Abstract
The crossbar $H$-mode (CH) cavity is an accelerating structure operated in the ${H}_{21(0)}$ mode. The robustness of the crossbar geometry allows one to realize room temperature as well as superconducting linac cavities. The shunt impedance characteristics of this structure are attractive to develop proton and heavy ion linacs in the low and medium beta range. A first room temperature eight-cell prototype has proven the feasibility of the crossbar design in terms of mechanical construction, copper plating, and cooling. An innovative rf coupling concept has been developed where two CH cavities are connected by a two gap ${E}_{010}$-mode resonator which, at the same time, provides transverse focusing by a quadrupole triplet. The concept has been applied in the design of the new FAIR proton linac and a scaled model of the second cavity of this injector has been built and tested too. The full scale prototype is now under construction at the University of Frankfurt. In this paper, the room temperature CH cavity development as well as the general layout of the FAIR proton injector (70 MeV, 325 MHz, 70 mA) is presented and discussed.
Highlights
During the past 15 years an increasing activity in linac development came up due to the capability of this kind of accelerator to deliver high intensity, high duty factor, proton and ion beams
The rf properties of the crossbar H-mode (CH)-drift tube linac (DTL) have been systematically investigated at the University of Frankfurt together with GSI showing promising results
A first model demonstrated the feasibility of the crossbar design in terms of mechanical stability, cooling, and copper plating
Summary
During the past 15 years an increasing activity in linac development came up due to the capability of this kind of accelerator to deliver high intensity, high duty factor, proton and ion beams. The interest towards pulsed operation has been active in designing high intensity injectors for synchrotrons, for instance at JPARC [10] and at CERN (LINAC4 [11]) Both projects base the front end on a 3 MeV radio frequency quadrupole (RFQ) cavity followed by a three tank Alvarez drift tube linac (DTL). The most useful alternative to the Alvarez linac in the low beta range is the interdigital H-mode linac (IH-DTL) which has been established as a standard cavity for ion acceleration in the frequency range from 30 to 220 MHz. The IH-DTL with quadrupole triplet focusing has different advantages, the higher shunt impedance and the low operation costs, a reduced number of quadrupoles, robustness against beam losses, a small overall size, and the simplified mechanical design which results in reduced fabrication costs [13,14].
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