Cavity magnetron
A magnetron is a vacuum tube runtime for generating electromagnetic radiation in the microwave range ( 0.3 to 300 GHz ) with an efficiency of up to 80 %. Magnetrons are very efficient, low-cost generators for high frequency. Power and frequency are largely determined by the design and are usually not changed. A distinction is continuously working ( CW ) magnetrons and Impulsmagnetrone. In continuous-wave operation and some kW in pulsed operation for more than 10 MW can be achieved. This electron tubes are briefly extremely overloaded.
Construction
The magnetron consists of a cylindrical thermionic cathode ( oxide or dispenser cathode ) in the center of the vacuum tube. Most of the heating wire is a directly heated cathode. This is usually surrounded by a massive, cylindrical anode block of copper. Cooling fins on the anode block allow cooling by free convection, a fan or water cooling. In the anode block are frequency-determining resonant cavities: mostly radiating, parallel to the filament extending slots (so-called Schlitzmagnetron ), which are open towards the central hole of the anode block, the so-called interaction space.
Other embodiments of cavity resonators are Lochresonator, Segmentresonator ( circle type) and Mehrfrequenzresonator ( Rising - Sun - type).
The magnetron requires an axial magnetic field, which is usually generated with permanent magnets. One of the cavities is connected to a loop or a hollow conductor and is used for power extraction.
Magnetron of a microwave oven in longitudinal section (magnets and fins removed )
Impulsmagnetron MI- 189W (approx. 160 mm wide, about 9 GHz, aircraft on-board radar, Soviet Union)
Impulsmagnetron MI -158 (approx. 110 mm high, 9.33 ... 9.42 GHz, 7 kW, 0.1 ... 1 microseconds, uaf aircraft on-board radar, Soviet Union 1970)
Operation
Electron orbits
In the interaction space between cathode and anode, electric and magnetic fields act simultaneously. The magnetic field lines are parallel to the cathode axis and pass through the interaction space. The voltage is applied between the anode and cathode, released by a hot cathode to the anode electrons are accelerated toward due to the electric field. However, the electric field formed by the magnetic field at a right angle, therefore, the electrons are deflected from their radial spiral path due to the Lorentz force. Thus they move around in the interaction space around the cathode. Only at a very high anode voltage it comes to the current flow - the electric field extends the path curvature from so far that the electron orbits the anode strips (green path in the figure).
Resonant anode shape
The slots or chambers of the anode form a closed ring delay line of cavity resonators: Electromagnetic oscillations in a resonant cavity may spread along the interaction space and the slots in the other cavity resonators. The result is a ring- closed multipolar electromagnetic resonant circuit. AC voltages occur between the ends of the anode segments and alternating currents on the inner surfaces of the diaphragm walls in him. The high-frequency field in the ring resonator will interact with the electrons. The resulting fields affect track and speed of the electrons. The result is that be slowed or accelerated electrons and thereby during their round form regions of higher and lower electron density. These electron clouds turn increase the high-frequency vibrations of the ring resonator - it takes a self-excitation. If the kinetic energy of an electron is too small, to the anode block it occurs. From the cathode constantly an excess of free electrons is re-stocked.
Electrical connection
For the liberation of electrons by thermionic emission magnetrons have an electrically heated thermionic cathode. This is directly heated and a heating terminal is connected to the cathode. Since the body has (anode block, magnet, waveguide flange or antenna pin ) to ground potential, the heating voltage supply for the operating voltage of the magnetron needs (several kilovolts ) to be executed well insulated - at the cathode is the negative with respect to ground operating voltage.
The illustration shows a typical microwave circuit with a magnetron in the half-wave mode: 2000- V high-voltage winding is grounded on one side and invites if you erdseitiges end forms the negative terminal on the semiconductor diode to the capacitor to about 2800 V, while the magnetron itself only the threshold voltage of the diode of about 0.7 volts. Hand, the other hand, in the next half-wave voltage in the high voltage winding around, now the voltages of the high-voltage winding and the series capacitor to a total anode voltage of about 5600 V, which briefly a current to flow through the magnetron add up. The combination of a capacitor and diode acts like a voltage doubler, and both these two as well as the insulation of the heating coil must be high voltages accordingly.
In the above pictures of Impulsmagnetrons the red-brown plastic body can be seen that insulate the heater voltage and cathode terminals to the metal body of the magnetron, which forms the anode.
Once the magnetron is put into operation drops a small part of the electrons on the cathode back, and it is heat energy. Therefore, the heater voltage for the cathode has to be reduced especially when continuously operating magnetrons to avoid overheating.
Applications
Applications of continuous wave magnetrons are mainly industrial heating and drying ( RF heating), plasma generation and the microwave oven.
In sulfur lamps and some ion source, a magnetron is used to generate the plasma.
Pulsed magnetrons are still often used in pulse - radar systems to generate the transmit pulse.
For sputtering, (English for spraying ), among other techniques, magnetrons are used.
In EMP weapons pulse magnetrons very high power use: An attempt is made by means of directional RF energy to destroy enemy electronics.
History
The physicist Heinrich Greinacher developed before 1912, a tube to measure the ratio of the charge of the electron to its mass, and presented the basic mathematical equations. However, the tube was not working due to insufficient vacuum in its interior and insufficient electron emission.
The physicist Albert W. Hull of the United States took advantage of the release of Greinacher, advanced the theory of the trajectories of electrons in the magnetron, improved the tube and gave it its name. Hull developed at the General Electric Company (GEC) in 1921, the first functioning magnetron, which consisted of several coaxial, cylindrical anode arranged walls ( engl. split- anode magnetron ) and a cathode. Interspersed is the arrangement of a longitudinal magnetic field of an external coil. The original goal was to build magnetically controlled relay or amplifier. You should make the control electrodes of the company Western Electric Co. competition. The possibility has been discovered to use the magnetron as a high frequency generator.
One of these independent development in 1921 by Erich Habann in Jena and August Žáček in Prague. Habann developed a magnetron with split anode cylinder, the generated frequency of 100 MHz. The main difference from the magnetron from Hull was that Habann used a DC magnetic field (as in today's magnetrons ). The conditions to cancel the damping (creating a negative differential internal resistance ), Habann could be accurately calculated. Žáček could reach frequencies of 1 GHz with a massive cylindrical anode. Through slits in the anode succeeded Kinjiro Okabe (冈 部 金治郎) at Tohoku University in Sendai ( Japan) in 1929 with frequencies of 5.35 GHz, the breakthrough for magnetrons in the centimeter - wave range.
On November 27, 1935 Hans Erich Hollmann registered his patent for the multi-chamber magnetron, which was granted on 12 July 1938. 1940 developed the British physicist John Turton Randall and Henry Albert Howard Boot an improved variant of Hollmann's multi-chamber magnetron by using a liquid - cooling system and the number of resonance chambers increased from four to six. This allowed them a hundredfold the output power. This allowed two years later, the development of very powerful Magnetronsender for radar equipment with a very short wavelength and thereby high resolution.