Superconducting RF (SRF) cavities are key components of modern linear particle accelerators. The National Superconducting Cyclotron Laboratory (NSCL) is building a 3 MeV/u re-accelerator (ReA3) using SRF cavities. Lightly loaded SRF cavities have very small bandwidths (high Q) making them very sensitive to mechanical perturbations whether external or self-induced. Additionally, some cavity types exhibit mechanical responses to perturbations that lead to high-order non-stationary transfer functions resulting in very complex control problems. A control system that can adapt to the changing perturbing conditions and transfer functions of these systems would be ideal. This paper describes the application of a control technique known as “Active Disturbance Rejection Control” (ARDC) to this problem.

/pdf/on-active-disturbance-rejection-based-control-design-for-401l7199yj.pdf

On active disturbance rejection based control design for superconducting RF cavities

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.

The Superferric Cyclotron Gas Stopper Magnet Design and Fabrication

Rare isotope beam (RIB) accelerator facilities provide rich research opportunities in nuclear physics in particular for nuclear structure physics, nuclear astrophysics and applied physics. The National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University (MSU) is constructing a RIB facility, called ‘ReA3‘. The facility will provide unique low-energy rare isotope beams by stopping RIBs produced in-flight and reaccelerating them in a compact linac. ReA3 comprises gas stopper systems, an Electron Beam Ion Trap (EBIT) charge state booster, a room temperature radio frequency quadrupole (RFQ), a linac using superconducting quarter wave resonators (QWRs) and an achromatic beam transport and distribution line to the new experimental area. Beams from ReA3 will range from 3 MeV/u for heavy ions to about 6 MeV/u for light ions, as the charge state of the ions can be adjusted by the EBIT. ReA3 will initially use beams from NSCL‘s Coupled Cyclotron Facility (CCF). Later ReA3 will provide reacceleration capability for the Facility for Rare Isotope Beams (FRIB), a new national user facility funded by the Department of Energy (DOE) that will be hosted at MSU. The ReA3 concept and status of ReA3 will be presented, with emphasis on the commissioning of the facility, which is underway.

ReA3 - the Rare Isotope Re-accelerator at MSU

A superconducting linac for re-acceleration of exotic ions is under development at Michigan State University. Two types of superconducting quarter-wave resonators (80.5 MHz, optimum β = 0.041 and 0.085) will be used for reacceleration to energies of up to 3 MeV per nucleon initially, with a subsequent upgrade path to 12 MeV per nucleon. Structural design is an important aspect of the overall cavity and cryomodule implementation. The structural design must include stiffening elements, the tuning mechanism, and the helium vessel. The main mechanical design optimization goal is to minimize the shift in the cavity’s resonant frequency due to the Lorentz force, bath pressure fluctuations, and microphonic excitation. Structural analyses of the MSU quarter-wave resonators are presented in this paper; stiffening measures are explored. The numerical predictions are compared to test results on prototype cavities.

Structural analyses of msu quarter-wave resonators

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.

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

The National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University (MSU) is currently constructing its new reaccelerated beam facility: ReA3. ReA3 will provide unique low-energy rare isotope beams by stopping fast rare isotopes in gas stopping systems and reaccelerating them in a compact linac. The main components, which will be explained in this paper, are a linear cryogenic gas cell to stop the fast beams produced by projectile fragmentation, an Electron Beam Ion Trap (EBIT) charge state booster, a room temperature Radio Frequency Quadrupole (RFQ) and a linac utilizing superconducting quarter wave resonators. An achromatic beam transport and distribution line towards the new experimental area will complete ReA3. Beams from ReA3 will range in energy from 0.3 to 6 MeV/u: the maximum energy is 3 MeV/u for heavy nuclei such as uranium, and 6 MeV/u for ions with A<50, as the charge state of the ions can be adjusted by the EBIT. ReA3 will provide pioneering beams for nuclear physics research, initially using beams from the Coupled Cyclotron Facility at NSCL and later providing reacceleration capability for the next-generation rare isotope facility FRIB that will be hosted at MSU. The ReA3 concept and status of ReA3 are presented, with particular emphasis on the superconducting linac.

/pdf/the-msu-nscl-re-accelerator-rea3-1dnw6zb7gs.pdf

THE MSU/NSCL RE-ACCELERATOR ReA3*

The Facility for Rare Isotope Beams (FRIB) is a high intensity, heavy ion SRF CW linac for nuclear science. FRIB SRF cryomodule design addressed four critical issues: high performance, stable operation, easy maintainability, and low cost construction, each presenting a unique challenge. FRIB SRF system design and R&D are almost complete. This paper presents unique R&D done in past 2-3 years for FRIB.

/pdf/superconducting-rf-development-for-frib-at-msu-2ez4zhxac4.pdf

Superconducting rf development for frib at msu

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THE MSU/NSCL RE-ACCELERATOR ReA3*

Superconducting rf development for frib at msu