Magnetron tubes

Typical microwave equipment consists of a magnetron tube (Fig. 3.4) [704]. Just as for other vacuum tubes, the anode has a higher potential with respect to the cathode (source of electrons). So, the electrons are accelerated to the anode in the electric field. The cathode is heated till the high temperature expulses electrons. Generally, the anode is close to earth potential and the cathode has a high negative potential. 

Microwave oven (Percy L. Spencer) The microwave oven grew out of the micro-wave generator, the magnetron tube, becoming more affordable. 

Until the advent of the magnetron tube, which changes electricity into microwaves, the only method of cooking was heat and, until about 100 years ago, when electricity was developed, this meant by a fire. 

The magnetron tube is a vacuum tube which can convert electricity into electromagnetic energy radiation. The micro-waves travel in straight lines. 

Fig. 138. Diagram of CC-MIP-MS system (1) four-port valve, (2) transfer line, (3) three-port gas inlet, TMoio cavity, (5) discharge tube, (6) waveguide, (7) magnetron tube, (8) electrical feedthrough, (9) skimmer cone, (10) extraction lenses, (11) swing gate valve, (12)...

Magnetron A crossed-field electromagnetic system where the path of electrons accelerated in an electric field is controlled by a magnetic field at an angle to the electric field. In a magnetron tube, the electron motion is used to generate microwave radiation (Klystron tube). 

The most dramatic evolution of a microwave power source is that of the cooker magnetron for microwave ovens (48). These magnetrons are air-cooled, weigh 1.2 kg, generate weU over 700 W at 2.45 GHz into a matched load, and exhibit a tube efficiency on the order of 70%. AppHcation is enhanced by the avaHabiHty of comparatively inexpensive microwave power and microwave oven hardware (53). The cost of these tubes has consistently dropped (11) since their introduction in the eady 1970s. As of this writing (ca 1995), cost is < 15/tube for large quantities. For small quantities the price is < 100/tube. 

Some power tubes can be operated without the need for a protective ferrite isolator. One example is the cooker magnetron (700 W) used in modern microwave ovens (57). At higher power levels, such as 25 kW, it is more common to employ a protective ferrite device, particularly in the form of a circulator (58), as shown in Figure 3. This results in a power loss equivalent to a few percentage points in system efficiency. The ferrite circulator prevents reflected power from returning to the power tube and instead directs it into an auxiHary dummy load. The pulling of tube frequency is thus minimised. 

High power microwaves are generated by vacuum tubes. The magnetron and klystron are the most commonly used tubes for the generation of continuous waves power for microwave processing. Power is normally launched from the microwave tube into a transmission line or waveguide, where it travels to a load or termination such an antenna or a microwave heating applicator. 

Fig. 14.5 A modified MW oven for microwave photochemistry experiments. A. magnetron, B. reaction mixture with the EDL and a magnetic stir bar, C. aluminum plate, D. magnetic stirrer, E. infrared pyrometer, F. circulating water in a glass tube, G. dummy load inside the oven cavity [88]. With permission from Elsevier Science.

The difference between the magnetron and other vacuum tubes is that the electron flow passes along a spiral this route is created by an external magnetic field B (Fig. 3.4). The electron cloud produces resonance cavities several times in its trip to the anode. These cavities work as Helmholtz resonators and produce oscillations of fixed frequency, determined by the cavity dimensions small cavities produce higher frequencies, large cavities smaller frequencies. The antenna in the right zone collects the oscillations. 

Safety features are essential to a microwave apparatus. An exhaust fan draws the air from the oven to a solvent vapor detector. Should solvent vapors be detected, the magnetron is shut off automatically while the fan keeps running. Each vessel has a rupture membrane that breaks if the pressure in the vessel exceeds the preset limit. In the case of a membrane rupture, solvent vapor escapes into an expansion chamber, which is connected to the vessels through vent tubing. To prevent excessive pressure buildup, some manufacturer use resealable vessels. A spring device allows the vessel to open and close quickly, releasing the excess pressure. 

The concept of rotatable target magnetron (C-Mag) sputtering is shown in Fig. 5.31a. The target material is a water-cooled tube, which is attached to... 

Porous structure of the outer support surface has been modified by deposition of the additional layer of metal Ni. Two vacuum condensation techniques have been used for nickel deposition dc ion magnetron sputtering and electron beam evaporation. To produce coatings on tubes additional installation for dc sputtering has been designed. 

Microwaves can be produced by four types of macroscopic cavity resonators that use the ballistic motion of electrons across a cavity opening the klystron, the magnetron, the traveling-wave tube (TWT), and the gyrotron. They can also be generated by field-effect transistors at low frequencies, by Gunn42 diodes, and by IMP ATT diodes. 

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