MAGNETRONS The Evolution & Operation of Chuck Hobson BA, BSc(hons)

1. MAGNETRONS The Evolution & Operation ofChuck Hobson BA, BSc(hons)

2. Introduction Who invented the magnetron? When I started to look into this, I soon realized that there was no simple answer to this question.   Basically, the magnetron is a simple electronic diode in a strong magnetic field. Electrons move from the cathode to the anode though a magnetic field, which is at right angles to the direction of electron motion. As such, the electrons experience another force at right angles to both their direction of motion and the magnetic field. This results in the electrons taking a curved path. The laws governing this motion are identical to the laws governing the rotation of a dc motor. The dc motor motor came about during the early mid 19th century. The oscilloscope made its entrance during the early 20th century. The effect of a magnet on an oscilloscope beam gave scientists a clue and something to investigate. It wasn’t long before scientists the world over were experimenting with electron beams in strong magnetic fields and observing oscillations. Up until WW2 these scientists were in communication with each other exchanging findings and experimental results.

3. Two such Scientists

4. EVOLUTION OF THEMAGNETRON

5. MAGNETRON TIME LINE • 1921 A. W. Hull invented magnetron. Cylindrical anode • 1927 Kinjiro Okabe at Osaka University introduced the split anode magnetron. Oscillated at 2.5gHz (12cm) • 1933 – 1945 Japanese Navy experimented with Okabe’s magnetron and various anode configurations • 1934 Posthumus at Philips developed 4 seg. Magnetron • 1934 A. L Samuel Bell Tele filed patent 4 cavity magnetron • 1935 Hans Hollmann Germany patented cavity magnetron • 1936 Cleeton & Wllliams reached 47gHz with split anode • 1937 Aleksereff and Malearoff 4 cavity magnetron • 1940 University of Birmingham & GEC developed high power µ-wave magnetron suitable for radar application

6. MAGNETRONS GERMANY 1920Heinrich Barkhausen 0.3 - 6.4gHz at 5W Electron cloud surrounds filament Pos. grid attracts electrons Electrons accelerate through grid Electrons near anode repelled back through grid. Electrons oscillate around grid RF taken off grid (glows white hot) Barkhausen Oscillator (not a magnetron) 1935 Hans Hollmann patented cavity magnetron in Berlin German military rejected it for radar application because of excessive frequency drift. However they used klystrons for their Wurzburg) radar. 5 – 11kW peak pwr. 2µsec pulse width

7. HULL’S 1921 MAGNETRON (US) • Cavity magnetron Coaxial configuration Electron path Frequency: 200kHz increasing to 10MHz 1925 Elder of GE (US) produced 8kW @ 30kHz 69% efficiency

8. OKABE’S 1927 SPLIT ANODE MAGNETRON • Plate and cathode enclosed in glass envelope Electron path cathode to anode Strong magnetic field parallel to cathode Oscillates at 2.5gHz (12cm)

9. MAGNETRON WAR TIME JAPAN Shimada Laboratory at the Technical Institute of the Japanese Navy had been carrying out experiments on high power microwaves since 1933 Below are some magnetron anode configurations involved. Frequency was 2.5cm (12gHz) Various configurations named after Japanese flowers C Kosumosu (Rising sun) U Umebachi (Apricot flower) Above information from paper by Professor Koichi Shimoda

10. MAGNETRON WAR TIME JAPAN Shimada Laboratory, Technical Institute of Japanese Navy, Shizuoka Prefecture in 1944

11. MAGNETRON RUSSIA 4 cavity magnetron Russia 1937 Aleksereff and Malearoff 300W CW @ 10cm 20% efficiency No record of Russian military using it in radars

12. MAGNETRON WAR TIME UK • 1938 Admiralty awarded GEC a development contract. • 1940 April, GEC bread-boarded a 25cm operating radar • Transmitter produced 25kW pulses using Hi-Pwr. Triodes • University of Birmingham: J. T. Randall and H. A. H. Boot • Literature on Magnetrons world-wide but unobtainable • 1940 Feb. Developed 9.4cm (3.91gHz) 400W CW Magnetron • GEC produced two magnetrons using R & B as a model • 1940 June Pulse powers of 10 to 40kW at 10cm achieved • 1940 Aug. Tizard and team brought magnetron to the U. S. • Sept. Mag. at MIT Labs. Bell Labs & Raytheon Co. x-rayed Mag. & reproduced it.By Nov. it was in mass production

13. MAGNETRON WAR TIME UK Experimental magnetron University of Birmingham Randall and Boot’s first experimental magnetron. Produced 400W CW at 3.91gHz (a true break through) The anode had six cavities ** and was water cooled Used 0.75mm tungsten rod as a filament for the cathode Tube was continuously pumped and placed between the poles of an electromagnet.

