Milestones:Cavity Magnetron

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Title

Development of the Cavity Magnetron, 1939-1941

Citation

In this building from 1939 to 1941, University of Birmingham researchers John Randall, Harry Boot, and James Sayers conceived and demonstrated fundamental ways to improve the output power, efficiency, and frequency stability of cavity magnetrons. Further developed and refined by others, these advances facilitated the Allies' deployment of microwave radar systems in World War II. Cavity magnetrons were later adapted for use in industrial heating and microwave ovens.

Street address(es) and GPS coordinates of the Milestone Plaque Sites

Poynting Building, University of Birmingham, Birmingham UK B5 7SW, Poynting Building, University of Birmingham, Birmingham UK B5 7SW

Details of the physical location of the plaque

standard external wall mounting at appropriate height

How the intended plaque site is protected/secured

These University buildings are open to the public during working hours.

Historical significance of the work

Summary

Following the independent development of radar in many countries from 1934-9 there was a need to efficiently achieve high power microwave energy in a lightweight generator to enable the advance from land based into airborne mapping radar. This was enabled by the development of the cavity magnetron. John Randall and Harry Boot at the University of Birmingham produced, in 1940, their first working example of a major advance in the design of cavity magnetrons which enabled, for the first time, the generation of hundreds of Watts of pulsed power at 10 cm wavelength. Shortly afterwards engineers at the General Electric Company, London, further engineered this design to generate well over a kilowatt of pulsed power, and by 1941 eventually on to over 100 kW. Jim Sayers engineering contribution was to stabilise the generated frequency by reducing the number of modes or individual frequencies in the generated oscillations. These high power microwave pulses could be transmitted from an antenna only tens of centimeters long, reducing the size of practical radar systems by orders of magnitude, enabling them to fit in an aircraft nosecone for the realisation of long range night-fighter and anti-submarine systems. Airborne radar enabled, for the first time, the detection of small targets such as surfaced submarines which was key to winning the battle of the Atlantic. As Britain at the time had limited funding and did not have the capability to mass-produce these magnetrons in the required volumes, Winston Churchill agreed that Sir Henry Tizard should offer this magnetron design to the Americans in exchange for financial and industrial help. Several examples of an early 10 kW version, built in England at the General Electric Company, were taken to America on the Tizard Mission in September 1940. This design was immediately adopted for mass production to enhance the operational capability of airborne radar systems for wartime use.

This application seeks to celebrate the following technical achievements in the Birmingham laboratory, by Randall, Boot, and Sayers: significantly increased power level; outstanding improvement in efficiency; and securing frequency stability; all of which were essential to the magnetron's eventual successful deployment in the Battle of the Atlantic.

Preamble

In December 1935, the British Treasury appropriated £60,000 for a five-station early radar system called Chain Home [1], covering the approaches to the Thames Estuary to identify the presence of incoming hostile aircraft and direct their interception by fighter aircraft. This system, which used high energy pulses generated at 20 or 30 MHz, was a rather basic radar design employing tall wooden and steel towers to support the physically large antennas. Randall and Boot spent the summer of 1939 visiting one of these Chain Home installations [4c] and this encouraged them to attempt to develop high power generators at microwave frequencies. To expand the Chain Home into an airborne radar required the use of much higher microwave frequencies to achieve the smaller sized transmitter and receiver antennae designs required for inclusion in an airframe. Early efforts at generating these microwave frequencies were often based on klystron and magnetron tubes but their power output was found to be far too small to achieve the required range to build an operational airborne radar system. In 1938 the shortest wavelength on which any significant power could be generated was ~ 1.5 m (200 MHz).

The Magnetron

The magnetron name originally arose out of work by Albert Hull in 1921 at GE Schenectady in New York on the use of magnetic fields to control the current in a vacuum tube.[2] The development of magnetrons with multiple cathodes was initially undertaken at Bell Telephone Laboratories in America from 1934, by Philips, the UK General Electric Company (GEC), Telefunken and others, but these devices were limited to typically modest 10 W output [2]. In the 1930s, the British expert on magnetron design was Eric Megaw at GEC who published reviews [5] of the mechanisms for their generation, including split-anode magnetron designs.

