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Development of GEC E1189 eight-cavity magnetron

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Forum » Valves / tubes - Semiconductors » VALVES/TUBES / SEMICONDUCTORS in RADIOMUSEUM » Development of GEC E1189 eight-cavity magnetron
           
Emilio Ciardiello
Emilio Ciardiello
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05.Oct.18 13:56
 
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Foreword: magnetron devices were described since 1921 by Hull and perfected in the years by many scientists worldwide. Among the others, we remember the work of the German Erich Habann, who first devised a split-anode structure in 1922, the Japanese Okabi and Yagi in the second half of twenties, Maurice Ponte at CSF, Klaas Posthumous at Philips and Eric Megaw at GEC, who in the thirties perfected the split-anode structure, adding multiple pairs of segments. In America we remember the works of Samuel at Bell and of Kilgore at RCA. Nevetherless until 1940 magnetrons were nothing more than laboratory oddities, capable of delivering few tens of watts at their best.

Fig. 1 - Early magnetron structures: A) CW10 was a split-anode magnetron designed by E.C.S. Megaw at GEC around the mid thirties. B) The RCA experimental A-103A with stabilizing electrodes, designed by Linder at RCA in 1936. C) RD4Ma was a Telefunken four segments CW magnetron capable of delivering about 14 W. D) A quite late GEC CV79, eight-segment interdigital magnetron. E) A GEC experimental 12-segment magnetron.

The credit for the revolutionary multicavity magnetron, the one that would generate hundreds or even thousands of kilowatts within months from its introduction, goes to Boot and Randall of the group leaded by Oliphant at the Birmingham University. Actually they devised an external anode structure, simple to operate and in which heat could easily be eliminated from the copper block by means of any suitable radiator. We will try to reconstruct the role of E.C.S. Megaw at GEC in the developmental steps of the first operational and reproducible device, the eight-cavity E1189 often referred to as the ‘British magnetron’.

 

The development E1189 eight-cavity magnetron at GEC

All the sources on the early story of the British multi-cavity magnetron actually talk of the six-cavity one, from the Boot and Randall prototype, first operated at Birmingham in February 1940. We know of the E1188 sealed-off design made at GEC by Megaw. E1188 was a CW water-cooled device, requiring a bulky electromagnet to operate. Thanks to Megaw, soon later E1188 evolved in two low-profile air-cooled E1189 variants, suitable to be fitted into the 38 mm air-gap of a 6 lbs magnet. The design started using a spiral-wound filament. In the meanwhile, in May 1940, Megaw received samples of the M-16 magnetrons developed at CSF and fitted with oxide-coated cathode. Laboratory tests under pulse conditions were encouraging and he decided to build a second sample of his low-profile magnetron fitted with oxide-coated unipotential cathode.

Fig. 2 - A) The prototype assembled by Boot and Randall at Birmingham. B) The sealed-off version, made by Megaw at GEC as E1188. C) The low profile, air-cooled variant designed by Megaw as E1189.

Peak power pulses in the order of 1 kW were obtained by both samples at the end of June 1940 at about 1040 oersted, in a standard 6 lb magnet. Within a couple of weeks power pulses in excess of 10 kW had been reached by the No.2 at 1400 oersted in the pole pieces of an electromagnet. The design was only partially successful. The six-cavity magnetron with oxide-coated cathode was an outstanding source of 10 cm waves at that time, but it could never operate efficiently in a magnet light enough to be used in airborne radar sets. Likely Megaw, thanks to his exceptional experience, realized right away that the design had to be re-calculated for eight-cavity. From those early days of July, all the available sources are silent on the eight-slot design, until it suddenly appears about three months later, when the E1189 No.12 is X-rayed in America. Where that 8-cavity sample sprung from? Not a word even from Megaw himself.

