II. The Space Charge Distribution in the Pre-Oscillation Magnetron

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Abstract
A description is given of a molecular beam apparatus which has been used to study the space charge distribution in a smooth anode cylindrical magnetron operating below cut-off. Relative values of the density have been measured as a function of both the radius and the anode voltage in magnetrons with anode to cathode radius ratios of 2·0 and 2·5. The results show that the density falls off gradually from the cathode, smoothly at first, and then in a series of small steps beyond the cloud radius predicted by the stream theories. The space charge has no definite edge. Increasing the anode voltage increases the radius of the cloud and the density at each point in the space charge. None of the quantitative theories predicts a distribution of this form and the implications of this are discussed in a companion paper.