Vii. Tube Research and Development A. Microwave Tubes 1. Electron Beam Studies A. Noise in Electron Beams

The experiments described by H. E. Rowe and C. E. Muehe. Jr. in previous Quarterly Progress Reports had to do with the noise characteristics of magnetically focused electron beams. Although the experiments were conducted with vacua of less than 10-6 mm Hg. the long drift tube insured almost complete neutralization of the electron beam by positive ions. Thus. it was never possible, for example. to achieve true Brillouin focusing. We decided to repeat the measurements, using a pulsed electron beam. With a pulse duration of 106 sec, a pressure of 2 X 107 mm Hg, a drift length of 40 cm, and with the assumption that no loss of ions occurs, the ion concentration at the end of the pulse would only amount to about 1 percent of the electron density. The apparatus used is that built by Muehe for his experiments. It has been modified by adding a motor drive to move the cavity smoothly along the beam, as shown in Fig. VII-I.