A. Donati

The Iseult/Inumac Whole Body 11.7 T MRI Magnet R&D Program

A neuroscience research center with very high field MRI equipments has been opened in November 2006 by the CEA life science division. One of the imaging systems will require a 11.75 T magnet with a 900 mm warm bore. Regarding the large aperture and field strength, this magnet is a real challenge when compared to the largest MRI systems ever built, it is being developed within an ambitious R&D program, Iseult, focused on high field MRI. The conservative MRI magnet design principles are not readily applicable, other concepts taken from high energy physics or fusion experiments, namely the Tore Supra tokamak magnet system, will be used. The coil will thus be made of a niobium-titanium conductor cooled by a He II bath at 1.8 K, permanently connected to a cryoplant. Due to the high level of stored energy, about 340 MJ, and a relatively high nominal current, about 1500 A, the magnet will be operated in a non-persistent mode with a conveniently stabilized power supply. In order to take advantage of superfluid helium properties and regarding the high electromagnetic stresses on the conductors, the winding will be made of wetted double pancakes meeting the Stekly criterion for cryostability. The magnet will be actively shielded to fulfill the specifications regarding the stray field. In order to develop the magnet design on an experimental basis, an ambitious R&D program has been set-up based on magnet prototypes, high field test facility (Seht) and stability experiments. The main results from these experiments and their impact on the Iseult magnet design will be discussed.

Manufacturing of the Iseult/INUMAC Whole Body 11.7 T MRI Magnet

As part the Iseult/Inumac project, a French-German initiative focused on very high magnetic-field molecular imaging, the Whole Body 11.7 T MRI Magnet currently under development is the worlds largest to-date. It is an actively shielded magnet system, manufactured from NbTi superconductor, with a homogeneous field level of 11.75 T within a 90 cm warm bore. It will operate at a current of 1483 A, in nonpersistent mode, in a bath of superfluid LHe at 1.8 K. The stored energy is 338 MJ and the inductance 308 H. The cryostat has external dimensions of 5 m in diameter and 5.2 m in length, the total weight of the magnet is 132 tons. The magnet is serviced by a separate cryogenic and electrical facility forming an integral part of the installation. It is currently being manufactured at Alstom Belfort under the supervision of CEA Saclay. Several reduced scale prototypes, each addressing a specific set of design and manufacturing risks, have been tested. Full-scale serial production of the 170 double pancakes that form the main coil has been finished by Alstom. The project plan includes finishing the cold mass and cryostat assembly in May 2014. Full tests and commissioning of the magnet at 1.8 K will be performed at the Neurospin center upon completion of assembly. The paper reviews the manufacturing status of the 11.7 T magnet and its dedicated equipment.

Design Status of the R3B-GLAD Magnet: Large Acceptance Superconducting Dipole With Active Shielding, Graded Coils, Large Forces and Indirect Cooling by Thermosiphon

The R3B-Glad superconducting Magnet provides the field required for a large acceptance spectrometer, dedicated to the analysis of Reactions with Relativistic Radioactive ions Beams. In the framework of the FAIR Project to GSI and within NUSTAR physics program, the technical study started in 2006, and the engineering design is undertaken. One main feature of this butterfly-like magnet with graded, tilted and trapezoidal racetrack coils is the active shielding. It makes it possible to decreasing the field by two orders of magnitude within a 1.2 m length, despite the large opening on the outlet side of the magnet (around 0.8 square meters). The fringe field is lower than 20 mT in the target area beside the entry, while the main field is larger than 2 teslas, out of 2 m length. The other principal characteristics are as follows: first, a high level of magnetic forces (300 to 400 tons per meter), with little place to block the coils, requiring a very specific mechanical structure; then, the magnet protection system that is based on an external dump resistor, coupled to a strong quenchback effect, to prevent any damage of the coils which could be caused by the 24 MJ of stored energy; lastly, the indirect cooling of the cold mass with a two-phase helium thermosiphon. The overall size of the conical cryostat will be around 3.5 m long, 3.8 m high and 7 m broad.

Quench Experiments in a 8-T Superconducting Coil Cooled by Superfluid Helium

Quench experiments were performed in the CEA Saclay facility on the Seht superconducting magnet. The Seht facility is part of the Iseult R&D program. Seht is an 8-T coil wound in sixty double pancakes using NbTi conductor. The coil is cooled by steady state superfluid helium at 1.8 K and 1.2 bar. Instrumentation, inside the coil and in the helium bath, includes voltage taps, pressure and temperature sensors, as well as flow meters. The major issues in the Seht experiments will be addressed here: the normal zone propagation in the coil during quench and the pressure and temperature rise in the helium.

Progress in Design and Construction of the

The R3B-Glad superconducting Magnet is a large acceptance dipole, dedicated to the analysis of Reactions with Relativistic Radioactive ions Beams. It takes part in the FAIR Project at GSI. As the superconducting NbTi Rutherford cable was under production, detailed studies of the mechanical structure (with both simulation and experiment on a half-scale mock-up) led to revise the magnet design and to abandon the grading of the coils in three stages. Due to the large magnetic forces (up to 400 tons/m), the maximum shear stress level of 20 MPa was impossible to meet in the coils. The main reasons consist in the orthotropic thermo-mechanical behavior of the coils together with the large differential thermal shrinkage between the Cu stabilized coils and their Al alloy casings. Indeed after several studies of different mechanical designs, we decided to simplify the magnet in order to cope with these difficulties. One innovative point is that the coils are not blocked at room temperature, but only at 4.5 K. This paper presents the magnetic calculations of this active shielded magnet, and shows how the new design features meet the specifications. Currently, the 22 tons magnet cold mass, i.e. the 6 coils and their integration in the casings, is ordered and under construction. Meanwhile, the design of the magnet cryostat has evolved into a shape of elliptical cylinder with a lateral satellite. The total weight is expected to be around 50 tons.

