Rudolf Tiede

RFQ AND IH accelerators for the new ebis injector at BNL

The new EBIS preinjector at BNL will accelerate ions from the EBIS source with specific mass to charge ratio of up to 6.25, from 17 keV/u to 2000 keV/u to inject into the Booster synchrotron, expanding experimental possibilities for RHIC and NASA experiments. The properties of the RFQ and IH accelerators and the status of the project will be discussed.

Development of a coupled CH structure for the GSI proton injector

The FAIR facility, under development at GSI, needs a new dedicated proton injector for the production of intense secondary p macr beams. This injector will accelerate protons from 3 to 70 MeV at a current of 70 mA, and due to the high voltage gain and shunt impedance will be based on CH cavities powered by 6 2.5 MW, 325 MHz klystrons. An innovative coupling cell containing one drift tube of length Nbetalambda was developed to combine multicell drift tube modules of the CH-type (H 211 mode). In order to study this coupling mechanism a 1:2 scaled model of the second resonator of GSI proton injector was fabricated at IAP and it ready for testing. The according full scale prototype, a 3 meter long coupled resonator from 11.7 MeV to 24.1 MeV is under development and will be power tested with a 2.5 MW klystron at GSI early in 2009. This paper describes in detail the coupled structure together with a general overview of the R&D results achieved on the CH-DTLs cavity.

High current ion beam RF acceleration and perspectives for an inertial fusion driver

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 − , 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-β 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 β = 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 fur 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 4+ , 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 A/q ≤ 65, 91-MV linac are reported. Beam dynamics calculations for Bi 1+ -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.

First Tests of the Superconducting CH-Structure

The CH-or Crossbar H-structure is a new H-mode drift-tube structure operating in the TE210mode. 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 optained 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 300 W cw and 2 kW pulsed. We present the first tests of the cavity as well as mechanical simulations.

End-to-end simulations of a superconducting deuteron CH-DTL for IFMIF

The IFMIF project (International Fusion Materials Irradiation Facility) requests two cw linacs operated in parallel. Each of them is designed to provide a 5 MW 125 mA deuteron beam at 40 MeV for the production of an intense neutron flux with an energy around 14 MeV. This paper presents an alternative linac design for this project. The acceleration is completely based on H-type cavities. The room temperature (rt) 4-Vane-RFQ and a short IH-DTL (Interdigital-H-DTL) are followed by 4 superconducting (sc) CH-DTL (Crossbar-H) cavities. The operating frequency is 175 MHz, the designed section lengths are 13 m for the RFQ (Radio-Frequency-Quadrupole) (5 MeV), 1 m for a compact MEBT (Middle Energy Beam Transport), 2 m for the IH-cavity (10 MeV) and 9 m for the sc CH-DTL (40 MeV). The structure parameters and end-to-end multiparticle beam dynamics calculations with and without DTL errors of the whole linac will be presented and the results will be discussed.

The superconducting linac approach for IFMIF

The IFMIF project requires a high current D+-linac operated in cw. Due to the cw operation a superconducting linac using CH-structures (see fig. 1) could be an alternative solution compared with the room temperature Alvarez reference design especially with respect to avoiding thermal problems and to reducing the operational costs.

Low energy DTL sections for intense Bi 1+ beams

The beam dynamics design of the low energy DTL section of a fusion driver linac based on the IH-structure is presented. The acceleration of a 209 Bi 1+ beam by an IH-DTL operated at 27 MHz (54 MHz) is investigated for two specific injection energies at 60 A keV and at 200 A keV, respectively. Both cases are optimized separately, with the goal to find out the maximum achievable acceleration gradient, beam current, as well as the most attractive field strength range of the quadrupole lenses. Calculations are performed on one beamlet, but the results can be applied to build up multibeam cavities. H-mode cavities are very well suited for this purpose and provide high acceleration efficiency, especially at low particle velocity. In addition, the “Combined 0° structure” (“Kombinierte Null Grad Struktur–KONUS”) beam dynamics concept allows grouping into modular units consisting of short, simple rf cavities and of multiaperture quadrupole triplet lenses located in the intertank sections.

