Very high field-gradient has become available by a new Magnetic Alloy (MA)-loaded cavity developed for high intensity proton synchrotrons. The available RF voltage per core is ten times larger than that of the ordinary ferrite core. The maximum voltage of 20 kV has been achieved by the High-Field Gradient Cavity(HGC) of 40 cm in length [1].Because the intrinsic Q-value of the MA core is low, acceleration without any tuning system also becomes possible. The first beam acceleration test using the HGC has been performed successfully at the HIMAC(Heavy Ion Medical Accelerator in Chiba). Furthermore, the dual harmonic RF and barrier bucket experiments have been carried out.Another advantage of the MA-loaded cavity is that it is easy to compensate the beam loading. The feed forward beam compensation was applied for both HGC on the test bench using electron beam and MA-loaded cavity installed in the AGS for the barrier bucket experiment. A new development for high-Q HGC using cut core configuration will be also reported.

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The microwave surface reactance of copper was measured over the temperature range from 300 to 14 K using the TE011 mode of a cylindrical cavity. The result was that with decreasing temperature the ratio of the reactive-to-resistive component remained at around 1 from 300 to 75 K, and increased to 1.32 at 14 K, in contrast to the theoretical extreme anomalous limit, equal to or higher than ∛. The discrepancy between the measured and theoretical values may be attributed to the quality of the material and the surface finishing. In this experiment an accurate measurement of the resonant frequency is important. Associated with the frequency, we discuss two thermal-expansion measurement techniques: cavity dilatometry and capacitance dilatometry. Further, the usefulness of the present data for studying high-Tc films will be mentioned.

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