The superconducting CH structure is a novel multicell cavity for the acceleration of protons and ions in the low and intermediate energy regime. The CH structure is a cross-bar-type cavity; it is the first superconducting low energy multicell cavity operated in an H mode. A superconducting CH-prototype cavity with 19 gaps at 0:1 has been built and tested successfully. Maximum surface fields of 25 MV=m and a corresponding effective cavity voltage gain of 3.7 MV have been achieved. In this paper the development and the tests of this new cavity are presented. This includes also tuning and coupling methods as well as a comparison with other low energy cavities.

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The actual situation with respect to the use of an RF linac driver for heavy ion inertial fusion~HIF! is discussed. At present, there is no high current heavy ion linac under construction. However, in the course of linac projects for e Ϫ , p, d, or highly charged ions several developments were made, which may have some impact on the design of a HIF driver. Medium-and low-b superconducting structures suited for pulsed high current beam operation are actually designed and investigated at several laboratories. A superconducting 40 MeV, 125 mA cw linac for deuteron acceleration is designed for the Inertial Fusion Material Irradiation Facility~IFMIF!. The Institute for Applied Physics~IAP! is developing a superconducting 350-MHz, 19-cell prototype CH-cavity for b ϭ 0.1. The prototype cavity will be ready for tests in 2004. A superconducting main HIF driver linac would considerably reduce the power losses. Moreover, it would allow for an efficient linac operation at a higher duty factor. The 1.4-AMeV room-temperature High Current Injector HSI at Gesellschaft für Schwerionenforschung~GSI! has been in routine operation for more than 2 years now. With a mass-to-charge ratio of up to 65, a current limit of 15 mA for U 4ϩ , and an energy range from 2.2 AkeV up to 1.4 AMeV, this linac is suited to gain useful experience on the way toward the design of a HIF RF driver. The status and technical improvements of that A0q Յ 65, 91-MV linac are reported. Beam dynamics calculations for Bi 1ϩ -beams show that powerful focusing elements at the linac front end are the bottleneck with respect to a further increase in beam current. Besides superconducting and pulsed wire quadrupoles, the potential of the Gabor-plasma lenses is investigated.

The CH-or Crossbar H-structure is a new H-mode drift-tube structure operating in the TE 210 mode. Due to its mechanical rigidity room temperature as well as superconducting cavities can be realized [1]. A superconducting version of the CH-structure has been development at the IAP in Frankfurt, Germany. To prove the promising results obtained by simulations a 19-cell, 352 MHz (β = 0.1) prototype cavity has been designed and built. This CH-prototype is the first superconducting low energy multi-cell cavity for the acceleration of protons and ions. The cavity has been tested at room temperature with an rf power of up to 300W cw and 2 kW pulsed. We present the first tests of the cavity as well as mechanical simulations. ) prototype cavity has been designed and built. This CHprototype is the first superconducting low energy multi-cell cavity for the acceleration of protons and ions. The cavity has been tested at room temperature with an rf power of up to 300 W cw and 2 kW pulsed. We present the first tests of the cavity as well as mechanical simulations.

High intensity accelerator concepts for Heavy Ion Inertial Fusion (HIIF) injectors require small emittance, high current and high energy beams. The improvement of brightness in such a driver linac is done by several funneling stages at low energies, in which two identically bunched ion beams are combined into a single beam with twice the frequency, current and brightness. For the Heavy-Ion Driven Ignition Facility (HIDIF) we have proposed the use of a two-beam accelerator structure which provides two beams within one cavity and a single r.f. deflector structure which bends the two beams to one common axis. The progress of the experiment and first beam-test results will be presented.