Cryogenic sample exchange NMR probe for magic angle spinning dynamic nuclear polarization

We describe a cryogenic sample exchange system that dramatically improves the efficiency of magic angle spinning (MAS) dynamic nuclear polarization (DNP) experiments by reducing the time required to change samples and by improving long-term instrument stability. Changing samples in conventional cryogenic MAS DNP/NMR experiments involves warming the probe to room temperature, detaching all cryogenic, RF, and microwave connections, removing the probe from the magnet, replacing the sample, and reversing all the previous steps, with the entire cycle requiring a few hours. The sample exchange system described here—which relies on an eject pipe attached to the front of the MAS stator and a vacuum jacketed dewar with a bellowed hole—circumvents these procedures. To demonstrate the excellent sensitivity, resolution, and stability achieved with this quadruple resonance sample exchange probe, we have performed high precision distance measurements on the active site of the membrane protein bacteriorhodopsin. We also include a spectrum of the tripeptide N-f-MLF-OH at 100 K which shows 30 Hz linewidths.National Institute for Biomedical Imaging and Bioengineering (U.S.) (Grant EB-002804)National Institute for Biomedical Imaging and Bioengineering (U.S.) (Grant EB-001960)National Institute for Biomedical Imaging and Bioengineering (U.S.) (Grant EB-001035)National Institute for Biomedical Imaging and Bioengineering (U.S.) (Grant EB-002026)National Institute for Biomedical Imaging and Bioengineering (U.S.) (Grant EB-003151)National Science Foundation (U.S.). Graduate Research Fellowship Progra

Resolution and Polarization Distribution in Cryogenic DNP/MAS Experiments

This contribution addresses four potential misconceptions associated with high-resolution dynamic nuclear polarization/magic angle spinning (DNP/MAS) experiments. First, spectral resolution is not generally compromised at the cryogenic temperatures at which DNP experiments are performed. As we demonstrate at a modest field of 9 T (380 MHz [superscript 1]H), 1 ppm linewidths are observed in DNP/MAS spectra of a membrane protein in its native lipid bilayer, and <0.4 ppm linewidths are reported in a crystalline peptide at 85 K. Second, we address the concerns about paramagnetic broadening in DNP/MAS spectra of proteins by demonstrating that the exogenous radical polarizing agents utilized for DNP are distributed in the sample in such a manner as to avoid paramagnetic broadening and thus maintain full spectral resolution. Third, the enhanced polarization is not localized around the polarizing agent, but rather is effectively and uniformly dispersed throughout the sample, even in the case of membrane proteins. Fourth, the distribution of polarization from the electron spins mediated via spin diffusion between [superscript 1]H–[superscript 1]H strongly dipolar coupled spins is so rapid that shorter magnetization recovery periods between signal averaging transients can be utilized in DNP/MAS experiments than in typical experiments performed at ambient temperature.National Institutes of Health (U.S.) (Grant EB002804)National Institutes of Health (U.S.) (Grant EB003151)National Institutes of Health (U.S.) (Grant EB002026)National Institutes of Health (U.S.) (Grant EB001965)National Institutes of Health (U.S.) (Grant EB004866)National Science Foundation (U.S.). Graduate Research Fellowship Progra

A novel wideband gyrotron travelling wave amplifier

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2003.Includes bibliographical references (leaves 193-214).We present the design and the experimental results of a novel wideband quasioptical Gyrotron Traveling Wave Tube (gyro-TWT) amplifier and the first Vacuum Electron Device (VED) with a Photonic Band Gap (PBG) structure. The theory and experimental results from two other quasioptical gyrotron oscillator experiments are also presented. The gyro-TWT amplifier at 140 GHz produced up to 30 kW of peak power in operation with 3 ts pulses at 6 Hz, 2.30 GHz unsaturated bandwidth, a peak linear gain of 29 dB and an efficiency of 12 %. This is the highest frequency gyro-TWT reported as yet. The use of a very high order operating mode in a novel mode selective interaction structure namely, a quasioptical open confocal waveguide makes the gyro-TWT potentially capable of up to 100 kW CW operation at 140 GHz and at 94 GHz in the W-band (75-110 GHz) if built to industrial standards. The theory and experimental results from the first VED with a PBG resonator, in the form of a high power high frequency gyrotron oscillator at 25 kW power level at 140 GHz in a high order mode, TE041 are also presented. The absence of any spurious mode excitation over 30 % variation in the magnetic field corresponding to an equal variation in the cyclotron frequency indicates that the PBG resonator was highly mode selective while operating in a higher order mode. This opens up promising avenues for building highly oversized resonators for generating Terahertz radiation with low voltage electron beams.(cont.) The successful operation of this high frequency gyrotron suggests that overmoded PBG resonators can also be used in conventional slow-wave devices to build moderate power (few hundreds of Watts) VEDs in the millimeter and sub-millimeter wave regime for a variety of applications. A gyrotron oscillator experiment with an overmoded open confocal resonator was also run at a peak power of up to 83 kW at 136 GHz with an efficiency of 16 %. The gyrotron oscillator experiment had excellent mode stability which makes an oversized quasioptical open confocal resonator a candidate for use in high frequency millimeter and sub-millimeter wave gyrotrons. A second harmonic confocal gyrotron experiment was also tried at 280 GHz but the poor quality electron beam prevented the generation of the second harmonic, however, the stronger fundamental mode interaction was suppressed thus validating the advantages of using confocal resonators in high harmonic gyrotrons to suppress the stronger fundamental interaction. The experiments on three millimeter wave VEDs with two novel kinds of highly oversized interaction structures described in this work have demonstrated a novel technique for building high power (> 1 kW) high frequency (> 100 GHz) VEDs.by Jagadishwar R. Sirigiri.Ph.D