Bowen tells that on 7 August he attended the briefing on the magnetron construction. Later he was allowed to select the best sample out of a batch of E1189 units, tested by Megaw himself. He was briefed on the six-cavity design, since he was not aware of eight-cavity variants until 7 October, when his sample was X-rayed at Bell. Bowen tells of his phone call to Megaw ‘At first he was vague, and disbelieving - he was obviously as puzzled as I was.’. Then Bowen tells what he was led to believe, that is the No.12 sample was just an odd experimental unit, built in the same batch of the six-cavity ones and that just by chance had operated on the test bench better than the others. This is not true. Fisk in his article tells that the magnetron was operated at Bell in a field of about 1100 gauss, giving pulses in excess of 15 kW. We know that the six-cavity type at 1100 gauss would have generated no more than one or two kilowatts before arcing. The experiment at Bell was then arranged with the right parameters for the eight-slot. Somewhere, likely on the test label of that unit, operating values were correctly indicated. What really happened before Bowen left GEC with his precious luggage?

First of all we should consider the historic background of the days preceeding the Tizard Mission, also observing the close relation between some key dates of that mission and the development of the magnetron at GEC. The British had correctly estimated that their industry would not be  able to produce the magnetron quantities required by military. We know that through 1941 GEC and BTH built about 2000 units of NT98s, while Raytheon alone would have produced some 2600 magnetrons a day.

On 8 July, few days that after the successful tests on the second E1189 sample, Lord Lothian, British ambassador in Washington, submits a direct appeal to Roosevelt: ‘The British Government have informed me ... that they would greatly appreciate an immediate and general interchange of secret technical information with the United States, particularly in the ultra short wave radio field. ...’. London was ready to send a small group of British military officers and civilian scientists to the United States ‘to give you the full details of any equipment or devices in which you are interested without in any way pressing you beforehand to give specific undertakings on your side”. The appeal concluded ‘for our part, we are probably more anxious to be permitted to employ the full resources of the radio industry in this country [the United States] with a view to obtaining the greatest power possible for the emission of ultra short waves than anything else.’ On 11 July Roosevelt accepts the British proposal. Likely the encouraging tests on six-cavity E1189 had given the green light to the mission that Tizard was planning for a long time.

From the acceptance of the British proposal, everything had to be carefully prepared to send a working sample of magnetron to America. It was, as we know, the most valuable secret that the British could offer to gain the unconditional help of America, the very purpose of the mission itself. Furthermore it was the prototype to be reproduced in volume by American industries for the future British needs.

The story told so far does not help to understand where the eight-slot design came from. The recent accidental finding of an E1189 prototype, likely coming from a British Marconi warehouse, sheds new light on what really happened at GEC in the days before August 6.

Fig. 3 - E1189 eight-slot prototype recently found. Believed to be the very early of the four samples listed by Megaw in his secret report and built at GEC in the Summer 1940. First operated around 30 July.

Examining the E1189 prototype and carefully reading today the few available documents of those days, the whole story becomes clear. It tells of a true miracle performed by Megaw and his team, designing the new eight-slot magnetron and then manufacturing, testing and characterizing a couple of laboratory prototypes, while assembling two complete units, all in about a couple of weeks. Megaw in his internal report on developmental magnetrons, dated 11 October, lists four units made of E1189, 8-segment type, 1050 gauss. He writes that three were still good after respectively 20, 30 and 60 hours of operation. The fourth unit worked for 210 hours, until opening of its heater. Official story tells that the E1189 No.12 was the very first eight-slot experimental magnetron made at GEC, hence these samples were hastily assumed as built sometime later for unknown reasons. Actually Megaw in his 1946 paper writes of two sealed-off samples, No.12 and 13, performance curves being reported for the last one.

Fig. 4 - Samples of E1189 built at the date of 11 October 1940, from the Megaw’s secret internal report.

Coming to our E1189 prototype, we observe that it does not have the usual finish, finned radiator, black paint and gold-sealed caps, as it was assembled in a hurry. The GEC experimental code 1189 is punched on the copper anode. There are still traces of sealing grease on the inside edge of anode block, as the unit was powered while continuously pumped. It also shows a partially faded marker writing, where the characters ‘210HR’ can be read. Cathode oxide shows signs of heavy usage and one of the heater ends is broken near the welding to the corresponding end baffle. Few doubts it is exactly the fourth eight-slot E1189 listed by Megaw, the one with heater open after 210 hours.

