously used values of emission current of 0.190 amperes and of the temperature of 1893 K, the value of A0 is 1.99/cm2deg2". PDF p.82 / 5-19, render read: "a reduction of 50 Celsius degrees decreases the loss by a factor of 3 ... An extrapolation of this data to lower operating temperatures would provide a factor of 27 in increased life if the temperature were reduced to 1850 degrees. If the factor of 27 is used to multiply the 75,000 hour life shown in Figure 5-8, the resulting predicted life is 230 years." "The corresponding predicted life for 1900 K and 1950 K, are 77 years, and 26 years, respectively ... individual lifetimes of up to 86,930 hours ... a total running time of 850,000 hours with no failures." PDF p.84 / 5-21, render read: "the life expectancy would be 700,000 hours or over 70 years, if decarburization of the cathode were the failure mechanism ... There is no comparable life data on any 'CW' magnetrons run on a low-ripple power supply at low cathode temperatures." SRC-560 PDF p.152 / 7-24, render read: "an anode current of 235 milliamperes ... 129 milliamperes per square cm ... an operating temperature of 1862 Kelvin ... If we assume 0.030 inch diameter tungsten wire that is 40% carburized ... life should be in excess of sixty (60) years." SRC-560 PDF p.32 / 1-6, render read: "at anode current levels 2/3 of the normal value, the temperature of the carburized thoriated tungsten cathode is so low as to give 10 to 20 years of cathode life as established from data on the life behavior of similar cathodes in high power triodes." PDF p.16 / 0-2: "it would be premature to start the development of the SPS space tube until all of these interactions and their importance are understood." PDF p.88 / 5-25 (SRC-559): "The deterioration process then increases rapidly and finally a point is reached where the tube can no longer sustain itself ... and the tube ceases to operate."
Basis: cathode temperature in K; years of life by extrapolation
Related records
- Answers question
- QST-002 not published in this releaseQST-017 not published in this release
Cite this
energyaperture.com/c/clm-1049
CLM-1049, Energy Aperture, as of 1981-02.
Connected records
Other published records linked to this one. A list, not a judgement.
- Also from SRC-559
- CLM-1044In 1980 the quiet oven magnetron was already beyond the instrument's reach: at least 155 decibels below carrier per hertz away from the...CLM-1045Raytheon's own numbers had the klystron just failing the interference limit at the band edge and the magnetron clearing it by 45 decibels...CLM-1047The oven magnetron's second harmonic came out about 70 decibels below its carrier, a tuned stub took another 40 off, and the antenna's own...CLM-1048A cheap oven magnetron run cold and hard converted 81.7 per cent of its DC input to microwaves, measured carefully by heating water...CLM-1050The interference limit Raytheon measured its tubes against is given as a CCIR requirement but never traced to a CCIR document, and one page...
- Also from SRC-560
- CLM-1043With the heater off and a tuning circuit on the cathode, one hand-picked oven magnetron was as quiet as the instrument could measure: about...CLM-1045Raytheon's own numbers had the klystron just failing the interference limit at the band edge and the magnetron clearing it by 45 decibels...CLM-1046The number Raytheon wrote into the design table for the satellite tube was about fifty times noisier than its best laboratory reading...CLM-1047The oven magnetron's second harmonic came out about 70 decibels below its carrier, a tuned stub took another 40 off, and the antenna's own...CLM-1048A cheap oven magnetron run cold and hard converted 81.7 per cent of its DC input to microwaves, measured carefully by heating water...CLM-1050The interference limit Raytheon measured its tubes against is given as a CCIR requirement but never traced to a CCIR document, and one page...