Presentation on theme: "MAGNETRONS The Evolution & Operation of Chuck Hobson BA, BSc(hons)"— Presentation transcript:
1 MAGNETRONS The Evolution & Operation of Chuck Hobson BA, BSc(hons)
2 IntroductionWho 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.
5 MAGNETRON TIME LINE1921 A. W. Hull invented magnetron. Cylindrical anode1927 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 configurations1934 Posthumus at Philips developed 4 seg. Magnetron1934 A. L Samuel Bell Tele filed patent 4 cavity magnetron1935 Hans Hollmann Germany patented cavity magnetron1936 Cleeton & Wllliams reached 47gHz with split anode1937 Aleksereff and Malearoff 4 cavity magnetron1940 University of Birmingham & GEC developed high power µ-wave magnetron suitable for radar application
6 MAGNETRONS GERMANY 1920 Heinrich Barkhausen 0.3 - 6.4gHz at 5W Electron cloud surrounds filamentPos. grid attracts electronsElectrons accelerate through gridElectrons near anode repelled back through grid.Electrons oscillate around gridRF taken off grid (glows white hot)Barkhausen Oscillator (not a magnetron)1935 Hans Hollmann patented cavity magnetron in BerlinGerman 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 pathFrequency: 200kHz increasing to 10MHz1925 Elder of GE (US) produced 30kHz 69% efficiency
8 OKABE’S 1927 SPLIT ANODE MAGNETRON Plate and cathode enclosed in glass envelopeElectron path cathode to anodeStrong magnetic field parallel to cathodeOscillates 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 1933Below are some magnetron anode configurations involved. Frequency was 2.5cm (12gHz)Various configurations named after Japanese flowersC 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 Malearoff300W 10cm 20% efficiencyNo record of Russian military using it in radars
12 MAGNETRON WAR TIME UK1938 Admiralty awarded GEC a development contract.1940 April, GEC bread-boarded a 25cm operating radarTransmitter produced 25kW pulses using Hi-Pwr. TriodesUniversity of Birmingham: J. T. Randall and H. A. H. BootLiterature on Magnetrons world-wide but unobtainable1940 Feb. Developed 9.4cm (3.91gHz) 400W CW MagnetronGEC produced two magnetrons using R & B as a model1940 June Pulse powers of 10 to 40kW at 10cm achieved1940 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 cooledUsed 0.75mm tungsten rod as a filament for the cathodeTube was continuously pumped and placed between the poles of an electromagnet.
15 Dr. Eric Stanley Megaw Born in Belfast Born in Belfast Educated at Queens UniversityAvid radio enthusiastTransmitted the first amateur Radio signals out of Ireland in First QSO’s with West Coast US and AustraliaWorked 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 oscillatorMegaw was awarded the MBE in 1951 for his µ-wave workBecame Director of Physical Research with the Admiralty
16 E-1189 MAGNETRONE-1189 GEC no. 12MegawPhoto of actual magnetron Tizard took to N. AmericaE-1189: The 1st GEC magnetron had 6 cavities **Subsequently modified to have 8 cavities (No. 12)Freq. 3297MHz peak Pwr. 12kW Peak anode current 7AMagnetic 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.
19 MAGNETRONS X-band magnetrons CV-208 glass enclosed probe which is inserted in wave-guide2J49, 725A, 730A shows x-band wave-guide outputs725A output 9375MHz at 60kWWestern Electric manufactured and delivered units to the British Empire during WW2
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 2.45gHzOutput power 650 – 1200W Efficiency ~ 65%Specialized Industrial applications
22 MAGNETRON CONSTRUCTION Typical S band 50 kW magnetron used in military radarsDriven by a 30kV 1.0µsec pulse.Efficiency ~ 30% (WW2) now ~ 65%Input peak power 167kW Peak current 5.6AWith 1000 repetition rate, average input ave. power 167W
23 MAGNETRON CONSTRUCTION Cutaway view of the magnetronOpen area between cathode & anode called Interaction spaceE & 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 anodeµ-wave energy is induced in all cavities by moving electronsCavities in series. Energy coupled to output loop as shown
26 MAGNETRON CONSTRUCTION One of 8 cavitiesEquivalent circuit of one cavityEight equivalent circuits shown in series Typical of German and Japanese magnetrons [Unstable]
27 MAGNETRON CONSTRUCTION Alternate cavities strapped together with solid copper ringsDr. Megaw’s addition to the Boot Randall magnetron configurationSchematic of eight strapped cavitiesNote that all cavities are connected in parallelThis insures that oscillations in all cavities are in phase
28 HOW DOES A MAGNETRON WORK? Various anode formsMagnetic field provided by strong permanent magnetProducing µ-waves can be subdivided into four phases:Production and acceleration of an electron beamVelocity-modulation of the electron beamForming of a “Space-Charge Wheel”Dispense energy to the ac field
29 MAGNETRON OPERATION PHASE 1 Cathode centre at high negative voltsAnode at zero voltsNo magnetic fieldElectrons move in straight lineMagnet addedNorth pole on topSouth pole at bottomElectrons curve to the rightElectrons curve more when the magnetic field is increased
30 MAGNETRON OPERATION PHASE 1 Green path Weak magnet. All cathode electrons reach anodeRed path Magnetic field increased to “critical” value. Anode current decreases to a small value.White path Magnetic field increased further. Anode current drops to zeroMagnetic 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 cavities2 electric fields, ac & dc in interaction spacePolarity is one instant of ac (µ-wave) fieldThe dc field extends radially from cavities to cathodeElectrons near cavities move tangentially to cavitiesElectrons approaching the positive sides are speeded upElectrons departing the positive side and approaching the negative side are slowed down.
32 MAGNETRON OPERATION PHASE 3 12 cavity magnetronRotating 6 spoke space chargeSpace charge gives µ-wave energy to the cavity keeping it oscillating8 cavity magnetron4 spoke wheel
33 MAGNETRON OPERATION PHASE 4 Assume dc field & rf fields on cavities (magnetron oscillatingElectron approaching cavity gives up energy to cavityElectron slows down accordinglyThen electron speeds up gaining energy from dc fieldElectron eventually reaches cavity (anode current)
34 MAGNETRON RADAR CIRCUIT PFN charges up to 12kV (dc resonance phenomena)Trigger switches thyratron onPFN discharges through transformer and thyratronDuring discharge PFN develops rectangular pulseTransformer steps negative 6kV pulse up to 30kVMagnetron oscillates for duration of pulse (~ 0.5 to 4µsec
35 Thank you for viewing my Magnetron presentation Thank you for viewing my Magnetron presentation. I hope you found it informative and enjoyable. Chuck Hobson BA, BSc(hons)Coments