A prototype 7.5 MHz Finemet(R) loaded RF cavity and 200kW amplifier for the Fermilab Proton Driver

A 7.5 MHz RF cavity and power amplifier have been built and tested at Fermilab as part of the proton Driver Design Study. The project goal was to achieve the highest possible 7.5 MHz accelerating gradient at 15 Hz with a 50% duty cycle. To reduce beam loading effects, a low shunt impedance (500/spl Omega/) design was chosen. The 46cm long single gap cavity uses 5 inductive cores, consisting of the nanocrystalline soft magnetic alloy Finemet, to achieve a peak accelerating voltage of 15 kV. The 95cm OD tape wound cores have been cut in half to increase the cavity Q and are cooled from both sides using large water-cooled copper heat sinks. The prototype cavity has a shunt impedance of 550/spl Omega/, Q=11, and is powered by a 200kW cw cathode driven tetrode amplifier. Both cavity and amplifier designs are described. Results from recent cavity tests coalescing beam in the Fermilab Main Injector are also presented.

1 INTRODUCTION

The RF group of the Fermilab Proton Driver Design Study has spent the past two years studying the RF requirements for new, fast cycling, high intensity proton synchrotrons in the 8–16 GeV energy range [1]. Initially, the group focused on a 3–16 GeV synchrotron design operating below transition at 15 Hz with an RF harmonic number h=12. A total beam current of 1E14 would be distributed in four equally spaced bunches. Early in the design study it was decided to construct a prototype RF cavity and power amplifier using these machine parameters. For the 3–16 GeV machine the RF cavity would operate between 7.17 and 7.586 MHz. The highest frequency, 7.586 MHz, although slightly above the 16GeV frequency was chosen to allow the prototype cavity to be tested as an h=84 system in the Fermilab Main Injector. To avoid any Robinson type instabilities [2] resulting from the heavy beam loading, the cavity would be tuned to the highest frequency 7.586 MHz and have a shunt impedance, . A Q=10 is necessary to span the operating frequency range.