HYBRID PULSED FIELD GRADIENT COIL DRIVER

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This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. To further enhance our instrumentation capability in pulsed ESR microscopy, where unlike c.w., the experimenter can directly measure many key parameters such as self diffusion, relaxation times of the magnetic probe, viscosity, accurate O2 concentration, etc., we have designed a programmable pulsed field gradient coil driver of advanced capability. Based on a hybrid topology consisting of our existing gated-haversine coil excitation scheme and a fast-switched constant-current source, the new gradient coil driver design can be operated in either a prgrammable "rectangular" mode to accommodate our need for time-invariant gradients during the pulse sequence, or the previous fast "haversine" gradient excitation mode. The main advantage of a fast pulsed rectangular gradient is that it can be turned on, for example, only during each imaging pulse sequence, which enables the operator to avoid the problem of a heat producing constant gradient in one of the dimensions for frequency-encoded imaging applications. Ultimately, with further development, we expect to be able to produce short rectangular gradient pulses of ca 100ns at 0.5% stability. Such gradients can be selectively turned on during a specific microwave pulse which would permit, e.g., the possibility for slice selection imaging capability. Furthermore, when operating in the mode of our present haversine current driver, the new hybrid design would be capable of stronger and shorter pulsed field gradients which are necessary to support ESRM studies of nitroxide radicals with T2's in the 100  400ns range. After the planned developments and improvements in the gradient drivers, it is anticipated that the imaging probes will be able to generate haversine pulsed field gradients with durations of ~80 ns. A two-channel prototype "hybrid" pulsed field gradient coil driver has been designed and constructed and is presently under test. During the coming year, this unit will replace the existing haversine-only coil driver system, and continuing developmental upgrades will be incorporated and evaluated in situ.