The resonant cavity magnetron design used the interaction of a stream of electrons with a magnetic field while moving past a series of open metal cavity resonators. These early cavity magnetron designs, which incorporated internal resonant elements to control the operating frequency, had been investigated and patented by several independent groups in the late 1930’s [2,3a,3b], such as Hansen at Stanford, Samuel at Bell Labs, Hollmann and Engberg at Telefunken, Posthumus at Philips, Ponte and Gutton in France, Alekseev and Malairov in Leningrad, Okabe and Nakajima in Japan etc. However, in spite of the many published papers and patents, not all the designs were actually built as prototypes and, generally, the resonator concept had been rejected as somewhat inflexible.[3a]

Nowhere in the world in 1939 was there a working, pulsed, cavity magnetron capable of generating 10 kW or more peak power at wavelengths of 10 cm or less, which had a compact portable size, used a small permanent magnet and which was readily capable of being manufactured at scale.[3a] Randall, Boot and Sayers were not the first innovators of the Cavity Magnetron as a microwave generator, but their engineering advances increased significantly the generated power to over a kW and further ensured its mass production which had a major wartime benefit. The patents included below [4a] were secured in recognition of their inventive contributions to the development of a manufacturable high power device.

Design Breakthrough

The radical improvement in power and manufacturability was achieved by the cavity design of John Randall and Harry Boot group, working under a British Admiralty contract, at the University of Birmingham, England in 1940 [4b,4c] and they secretly patented their device that August, along with their Admiralty sponsor.[4a] As mentioned above, cavity magnetron designs were really “a simultaneous invention” in many different nations [3b] but dissemination of the various design details was somewhat patchy! Randall arrived at the inspirational idea of using concentric cavities when he researched the design of the original Hertz oscillator, which was an open single ring. Randall had earlier visited, while on holiday, the University College bookshop in Aberystwyth where he found and acquired a copy of Jones's translation of Hertz's "Electric waves".[3a,4c]

It is somewhat difficult to establish is precisely how much Randall and Boot were inspired by the cavity klystron of Hansen and other cavity-anode ideas that had existed since 1934? In his Guthrie lecture [3c] to the Physical Society in 1945 Randall [4b] described the sequence that first led them to the possible use of cavity resonators to enhance the power output of the simple magnetron and then to envisage a three-dimensional equivalent of the Hertz resonant wire loop. It is stated by the group of researchers at the University of Birmingham that they were not fully aware of other developments. [4b,4c] Thus the Birmingham group are now largely credited with implementing the first high-power version of this microwave device which was easily reproducible and readily adapted for mass production.

Randall and Boot developed their cavity magnetron design in November 1939 and they showed their first copper block to Lawrence Bragg and Edward Appleton when they paid a visit to the Birmingham laboratory during that month.[3a] In late 1939 Randall and Boot had converged on a 6 cylindrical resonator geometry, slotted so that the slots were parallel to the cathode axis, which they opened into the anode-cathode space as a cylindrical extension of the original Hertzian dipole. The resonating chambers were thus built with a physical size which was matched to the wavelength of the operating microwave frequency, to boost the signal. Randall’s structure is claimed in [2] to differ from the prior cavity designs (hollow spheres, cubes and “doughnut” shaped cavities) as he was thought to be the first person to introduce the cylindrical symmetry into the magnetron design, but [3b] somewhat contradicts this claim! However, a significant difference between the Randall, Boot and Sayers cavity magnetron design and those previously patented was that nearly all the others had their anode system inside a glass envelope containing a vacuum whereas the Birmingham valve had its vacuum system inside the anode structure. This was the design feature which achieved much more efficient cooling of the anode system to permit the higher power dissipation and generate the larger output power.

When initially switched on Randall’s device built within a cylindrical copper block a series of three-dimensional resonators which, with the cathode along the axis of the cylinder and in an appropriate magnetic field, produced, on February 21 1940, about 400 W continuous wave at ~10cm wavelength. Randall and Boot provide their reminiscences on this development in [4c] with [5] covering subsequent industrial developments and [2,3a,3b] assesses their achievements in the context of the other innovators. Randall, Boot and Sayers technical innovation is often seen as an unprecedented achievement compared to all the pre-existing magnetron designs.[6] In retrospect Randall, Boot and Sayers design is viewed as a major technological revolution as they made the novel, innovative steps which paved the way for generations of magnetron devices at exactly the right time for the war effort. However, their cavity magnetron device was a laboratory prototype, not suited for field operation.