The KONUS IH-DTL proposal for the GSI UNILAC poststripper linac replacement

Motivated by the necessary replacement of the GSI UNILAC poststripper linac, a compact and efficient linac design based on IH-type cavities has been developed. Using KONUS beam dynamics, it was possible to design a linac consisting of only five cavities that can be operated by the existing UNILAC RF amplifier structure. The transversal focusing scheme is based on magnetic quadrupole triplet lenses. The optimized design provides full transmission and low emittance growth for the design current of 15 emA U28+, accelerating the beam from 1.4 MeV/u to 11.4 MeV/u. Extensive error studies were performed to define tolerances and verify the stability of the design with respect to misalignment and injection parameters. The design provides a compact and cost effective alternative to a new Alvarez linac. With a total length of just 22.8 meters it will leave room for future energy upgrades in the UNILAC tunnel.

Efficient Heavy Ion Acceleration with IH-Type Cavities for High Current Machines in the Energy Range up to 11.4 MeV/u

We propose an efficient design for heavy ion acceleration from 1.4 to 11.4 MeV/u with a design current of 15 emA for a U28+ beam as a possible injector for FAIR. The proposed linac is based on IH-DTL cavities and quadrupole triplet focusing. The KONUS beam dynamics concept [1] is used to achieve high acceleration efficiency. By optimization of the transverse focusing scheme and the longitudinal bunch center motion, low emittance growth for the entire linac is achieved. Beam dynamics simulations were performed along with 3D rf simulations of all cavities. The cavities are designed for 108.408 MHz, reaching an effective shunt impedance of 100-200 MΩ/m. The overall length of the linac is just 22.6 m which is almost a third of an alternative Alvarez layout. A mechanical realization concept employing a modular tank design is presented. The proposed design is a viable option for the GSI UNILAC poststripper linac replacement, leaving free space in the UNILAC tunnel for future energy upgrades. LAYOUT AND COMPONENTS The whole linac structure was developed with LORASR and CST Microwave Studio. The design comprises five 108 MHz IH-DTL cavities and seven quadrupole triplet lenses [2]. The linac is divided into three mechanically rigid sections (see Fig. 1). Figure 1: Layout of the IH-DTL linac. Additionally, the cavities are divided into short modules (as shown in Fig. 2) to allow copper plating and easy alignment of the modules with drift tubes and lenses. The layout features phase probes for all lenses except L1 and L2 and beam steerers between L4 and L5 to ensure optimal beam transport. The power requirements of the cavities were estimated using CST simulations of all sections. The overall consumed power per cavity is 0.82 MW at the beginning of the linac and reaches 1.22 MW towards the end of the linac (see Table 1). At a duty factor of 0.2 % the thermal losses are in the order of 2 kW and could be managed with simple cooling techniques. Significantly higher duty factors are also possible, but would require extensive cooling of the structures. Figure 2: Mechanical design of the modular sections for the first IH-DTL cavity (courtesy of D. Bänsch At 108.408 MHz the IH-cavities are 0.7-0.8 m in diameter. The design limits were chosen to be state of the art values to provide a reliable and durable machine. On axis electric field in the IH-cavities is at maximum just above 11 MV/m which is a value considered safe for an IH-structure at this frequency. The quadrupole magnets are now limited by < 1.1 T at the pole tip. This provides some operational margin, since tip fields of 1.3 T are possible using current magnet technology. The average accelerating gradient of the whole linac is 3.76 MV/m. Table 1: Cavity Properties Cav. [m] [ ] [MW] 1 4.9 19.6 0.8 2 2.8 17.2 1.0 3 3.6 18.7 1.2 4 3.7 18.7 1.2 5 3.9 16.8 1.2

A Coupled RFQ-IH-DTL Cavity for FRANZ: A Challenge for RF Technology and Beam Dynamics

For the ‘Frankfurt Neutron Source at the Stern-GerlachZentrum’ (FRANZ) facility an inductively coupled combination of a 4-Rod-type Radio-Frequency-Quadrupole (4Rod-RFQ) and an 8 gap interdigital H-type (IH-DTL) structure will provide the main acceleration of an intense proton beam from 120 keV to 2.0 MeV. The RFQ-IH combination with a total length of about 2.3 m will be operated at 175 MHz in cw mode. The expected total power need is around 200 kW. Due to the internal inductive coupling only one RF amplifier is needed, which significantly reduces the investment costs. At present the RFQ is installed separately in the beam line for conditioning up to the design rf power and for measuring the beam quality behind the RFQ. In parallel, the IH-DTL is rf tuned together with a dummy RFQ outside the FRANZ cave. This paper will present the status of the project with emphasis on key questions like beam dynamics constraints, rf tuning issues and technological challenges resulting from the high thermal load in cw operation.