Fig. 5 - Close-up views of the E1189 prototype. A) The code 1189 C 528 is punched on the anode block. C could indicate the third revision, after the six-slot designs with tungsten filament and with oxide-coated cathode. B) In the marker writing on the side characters ‘HR210’ are still well visible. C) The oxide layer is partially swollen and detached, as caused by overheating or internal arcing. D) One of the heater ends is broken just at the edge of the end baffle.

Now let us go back to the parallel stories as told by Megaw himself, by Sir Paterson in his diary and by Bowen. New samples had to be made since, according to Megaw, the E1189 No.2 had been stressed up to arcings during tests. We know that the sample No.1 had been superseded by the No. 2, ending its life into the experimental microwave set installed at GEC. The sample No.2, after the test sessions, was delivered to TRE at Worth Matravers on 19 July. The Paterson’s diary reports that in the CVD meeting on 17 July the discussion dealt with a new ‘air-cooled low field magnetron’, obviously the eight-slot one, followed by ‘General demand for specimens’. Assuming that a small batch of magnetrons had to be built for the mission and for further experiments by military, their planning was likely discussed in that meeting. Eight-slot magnetron could be the desirable if ready; in the meanwhile all the programs had to go on with the six-slot type. Consequently the decision was taken of building a batch of the 6-cavity E.1198 for the most urgent needs, while trying the venture with the 8-cavity variant proposed by Megaw. The sample that Bowen was to bring to America had anyway to be produced in volume there.

Then the plan of Megaw looks obvious. He launched the production of eight six-cavity units and in parallel he started the rush production of four eight-slot anode blocks with their matching cathode subassemblies. Two eight-slot samples were partially assembled in a hurry, no radiator, no sealed end caps, operated on the bench as soon as ready. In the meanwhile, the other two units were completely assembled and serialized as E1190 No.12 and No.13. On 6 August the eight-slot designs review, still experimental until the morning, was officially approved by ‘the crowd of visitors including Oliphant, Randall and Ellis’ described by Paterson, to replace the six-slot design.

Looking at the worked time of the three still good, 20, 30 and 60 hours, we must assume that tests were interrupted all together at a given date. Assuming they were left running about 10 hours per day and tests were all interrupted after the approval of Oliphant, Randall and Ellis, towards the end of August 6, we can conclude that three units started oscillating approximately on 5, 4 and 1 August, respectively.

Likely the last two units, those operated for 30 and 20 hours, were the E1189 No 12, brought to America by Bowen, and No. 13, used by Megaw. The fourth sample, later used in the endurance test until opening of the heater, certainly was the first unit to operate while continuously pumped, just completed with anode block and cathode subassembly. We can assume that it started oscillating since July 30, or 31 at worst, so giving Megaw the full confidence in the design while other units, including the No.12, were in process to be assembled.

After the meeting of August 6, only Megaw was aware that the sample No.12 just approved was built to the latest design review, still to be filed. GEC employees had access to production documents of the six-slot E1189. Likely on August 7 Megaw was too relaxed to think warning of the change his employees and even Bowen. Due to a small lapse, on 7 August Bowen was briefed on the six-slot magnetron and later, on 11 August, he was given blueprints and production details of the six-slot design with the sample of the latest eight-slot E1189. Here a table with the most significant steps.

Development steps at GEC of the eight-cavity E1189 magnetron

29 June 1940

Six-cavity E1189 No.1 and soon later the No.2, with oxide-coated cathode, start operating at about 1000 oersted, giving 1 kW pulses. In a few days, using an electro-magnet to increase the field to 1400 oersted, output power of the No.2 will raise to more than 10 kW.

8 July

The Tizard Mission starts officially, based upon the early successful operation of the E1189 No.2. Three days later America accepts the British proposal. Likely at that date Megaw had already realized that the design had to be changed to eight-cavity.

17 July

Paterson writes that in the meeting with CVD, part of the discussion deals with the ‘Megaw’s air-cooled low field magnetron’. There is a ‘general demand for specimens’. This is the official start for Megaw to launch his eight-cavity design review together with the six-cavity production batch

30 July

Likely the first prototype of the eight-slot E1189 starts operating while continuously pumped.

