T.L. Grimm

Niobium cavity development for the high-energy linac of the Rare Isotope Accelerator

This paper will cover the design of a 6-cell /spl beta//sub g/ = 0.47 cavity for the Rare Isotope Accelerator, as well as the fabrication and RF testing of single-cell prototypes. Single-cell prototypes were chosen as a first step, as they provide a quick and inexpensive way to find out whether the desired field level and Q can be reached, and to check for problems with multipacting. An accelerating gradient of 8 MV/m was chosen as a goal for the /spl beta//sub g/ = 0.47 cavity.

New directions in superconducting radio frequency cavities for accelerators

Superconducting radio frequency (SRF) cavities used in present-day accelerators for the acceleration of charged particles near the speed of light are based on the axially symmetric TM010 mode of a pillbox cavity. Future accelerators such as the Linear Collider require high accelerating gradients to limit the length of the linac. Two techniques to improve the gradient are being explored: a cavity that is half reentrant to improve the electromagnetic characteristics, and improved heat transfer via cooling channels and surface modification at the helium interface. These changes could potentially increase the gradients and reduce the cryogenic losses. For other applications more important criteria are simplicity, acceleration of high beam current, or the ability to use advanced materials such as Nb/sub 3/Sn or high-T/sub c/ superconductors. A new type of cavity based on the TM01p pillbox mode with p>0 offers such improvements.

Cryomodule design for the Rare Isotope Accelerator

The Rare Isotope Accelerator (RIA) driver linac will produce >400 MeV/u proton through uranium beams using many types of superconducting accelerating cavities such as quarter wave, spoke, and elliptical cavities. A cryomodule design that can accommodate all of the superconducting cavity and magnet types is presented. Alignment of the cold mass uses a titanium rail system, which minimizes cryomodule size, and decreases both the tunnel cross-section and length. The titanium rail is supported from the top vacuum plate by an adjustable tri-link, which is similar to existing Michigan State University magnet technology. A prototype cryomodule is under construction for testing 805 MHz, v/c=0.47, six-cell niobium cavities in realistic operating conditions. Details of the design and progress to date are presented.

Status report on multi-cell superconducting cavity development for medium-velocity beams

The Rare Isotope Accelerator (RIA) is being designed to supply an intense beam of exotic isotopes for nuclear physics research. Superconducting cavities are to be used to accelerate the CW beam of heavy ions to 400 MeV per nucleon, with a beam power of up to 400 kW. Because of the varying beam velocity, several types of superconducting structures are needed. This paper covers the fabrication of three prototype RIA 6-cell /spl beta//sub g/ = 0.47 cavities and the RF tests on the first and second of these cavities.

Experimental study of a 322 MHz v/c=0.28 niobium spoke cavity

The Rare Isotope Accelerator (RIA) will accelerate heavy ions to >400 MeV/u using an array of superconducting cavities. A proposed linac design based on harmonics of 80.5 MHz will require six cavity types to cover the entire velocity range: three quarter wave resonators, one spoke cavity (half wave resonator), and two 6-cell elliptical cavities. A prototype 322 MHz niobium spoke with optimum velocity of 0.28 c has been fabricated. Each spoke would generate over 1 MV at 4 K for acceleration from v/c=0.20 to 0.40. Details of the design and experimental study are presented.

Cryomodule Design for A Superconducting Linac with Quarter-Wave, Half-Wave and Focusing Elements