Design of an overmoded W-band TWT

We report on the design and cold test validation of an overmoded TWT capable of producing power in excess of 100 Watts in the W-band and above. The TWT operates in the TM31 mode of a rectangular cavity and has transverse dimensions three times larger than a conventional ladder TWT. Dielectric loading of a resonant cavity was utilized to suppress lower order modes and prevent parasitic oscillations. HFSS and MAGIC3D codes were used to predict performance. An X-Ku band scaled down version of the interaction structure was built and cold tests performed on it showed excellent agreement with HFSS simulations.United States. Air Force Office of Scientific Researc

Demonstration of a 140-GHz 1-kW Confocal Gyro-Traveling-Wave Amplifier

The theory, design, and experimental results of a wideband 140-GHz 1-kW pulsed gyro-traveling-wave amplifier (gyro-TWA) are presented. The gyro-TWA operates in the HE [subscript 06] mode of an overmoded quasi-optical waveguide using a gyrating electron beam. The electromagnetic theory, interaction theory, design processes, and experimental procedures are described in detail. At 37.7 kV and a 2.7-A beam current, the experiment has produced over 820 W of peak power with a -3-dB bandwidth of 0.8 GHz and a linear gain of 34 dB at 34.7 kV. In addition, the amplifier produced a -3-dB bandwidth of over 1.5 GHz (1.1%) with a peak power of 570 W from a 38.5-kV 2.5-A electron beam. The electron beam is estimated to have a pitch factor of 0.55-0.6, a radius of 1.9 mm, and a calculated perpendicular momentum spread of approximately 9%. The gyro-amplifier was nominally operated at a pulselength of 2 mus but was tested to amplify pulses as short as 4 ns with no noticeable pulse broadening. Internal reflections in the amplifier were identified using these short pulses by time-domain reflectometry. The demonstrated performance of this amplifier shows that it can be applied to dynamic nuclear polarization and electron paramagnetic resonance spectroscopy.National Institutes of Health. National Institute for Biomedical Imaging and Bioengineering (Contract EB001965

Active real-time imaging system employed with a CW 460-GHz gyrotron and a pyroelectric array camera

We report experimental testing of an active real-time imaging system useful for many practical applications, such as fast security check, food safety inspection, etc. The system consists of a 460-GHz gyrotron capable of producing 16 W in continuous wave operation and a pyroelectric array camera with 124-by-124 pixels. The detailed results obtained from the proof-of-concept experiment with the system will be presented.National Institute of Biomedical Imaging and Bioengineering (U.S.) (contract EB004866)National Institutes of Health (U.S.

Design study of critical components of second harmonic a 100 kW, 0.4 THz gyrotron oscillator

We present simulation result of the critical components for a second harmonic 0.4 THz, 100 kW gyrotron. The design study of the MIG electron gun has been done by three different simulation codes, CST Particle studio, MICHELLE, EGUN code. Analysis of an interaction cavity and a mode converter will be presented

Design study of a MIG electron gun of a 100 kW, 0.4 THz gyrotron oscillator

We present simulation result of an MIG electron gun for a 0.4 THz, 100 kW gyrotron. The gyrotron is a device to generate very high power at very high frequencies. The magnetron injection gun (MIG) is a critical component in the gyrotron[1]. The design study of the MIG electron gun has been done by three different simulation codes. The EGUN code for 2D has been used for the initial geometry optimization with a 70 kV and 13 A of the electron beam. The MICHELLE code and the CST Particle Studio have been also used and results are compared

Observation and Study of Low-Frequency Oscillations in a 1.5-MW 110-GHz Gyrotron

We report the observation of low-frequency oscillations (LFOs) in the range 165-180 MHz in a 1.5-MW 110-GHz gyrotron operating in 3-mus pulses. The oscillations have been measured by a capacitive probe located just before the entrance to the cavity. The LFOs are observed only in a narrow region of beam parameter space, at voltages between 45 and 60 kV, where no microwave emission occurs. When the gyrotron operates near 96 kV, with high output power, they are not seen. The variation of the frequency of the oscillations with electron beam voltage and magnetic compression was measured, and the results are reported. Time-domain analysis of the probe signal shows the influence of the beam current and cathode voltage on the time of onset of the oscillations. The amplitude of the time-domain signal indicates that the trapped electron current associated with the LFOs represents a few percent of the total electron current.U.S. Department of Energy, Office of Fusion Energy Science

Design and testing of an internal mode converter for a 1.5 MW, 110 GHz gyrotron with a depressed collector

We report experimental results on a 1.5 MW, 110 GHz, 3 microsecond pulsed gyrotron with a single-stage depressed collector. A simplified mode converter with smooth mirror surfaces has been installed in the tube. The converter was designed with the code SURF3D. We present the hot and cold test results of the internal mode converter. The hot and cold test measurements show good agreement.United States. Dept. of EnergyUnited States. Dept. of Energy. Office of Fusion Energy Science