In April 1940, the British Admiralty signed a contract [3b] with the General Electric Company (GEC) at Wembley, England, to produce an operational pulsed device. The GEC device was required to operate with neither vacuum pumps nor an external generator of the magnetic field. The GEC engineers, led by Eric Megaw, performed the required further development on the academic prototype device by deploying their industrial techniques to improve the design of the vacuum seals and thus remove the pump requirement.

Megaw had benefited from 5 years of collaborations, with French engineers, on magnetron design in particular in developing oxide coated cathodes. Megaw thus transformed the Birmingham magnetron design into a sealed-off version capable of manufacture in quantity. The GEC engineers extended the resonator system of Randall, Boot and Sayers to 8 cavities with a compact sealed-off all-metal and air-cooled housing, a reduced axial dimension minimising the air gap for the magnet, and an enlarged thoriated-tungsten spiral cathode. In June 1940 the GEC improved design gave a pulsed output power of 3 kW when employing a 1000 Oersted permanent magnet [3b]. The pulsed power output was immediately improved to 10 kW, within months to 25 kW, and on to over 100 kW by 1941!

Sayers contribution to the Birmingham development was that he brought his prior knowledge of vacuum tube design and noted the excessive frequency noise. He thus suggested in August 1941 [4d] strapping the cavities to constrain them to generate oscillations in one or more particular modes, to the exclusion of others over a wide range of operating conditions, which provided the major improvement in frequency stability. The number of possible modes or frequencies in the generated oscillations is limited by the provision of electrical connections or "straps" between selected points on the resonator system and his “strapping” technique continues in use and in patent awards today. [4a2,4a3] Sayers technical innovation was acknowledged when he joined Randall and Boot in securing their 1949 award from the “Royal Commission on Awards to Inventors 1946”.

Airborne Radar

Air-to-Surface Vessel, (ASV) airborne military surveillance radar [10], was developed immediately prior to the start of World War 2 specifically for RAF Coastal Command and the Fleet Air Arm. These radar systems, which were developed by among others Edward (Taffy) Bowen, operated at a somewhat higher (~200 MHz) frequency than in Chain Home, and they employed dipole and Yagi transmit and receive antennae fixed to the fuselage. Although these ASV Mk I and II radars could detect large ships at range up to 60 miles, the ASV radar signals could be readily detected at long range with the Metox radar warning receiver and this permitted target submarines to safely dive. Key wartime operational requirements were to advance to higher microwave frequencies to avoid radar signal detection, improve target resolution, and combine this with a compact antennae which can be readily rotated or scanned to further provide a mapping capability.

The required high power magnetron pulses were generated from a device the size of a small book (plus the permanent magnet) and radiated from an antenna only a few tens of 10's of cm long, reducing the size of practical radar systems by orders of magnitude. The cavity magnetron thus enabled new compact radars to be designed for deployment on aircraft such as submarine hunters and night-fighters and also on the smallest of escort ships. A 10 cm wavelength radar achieves superior angular resolution and different objects have very different radar signatures; water, open land and built-up areas of cities and towns all produced quite distinct returns and this enables the radar to map the ground below the aircraft to assist navigation as well as targeting munitions delivery, even through cloud.

An early test of a complete microwave radar transmit/receive system took place in May 1940 at the Telecommunications Research Lab in Swanage, England, where a submarine periscope was reported as being detected at 11 km distance [4c] (presumably in a calm sea).

The first operational deployment of an airborne microwave or centimetric radar was the early 1942 Airborne Interception (AI) radar which used the British manufactured CV64 magnetron. Another deployments was in the ASV Mk. III sea-surface search submarine detection radar. The most widely-deployed British cavity magnetron based radar was the H2S radar. H2S Mk. I, which operated at S-band (around 3 GHz), entered service in early 1943 and was designed as a ground mapping bombing aid.

Early in the war, U-boats routinely ran on the surface as their speed and range were both very limited when submerged. They normally submerged only when conducting daytime torpedo attacks or when they were themselves being attacked. H2S Mk. II was used for hunting these surfaced U-boats. Later in the war U-boats operated on the surface mostly at night in order avoid detection by either ships or aircraft. Following the advent of airborne microwave radar, surface operation became dangerous and they had to resort to operating submerged, but still close to the sea surface, by deploying a schnorkel. The schnorkel permitted battery charging while submerged close to the surface and the radars at that time struggled to detect the schnorkel except in a calm sea, and then only at up to ranges of 4 miles or so.

