Radiation damping in high resolution liquid NMR: A simulation study

Abstract

Radiation damping generates surprising new features in two-dimensional and spin–echo experiments. The theoretical interpretation of the unexpected results has led to some controversy. The purpose of the present paper is to show, by computer simulations, that the experimental results can be completely understood by describing radiation damping through a classical magnetic field caused by the current induced in the sample coil (resonantly enhanced by the high quality factor of the tuned circuit), and acting back on the spins. We examine a variety of two-dimensional and ‘‘multiple quantum’’ experiments in homogeneous fields, and spin-echo experiments in a fixed field gradient (with and without molecular diffusion).

Nuclear spin noise in NMR revisited

“…Finally, the general demonstration and calculation carried out here, prove that the spin-dynamics interaction between the magnetization and the electronic circuit can be modeled by a simple equivalent RLC circuit [9]. Obviously, the effective quality factor is not given by the quality factor of the coil = / but depends on all electronic components of the reception circuit, explaining the appearance of the modified radiation damping rate ′ in Equations (3.10) and (5.1) and the introduction of an apparent quality factor ′ = / �1 + �, also introduced experimentally for explaining the difference between the extracted parameters and the ones claimed by the probe manufacturers [8,28,29,36].…”

“…It corresponds to an equivalent impedance, 6 , at the reference plan #6, which is purely real. In terms of radiation damping physics this is equivalent to a current in phase with the source voltage produced by the precessing magnetization [9], inducing a feedback field in quadrature to the precessing magnetization. Mathematically the FSTO condition corresponds to Equation (3.9) which appears to be dependent on the tuning capacitance but also on all the other components of the electronic reception circuit.…”

“…Classical theory of nuclear spin-noise line-shape Classically [9], the detection circuit is modeled by a coil ( , ) with a capacitor chip in parallel and the voltage is measured at the position of plane 3 of Figure 1. This measurement condition implicitly assumes a very high preamplifier input impedance, , directly connected at position of plane 3 in parallel to the capacitor chip , i.e.…”

“…This feedback field is expected to be in quadrature to the precessing magnetization for a perfectly tuned electronic circuit [9], inducing only broadening of the observed NMR signals for small flip-angle excitation pulses. Electronic mistuning lifts the perfect quadrature between the feedback field and the transverse magnetization whose NMR signatures are the previously mentioned signal broadening and a frequency shift of the observed NMR resonance frequency (frequency pushing) [10,11].…”

NMR Time Reversal as a Probe of Incipient Turbulent Spin Dynamics

“…This approximation is justified for many -but certainly not all -practical applications. One of the most well known and pervasive counter examples is the phenomenon of radiation damping [1], in which the emf induced in the pickup coil by M drives a current; the magnetic field associated with this current in turn acts on M, driving it into alignment with the static field B 0 in a nonlinear manner. A less common but more insidious problem arises when the magnetic field produced by the magnetized sample becomes large enough to directly influence spin dynamics [2,3,4,5,6].…”