RÉSUMÉ
Spherical rotors in magic angle spinning (MAS) experiments have significant advantages over traditional cylindrical rotors including simplified spinning implementation, easy sample exchange, more efficient microwave coupling for dynamic nuclear polarization (DNP), and feasibility of downscaling to access higher spinning frequencies. Here, we implement spherical rotors with 4â¯mm outside diameter (o.d.) and demonstrate spinning >28â¯kHz using a single aperture for spinning gas. We show a modified stator geometry to improve fiber optic detection, increase NMR filling factor, and improve alignment for sample exchange and microwave irradiation. Higher NMR Rabi frequencies were obtained using smaller radiofrequency (RF) coils on small-diameter spherical rotors, compared to our previous implementation of MAS spheres with an o.d. of 9.5â¯mm. We report nutation fields of 110â¯kHz on 13C with 820â¯W of input power and 100â¯kHz on 1H with 800â¯W of input power. Proton decoupling fields of 78â¯kHz were applied over 20â¯ms of signal acquisition without any sign of arcing. Compared to our initial demonstration of a split coil for 9.5â¯mm spheres, this current implementation of a double-saddle coil inductor for 4â¯mm spheres not only intensifies the RF fields, but also improves RF homogeneity. We achieve an 810°/90° nutation intensity ratio of 0.84 at 300.197â¯MHz (1H). We also show electromagnetic simulations predicting a nearly 3-fold improvement in electron Rabi frequency of 0.99â¯MHz (with 4â¯mm spheres) compared to 0.38â¯MHz (with 3.2â¯mm cylinders), with 5â¯W of incident microwave power. Further improvements in magnetic resonance spin control are expected as RF inductors and microwave coupling are optimized for spherical rotors and scaled down to the micron scale.