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Localized regions of enhanced surface resistance cause a strong mode dependence of the unloaded quality factors in multi-cell superconducting rf cavities. An analysis of the measured dependence of quality factors on modes at low temperatures allows to localize cells of high surface resistance. Similarly cells where magnetic break-down occurs may be identified. This analysis is based on the field distributions inside the rf cavity as obtained from simple field-perturbation measurements in a given frequency passband.

The use of controlled superconducting devices in linear amplifier applications is discussed. Static characteristics are presented and the location of the cut-off regions is developed. Linear equivalent circuits are developed for the low-frequency case. Several amplifier circuits are discussed. The theoretical efficiencies of controlled-superconductor amplifiers are determined for various classes of operation.

Resonant frequency of superconducting radio frequency (RF) cavities, which are used in linear accelerator research, may rapidly shift over many bandwidths because of vibration. In this paper an all electrical method is analyzed which can correct for these frequency shifts. The method depends upon the use of a voltage-variable reactance to control frequency. The general characteristics of control are first determined. Power distribution in the system is then found.

The surface conductance of superconductors in the frequency region of microwaves is by several orders of magnitude higher than that of normal conducting copper. This can be well understood from the two-fluid model of the superconductors and the non-local behaviour of the metallic electrons at low temperatures.

For more than a decade, superconductive magnets have been used increasingly in complex experimental facilities in support of high-energy physics. In particular, accelerator technology and plasma research are areas where superconductive magnets have become almost indispensible.

The author has made the first measurements of radiation damage in a superconducting niobium cavity operated at microwave frequencies. It is shown that even a rather low radiation level of 1015 protons/cm2 with an energy of 175 MeV increases significantly the residual surface resistance at high power levels. These data are very pertinent to the construction of superconducting linear accelerators.

Measurements of the circuit Q of a self-resonant coil of niobium stannide, a Type II superconductor, have been made at radio frequencies (26.8 MHz). Unloaded Q's of 3.7 million at 4.2°K, and near 450 000 up to 16.2°K, were obtained. The above values of Q were limited by dielectric and radiation loss, rather than by the superconductor.

The extremely high Q of superconducting microwave cavities makes them very attractive for use in many applications. In some of these applications the resonant frequency of the superconducting cavity must be adjustable while the cavity is at liquid‐helium temperatures. Because of the requirement that the losses in the tuning mechanism be very low, in order that the Q of the cavity not be degraded, the usual methods of tuning microwave cavities are not always applicable to superconducting devices. One example of a case in which new techniques have had to be developed is the superconducting linear accelerator which is being constructed at Stanford University. In this accelerator, 25 superconducting sections, each 20 ft long, will have to resonate at the same frequency to within about 1 part in 108. The sections are being constructed to be tunable at liquid‐helium temperatures over a range of 1 part in 104, with a resolution of 1 part in 104.