Imaging, Beam-acceptance and Beam-discharge Lag in Camera Tubes

Abstract

In the paper presented at the Fifth Symposium on Photoelectronic Image Devices particular consideration was given to the relationship between the imaging system and the shape of the beam-acceptance curve in camera tubes. This is only a small part of the information collected in a paper which was published a few months after the Symposium.1 For references to the subject and detailed information reference is made to that paper, which—for obvious reasons—is presented below in abstract form only. In theoretical models for calculating the beam-discharge lag it has always been assumed that the current ia accepted by the target (which is stabilized near cathode potential) is given by where ib V and T are the current of the operating beam, the target potential and the cathode temperature, respectively. However, because (i), the initial-energy distribution of the electrons contributing to the operating beam and the beam-apex angle and (ii), Coulomb interactions in the cross-over region are essential to the shape of the beam-acceptance curve, Eq. (1) applies only under certain specified conditions. This is made clear by considering three different imaging systems. Because, compared with Eq. (1), the high current contributing to the cross-over in usual camera tubes gives rise to an excess of high-energy electrons generated by Coulomb interactions, the three systems are chosen such that the cross-over current is low. Then, again compared with Eq. (1), the known experimental deviation for relatively small values of |V| remains. It is shown that, apart from landing errors caused by the deflection system, for instance, its amount depends on the design of the imaging system. Technological difficulties which have yet to be overcome before the three systems can be applied in practice are left out of consideration. It would be of minor practical importance to compare the beam acceptance in different systems without considering the quality of the imaging. Therefore the effect of beam interception on the initial-energy distribution of the electrons contributing to the operating beam and on the imaging in camera tubes is examined. Moreover, it is shown that the imaging properties of different imaging systems can be compared mutually by calculating their figure of merit which is defined as the product of the beam-apex angle and the spot diameter. Because the figure of merit is determined by the ratio : (current of operating beam)/(cathode loading), both the beam current and the cathode loading must have the same values in the three systems. Then, both the beam-apex angle and the spot diameter can be made the same in the three systems. The imaging properties of these systems are compared. Due to the differences in the design of the three systems, the initial-energy distributions of the electrons contributing to the operating beam are different. This gives rise to different beam-acceptance curves which are calculated. These curves can be represented by rather simple analytic expressions and conditions under which Eq. (1) applies are specified. It is shown that a space-charge minimum between mesh and target, which would affect the beam acceptance, does not occur for normal values of the mesh potential and the distance between mesh and target. The influence of increasing the beam-apex angle on the beam acceptance is calculated and found to give rise to a nearly parallel shift of the acceptance curve which looks like the effect of an increasing contact-potential difference. The differences in beam acceptance give rise to differences in the beam-discharge lag in the three systems which are calculated. Their importance depends on the layer capacitance and other operating conditions.