Numerically optimized modulations for adiabatic pulses in surface nuclear magnetic resonance

Adiabatic pulses, which provide an effective means of generating a large-amplitude nuclear magnetic resonance (NMR) signal in the presence of a heterogeneous magnetic field, have the potential to greatly improve the signal-to-noise ratio of the surface NMR experiment. To ensure efficient implementation of adiabatic pulses into the surface NMR framework, a numerically optimized modulation (NOM) approach is used to design adiabatic pulses specifically intended for application in surface NMR. The scenario in which the frequency response of the tuned transmit coil is used to modulate the current amplitude is considered. The performance of a NOM pulse is contrasted against two alternative adiabatic pulses (described by a linear frequency sweep and a hyperbolic tangent sweep) that are currently implemented with the existing hardware. The NOM approach provides equivalent excitation as the chirp and hyperbolic tangent pulse while shortening pulse durations and reducing power consumption. Furthermore, the NOM approach also provides sharp resolution and large signal amplitudes. Considerations for the design of the NOM adiabatic pulse for surface NMR are given, as well as a discussion about their implementation into the surface NMR experimental framework.

Adiabatic pulses enhance surface nuclear magnetic resonance measurement and survey speed for groundwater investigations

Surface nuclear magnetic resonance (surface NMR) is an extremely powerful tool for groundwater resource investigations. However, the technique suffers from an inherently low signal-to-noise ratio (S/N), which commonly necessitates extensive signal averaging, resulting in very long measurement times. Previous approaches to improve S/N and measurement efficiency have focused primarily on reducing noise, through hardware and processing advancements. We introduce a new and divergent approach to actually increase the signal amplitude by modifying the form of the transmitted pulse used to excite the groundwater signals. An on-resonance pulse, the only form of excitation pulse previously used in surface NMR, has a fixed frequency and induces coherent excitation over a narrow range of transmit field strengths. Given spatially inhomogeneous fields underlying the surface coil, an on-resonance pulse excites water, a limited volume of water, producing a similarly limited signal amplitude. An adiabatic pulse, one of many pulse forms used for medical imaging and chemical spectroscopy, modulates pulse frequency and provides excitation over a much larger range of transmit field amplitudes. Numerical simulations of surface NMR with adiabatic pulses demonstrate almost a factor of three improvement in the peak signal amplitude compared to an on-resonance pulse. Simulations also show that a single measurement using an adiabatic pulse with high transmit current provides sensitivity to water over a wide range of depth. In contrast, multiple on-resonance measurements using a range of transmit currents are required to span sensitivity over a similar range of depths. Numerical simulation results are validated by the first field experiments comparing on-resonance and adiabatic pulses. We have considered how improvements in S/N can be used for dramatically improved measurement speed and how other advantages of adiabatic pulses may more generally be used to enhance surface NMR measurements.

The Application of Simple and Easy to Implement Decoupling Pulse Scheme Combinations to Effect Decoupling of Large J Values with Reduced Artifacts

Of the various problems in decoupling one nucleus type from another using standard decoupling pulse schemes for broadband decoupling, a particular challenge is to effect full, artifact-free decoupling when the size of the coupling constant is very large. Herein it is demonstrated that 1H decoupling of the 31P NMR spectrum of diethyl phosphonate {HP(=O)(OCH2CH3)2} can be accomplished with reduced artifacts despite the large JH,P1 value of 693 Hz by using a combination of decoupling pulse schemes involving continuous-wave (CW) irradiation and either adiabatic-pulse decoupling (APD), MPF decoupling, or traditional composite-pulse decoupling (CPD) schemes such as WALTZ or GARP. The considered strategy is simple, efficient, and easy to implement on most instruments. The best result was attained for a combination of CW and CPD using GARP with a standard pulse width of 60 μs. Altogether, the advantages of the methodology include low power requirements, complete decoupling, tolerance of a range of large J values, large bandwidth for normal-sized J values, and the suppression of sidebands.

Relativistic channeling of a linearly polarized laser pulse in overdense plasma

We investigated a dynamic procedure for relativistic channeling by a linearly polarized ultraintense laser pulse in overdense plasma, subsequently determining a phenomenological formula for the channel-digging speed. Channeling of the linearly polarized pulse usually results in a sharp-cut (non-adiabatic) pulse front, since the pulse is continuously reflected on the transparency-opacity interface during the channeling process. Using the novel formula for the channel-digging speed, it was possible to analytically predict where such a sharp-cut occurs longitudinally.