H2S was last used in anger during the Falklands War in 1982. Some H2S units remained in service for more than 50 years, until 1993. The US equivalent operated at X-band (around 10 GHz), was denoted H2X, and was in service from October 1943.

From 1940 onwards the Allies of World War 2 held a technical lead in radar systems that their counterparts in Germany and Japan were never able to close, even though they knew about and had previously patented earlier versions of these magnetron devices. The development of the cavity magnetron was so sensitive that aircraft were not permitted to fly over Germany until 1943 and they were fitted with explosives to destroy the magnetron if the plane was shot down, to ensure that the enemy were unable to learn about the device. In February 1943 a Stirling bomber carrying a cavity magnetron powered 10 cm H2S radar crashed near Rotterdam when the Germans finally acquired a complete H2S system design and they quickly copied it but it was too late to have a significant effect on the outcome of the war. By the end of World War 2, practically every Allied radar was based on a cavity magnetron. After the war in 1947 British cavity magnetron production was taken up at EEV in Chelmsford and they lobbied for additional financial reward for the inventors. Today the cavity magnetron is located in the microwave oven in 93% of UK households, with US sales exceeding 10 million new microwave ovens p.a.

Technology Transfer

Because France had just fallen in World War 2 and Britain was strapped for the funding and manufacturing capability for the cavity magnetron on the required massive scale, Winston Churchill agreed that Sir Henry Tizard should offer the magnetron to the Americans in exchange for their financial and industrial help. The 12th device, a 10 kW version, built in England by the General Electric Company London, was taken on the Tizard Mission in September 1940.[7,8] This cavity magnetron was a small, air cooled, compact device and, from no. 12 onwards, they were designed with 8 cavity resonators.

As the discussion turned to radar, the US Navy representatives detailed the problems with their short-wavelength systems, complaining that their klystrons could only produce 10 W. One of the British mission members, Edward (Taffy) Bowen, a Welsh physicist and radar pioneer, pulled out from his briefcase a cavity magnetron and explained that it could already produce 1000 times that power. A meeting report stated: "The atmosphere was electric - the US experts found it hard to believe that such a small device could produce so much power, and that what lay on the table in front of us might prove to be the salvation of the Allied cause."

Subsequent testing at Bell Labs showed that the new design produced 10 times the output power at 5 times the frequency of the best performing American triodes! [2] Bell Labs quickly began making copies, and before the end of 1940, the Radiation Laboratory was set up from 1940-1945 on the campus of the Massachusetts Institute of Technology to develop various types of radar system using the principles of this cavity magnetron design. The 1940 cavity magnetron thus became the heart of more than 150 new radars of all categories designed between 1941 and 1944. [3b]

Subsequent Recognition

In 1945 Randall, Boot and Sayers were awarded the Duddell Medal and Prize by the Physical Society of London.

The “Royal Commission on Awards to Inventors 1946” was formed after the war to judge how much UK Civil Servants should receive if the “British Crown" profited during wartime from their inventions. Randall, Boot and Sayers initially shared in 1949 such a £30,000 award and they received additional financial personal benefit after English Electric Valve (EEV) Chelmsford took up their cause.

Randall and Boot's innovative development of the magnetron into a high power manufacturable device was "massive technological breakthrough" and "deemed by many, even now, to be the most important invention that came out of the Second World War"!

Further, the official historian of the Office of Scientific Research and Development, James Phinney Baxter III, wrote: "When the members of the Tizard Mission brought the cavity magnetron to America in 1940, they carried the most valuable cargo ever brought to our shores." [8]

Further discussion of the significance of these technical developments are contained in the reminiscences by Churchill’s wartime director of scientific intelligence, R V Jones, latterly Professor of Natural Philosophy at the University of Aberdeen. [9]

Some Parallel Developments

We know that Ponte had brought to GEC Wembley in May 1940 [3b] two samples of the French “M¬16” magnetron device which he had developed with Gutton. This device incorporated an oxide-coated 0.45 cm diameter cylindrical cathode to replace the spiral filament and this provided the much longer device lifetime. Ponte was not shown on this visit to GEC the secret Birmingham cavity design, but he provided Megaw with significant technical information which was incorporated into their GEC E-1189 device design. The GEC E-1189-b device, as tested in June 1940, gave an increased peak power of 15 kW, with a lifespan, which exceeded all expectations [3b].

