At one of the flagship facilities being constructed by the DOE Office of Nuclear Physics — the Facility for Rare Isotope Beams (FRIB) at Michigan State University — everything begins with a high-performance source of heavy ions, the electron cyclotron resonance (ECR) source. The Berkeley Center for Magnet Technology has designed, built, and now delivered, a key component of the ECR source: an advanced superconducting magnet configured to produce a sextupole field embedded within three solenoids.
The ECR source will provide beams of ions as heavy as uranium. It must be capable of high current and high charge states (13.5 particle microamperes (pµA) and 33+, respectively, for uranium). This difficult task was a natural for LBNL, where the ECR source VENUS (Versatile ECR Ion Source for Nuclear Science) had already been built for the LBNL Nuclear Science Division’s 88-Inch Cyclotron. The performance of VENUS in producing intense high charge state beams was crucial in demonstrating the feasibility of the FRIB design concept.
The FRIB ECR magnet provides the combination of strong magnetic fields needed for plasma confinement: a 2-tesla sextupole field in the plasma chamber with a superimposed solenoidal field profile (4 T – 0.8 T – 3 T) produced by three solenoids. The project serves as an example of advanced magnet system design and construction that the Berkeley Center for Magnet Technology (BCMT) provides for the DOE Office of Science. BCMT brings together the Accelerator Technology and Applied Physics Division (ATAP) and the Engineering Division as a center for magnetics expertise.
The design phase was led by Dr. Helene Felice, then a scientist in the ATAP division at LBNL, leveraging in-depth expertise in advanced magnet structures from the HEP-funded high-field magnet program. Construction got underway after a successful design review in September 2014. BCMT staff, led by Dr. Diego Arbelaez of the Engineering Division, worked closely with the FRIB Front End team, led by Dr. Eduard Pozdeyev, and the FRIB ion source group, led by Dr. Guillaume Machicoane, to bring the project to fruition and ensure proper integration of the ECR magnet in its environment.
The design and realization of the magnetic fields is crucial because the performance of an ECR ion source relies primarily on the plasma density and confinement time. The plasma is produced within the magnet bore by heating of the electrons through the ECR phenomenon, driven by radiofrequency (RF) power. To build up the plasma density, strong confinement with a magnetic field is required. The strength of the confinement field has to increase with the ECR heating frequency. High intensity sources require correspondingly high frequencies (28 GHz in this case) and thus high magnetic fields. The combination of the solenoidal and sextupolar fields will provide a closed isomagnetic surface of at least 1.75 T in the magnet aperture.
“This magnet was a natural match for BCMT,” observed ATAP Division Director Wim Leemans, adding, “They bring depth and breadth of experience in the science and technology of magnets relevant to accelerators, and are especially well known for their know-how and leadership in the development and deployment of advanced superconducting magnets.”
The program offers expertise “from mesoscale to magnet” (that is, at all stages from the metallurgy of superconducting wire to the construction and testing of magnets) and an unequalled degree of integration of computerized design tools.
“This was an interesting challenge from both a magnetic-design and an engineering standpoint,” notes BCMT Director Soren Prestemon. The coils are made from niobium-titanium (NbTi), a familiar product used widely in accelerator applications.
The structure incorporates an innovation that has been extensively demonstrated since the days of the VENUS design: “key and bladder” assembly, which uses water-inflatable metal bladders (removed after assembly) and load keys inside the structure to provide compression to the sextupole structure at room temperature. This allows tunable pre-loading of the magnet to minimize conductor motion during magnet excitation. Such motion could result in quenches (sudden losses of superconductivity, which would trigger shutdowns). Differential thermal contraction of the support structure components during cool-down to the operating temperature (4.2 K) completes the preload. Thanks to this concept, the assembly of the sextupole magnet is fully reversible, allowing repair or replacement of a sextupole coil if necessary.
“Soren’s team has designed it not only to perform, but to be reliable and easily sustainable in a user facility,” says Henrik von der Lippe, Director of Berkeley Lab’s Engineering Division. “A product like this sums up what we’ve learned from everything else we’ve done.”
“I am delighted that LBNL has built, successfully tested, and delivered the superconducting cold mass magnet for the FRIB ECR,” adds Thomas Glasmacher, FRIB Project Director. “It has been a very good experience for us to work with the LBNL team on this magnet. I particularly appreciate the transparency with which the LBNL team has communicated with the FRIB team, and LBNL’s commitment to a high-quality product that was delivered within cost.”
A vision many years in the making
“FRIB has been a generation in the making for nuclear physics and nuclear astrophysics,” says James Symons, LBNL’s associate laboratory director for the Physical Sciences Area, which encompasses the ATAP, Engineering, Nuclear Science, and Physics Divisions. While the field is now firmly established at MSU, many of its roots can be traced through Berkeley Lab.
While chairing the joint DOE and National Science Foundation Nuclear Science Advisory Committee, Symons played a key role in the genesis of FRIB, which was given top priority for new construction in the committee’s 2001 Long Range Plan. Symons also chaired the committee that laid out the science case for the final design chosen for the facility design in 2006. The idea of a dedicated user facility can be traced three decades earlier, when the principal technique for producing the rare isotopes — fragmentation of heavier ones — was pioneered at LBNL’s Bevalac. Subsequently, the late LBNL nuclear scientist Mike Nitschke and Yale physicist Rick Casten championed the construction of a Rare Isotope Accelerator with which intense beams of short-lived nuclides not normally found in nature could be studied.
When it comes online in 2022, FRIB will bring capability to these experiments that was undreamed-of in the late 1970s when the Bevalac experiments were performed. “The secondary beams of the rare isotopes will be more powerful than the primary beams we had in those days,” says Symons.
Besides the ECR ion source concept and the magnets for it, LBNL is also leading one of the principal efforts at the other end of the facility: a detector called GRETA, the Gamma Ray Energy Tracking Array. The GRETA concept was prototyped as GRETINA, which was initially commissioned at LBNL’s 88-Inch Cyclotron, after which it was used at MSU’s National Superconducting Cyclotron Laboratory, and later at Argonne National Laboratory.