Backward wave oscillator tube utilizing successive delay line sections for increased power

A backward wave oscillator which comprises:

• a source of a beam of electrons;
• means for focusing said beam of electrons;
• a collector spaced from said source for receiving said beam of electrons;
• and
• a delay line exhibiting an attenuation factor per unit length, disposed intermediate said collector and said electron beam source, along which delay line an electromagnetic wave propagates in operation and interacts with the beam, characterized in that:
• said delay line comprises a first and a second delay line section;
• said first delay line section modulating the electron beam as it passes thereunder and having a length such that due to its attenuation factor per unit length substantially no energy is available at the end thereof closest to said electron beam source as a result of said interaction, the only result of said interaction being a modulation of the electron beam;
• said second delay line section substantially shorter than said first delay line section being incapable of sustaining oscillations in the absence of the priorly modulated electron beam, the length of said second delay line section being such that due to its attenuation factor per unit length oscillations under interaction with the electron beam priorly modulated by the first delay line section are generated,
• said second delay line section having a structure homothetic with the first in the ratio 1/n, where n is an integer, said backward wave oscillator further comprising;
• means, coupled to said second delay line section, for extracting oscillatory energy from the backward wave oscillator, the oscillator supplying in that means an electromagnetic energy on the n.sup.th harmonic of the frequency of the predominant component of the wave in the first line section.