After World War 2, British scientist were able to access the 1943 German report [3c] on the H2S aircraft crash in Belgium, which had been prepared by Otto Hachenberg but was only discovered sometime after his death in 2001. This revealed that the principle of the cavity magnetron was already known in Germany at Telefunken during World War 2, based on work previously published in Leningrad by Alekseev and Malairov [3d]. They had constructed a four-segment cavity magnetron that produced 300 W at a wavelength of 9 cm (3.3 GHz) but this development was not known about in England in 1939 as these results were only openly published first in Russian in 1940 and later in English in 1944.[3d]

In April 1953, Nakajima discovered with great surprise on a visit to the UK the E-1188 GEC magnetron design in the London Science Museum. On examining this he claimed [3b] that: the dimensions of glass covering the vacuum; the water-cooling system around the anode; and the anode mechanism, very strong similarity with his eight-cavity “M3” magnetron, Japanese design from April 1939, which generated 500 W at 10 cm, somewhat earlier than the British designs.

So it appears in retrospect that the Russian, German and Japanese scientists all possessed cavity magnetron technology, with somewhat reduced output power compared to the British developments at Birmingham University and GEC, but they all failed to recognise the true significance of this for developing microwave airborne radar system with capability for mapping, munitions delivery and small target submarine and aircraft detection which became the main contributor to the successful end of the Battle of the Atlantic.

Today cavity magnetrons are located in the microwave oven in 93% of UK households, with US sales exceeding 10 M new microwave ovens each year. Thus, almost everyone now relies on these wartime innovations for fast, efficient, low power cooking. In industry high power microwave heating and drying systems are widely deployed for food, liquids, and materials processing. [11]


Justification for Inclusion of Names in the Citation:

The technical advances in developing the high power cavity magnetron was acknowledged by the British Government Commission in 1949 after WW 2 when John Randall, Harry Boot, and James Sayers secured their “Royal Commission on Awards to Inventors”. This Commission was responsible for judging how much British Civil Servants should receive if and when the “British Crown" profited from their inventions. The Commission initiated the very significant £30,000 prize (worth more than $0.8 M today) to John Randall, Harry Boot and James Sayers. Although many researchers round the globe worked on cavity magnetron designs John Randall, Harry Boot, and James Sayers were the actual individuals who implemented the first high-power cavity magnetron which was easily reproducible and readily adapted for mass production.

Two expert reviews confirming the veracity of this evidence and providing support for including the names, as proposed in the citation, are provided by two distinguished Fellows of the Royal Society: Cyril Hilsum (Past Director of Research at the General Electric Company, IEEE David Sarnoff Medal in 1981, IEEE Third Millenium Medal, Foreign member of the National Academy of Engineering and Past President of the Institute of Physics); and Professor Hugh Griffiths (President of the IEEE AES Society 2012-13, IEEE AES Distinguished Lecturer, AES Radar Systems Panel chair from 2007-09, and chair of the Working Group which revised the IEEE Radar Definitions Standard P686 and reaffirmed the Radar Letter Band Standard. He was awarded the 2018 IEEE Pickard Medal).

Features that set this work apart from similar achievements

Nowhere in the world in 1939 was there a working, pulsed, cavity magnetron capable of generating 10 kW or more peak power at wavelengths of 10 cm or less, which had a compact portable size, used a small permanent magnet and which was readily capable of being manufactured at scale. This was the design breakthrough achieved at Birmingham University.

Their magnetron design achieved over 90% efficiency which was the major advance over the 30% efficiency achievable with klystrons, triodes, etc. This high efficiency enabled the achievement of kW's of output power.

Significant references

[1] Hugh Griffiths, “Some Reflections on the History of Radar from its Invention up to the Second World War”, Maxwell Foundation Newsletter No 8, Spring 2017. ISSN2058-7511 https://www.clerkmaxwellfoundation.org/Newsletter_2017_Spring.pdf

[2] Proceedings International Conference on the ‘Origins and Evolution of the Cavity Magnetron’, IEEE Cat. No. CFP10771-CDR, ISBN: 978-1-4244-5610-9, Library of Congress: 2009938875, 19-20 April 2010.