1 August

The second eight-slot laboratory prototype starts operating around this date.

4 August

The third eight-slot E1189 is sealed and starts running on the test bench. It is the No.12.

5 August

The fourth unit listed by Megaw, the E1189 No.13, starts running.

6 August

From the Paterson’s diary: ‘A crowd of visitors at GEC, including Oliphant, Randall and Ellis. The Megaw’s improved 10 cm magnetron with eight chambers appears up to expectations’. Sir Paterson takes the picture of the official approval of the eight-slot E1189 review, based upon the successful tests on the four samples. Tests are stopped all together once the decision is taken.

7 August

Bowen is briefed on the construction details of the six-slot E1189. Then he is allowed to select the best performing sample in the batch tested by Megaw himself. It is the No.12. Megaw forgets to warn the briefer and the same Bowen that the sample is eight-slot. Likely he is too busy with his latest achievement, arranging the endurance test on the very early eight-slot prototype and the complete performance characterization on the sealed sample No. 13.

11 August

Bowen is again at GEC to pick his sample and the envelope with the blueprints of the E1189. The No.12 leaves GEC and begins its travel to America.

6 October

E1189 No.12 is powered at Bell, delivering about 15 kW RF pulses at 1100 gauss.

7 October

E1189 No.12 is X-rayed at Bell unvealing its eight-slot structure

The recent discovery of the E1189 prototype, jointly with the Megaw’s internal report, his 1946 paper on the magnetron development and the diary of Sir Paterson contributed to clarify the story of the British eight-cavity magnetron development. It was not the result of mistakes, confusion and lucky coincidences. On the contrary, as other British achievements, it was the result of carefully planned and coordinated work of a brilliant person, Megaw, and his team. They succeeded in a true miracle, designing, building, testing and fully characterizing the new magnetron in a couple of weeks or so, while manufacturing the backup batch of the previous type.

Soon we will see the huge impact that the GEC E1189 design will have in England, in America and soon later even in the rest of the world and in the everyday life of us all. 

 

References

  1. Metres to Microwave, Callick
  2. Notes on magnetron development programme, E. Megaw, 11 October 1940
  3. Radar Days. Bowen
  4. The high-power pulsed magnetron: a review of early developments, E. Megaw, February 1946
  5. The Magnetron as Generator of Centimeter Waves, Fisk et al, BSTJ April 1949
  6. The Tizard Mission, Stephen Phelps
  7. Direct observation of the E1189 prototype        

All the tubes photographed in color are from the collection ase-museoedelpro

This article was edited 05.Oct.18 21:18 by Emilio Ciardiello .

Emilio Ciardiello
Emilio Ciardiello
Editor
I  Articles: 522
Schem.: 165
Pict.: 778
16.Oct.18 12:46

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Part II, early evolutions of the design in 1941

We know that the early eight-cavity samples of E1189 were ready by August 1940. Unfortunately the magnetron was just one of the components needed to build a microwave radar. Other new components had to be designed and tested, as they were, in 1940. Among the others devised in UK we remember the ‘Sutton’ reflex klystron, the silicon diode mixer and suitable switches for the pulse modulator. TR switches appeared quite late in 1941, since early microwave radar sets used two separate antennas for transmitter and receiver.

The British development

The preliminary specs for the Naval radar Type 271 and the Army radar GL3 were defined around November 1940. Copy of the GL3 preliminary specs was also submitted to Canadian REL for local productions based upon procurements of American components. We know that the first experimental radar Type 271 was installed in March 1941 aboard of the HMS Orchis. The radar became operational by July 1941 and the magnetron, simplified in its radiator, was standardized as NT98. This magnetron was manufactured by both GEC and BTH. We know that within the end of 1941 about 2000 units had been built. Quite soon sealed-off units were selected according to their frequency of oscillation, grouped into four variants A to D. Damaged units had to be returned to repair shops for rebuilding, if feasible, or to recovery precious and strategic metals, gold, copper, nickel and tungsten.

