J.R. Delayen

rf properties of an oxide‐superconductor half‐wave resonant line

Cylindrical rods of YBa2Cu3O7−δ from 0.6 to 1.9 mm in diameter and up to 565 mm long have been used as the center conductor of a half‐wave resonant coaxial line. The resonant line can be immersed in liquid nitrogen, which provides excellent cooling of the current‐carrying superconducting surface, even in the presence of substantial rf losses. Such superconducting resonant lines have been operated at frequencies of a few hundred MHz and at rf surface magnetic fields up to 150 G at 77 K.

Power and magnetic field‐induced microwave absorption in Tl‐based high Tc superconducting films

The increase in the microwave surface resistance Rs of high Tc superconductors at elevated microwave power levels is reported for both oriented and unoriented Tl‐based films as a function of rf magnetic field at 820 MHz and 18 GHz. The application of dc magnetic fields produces qualitatively similar increases in Rs and in the surface reactance Xs. The increase in Rs with dc field is shown to arise from simple decoupling of grains by intergranular magnetic flux. The increase in Rs with microwave power, on the other hand, is a consequence of hysteretic intergranular processes.

Radio frequency surface resistance of large‐area Bi‐Sr‐Ca‐Cu‐O thick films on Ag plates

Films nominally 80 μm thick of Bi‐Sr‐Ca‐Cu‐O on silver substrates with surface areas from 1.25 to 182 cm2 were fabricated using two different processing procedures. Their rf surface resistances were measured as functions of temperature in the frequency range 2.65–29.2 GHz at low rf field amplitudes using cylindrical resonant cavities. The critical temperatures of the films were in the range 81–83 K. At X‐band frequencies, the surface resistances matched that of room‐temperature copper. The results were comparable to recent data on small pellets of bulk Bi‐Sr‐Ca‐Cu‐O.

Measurements of the surface resistance of high-T c superconductors at high RF fields

We report measurements of the r.f. surface resistance of a wide variety of high-Tc superconductors in bulk form and as films on silver substrates. Several apparatuses have been constructed and used for measurements at frequencies from 0.15 to 40 GHz and r.f. surface magnetic fields as high as 640 G. In every case in which the field dependence of the surface resistance was measured, the surface resistance increased monotonically with field amplitude through a transition region characterized by a strong field dependence. It then saturated at high field at a value of a few percent of the normal-state surface resistance just aboveTc. In the presence of this field dependence, the frequency dependence of the surface resistance changed from quadratic to less than linear.

Application of rf superconductors to linacs for high-brightness proton beams

Abstract A series of superconducting structures for the acceleration of high-current ion beams is being developed, and two prototype Nb cavities are under construction. These cavities operate in a frequency-velocity range which, for superconducting structures, has been little explored: frequencies of 0.4 GHz to more than 1 GHz, and velocities of 0.1 c to 0.5 c . Issues discussed include: need for strong beam loading (∽10 4 ), need for strong focusing elements located close to the cavities, minimization of beam impingement and beam instabilities.

Design considerations for high-current superconducting ion linacs

Superconducting linacs may be a viable option for high-current applications such as fusion materials irradiation testing, spallation neutron source, transmutation of radioactive waste, tritium production, and energy production. These linacs must run reliably for many years and allow easy routine maintenance. Superconducting cavities operate efficiently with high cw gradients, properties which help to reduce operating and capital costs, respectively. However, cost-effectiveness is not the sole consideration in these applications. For example, beam impingement must be essentially eliminated to prevent unsafe radioactivation of the accelerating structures, and thus large apertures are needed through which to pass the beam. Because of their high efficiency, superconducting cavities can be designed with very large bore apertures, there by reducing the effect of beam impingement. Key aspects of high-current cw superconducting linac designs are explored in this context.<<ETX>>

RF properties of high-T/sub c/ superconductors

An investigation was conducted of the RF properties of high-T/sub c/ superconductors over a wide range of temperatures, frequencies and RF field amplitudes. Both bulk polycrystalline samples and thick films on silver substrates were tested. At 150 MHz and 4.2 K, surface resistances of 18 mu Omega at low RF field and 3.6 m Omega at an RF field of 270 G were measured. All samples showed a strong dependence of the surface resistance on RF field. However, no breakdown of the superconducting state was observed up to the highest field achieved (320 G). >

Tests of a superconducting rf quadrupole device

High surface electric fields have been obtained in the first tests of a superconducting rf quadrupole device. The rf quadrupole fields were generated between niobium vanes 6.5 cm in length, with an edge radius of 2 mm, and with a beam aperture of 6 mm diameter. In tests at 4.2 K, the 64 MHz device operated cw at peak surface electric fields of 128 MV/m. Virtually no electron loading was observed at fields below 100 MV/m. It was possible to operate at surface fields of 210 MV/m in pulses of 1 ms duration using a 2.5 kW rf source. For the vane geometry tested, more than 10 square centimeters of surface support a field greater than 90% of the peak field. The present result indicates that electric fields greater than 100 MV/m can be obtained over an appreciable area, sufficient for some accelerator applications. It also shows that superconducting rf technology may provide an extended range of options for rf quadrupole design.

Apparatus for measurement of surface resistance versus rf magnetic field of high- Tc superconductors

A coaxial niobium quarter‐wave resonant cavity has been fabricated and used for measurement of the surface resistance of disk‐shaped samples of high‐Tc superconductors at 820 MHz and 4.2 K, and at surface rf magnetic fields ranging to 300 G. The cavity can accommodate samples as large as 36 mm in diameter. A measurement procedure has been developed with which rf surface resistances less than 20 μΩ can be resolved for a 24‐mm‐diam sample at 820 MHz. This cavity is useful in determining the suitability of high‐Tc superconductors for high‐power rf applications.

Application of rf superconductivity to high-brightness ion-beam accelerators

Abstract A development program is underway to apply rf superconductivity to the design of cw linear accelerators for high-brightness ion beams. The key issues associated with this endeavor have been delineated in an earlier paper. Considerable progress has been made both experimentally and theoretically to resolve a number of these issues. In this paper we summarize this progress. We also identify current and future work in the areas of accelerator technology and superconducting materials which will confront the remaining issues and/or provide added capability to the technology.