{\hbox {R}}^{3}{\hbox {B}}

The Whole Body 11.7 T MRI Magnet is an actively shielded magnet system, with a stored energy of 338 MJ and an inductance of 308 H. Operating at a homogeneous field level of 11.75 T within a 90 cm warm bore, the cryostat has external dimensions of 4.8 m in diameter and 5.0 m in length. It is part of the Iseult/Inumac project, a French-German initiative focused on very-high-magnetic-field molecular imaging to improve sensitivity, spatial, temporal, and spectral resolution for preclinical and/or clinical MR systems. After the qualification of two first unit lengths of 820 m, the NbTi conductor with a current of 1483 A is now being produced at Luvata Waterbury. Winding of the main coil, made of 170 double pancakes, is starting at Alstom Belfort. Several pieces of equipment have already been delivered to the Neurospin site, CEA Saclay,; including the main refrigerator produced by Air Liquide. Several prototypes have been tested and confirmed the soundness of the magnet design. This paper describes the 11.7 T magnet and the latest progress in its design and fabrication.

-GLAD Large Acceptance Superconducting Dipole Spectrometer for GSI-FAIR

A neuroscience research center with very high field MRI equipment was opened in November 2006 by the CEA life science division. One of the imaging systems requires a 11.75 T magnet with a 900 mm warm bore, the so-call Iseult/Inumac magnet. Regarding the large aperture and field strength, this magnet is a challenge as compared to the largest MRI systems ever built, and will be developed within an ambitious R&D program. With the objective of demonstrating the possibility of achieving field homogeneity better than 1 ppm using double pancake windings, a 24 double pancakes model coil, working at 1.5 T has been designed. This model magnet was manufactured by Alstom MSA and tested at CEA. It has been measured with a very high precision, in order to fully characterize the field homogeneity, and then to investigate and discriminate the parameters that influence the field map. This magnet has reached the bare magnet field homogeneity specification expected for Iseult and thus successfully demonstrated the feasibility of building a homogenous magnet with the double pancake winding technique.

Latest Progress on the Iseult/INUMAC Whole Body 11.7 T MRI Magnet

In the frame of the W7X stellarator project, a cooperation agreement between the Max-Planck-Institut fur Plasmaphysik and CEA has been set-up in order to perform the acceptance tests of all the 70 superconducting coils that compose the W7X magnet system. The main purpose of these tests is to demonstrate that each coil can work at nominal operating conditions, with enough margin to ensure the coil safety during the stellerator operations. For that purpose, CEA has built a new test facility at Saclay. This paper presents a general overview of the test facility. It is mainly composed of two large cryostats (useful space of 5 m diameter and 4.2 in height), a cryogenic source to produce supercritical helium at 4.5 K and 6 bar with a power rating of 200 W, and an electrical power supply of 25 kA. Each cryostat can contain two coils. It is then possible to cool down two coils at the same time, and to warm up two others. But only one coil can be energized at the same time. As the assembly of the facility is now nearly completed, the first cryogenic tests with the prototype coil (DEMO) have started. The first conclusions of these tests and the facility performances will also be discussed in this paper.

Tests of a Prototype for Assessing the Field Homogeneity of the Iseult/Inumac 11.7 T Whole Body MRI Magnet

A neuroscience research center with very high field MRI equipments was opened in November 2006 by the CEA life sciences division. Three MRI systems operating at 3, 7 and 17 T have been already installed. One of the imaging systems will require a 11.75 T magnet with a 900 mm warm bore. The large aperture and high field strength of this magnet provide a substantial engineering challenge compared to the largest MRI systems ever built. This magnet is being developed within an ambitious R&D program, Iseult, whose focus is high field MRI. Traditional MRI magnet design principles are not readily applicable and thus concepts taken from high energy physics or fusion experiments, namely the Tore Supra tokamak magnet system, will be used. The coil will be made of a niobium-titanium conductor cooled by a He II bath at 1.8 K, permanently connected to a cryoplant. Due to its design the magnet will be operated in a non-persistent mode. As the field stability needed for MRI imaging requires a field drift of less than 0.05 ppm/h, it is hardly feasible to directly transpose these requirements in the power supply specification. Two existing solutions developed for other applications have been selected: one using a semi-persistent mode, and the other using a short-circuited superconducting coil in the inner bore. In order to make a decision on experimental basis, an ambitious R&D field stability program has been set-up based on magnet prototypes, high field test facility (Seht, a 44 H and 8 T magnet with a warm bore to 600 mm). We will present development and experimental results of the two stabilization solutions. In conclusion, the stability solution selected for the Iseult magnet is given.

Overview of a new test facility for the W7X coils acceptance tests

In order to measure high current (several tens of kilo Ampere) at low temperature (down to 4.2 K), we have developed collaboration with Hitec Power Protection (Netherlands ex. Holec). This company sell the Macc+ (a low cost Direct Current Control Transformer) which measures up to 600 A (AC and DC) at 300 K. Several limitations of the standard Macc+ will be pointed out to find the adapted solutions for the different user conditions (temperature, current level and background magnetic field). With some minor modifications to the standard produce, we could place the torus sensor at low temperature and we measured up to RA in liquid helium at low field. Analysis of the behavior of the device in nonstandard conditions and experimental results will be reported. With more modifications, we could measure up to 38 kA at 4.2 K in a 0.5 T-background magnetic field.