14. J. T. Randall & H. T. Boot

15. Born in Belfast Dr. Eric Stanley Megaw Born in Belfast Educated at Queens University Avid radio enthusiast Transmitted the first amateur Radio signals out of Ireland in 1924. First QSO’s with West Coast US and Australia Worked for GEC for 16 years. Headed group which took the Boot and Randell magnetron design and developed the E-1189 Magnetron. This included improvements making it suitable for airborne radar use. It was actually Megaw who added the straps which made the magnetron a stable µ-wave oscillator Megaw was awarded the MBE in 1951 for his µ-wave work Became Director of Physical Research with the Admiralty

16. E-1189 MAGNETRON E-1189 GEC no. 12 Megaw Photo of actual magnetron Tizard took to N. America E-1189: The 1st GEC magnetron had 6 cavities ** Subsequently modified to have 8 cavities (No. 12) Freq. 3297MHz peak Pwr. 12kW Peak anode current 7A Magnetic field 1050 gauss (0.105 Tesla) ** Dr. Boot used a Colt 45 revolving chamber as a drill fixture at U. of Birmingham for his first magnetron.

17. E-1198 MAGNETRON E-1198 8 cavity 12.5kW 3gHz (10cm) 1500 Oersteds

18. MAGNETRONS CV 38 E-1198 8 cavity magnetron Fil. 6V Nom. Freq 3297MHz Pk. Pwr. 7kW Magnet 1050 gauss

19. MAGNETRONS X-band magnetrons CV-208 glass enclosed probe which is inserted in wave-guide 2J49, 725A, 730A shows x-band wave-guide outputs 725A output 9375MHz at 60kW Western Electric manufactured and delivered 89000 units to the British Empire during WW2

20. MAGNETRON OPERATION

21. MAGNETRON APPLICATION • Magnetrons are used primarily in: • Radar Transmitters (pulsed) • Peak power from ~10kW to 3MW + • Frequency from ~600MHz to 47gHz + • Microwave Ovens (CW} • Frequency 2.45gHz • Output power 650 – 1200W Efficiency ~ 65% • Specialized Industrial applications

22. MAGNETRON CONSTRUCTION Typical S band 50 kW magnetron used in military radars Driven by a 30kV 1.0µsec pulse. Efficiency ~ 30% (WW2) now ~ 65% Input peak power 167kW Peak current 5.6A With 1000 repetition rate, average input ave. power 167W

23. MAGNETRON CONSTRUCTION Cutaway view of the magnetron Open area between cathode & anode called Interaction space E & H fields interact on electrons to get µ-waves in cavities

24. MAGNETRON CONSTRUCTION Another cutaway view of the magnetron

25. MAGNETRON CONSTRUCTION Magnetron eight cavity anode µ-wave energy is induced in all cavities by moving electrons Cavities in series. Energy coupled to output loop as shown

26. MAGNETRON CONSTRUCTION One of 8 cavities Equivalent circuit of one cavity Eight equivalent circuits shown in series Typical of German and Japanese magnetrons [Unstable]

27. MAGNETRON CONSTRUCTION Alternate cavities strapped together with solid copper rings Dr. Megaw’s addition to the Boot Randall magnetron configuration Schematic of eight strapped cavities Note that all cavities are connected in parallel This insures that oscillations in all cavities are in phase

28. HOW DOES A MAGNETRON WORK? Various anode forms Magnetic field provided by strong permanent magnet • Producing µ-waves can be subdivided into four phases: • Production and acceleration of an electron beam • Velocity-modulation of the electron beam • Forming of a “Space-Charge Wheel” • Dispense energy to the ac field

29. MAGNETRON OPERATION PHASE 1 Cathode centre at high negative volts Anode at zero volts No magnetic field Electrons move in straight line Magnet added North pole on top South pole at bottom Electrons curve to the right Electrons curve more when the magnetic field is increased

30. MAGNETRON OPERATION PHASE 1 Green path Weak magnet. All cathode electrons reach anode Red path Magnetic field increased to “critical” value. Anode current decreases to a small value. White path Magnetic field increased further. Anode current drops to zero Magnetic field adjusted to where electrons just fail to reach the anode, the magnetron can oscillate

31. MAGNETRON OPERATION PHASE 2 Interaction space between cathode and cavities 2 electric fields, ac & dc in interaction space Polarity is one instant of ac (µ-wave) field The dc field extends radially from cavities to cathode Electrons near cavities move tangentially to cavities Electrons approaching the positive sides are speeded up Electrons departing the positive side and approaching the negative side are slowed down.

32. MAGNETRON OPERATION PHASE 3 12 cavity magnetron Rotating 6 spoke space charge Space charge gives µ-wave energy to the cavity keeping it oscillating 8 cavity magnetron 4 spoke wheel

33. MAGNETRON OPERATION PHASE 4 Assume dc field & rf fields on cavities (magnetron oscillating Electron approaching cavity gives up energy to cavity Electron slows down accordingly Then electron speeds up gaining energy from dc field Electron eventually reaches cavity (anode current)

34. MAGNETRON RADAR CIRCUIT • PFN charges up to 12kV(dc resonance phenomena) • Trigger switches thyratron on • PFN discharges through transformer and thyratron • During discharge PFN develops rectangular pulse • Transformer steps negative 6kV pulse up to 30kV • Magnetron oscillates for duration of pulse (~ 0.5 to 4µsec

35. Thank you for viewing my Magnetron presentation.I hope you found it informative and enjoyable.Chuck Hobson BA, BSc(hons) Coments