Robert I. Hunter

A kilowatt pulsed 94 GHz electron paramagnetic resonance spectrometer with high concentration sensitivity, high instantaneous bandwidth, and low dead time

We describe a quasioptical 94 GHz kW pulsed electron paramagnetic resonance spectrometer featuring pi/2 pulses as short as 5 ns and an instantaneous bandwidth of 1 GHz in nonresonant sample holders operating in induction mode and at low temperatures. Low power pulses can be as short as 200 ps and kilowatt pulses as short as 1.5 ns with timing resolution of a few hundred picoseconds. Phase and frequency can be changed on nanosecond time scales and complex high power pulse sequences can be run at repetition rates up to 80 kHz with low dead time. We demonstrate that the combination of high power pulses at high frequencies and nonresonant cavities can offer excellent concentration sensitivity for orientation selective pulsed electron double resonance (double electron-electron resonance), where we demonstrate measurements at 1 microM concentration levels.

W-band PELDOR with 1 kW microwave power: molecular geometry, flexibility and exchange coupling.

A technique that is increasingly being used to determine the structure and conformational flexibility of biomacromolecules is Pulsed Electron-Electron Double Resonance (PELDOR or DEER), an Electron Paramagnetic Resonance (EPR) based technique. At X-band frequencies (9.5 GHz), PELDOR is capable of precisely measuring distances in the range of 1.5-8 nm between paramagnetic centres but the orientation selectivity is weak. In contrast, working at higher frequencies increases the orientation selection but usually at the expense of decreased microwave power and PELDOR modulation depth. Here it is shown that a home-built high-power pulsed W-band EPR spectrometer (HiPER) with a large instantaneous bandwidth enables one to achieve PELDOR data with a high degree of orientation selectivity and large modulation depths. We demonstrate a measurement methodology that gives a set of PELDOR time traces that yield highly constrained data sets. Simulating the resulting time traces provides a deeper insight into the conformational flexibility and exchange coupling of three bisnitroxide model systems. These measurements provide strong evidence that W-band PELDOR may prove to be an accurate and quantitative tool in assessing the relative orientations of nitroxide spin labels and to correlate those orientations to the underlying biological structure and dynamics.

Design of High-Performance Millimeter Wave and Sub-Millimeter Wave Quasi-Optical Isolators and Circulators

Faraday rotators using permanently magnetized ferrite materials are used to make quasi-optical isolators and circulators at millimeter wave and sub-millimeter wave frequencies that have far higher performance than their waveguide equivalents. This paper demonstrates state-of-the-art performance for four-port quasi-optical circulators with 60-dB isolation, 0.2-dB insertion loss, and better than 80-dB return loss for devices centered at 94 GHz. A method is presented for the accurate characterization of the complex permeability and permittivity of permanently magnetized ferrites via a series of frequency and polarization dependent transmission and reflection measurements. The dielectric and magnetic parameters for the sample are determined by fitting theoretical curves to the measured data. These fitted parameters are then used in a model for a complete quasi-optical Faraday rotator, including matching layers, allowing the accurate design and fabrication of these devices for any specific operational frequency band in the millimeter wave and sub-millimeter wave regime. Examples are given showing typical results and demonstrating how temperature cycling can significantly improve the temperature stability of these devices, while allowing fine tuning of the center frequency. We also indicate the performance possible at higher frequencies to above 1 THz and outline performance of truly planar isolators where lossy polarizer material is built into the Faraday rotator matching structure

Compact Wideband Corrugated Feedhorns With Ultra-Low Sidelobes for Very High Performance Antennas and Quasi-Optical Systems

The corrugated or scalar feedhorn has found many applications in millimeter wave and sub-millimeter wave systems due to its high beam symmetry, relatively low sidelobe levels and strong coupling to the fundamental mode Gaussian beam. However, for applications such as millimeter wave cosmology, space-based experiments, or even high performance imaging, there is a generic requirement to reduce the size of horns whilst maintaining very high levels of performance. In this paper we describe a general analytic methodology for the design of compact dual-profiled corrugated horns with extremely low sidelobe levels. We demonstrate that it is possible to achieve

340-GHz 3D radar imaging test bed with 10-Hz frame rate

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DEER Sensitivity between Iron Centers and Nitroxides in Heme-Containing Proteins Improves Dramatically Using Broadband, High-Field EPR.

dB sidelobe levels, over wide bandwidths with short horns, which we believe represents state-of-the-art performance. We also demonstrate experimentally a simple scalar design that operates over wide bandwidths and can achieve sidelobes of better than

High power pulsed dynamic nuclear polarisation at 94 GHz

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Geometric model-based fitting algorithm for orientation-selective PELDOR data

dB, whilst maintaining a frequency independent phase center. This design methodology has been validated experimentally by the successful manufacture and characterization of feedhorns at 94 GHz and 340 GHz for both radar and quasi-optical instrumentation applications.

An Output Coupler for a W-Band High Power Wideband Gyroamplifier

We present a 340 GHz 3D radar imaging test bed with 10 Hz frame rate which enables the investigation of strategies for the detection of concealed threats in high risk public areas. The radar uses a wideband heterodyne scheme and fast-scanning optics to achieve moderate resolution volumetric data sets, over a limited field of view, of targets at moderate stand-off ranges. The high frame rate is achieved through the use of DDS chirp generation, fast galvanometer scanners and efficient processing which combines CPU multi-threading and GPU-based techniques, and is sufficiently fast to follow smoothly the natural motion of people.

Optimization and Measurement of a Smoothly Profiled Horn for a W-Band Gyro-TWA

This work demonstrates the feasibility of making sensitive nanometer distance measurements between Fe(III) heme centers and nitroxide spin labels in proteins using the double electron–electron resonance (DEER) pulsed EPR technique at 94 GHz. Techniques to measure accurately long distances in many classes of heme proteins using DEER are currently strongly limited by sensitivity. In this paper we demonstrate sensitivity gains of more than 30 times compared with previous lower frequency (X-band) DEER measurements on both human neuroglobin and sperm whale myoglobin. This is achieved by taking advantage of recent instrumental advances, employing wideband excitation techniques based on composite pulses and exploiting more favorable relaxation properties of low-spin Fe(III) in high magnetic fields. This gain in sensitivity potentially allows the DEER technique to be routinely used as a sensitive probe of structure and conformation in the large number of heme and many other metalloproteins.