[3a] Mike Diprose, ‘The cavity magnetron: known knowns, known unknowns, but are there any unknown unknowns?’ 11th South Yorkshire Network IET Annual Radio/Radar lecture, South Yorkshire Aircraft Museum, Doncaster, UK, October 2015, PDF supplied.

[3b] Yves Blanchard, Gaspare Galati and Piet van Genderen, “The Cavity Magnetron: Not Just a British Invention”, IEEE Antennas and Propagation Magazine, Vol. 55, No. 5, pp. 244-254, October 2013. doi‎: ‎10.1109/MAP.2013.6735528

[3c] Bernard Lovell, “The Cavity Magnetron in WW II: was the Secrecy Justified”, Notes Rec. Royal Society London 58 (3), 283–294 (2004) doi 10.1098/rsnr.2004.0058

[3d] N. F. Alekseev and D. E. Malairov, ‘Generation of high-power oscillations with a magnetron in the centimeter band’, Proceedings Institution Radio Engineers, 32, 136–139 (1944).

[4a1] J.T. Randall, H.A.H. Boot, and C.S. Wright [Director of Scientific Research, Admiralty, London], “Magnetron: Improvements in high frequency electrical oscillators”, UK Patent GB 588185A, filed Aug. 22, 1940. https://patents.google.com/patent/GB588185A/en?oq=GB+588185

[4a2] C.S. Wright, J.T. Randall, J. Sayers, H.A.H. Boot, and R.H.V.M. Dawton, “Improvements in and relating to high frequency electrical oscillators”, UK Patent GB 588917A, filed Dec. 17, 1941.

[4a3] J. Sayers and C.S. Wright, “Improvements in high frequency electrical oscillators”, UK Patent GB 588916A, filed Oct. 3, 1941.

[4a4] J.T. Randall and H.A.H. Boot, “Magnetron”, US Patent US 2648028A, filed 1947.

[4b] H.A.H Boot and J. T. Randall, “The cavity magnetron”, Journal Institution of Electric Engineers, vol. 93, Issue 5, pp. 928-938, 1946. doi: 10.1049/ji-3a-1.1946.0183 also J. T. Randall, ‘The cavity magnetron’, Proceedings Phys. Society London 58, 247–252 (1946).

[4c] H.A.H. Boot and J.T. Randall, “Historical notes on the cavity magnetron,” IEEE Transactions on Electron Devices, vol. 23, no. 7, pp 724 – 729, Jul 1976. doi: 10.1109/T-ED.1976.18476

[4d] J. Sayers, “High Frequency Electrical Oscillator”, US Patent 2546870 awarded 27/03/1951 on priority 03/10/1941. https://patents.google.com/patent/US2546870A/en?oq=US2546870

[5] E.C.S. Megaw, “The High-Power Pulsed Magnetron, a Review of Early Developments”, Journal of the Institution of Electrical Engineers, pp. 977 – 984, vol. 93, Issue 5, 1946. doi: 10.1049/ji-3a-1.1946.0187

[6] Allison Marsh, “From World War II Radar to Microwave Popcorn, the Cavity Magnetron Was There”, IEEE Spectrum, November 2018. https://spectrum.ieee.org/tech-history/dawn-of-electronics/from-world-war-ii-radar-to-microwave-popcorn-the-cavity-magnetron-was-there

[7] Angela Hind, "Briefcase 'that changed the world'". BBC News February 5, 2007

[8] B. Schroter, "How important was Tizard's Box of Tricks?", Imperial Engineer, 8: 10, Spring 2008. http://www3.imperial.ac.uk/pls/portallive/docs/1/44009701.

[9] R.V. Jones, “Most Secret War”, Hamish Hamilton, London, 1978 ISBN: 9780241897461

[10] S. Watts, “Airborne Maritime Surveillance Radar: Volume 1, British ASV Radars in WWII 1939-1945”, Morgan & Claypool, 2018 ISBN: 9781643270661.

[11] A.C. Metaxas, R.J. Meredith, “Industrial Microwave Heating”, Institution of Engineering and Technology, 1988. ISBN-13: 978-0-906048-89-4 or ISBN: 0-906048-89-3

Supporting materials

Media:Cavity_Magnetron_figures.pdf<br><br> Media:Diprose_York_Magnetron_lecture.pdf<br><br>