[PAST EVENT] Applied Science and Chemistry Informational Session - Dr. Timothy Cross

November 9, 2017
12:30pm - 1:30pm
Access & Features
  • Open to the public

Title of Talk: "Magnet Technology for Nuclear Magnet Resonance from DC Powered Magnets to Prospects for High Temperature Superconducting Magnets."

Timothy A. Cross, Ph.D.
National High Magnetic Field Lab

Preamble: I am not an Engineer but a Biophysicist who has worked with the Magnet Science and Technology Engineers at the MagLab for the past 28 years to obtain funding for the design and construction of three magnets for NMR spectroscopy & MR imaging users.

Abstract: Today?s NMR spectroscopy and MR imaging is almost exclusively performed in superconducting magnets that have rock solid stability the enough homogeneity to meet the highest demands of the spectroscopist and imager. Indeed, the first magnet for NMR and MRI designed and constructed at the MagLab utilized standard low-temperature superconductors of NbTi and Nb3Sn filaments in a Cu matrix. These wires were wound in monolith coils supported by extensive stainless steel overbanding storing 38 MJ of energy. The magnet brought to field in 2004 remains the widest bore (105 mm) highest field (21.1 T or 900 MHz for 1H) magnet used for animal imaging in the world. The second magnet constructed for NMR at the MagLab was a 25T DC magnet funded by the Keck Foundation. This magnet was powered with 20 MWatts and cooled with chilled deionized water. Making very long coils as in superconducting magnets is impractical due to dissipating a lot more power, consequently enhanced homogeneity in this first-of-a-kind DC magnet used a current density grading system to achieve enhanced homogeneity. Unfortunately, the multiples of 60 Hz noise for the DC power supplies generated temporal instabilities that were a great challenge to control. However, this magnet helped to lay an engineering foundation for the development of the 36T Series Connected Hybrid Magnet that reached field for the first time in November, 2016 and has been operating in a commissioning phase for both the magnet operations team and the solid state NMR spectroscopy team this past year. The series connection between the outer superconducting coil cooled with supercritical liquid He and the resistive Florida-Bitter stacks in the interior cooled with 1800 gallons/min of chilled water contributes enough inductance, such that the multiples of 60 Hz are essentially completely suppressed. A combination of ferro-shimming and resistive shimming permits the acquisition of high resolution solid state NMR spectroscopy at a field strength that is 50% higher than any other magnet on the planet. Today, we have demonstrated a wide range of spectroscopy in biological and materials science.  The science from this magnet is now laying the scientific justification for the development of the next generation of superconducting magnets using one of several high temperature superconductors. Prospects, challenges and preliminary results for such magnets will be described.