The W-band (95 GHz) EPR spectrometer at Illinois EPR Research Center was designed as a multi-purpose scientific instrument. The capabilities of the spectrometer have been extended by adding a fast magnetic field sweep system (maximum 1000G sweep in a minimum of 1 sec.) and a specially designed cryostat for variable temperature studies (2-300 K) with a vacuum-tight microwave window. The fast sweep system employs a room-temperature water-cooled solenoid and a modified Techron (Techron, IN) power supply. The sweep system features a digital scan with 14 bit resolution. In addition, a new water-cooled coil for scanning the magnetic field was designed and manufactured at the University of Illinois. The new coil produces 18% larger scans of magnetic field at the same current than does the original Oxford design. To obtain high-precision, reproducible sweeps around the center field, we have modified the original sweep coil supplied by Oxford. First, we have replaced the original sweep coil with our own design having a sweep range of +/- 600 Gauss, instead of the original +/-500 Gauss. Second, because high-power, high-stability bipolar power supplies are difficult to obtain, we added a Danfysik Ultrastab current transducer onto the original amplifier supplied by Oxford. The Danfysik transducer operates on the zero flux principle and enables current measurement and regulation without an ohmic connection with their inherent temperature stabilization problem at high current. A 16-bit D/A converter allows sweeping the field in steps as fine as 15 mG. Measured resettability is better than 100 fA and the stability is better than 10 ppm. The W-band system is a part of a NIH Biomedical Technology Research Resource (supported by grant P41-RR01811) and is accessible to others for collaborative or service use.
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