Design and Measured Characteristics of Minimum Loss Low-Velocity Helix Resonators

The design and performance of helix resonators operating at room temperature are described. The resonators are analogous to ¿/2 transmission lines shorted at both ends, and they oscillate at a frequency of 50 MHz. If slightly flattened 1/4 in. diam tubing is used to construct the helix, the resonators combine high shunt impedance with adequate coolant flow for high duty factor operation. Calculated and measured shunt impedances for different resonator phase velocities are given. The highest measured shunt impedance was 31 M¿/m, for a resonator with a phase velocity equal to 0.04 c. For a steadily applied field of 1.25 MV/m, and a water pressure of 1500 psi, various helix designs have cooling water temperature rises of 10-22 C. Two coupling arrangements which provide 50 ohm resistive input impedance at resonance are described. The resonant frequency can be tuned by a threaded 3 in. diam copper plug mounted in the cylindrical wall. A facility for testing resonators at full power is described.