PRECISION FAST PULSED FIELD GRADIENT DRIVER FOR ESR MICROSCOPY

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This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. In our pulsed ESR microscopy research and development work, we plan to reduce coil dissipation at high collection rates by providing precise, fast constant-current pulses to the resonator gradient coil sets. In practice, this objective is difficult to achieve because the gradient coils represent a substantially inductive load, requiring that the pulse driver be capable of charging the coil's magnetic field to the requisite energy level in several tens of nanoseconds or so and also that the coil's inductance not resonate at high "Q" with parasitic system driver and cable capacitances. Also, during the gradient "on" time, the pulse amplitude variation must be held to less than 1%, in order to avoid degrading the microimage resolution. Finally, the driver must rapidly "dump" the stored coil energy following the end of the gradient pulse period. To address these issues, we have developed several possible implementation schemes for the fast pulsed gradient driver. Currently, we are evaluating a novel fast constant-current driver core as the basis for our present design options. The driver core network consists of a current-programmed "cascode" configuration, essentially a series-connected, open-loop MOSFET topology. This type of wideband driver stage is inherently current-regulating and therefore does not rely on a discrete closed-loop feedback network for its constant-current characteristic. As a result, its bandwidth and stability margin are relatively high and handily exceed the requirements for our application. We are presently evaluating this core network to determine if, in its open-loop form, it will adequately maintain the pulse amplitude regulation that our microscope application requires while driving a representative load impedance over the full programmed-output range. If the open-loop driver amplitude variation exceeds our specification, we will then explore the addition of a gated fast current feedback loop to further reduce the amplitude variation. Work on this project will continue through the 2005-6 year.