95 GHZ PULSE SPECTROMETER DEADTIME REDUCTION

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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. Deadtime in the ACERT 95 GHz pulse spectrometer is presently around 40 nanoseconds, which carries substantial impact on the system SNR, and therefore spin sensitivity. This limitation is significant when collecting data on biological samples of low concentration. Our estimate is that the spectrometer's present intrinsic deadtime can be reduced to 15 ns or less by a.) modifications to the Extended-Interaction klystron Amplifier (EIA) modulator and b.) reduction of the quasioptical bridge corrugated waveguide length. With regard to the latter contribution we note that the ringdown time of the present 900mm corrugated waveguide section is predicted to be ca 20ns, a figure that is in good agreement with our low power evaluation data. Installation of the Cryogenic, Ltd. 9T magnet to replace the present Oxford 6T magnet, scheduled for late August, will permit substituting a 300mm section for the present corrugated waveguide section, thereby reducing the waveguide length and ringdown time by a factor of 3. The resulting waveguide-related deadtime contribution in attaining the spectrometer receiver noise floor of approximately 1 nW from an initial level of 1KW will accordingly fall from 20ns to 6.7ns. Regarding the modulator timing, the modulator internal timing hardware was initially designed to operate for a fixed retriggerable interval of approximately 1 ¿s to simplify the original external timing system configuration consisting of discrete timing modules. This "triggerable" hardware option, however, has the unwanted side effect of introducing around 10ns r.m.s. modulator turn-off jitter, which contributes about 20ns of additional deadtime. The solution to this minimizing this deadtime contribution is straightforward. Our 95 GHz spectrometer timing system has been upgraded with a commercial multichannel programmable card and companion high-resolution (1ns) card of ACERT design. A suitably programmed timing control line, with our planned modulator internal logic modifications, will permit us to operate the EIA modulator in a "strobed" mode, where the modulator turn-off time will then reflect the system's intrinsic resolution of 1ns and a deadtime improvement of about 20ns. We estimate that the two system improvements described in this Subproject will reduce the 95 GHz deadtime to ca 15ns, from the present value of ca 40ns.