Flux Regulation in Powered Magnets: Enabling Magnetic Resonance Experiments with Pulsed Field Gradients

Open Access
Author:
Mcpheron, Benjamin David
Graduate Program:
Electrical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
July 10, 2014
Committee Members:
  • Jeffrey Louis Schiano, Dissertation Advisor
  • William Evan Higgins, Committee Member
  • Thomas Neuberger, Committee Member
  • Constantino Manuel Lagoa, Committee Member
Keywords:
  • Field fluctuations
  • Cascade Regulation
  • Field Regulation
  • NMR
  • Powered Magnets
  • Pulsed Field Gradients
Abstract:
High magnetic fields can significantly improve the resolution and sensitivity of nuclear magnetic resonance (NMR) spectroscopy measurements, which presents exciting research opportunities in areas of chemistry, biology, and material science. Powered magnets are capable of generating significantly higher magnetic fields than persistent superconducting magnets, but suffer from temporal magnetic field fluctuations introduced by power supply ripple and variations in temperature and flow rate of cooling water systems. These field fluctuations make powered magnets inviable for high resolution NMR experiments. Previous work by other researchers have shown that by feeding back measurements of field fluctuations, it is possible to significantly reduce magnetic field variations. The most recent feedback design uses two sensors, including a pickup coil sensor that is effective to measure and reduce field fluctuations introduced by power supply ripple, and an NMR sensor to measure and reduce field fluctuations originating in variations in the cooling water temperature and flow rate. Despite these advances in reducing temporal magnetic field fluctuations, the feedback design has two problems. The first problem is that the feedback design requires further reduction in field fluctuations to allow high resolution NMR experiments. The second problem is that the feedback scheme does not accommodate the use of pulsed field gradients, which are necessary in many NMR experiments. This dissertation is presented to address both of these problems with the feedback scheme. The first goal of this dissertation is to modify and test the feedback design to further reduce magnetic field fluctuations to enable 2D NMR in a powered magnet. The second goal of this dissertation is to design, synthesize and verify a control method that accommodates the use of pulsed field gradients with the feedback field regulation system. Challenges to meeting these goals include limited experiment time in powered magnets, insufficient processing power for real-time implementation of algorithms, and coupling of hardware noise into the physical system. %Powered magnets are capable of producing high fields that can significantly improve the resolution and sensitivity of magnetic resonance (MR) spectroscopy measurements. With this improvement in MR measurements, new research opportunities are presented in focus areas of chemistry, biology, and material science. Unfortunately, temporal magnetic field fluctuations are introduced into powered magnets by power supply ripple and variations in temperature and flow rate of the cooling water systems, which causes powered magnets to be inviable for most high resolution MR measurements. The application of cascade feedback control techniques can greatly reduce temporal field fluctuations in powered magnets. Additionally, many classes of MR experiments, such as diffusion measurements, magnetic resonance imaging (MRI), and solvent suppression require the use of pulsed field gradients. The recent iterations of the feedback control system used to attenuate temporal field fluctuations also measures and responds to pulsed field gradient signals. Not only is the pulsed field gradient signal attenuated by the control system, the controller response to the gradient signal introduces new field fluctuations into the magnetic field. These ancillary effects of pulsed field gradients on the feedback control system render it impossible to perform pulsed field gradient MR experiments with the field regulation system. In addition, the cascade field regulation system, composed of a fast inner loop and a slow outer loop, requires further field fluctuation reduction allow more complex MR measurements such as two-dimensional nuclear magnetic resonance (2D NMR) in powered magnets. The fast inner loop must be modified so that it has increased disturbance rejection over a large frequency band so that it better serves as an antialiasing filter for the outer loop, and so that it does not attenuate low frequency field fluctuations because the inner loop sensor does not provide and accurate measure of field fluctuations in this frequency band. The outer loop requires increased disturbance rejection at low frequencies and add derivative action to cause the field reach steady state more quickly.