D. Lawton

The low-energy-beam and ion-trap facility at NSCL/MSU

Abstract The goal of the low-energy-beam and ion-trap (LEBIT) project is to convert the high-energy exotic beams produced at NSCL/MSU into low-energy low-emittance beams. This beam manipulation will be done by a combination of a high-pressure gas stopping cell and a radio-frequency quadrupole ion accumulator and buncher. The first experimental program to profit from the low-energy beams produced will be high-accuracy mass measurements on very short-lived isotopes with a 9.4 T Penning trap system. The status of the project is presented with an emphasis on recent stopping tests range of 100 MeV/A 40Ar18+ ions in a gas cell.

A second-generation ion beam buncher and cooler ☆

A radiofrequency quadrupole (RFQ) ion accumulator and buncher has been designed for the low-energy beam and ion-trap (LEBIT) facility which is being set up at the NSCL/MSU. The LEBIT buncher will be a cryogenic system. Compared to room-temperature systems an improved beam quality and overall efficiency are expected. It will feature a novel electrode structure with a drastically reduced number of electrodes for simplified operation. Its design is presented and Monte-Carlo type ion-trajectory calculations are discussed which predict excellent beam quality and high performance.

The Superferric Cyclotron Gas Stopper Magnet Design and Fabrication

The Facility for Rare Isotope Beams under construction and the existing National Superconducting Cyclotron Laboratory at Michigan State University will provide exotic low-energy rare isotope beams (KeV-MeV) by stopping relativistic fragments produced by projectile fragmentation at high energies (<; 50 MeV/u). The stopped radioactive ions using the cyclotron gas stopper magnet system will feed the existing program centered on precision mass measurements of exotic nuclei and laser spectroscopy. Later on, stopped radioactive ions will be available as reaccelerated low-energy beams ( <; 15 MeV/u) using compact linear accelerator currently under construction. The cyclotron gas stopper magnet is a warm iron superconducting cyclotron sector dipole. The maximum field in the gap (0.18 m) is 2.75 T. The outer diameter of the magnet yoke is 4.0 m, with a pole radius of 1.1 m and Br = 1.8 T m. The desired field shape is obtained by a pole profile. Each coil of the two halves is in a separate cryostat and connected in series through a warm electrical connection. The entire system is mounted on a high voltage platform, and will be cooled by six cryocoolers. This paper presents the magnet design and discusses various design aspects of the magnet.

Design of superferric magnet for the cyclotron gas stopper project at the NSCL

We present the design of a superferric cyclotron gas stopper magnet that has been proposed for use at the NSCL/ MSU to stop the radioactive ions produced by fragmentation at high energies (~140 MeV/u). The magnet is a split solenoid-dipole with three sectors (Bave~ 2.7 T at the center and 1.7 T at the pole-edge.) The magnet outer diameter is 3.8 m, with a pole radius of 1.1 m and B*rho= 1.7 T-m. The field shape is obtained by extensive profiles in the iron. The coil cross-section is 80 mm times 80 mm and peak field induction on the conductor is about 2.05 T. The upper and lower coils are in separate cryostats and have warm electrical connections. We present the coil winding and the protection schemes. The forces are large and the implications on the support structure are presented.

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.

Radiation Simulations and Development of Concepts for High Power Beam Dumps, Catchers and Pre-Separator Area Layouts for the Fragment Separators for RIA

The development of high-power beam dumps and catchers, and pre-separator layouts for proposed fragment separators of the Rare-Isotope Accelerator (RIA) Facility are important in realizing how to handle the 400 kW in the primary beam. Examples of pre-conceptual designs of the pre-separator area and components, along with examples of ongoing radiation simulations with results characterizing the secondary radiation are given. These initial studies will yield insight into the impact of the high-power dissipation on fragment separator design, remote handling concepts, nuclear safety and potential facility hazard classification, shielding design, civil construction design, component design, and material choices. Furthermore, they will provide guidance on detailed radiation analyses as designs mature.

Progress report on the small isochronous ring project at NSCL

The small Isochronous Ring (SIR), whose main objectives are experimental studies of space charge effects in the isochronous regime and validation of space charge codes, is under development at the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University (MSU). The ring is a small-scale experiment that simulates the dynamics of intense beams in large-scale accelerators. It will store hydrogen and deuterium ions at energies of approximately 20-30 keV for a few tens of turns. The low beam energy and the small scale of the experiment provide a unique opportunity to perform accurate experiments on space charge dominated beams that are difficult to conduct in large-scale accelerators because of power and timing limitations imposed on beam diagnostics. The paper reports the status of the project and describes the progress in the development of ring subsystems.

Slow Current Discharges and Quenching of the MSU Superconducting Cyclotron Gas-Stopper Magnet

The Michigan State University cyclotron gas-stopper magnet generates a strong focusing cyclotron magnetic field that permits an ion beam of up to 100 MeV/u to enter the magnet and have most of its energy reduced by low-pressure helium gas. The magnet has two superconducting coil cryostats mounted within warm iron poles and a warm split iron return yoke. The coils are connected in series using room temperature cables. The peak magnet stored energy is 3.5 MJ. The magnet has been tested to its full operating current of 180 A. During tests, the magnet discharged inadvertently three times. Two low-current discharges caused the magnet current to decay through the power supply system. A discharge at 180 A quenched the magnet, because the quench protection system caused the magnet to discharge across a 1.25-Ω resistor. This fired the cold quench protection diodes within the cryostat, quenching both coils. The magnetic field in the gap was measured as the magnet discharged. For the two low-current discharges, the magnetic field followed the current in the external magnet circuit. During a quench, the magnetic field decay rate was much slower than the external circuit current decay rate.

Cold Mass Support System for the MSU Superconducting Cyclotron Gas-Stopper Magnet

The cyclotron gas-stopper magnet at Michigan State University consists of two superconducting coils, each in its own cryostat. The two cryostats are mounted in the two warm iron poles of a sector cyclotron magnet used to control the orbit of heavy ions as the particle energy is being removed by circulating the ions through helium gas. Because the two poles of the magnet must be separated to install the gas chamber and beam extraction system, the magnet coils cannot be connected together. As a result, the magnet cold mass support system must carry the forces pushing the magnet into the iron pole as well as any decentering forces that occur from coil placement errors. The cold mass support system for each magnet coil consists of six compression supports that support magnet forces in the axial direction. In addition, there are three radial supports to center the coil axis coaxial with the axis of the iron poles. This paper presents an analysis of the superconducting magnet cold mass support system, which must be designed to have a spring constant that is higher than the magnet force constant at the full design current for the magnet.

Fabrication of the Superferric Cyclotron Gas-stopper Magnet at NSCL at Michigan State University

The magnet for the cyclotron gas stopper is a newly designed, large warm-iron superconducting cyclotron sector gradient dipole. The maximum field in the centre (gap = 0.18 m) is 2.7 T. The outer diameter of magnet yoke is 4.0 m, with a pole radius of 1.1 m and B*ρ = 1.8 T m. The fabrication and assembly of the iron return yoke and twelve pole pieces is complete. Separate coils are mounted on the return yokes that have a total mass of about 167 metric tons of iron. This paper illustrates the design and the fabrication process for the cyclotron gas-stopper magnet that is being fabricated at MSU.