The Rare Isotope Accelerator (RIA) driver linac is designed to accelerate heavy ions up to 400 MeV/u (β = v/c = 0.72) with a beam power up to 400 kW [1]. To obtain these intensities, partially stripped ions are accelerated in a 1400 MV superconducting linac. A design based on the 80.5 MHz harmonic requires six cavity types. A rectangular cryomodule design with a cryogenic alignment rail can accommodate all of the superconducting cavity and magnet types for RIA. A prototype 2-cavity cryomodule for the RIA elliptical cavities was designed in 2003 [2] and tested in 2004 [3]. This cryomodule design is suitable for all 3 elliptical cavity types. A similar cryomodule design has been developed for the lower-β quarter-wave and half-wave cavities for RIA. The cavities are interspersed with superconducting magnets for focusing, with 2 cavities between magnets for the quarter-wave cryomodules and 4 cavities between magnets for the halfwave cryomodules. A prototype low-β cryomodule was designed and is now under construction. The prototype module is large enough for 2 cavities and 2 magnets. The cryomodule design will be presented in this paper, along with the current status of assembly and testing of the cavities, magnets, and cryomodule.

Proposed upgrade of the NSCL

The present nuclear physics program at the National Superconducting Cyclotron Laboratory (NSCL) is based on an ECR-ion-source-injected K1200 superconducting cyclotron. We propose to significantly increase the facilitys output intensity for light ions and energy for heavy ions by coupling the existing superconducting K500 cyclotrons output to the K1200. The improved acceleration chain will consist of an ECR-ion-source injected K500 cyclotron to accelerate ions to /spl les/17 MeV/nucleon followed by radial, charge-stripping injection into the K1200 for final acceleration to 100-200 MeV/nucleon.

Mechanical Properties of High RRR Niobium With Different Texture

High purity bulk niobium RRR ~ 300 is the standard material used for SRF cavities since this has been the most promising material for over 10 years. Mechanical properties of the high RRR niobium play a critical role in the physical integrity of these structures. It is well known that the mechanical properties of polycrystalline high RRR niobium are affected by a variety of possible contributors: crystallographic texture, grain size, and impurity concentration. Tensile tests were carried out using three groups of niobium samples from MSU SRF cavity and Fermi International Linear Collider projects with different texture and grain size, which came from two different manufacturers. For some of the groups the tensile tests were done with different orientations with respect to the sheet. Orientation Imaging Microscopy (OIM) data were collected in these samples. The relationships between mechanical properties, texture, and grain size in these samples are analyzed.

Mechanical properties, microstructure, and texture of electron beam butt welds in high purity niobium

The effects of Electron Beam Welding on solidification microstructure, texture, microhardness and mechanical properties were investigated in high purity niobium weld specimens. The welds have an equiaxed microstructure with a 1 mm grain size in the fusion zone, 100 /spl mu/m in the heat affected zone (HAZ) and 50 /spl mu/m in the parent metal. The fusion zone had slightly higher microhardness values despite having a large grain size, while the unaffected material had the lowest microhardness. The texture in the weld consisted of a strong {111} fiber texture in the center and a mix of {111} - {100} components on the surface. Tensile tests of specimens gave /spl sigma//sub y/ = 60 MPa, but the UTS and elongation for weld specimens were lower than the parent material (137 vs. 165 MPa, 32% vs. 58%). The properties and microstructure of the weld are discussed in terms of optimizing the SRF cavity.

Beam dynamics studies for the reacceleration of low energy ribs at the NSCL

Rare Isotope Beams (RIBs) are created at the National Superconducting Cyclotron Laboratory (NSCL) by the in-flight particle fragmentation method. A novel system that stops the RIBs in helium gas and reaccelerates them is proposed to provide opportunities for an experimental program ranging from low energy Coulomb excitation to transfer reaction studies of astrophysical reactions. The beam from the gas stopper [1] will first be brought into a Electron Beam Ion Trap (EBIT) charge breeder [2] on a high voltage platform to increase its charge state, and then accelerated up to about 3 MeV/u by a system consisting of an external multi-harmonic buncher and a radio frequency quadrupole (RFQ) followed by a superconducting linac. The superconducting linac will use quarter-wave resonators with optimum acceleration for particle velocities as a fraction of the speed of light (betaopt) of 0.041 and 0.085 for acceleration and superconducting solenoid magnets for transverse focusing. The accelerator system design and the end-to-end beam dynamics simulations are presented.