Magnetron

An electron tube for ultra-high frequency oscillation in the GHz range. The oscillation principle is shown in the diagram. Assume that a uniform electric field E is applied between parallel anode and cathode, and in addition, a magnetic field B is applied perpendicular to the electric field. Electrons emitted from point P on the cathode are attracted to the anode by the electric field E , but due to the magnetic field B they receive a force proportional to their speed, and move in an arc as shown in the diagram. In this case, if the magnetic field B is small, the electron will move as shown by _l1 and reach the anode, but if B is large, it will not reach the anode as shown by _l2 , and will move sideways in an arc. The limiting magnetic field is

B =
where m is the mass of the electron and e is its charge. At this time, the electron rotates at ω _c (the cyclotron angular frequency) and moves laterally at a speed of v _c . A voltage is induced in the electrodes due to the cyclotron motion of the electrons, and oscillation occurs between this voltage and an external resonant circuit (A-type oscillation). This electron motion also induces a voltage between the split anodes, causing oscillation in the external resonant circuit (B-type oscillation). Using this principle, A-type and B-type magnetrons are created.

Source: Morikita Publishing "Chemical Dictionary (2nd Edition)" Information about the Chemical Dictionary 2nd Edition

GHz 帯の超高周波発振用の電子管．発振原理を図に示す．平行な陽極と陰極との間に一様な電場Eが与えられ，そのうえ電場に直角に磁場Bが加えられているとする．陰極のP点から発した電子は電場Eによって陽極に引かれるが，磁場Bのため速度に比例した力を受け，図のように円弧を描いて移動する．この際，磁場Bが小さければ電子は l₁ のように移動して陽極に達するが，Bが大きくなると l₂ にように陽極に達しないで，円弧を描いて横に移動していく．その限界磁場は

B ＝
であり，mは電子の質量，eはその電荷である．このとき，電子は ω_c(サイクロトロン角周波数)で回転し，v_c の速度で横に移動する．電子のサイクロトロン運動によって電極には電圧が誘起されるので，この電圧と外部共振回路との間で発振が生じる(A型振動)．また，この電子の運動は，分割された陽極間に電圧を誘起して外部共振回路で発振する(B型振動)．この原理で，それぞれA型とB型のマグネトロンがつくられる．

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