A preliminary design for a one GHz NMR spectrometer magnet

Abstract

Abstmct --We have performed a preliminary design for a persistent GHz NMR magnet at 23.5 T and 1.8 K operating conditions. In this paper we shall address the issues of realistic conductor selection, the coil design, the magnetic and mechanical analysis of the coil, and the required field uniformity. In addition, we shall describe the GHz magnet cryostat with a practical 1.8 K J-T refrigerator system. Finally vibration isolation system and field shield design and its associated field harmonics will be analyzed. I INTRODUCTIONWe have carefully evaluated all available conductors and have selected those that are best for each field regions. Our design is based on realistic Jc values which have been demonstrated on long lengths of conductor and confirmed in our conductor evaluation program.A persistent NMR magnet has a highly stable magnetic field. If the field drift is very small, changes in the magnetic field are easily corrected by means of an internal lock system based usually on a deuterium resonance line of the solvent. In a powered solenoid with feedback control on the power supply, the magnetic field stability is not so good, so that a strong lock signal is required. Whereas a persistent system can be operated for short periods of time in an unlocked mode, this is not possible for a powered solenoid (an unlocked mode provides a convenient way of carrying out deuterium decoupling experiments). Also, since modem complex multipulse 2D and 3D NMR experiments require great stability, some of these experiments would suffer in performance on a driven-solenoid NMR spectrometer.If the magnet is not persistent, the helium boiloff will be much larger. Therefore, either liquid helium must be frequently transferred or a helium liquefier must be provided; these can add more vibration, maintenance problems and higher cost.The basic information that one needs to design a persistent 23.5 T-1.8 K NMR magnet is the conductor critical current density Jc and conductor index N. We need to take a conservative approach to the design of a persistent 23.5 T GHz NMR magnet because index N is small at 23.5 T and 1.8 K .We must also reserved additional margin for Jc reduction due to tensile and transverse strains.We have selected 8th order coils to maximize field uniformity and to minimize the ratio of peak field to central field.We have designed a structurally reinforced coils to reduce the strain level to 0.25% and to eliminate the chance of irreversible conductor damage. I1 MAGNET DESIGN 11-1 Magnetic Analysis of R e 65 mm Bore SolenoidThe GHz solenoid is composed by 15 coils : the first 11 coils are made of Nb3Sn and the others are made of NbTi. All the coils are vacuum impregnated with epoxy.When the current in each turn is 140 A the central field is 23.5 T. The field harmonics have been computed with a double precision code for harmonic analysis: 0018-9464/94$04.00 0 1994 IEEE