S. Chouhan

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.

Superconductive undulators with variable polarization direction

In the past planar superconductive undulators have been successfully developed and tested with beam. They produce linearly polarized light (X-rays) and allow to tune the emitted wavelength electrically. In this paper a novel type of superconductive undulators is introduced which allows to tune electrically in addition to the wavelength the polarization direction. A short prototype was built and tested in a LHe bath.

Radiation Resistant Superferric Quadrupole Magnets With Warm Iron

The Facility for Rare Isotope Beams to be built at Michigan State University will be capable of providing beams of any element at energies of at least 200 MeV/u at beam powers of 400 kW. The fragment production target is in close proximity to the superconducting quadrupole magnets. These magnets have to operate in the high-radiation environment, with calculated radiation doses of more than 10 MGy per year. The design of a large bore, superferric quadrupole magnet that provides high field gradient of 15 T/m and effective length of 0.6 m is presented. The current design is “warm-iron” with a nominal yoke length of 520 mm and a pole radius of 135 mm. A coil design based on spinel insulated cable-in-conduit conductor that provides ample current margin provides significant long lifetime against radiation damage. The major challenges are the tight geometry, high forces and remote handling of an irradiated magnet. This paper presents the magnet design including coil forces and coil restraint system. In addition, coil properties, conductor stability and full mechanical details are also presented.

Design and Engineering of an HTS Dipole in the FRIB Fragment Separator

One of the challenges in the Facility for Rare Isotope Beams at Michigan State University is the 30 degree bending dipoles in the fragment separator operating in a high radiation environment. It is known that high temperature superconductors (HTS) have a much larger thermal margin due to high critical temperature > 90 K and high upper critical field > 100 T, which allows HTS magnets to operate stably so as to tolerate very high heat loads due to radiation. The HTS dipole magnets will utilize ReBCO conductor technology and operate at 38 K cooled by helium gas. High radiation deposits a large amount of heat into the iron yoke, cryostat, bobbin and HTS coil itself. For certain beams, over-bent particles will hit the cryostat with high intensity in the beam down-stream. Another difficulty is that the dipole coils generate significant Lorentz forces that need to be contained. All of these challenges have been analyzed separately and then integrated to find novel approaches. These approaches have been applied to optimize the magnet structure and enhance the 38 K helium gas cooling system. We present project status and progress of this HTS ReBCO dipole magnets and lay out a plan for magnet manufacturing.

Lessons learned from the cool down of a superconducting magnet using a thermal-siphon cooling-loop

The two Michigan State University (MSU) cyclotron gas-stopper magnet superconducting-coils were designed to be cooled down and to be kept cold using three pulse-tube coolers per coil cryostat. These coolers are designed to produce from 1.3 to 1.7 W per cooler when the cooler first-stage is at 40 K. The cyclotron gas stopper coils can be separated while cold, but unpowered. The two coil cryostats were cooled down separately in 2014, and room temperature helium gas was liquefied into the coil cryostats. The magnet temperature at the end of the cool-down was 4.55 K for one coil and 4.25 K for the other with and added 1.6 W of heat. The coil-down time for the coils was three and a half times longer than expected. The time to liquefy the helium was also much longer. The reasons for the disparity between the calculated cool-down time and measured cool-down time are discussed in the paper.

Magnetic Measurement System for the NSLS Superconducting Undulator Vertical Test Facility

One of the challenges of small-gap superconducting undulators is measurement of magnetic fields within the cold bore to characterize the device performance and to determine magnetic field errors for correction or shimming, as is done for room-temperature undulators. Both detailed field maps and integrated field measurements are required. This paper describes a 6-element, cryogenic Hall probe field mapper for the NSLS superconducting undulator Vertical Test Facility (VTF) [1]. The probe is designed to work in an aperture only 3 mm high. A pulsed-wire insert is also being developed, for visualization of the trajectory, for locating steering errors and for determining integrated multi-pole errors. The pulsed-wire insert is interchangeable with the Hall probe mapper. The VTF and the magnetic measurement systems can accommodate undulators up to 0.4 m in length.

Simulation of the Quenching of the Cyclotron Gas Stopper Magnet at MSU

The cyclotron gas stopper magnet at Michigan State University (MSU) 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. The two coils are connected in series to 200-A power supply. The magnet current leads are protected by cold silicon diodes in series with a resistor. The coils are attached to a stainless steel mandrel. As a result there is no magnet quench-back from the mandrel. There is quench-back from the resistor in series with the diodes that is wound on the outside of the solenoid coil. This causes the magnet coils to quench more rapidly reducing both the hot-spot temperature and the peak voltages in the circuit. This report presents a simulation of the quench process for this magnet.

Voltage Characteristics of Diodes Used for Passive Quench Protection of Low-Current Magnets

Cold diodes and resistors in parallel with the coils are keys components in the passive quench protection system of low-current (high inductance) superconducting magnets. The diodes prevent current from flowing in the circuits that are in parallel with sections of the superconducting coil when the magnet is being charged or discharged. The use of diodes and resistors in parallel with the superconducting coil sections reduce the peak voltages to ground and, in some cases, the hot-spot temperature in the magnet during a quench. This paper presents measurements of the onset (low current) forward voltage of several power diodes as a function of temperature and magnetic field.

A Cyclotron Magnet Case Study: Would Replacing the LTS Coils With HTS Coils Make Sense?

The cyclotron gas-stopper magnet (CGSM) at Michigan State University (MSU) was used as model for a study for whether HTS conductor is feasible for use in cyclotrons. The outside diameter of the CGSM split warm iron magnet yoke is 4 m, with a pole radius of 1.1 m. The desired field shape is obtained by shaping the iron pole profile. Each superconducting coil is in a separate cryostat. The two coils are connected in series through the warm electrical connection. Each coil is wound and potted within an 80 mm × 80 mm cross section and mounted within a 304 stainless steel helium vessel that is designed to carry magnetic forces to the magnet cold mass supports without excessive deflection. The iron is split, so that the cyclotron chamber and RF extraction system can be mounted between the two poles. This magnet has been successfully cooled down and tested at MSU to its 180-A operating current. One can ask the following question, “If one built a cyclotron magnet the size of the MSU cyclotron gas-stopper magnet, would replacing the existing LTS coils with HTS coils using a standard second generation HTS tape make technical sense or economic sense?” The report will explore the technical and economic issues of replacing an existing LTS coil with an HTS coil of the same size and shape, using 2G YBCO tape conductor commercially available in 2015 in place of the Nb-Ti used in the magnet.