Lattice Design Research

Background

Lattice design research is the study of beam dynamics in beam transport systems, linear and circular accelerators, and experimental devices such as NSCL’s S800 magnetic spectrograph or the A1900 fragment separator.

The Role of NSCL

At NSCL, we use computer simulations and beam experiments to study the particle beam behavior with different magnetic and electrostatic elements and to find a “lattice design” to accelerate or transport beam from one location to another. Beyond satisfying the existing geometric constraints, the systems we design must provide adequate focusing for the beam to avoid beam loss, match the specific beam characteristics requirements for different accelerators and nuclear physics experimental equipment, and consist of feasible hardware at a reasonable cost.

In NSCL’s recently completed Coupled Cyclotron Facility, our lattice design research focused on three important areas of the project: the K500 Injection Beam Line, the K500-to-K1200 Coupling Line, and the A1900 Fragment Separator.

The K500 Injection Line transports beams from the ECR ion sources to the central region of the K500 cyclotron. It maintains the beam brightness by controlling the space-charge effect and matches specific beam characteristics (the transverse phase space) from the ion source to the requirements for injection into the K500 cyclotron. Since the beam energy in the Iinjection Lline is very low, solenoid magnets and electrostatic elements are used for focusing and bending of the beam.

The K500-to-K1200 Coupling Line is designed to transport the beams extracted from the K500 cyclotron to the radial injection port of the K1200 cyclotron. In order to achieve the high transmission efficiency, specific beam properties (the transverse phase spaces) in the two cyclotrons must be determined and matched properly. The beam energies in the coupling line can range from 6.9 MeV/nucleon for heavy ions like uranium-238, to 16.7 MeV/nucleon like oxygen-16. Superconducting quadrupole and dipole magnets are used in the system to provide the focusing and bending for the beam.

At NSCL, secondary beams of often short-lived rare isotopes are produced by the technique of projectile fragmentation, where a primary beam from the K1200 cyclotron impinges on a thin target to produce a broad range of isotopes for nuclear physics research. The A1900 fragment separator is then used to select the particular isotope required for a specific experiment. Since the secondary beams produced by this technique are highly energetic and have a relatively large angular “divergence” (spread), the A1900 uses large-aperture, high-field superconducting quadrupole and dipole magnets to obtain the large acceptance needed to catch most of desired isotopes. Additional sextupole and octupole magnets are installed in the A1900 to correct the beam distortions that result from to the large beam divergence and broad beam-energy spectrum.

In recent years, our lattice design team has collaborated with a number of other institutions, including the University of Maryland, Lawrence Berkeley National Laboratory, and the Canadian TRIUMF Laboratory. Collaborative research projects included the University of Maryland Electron Ring, a Heavy Ion Fusion Final Focus System, and the Canadian Light Source.

In the proposed RIA project, multi-charge beam acceleration and beam transport are required to achieve the required high beam energy and power, especially for heavy ions like uranium-238. This will provide new challenges in the designs of the Linac focusing structure, charge-stripping section, ECR front-end system, and final beam switchyard. In addition, the required high-acceptance and high-resolution fragment separators will need significant effort in lattice design research.