Crossed-field amplifier

A crossed-field amplifier (CFA) is a specialized vacuum tube, first introduced in the mid-1950s and frequently used as a microwave amplifier in very-high-power transmitters. A CFA has lower gain and bandwidth than other microwave amplifier tubes (such as klystrons or traveling wave tubes); but it is more efficient and capable of much higher output power. Peak output powers of many megawatts and average power levels of tens of kilowatts can be achieved, with efficiency ratings in excess of 70 percent.

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The electric and magnetic fields in a CFA are perpendicular to each other ("crossed fields"). This is the same type of field interaction used in a magnetron; as a result, the two devices share many characteristics (such as high peak power and efficiency) and they have similar physical appearances. However, a magnetron is an oscillator and a CFA is an amplifier; a CFA's RF circuit (or slow-wave structure) is similar to that in a coupled-cavity TWT.

Raytheon engineer William C. Brown's work to adapt magnetron principles to create a new broadband amplifier is generally recognized as the first CFA, which he called an Amplitron. Other names that are sometimes used by CFA manufacturers include Platinotron or Stabilotron.