According to Megaw, the development of high-power magnetrons started right after the delivery of E1189 early samples. He reports that a sample of a 100 kW magnetron was nearly to completion at the date of 11 October 1940. It originated the CV41, manufactured in small quantities in 1941.

The frequency variant E1198, operating at 9.1 cm, was ready by September 1940 for airborne applications. It was later approved as CV38.

Fig. 2.1 - Draft of the eight-cavity E1189 magnetron. Right, two frequency selections, C and D, of NT98.

In July 1941 J. Sayers at Birmingham University devised the strapping technique to prevent moding of magnetrons. Before, at high power levels, mode jumping resulted in low efficiency and could cause the destruction of the cathode. Strapping forced the magnetron to oscillate with phase shift of 180 degrees between adjacent cavities, resulting in stable and efficient operation which opened the way to virtually unlimited increase of power. CV56 and CV64 were the early strapped magnetrons, available by the late 1941. CV56, replacing the NT98, was rated for 80 to 100 kW output pulses with about 40 % efficiency. It was followed by the more powerful CV76, capable of generating 450 kW pulses. CV64 was developed by BTH to replace CV38. In its first airborne applications its output pulse power was limited to about 40 kW, but it operated at 160 kW pulse power when a more powerful modulator based upon the new CV125 trigatron was available.

Fig. 2.2 - A) and B), two images of an experimental strapped magnetron, likely built by the Sayers group at Birmingham in the summer 1941. C) An early sample of CV56 strapped magnetron, likely build shortly after the experimentation at Birmingham and before the frequency selections was decided. D) A sample of CV56A, one of the four frequency selections. Note the suffix and the serial number added manually, actions likely performed at the completion of the test procedures.

 

Developments in America and in countries of Commonwealth

The E1189 No.12, brought to America by the Tizard Mission, originated a small batch of test and qualification samples, exact copies of the British prototype. From November 1940 Western Electric delivered some 60 units with the developmental code D-160052, part of which went to the MIT Radiation Laboratory for experimentation. This developmental magnetron was followed in 1941 by two separate productions from sister companies: in Canada the Northern Electric built for REL the 3D, while in Australia the Standard Telephone and Cables built the NTA98.

Fig. 2.3 - A) The E1189 No. 12 brought to America by the Tizard Mission. Today on display at Canada Science and Technology Museum, Ottawa. B) The Western Electric D-160052 was the Bell reproduction of the above sample. The unit S/N 56 coming from the closure of MIT Radiation Laboratory was in the collection of the late Jerry Vaniceck. C) REL 3D, made by the Canadian Northern Electric, was equivalent to the British NT98. Northern Electric was a Bell owned company. D) REL 3C was the equivalent of the British frequency variant made for Air Ministry, the E1198 approved as CV38.  

In America the first magnetron family designed at Western Electric in 1941, the 700 A to D, considerably departed from the British E1189. It was a family of six-cavity low-frequency magnetrons, which included four frequency variants around 700 MHz. This because Bell at the time of the Tizard Mission was working at its Whippany radio laboratory to the development of the fire-control radar CXAS for the Navy, operating between 500 and 700 MHz. When in October 1940 Bell arranged the test of the British E1189 magnetron, their transmitter, based upon their best doorknob triodes, generated a kilowatt or so in output. They saw the immediate opportunity for switching to the new device. The first magnetron designed at Western Electric was readily integrated in the transmitter of CXAS, raising output pulses to 40 kW.

Fig. 2.4 - Left, the transmitter of the CXAS radar. The RF oscillator is on the top. Center, the 700A magnetron which gave a forty fold increase of the pulse power. Right, 706 was the first 10 cm magnetron designed by Western Electric. With respect to the British E1189, the WE design had a more efficient finned radiator and side fixing brackets. It was rated for about 25 kW output pulses. The sample in the photo is a later strapped unit, rated for output pulses exceeding 200 kW.

We will see elsewhere subsequent developments of the magnetron E1189 in England, in America and from 1943 also in Germany.

 

References:

 

This article was edited 16.Oct.18 12:52 by Emilio Ciardiello .

  
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