95GHZ SPECTROMETER PULSED MODULATOR DESIGN UPGRADES

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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. The 95 GHz CW/pulse spectrometer completed during the first years of the initial center grant is a unique instrument in the current ACERT ensemble of HF ESR spectrometers is. Operating at a peak output power of 1.2 KW, the spectrometer's high-power pulse section consists of an extended-interaction klystron (EIK) from CPI, Inc., a fast high-voltage pulse modulator designed and constructed by members of the ACERT engineering group, and a timing pattern generator of 5ns resolution in 8 utility channels and a single high-resolution channel of 1ns resolution (SpinCore ESR PRO-300, with ACERT custom high-resolution card). This Subproject addresses both longstanding and more recently-identified upgrades that we have in the past month (spring 2009) incorporated into the modulator section. Longstanding improvements to be made were 1) modification of the modulator internal beam-gating logic to permit generation of beam "on" time patterns that minimize both dissipation and deadtime; 2) increasing the modulator h.v. switch dissipation-limited repetition rate, and 3) to provide enhanced electromagnetic hermiticity for the modulator enclosure. A recent period of utilizing the main 95 GHz spectrometer exclusively in CW mode allowed us to remove and dismantle the high-power modulator section for the extensive upgrade modifications. The high-voltage switch drive logic has been converted to operation in gated mode and suitable fast timing algorithms have been hardwired into the gated logic for EIK protection. To significantly increase the high-voltage switch power dissipation limit, electrically isolated external heat-sinks were added to the switch module housing and the module was then incorporated into a fan shroud for forced-SF6 gas cooling of the switch output devices. EMI issues due to pickup and external conduction by coax shields within the modulator box have been corrected with improved interface transition grounding. Bench and initial in situ tests of the modulator after reassembly indicate that we have successfully addressed all the above technical issues and have reduced the EIK output power droop to nearly zero. Measurement of the actual pulse mode deadtime reduction will be made when the timing software scripting has been completed (see Subproject 211).