[Milsurplus] Magnetron Anode Fabrication

[J. Forster]

This was just received in reply to the question of Magnetron Anode
Fabrication.

-John

=================

---------------------------- Original Message ----------------------------
Subject: Magnetron Anode Fabrication
From:    Bob Edwards  *******
Date:    Wed, July 29, 2009 1:42 pm
To:      jfor at quik.com
Cc:      norman nkrim  *******
--------------------------------------------------------------------------

John,
I really must apologize for being so late in responding to your inquiry
about magnetron anodes. I 've been tied up with ***  medical treatments,
etc. Finally I have time to sit down and reply in some detail.

Yes, as Norman has told you, I worked as a Development Engineer and
Manager, much of it on magnetrons, for 43 years at Raytheon in Waltham
(October 1946 - January 1990). When the British brought their sample to
the U.S. in the Fall of 1940, it was a rather crude experimental sample,
but one which produced what was at that time the astounding power of about
10 kilowatts peak pulsed power.at S-band. Neither we nor the British knew
then about the Japanese work at even higher frequencies and more power
(which was not appreciated and was largely ignored by the Japanese
military).

The British tubes were made by laborously machining the intricate copper
anode block, with either six or eight vanes (two versions), out of solid
copper bars or rods.  They wanted help from the U.S. in making more tubes
for higher frequency radars than they had been using, for greater
precision in target tracking. Raytheon's Percy Spencer quickly devised the
improved method of laminating the anodes using replicas of the anode vanes
stamped from thin copper sheet stock . These were then prepared for
brazing by one of two techniques: either by silver plating each
lamination, or placing a stamped silver solder lamination between each
copper lamination. Then the laminations were stacked on a stainless steel
brazing jig for alignment (and the jig oxizidized to prevent silver solder
from adhering to it), and run slowly through a high temperature brazing
furnace with a hydrogen atmosphere inside to prevent oxidation of the
copper parts.

The technique I just described is used mostly for tubes at frequencies
from L-band (1200 MHz) up through X-band (9-10,000 MHz). At frequencies
above this, the anode vane sizes become so small that accurate dimensions
are hard to obtain.

A second method of making magnetron anodes is to stamp out individual
vanes, silver plate them, and braze them inside of a round, hollow
cylinder of copper, using a stainless steel jig to align them in the
proper positions. This method has been used since the early days of
magnetron constuction too, though it is only useful for the wedge-shaped
cavity designs, and of course does not adapt to the hole-and slot style
cavity designs. (like the first British tubes). This approach has been
used on tubes from 400 MHz up to 10,000 Mhz.

A third system is to "hob" the anode vane structure into a solid copper
block using a hydraulic press. A steel hob, or negative of the vane system
is ground on a precision grinding machine. It looks like the inverse of
the anode-cavity system, with radial "vanes" representing the cavities.
and is usually a few inches long. The copper block is carefully anealed to
a certain hardness, and then several tons of hydraulic force are used to
push the hob into the block, and extract it. The excess copper is easily
machined away, yielding a usable anode of good precision. Generally, this
method is used for anodes at frequencies of X-band (9,000 MHz) through
K-band (24,000 MHz) but sometimes even up as high as 70,000 MHz (very tiny
anodes)!! The newer design tubes called "coaxial magnetrons" mostly have
anodes made in this fashion. because they have most applications at 9,000
MHz and up, where the vane sizes are small.

We were all quite amazed to find that the Japanese had been building
somewhat successful magnetrons as far back at the mid-1930's. A couple of
papers written by Dr. Nakajima describe various designs a styles of tubes
that they were exploring. Unfortunately (for them) the Japanese military
did not realize the importance of radar, or of the magnetron's power
capability, until late in WW II.  The British, I believe, should get
credit for adapting the Klystron cavity configuration for use in
magnetrons (the hole-and-slot design), and applying the tubes to practical
radar use. Raytheon gets credit for finding a way to mass-produce the
devices.

There a couple of books which I can recommend that describe magnetron
design and construction, though perhaps not the gritty manufacturing
details that I've presented:

"Microwave Magnetrons", by Collins. This is one of the MIT Radiation Lab.
books printed shortly after WW II, and is long out of print. However,
there are a couple of copies in the Raytheon Archives, probably now in
Andover. Also, the MIT Library probably has copies, or you might even try
E-Bay.

"Crossed-Field Devices", Volumes I and II, by Ernest Ocress. These were
written more recently, perhaps in the 1970-80 time period. Once again,
either MIT or E-bay might yield copies.

I  hope this information is useful to you. Let me know if you need
anything further.

Bob Edwards

==========

Thank you very much, Bob.