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MAGNETRONS The Evolution & Operation of

Chuck Hobson BA, BSc(hons)

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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.

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EVOLUTION OF THE

MAGNETRON

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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

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MAGNETRONS GERMANY 1920 Heinrich Barkhausen 0.3 - 6.4gHz at 5W

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

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)

G0MDK7HULL’S 1921 MAGNETRON (US)

• Cavity magnetron Coaxial configuration

Frequency: 200kHz increasing to 10MHz

1925 Elder of GE (US) produced 8kW @ 30kHz 69% efficiency

Electron path

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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)

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MAGNETRON WAR TIME JAPAN

Various configurations named after Japanese flowers

C Kosumosu (Rising sun)

U Umebachi (Apricot flower)

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)

Above information from paper by Professor Koichi Shimoda

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MAGNETRON WAR TIME JAPAN

Shimada Laboratory, Technical Institute of Japanese Navy, Shizuoka Prefecture in 1944

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MAGNETRON RUSSIA

4 cavity magnetron Russia 1937

Aleksereff and Malearoff

300W CW @ 10cm 20% efficiency

No record of Russian military using it in radars

G0MDK12MAGNETRON WAR TIME UK

4. University of Birmingham: J. T. Randall and H. A. H. Boot

5. Literature on Magnetrons world-wide but unobtainable

6. 1940 Feb. Developed 9.4cm (3.91gHz) 400W CW Magnetron

7. GEC produced two magnetrons using R & B as a model

8. 1940 June Pulse powers of 10 to 40kW at 10cm achieved

9. 1940 Aug. Tizard and team brought magnetron to the U. S.

10.Sept. Mag. at MIT Labs. Bell Labs & Raytheon Co. x-rayed Mag. & reproduced it.By Nov. it was in mass production

1. 1938 Admiralty awarded GEC a development contract.

2. 1940 April, GEC bread-boarded a 25cm operating radar

3. Transmitter produced 25kW pulses using Hi-Pwr. Triodes

G0MDK13MAGNETRON WAR TIME UK

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.

Experimental magnetron

University of Birmingham

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J. T. Randall & H. T. Boot

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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

Born in Belfast

G0MDK16E-1189 MAGNETRON

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.

MegawE-1189 GEC no. 12

G0MDK17E-1198 MAGNETRON

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

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MAGNETRONS

CV 38 E-1198

8 cavity magnetron

Fil. 6V

Nom. Freq 3297MHz

Pk. Pwr. 7kW

Magnet 1050 gauss

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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

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MAGNETRON OPERATION

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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

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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

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MAGNETRON CONSTRUCTIONCutaway view of the magnetron

Open area between cathode & anode called Interaction space

E & H fields interact on electrons to get µ-waves in cavities

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MAGNETRON CONSTRUCTIONAnother cutaway view of the magnetron

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MAGNETRON CONSTRUCTIONMagnetron eight cavity anode

µ-wave energy is induced in all cavities by moving electrons

Cavities in series. Energy coupled to output loop as shown

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MAGNETRON CONSTRUCTION

Equivalent circuit of one cavity

Eight equivalent circuits shown in series Typical of German and Japanese magnetrons [Unstable]

One of 8 cavities

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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

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HOW DOES A MAGNETRON WORK?

Producing µ-waves can be subdivided into four phases:

1. Production and acceleration of an electron beam

2. Velocity-modulation of the electron beam

3. Forming of a “Space-Charge Wheel”

4. Dispense energy to the ac field

Various anode forms

Magnetic field provided by strong permanent magnet

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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

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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

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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.

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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

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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)

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MAGNETRON RADAR CIRCUIT

1. PFN charges up to 12kV (dc resonance phenomena)

2. Trigger switches thyratron on

3. PFN discharges through transformer and thyratron

4. During discharge PFN develops rectangular pulse

5. Transformer steps negative 6kV pulse up to 30kV

6. Magnetron oscillates for duration of pulse (~ 0.5 to 4µsec

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Thank you for viewing my Magnetron presentation.I hope you found it informative and enjoyable.

Chuck Hobson BA, BSc(hons)

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