Shmaryu M. Shvartsman

Design of actively shielded main magnets: an improved functional method

An improved functional approach for designing MRI (magnetic resonance imaging) main magnets with active shielding is presented. By nulling one or two external moments as well as a certain series of internal moments of the magnetic field, new designs with improved shielding in combination with or without shorter magnet lengths are obtained. The improved method can be employed to design short and practical superconducting magnets at any given field strength. The resulting designs yield the desired field homogeneity inside the region of interest without using superconducting shim coils. This approach requires only a modest amount of computing power. One of the design steps, a contour plot of the continuous current solutions, can be utilized to study stretch goals for favorable design parameters.

A comparison of two design methods for MRI magnets

Designs of magnetic resonance imaging (MRI) main magnets obtained from both a functional method and a genetic algorithm method have been compared. While most features in the two approaches are similar, there are several important differences. The functional method leads to fewer coil bundles and a reduced total current, i.e., total ampere turns, (e.g., 6-8 MA) that can be as much as 70% of the total current found with the genetic analysis. While the conclusion about stress is that it is a sensitive function of the choice of wire current density, the designs found with the functional method have a larger hoop stress than that of the genetic design, which may require new or refined manufacturing techniques. Furthermore, the functional approach requires much less computing power (i.e., a personal computer is quite sufficient) while the genetic algorithm method in general demands massively parallel computing techniques. However, in order to search for the optimal magnetic resonance design at a given field strength, it is likely that a combination of these two methods will lead to the best results.

Role of zero modes in the canonical quantization of heavy-fermion QED in light-cone coordinates

Four-dimensional heavy-fermion QED is studied in light-cone coordinates with (anti)periodic field boundary conditions. We carry out a consistent light-cone canonical quantization of this model using the Dirac algorithm for a system with first- and second-class constraints. To examine the role of the zero modes, we consider the quantization procedure in the zero mode and the nonzero-mode sectors separately. In both sectors we obtain the physical variables and their canonical commutation relations. The physical Hamiltonian is constructed via a step-by-step exclusion of the unphysical degrees of freedom. An example using this Hamiltonian in which the zero modes play a role is the verification of the correct Coulomb potential between two heavy fermions.