The design and the operation of a frequency-tunable continuous-wave (CW) 330-GHz gyrotron oscillator operating at the second harmonic of the electron cyclotron frequency are reported. The gyrotron ...has generated 18 W of power from a 10.1-kV 190-mA electron beam working in a TE -4,3 cylindrical mode, corresponding to an efficiency of 0.9%. The measured start oscillation current over a range of magnetic field values is in good agreement with theoretical start currents obtained from linear theory for successive high-order axial modes TE -4,3,q , where q = 1-6. Moreover, the observed frequency range in the start current measurement is in reasonable agreement with the frequency range obtained from numerical simulations. The minimum start current was measured to be 33 mA. A continuous tuning range of 1.2 GHz was experimentally observed via a combination of magnetic, voltage, and thermal tuning. The gyrotron output power and frequency stabilities were assessed to be ±0.4% and ±3 ppm, respectively, during a 110-h uninterrupted CW run. Evaluation of the gyrotron output microwave beam pattern using a pyroelectric camera indicated a Gaussian-like mode content of 92% with an ellipticity of 28%. The gyrotron will be used for 500-MHz nuclear magnetic resonance experiments with sensitivity enhanced by dynamic nuclear polarization.
We present a conceptual design of a conduction-cooled 15 T/220 mm warm-bore superconducting magnet for use in a 350 GHz, 1 MW gyrotron. Gyrotrons can generate MW class of continuous wave power per ...device in the millimeter wave band and thus make them suitable for heating plasmas and driving current in magnetically confined fusion devices. The 12 T class tokamaks require a heating frequency of ∼350 GHz and a power level of 1-2 MW per gyrotron, as part of a 20-40 MW electron cyclotron resonance heating system. Gyrotrons at 350 GHz and 1 MW require superconducting magnets with ∼15 T field with a specific field profile and a warm bore diameter of 220 mm. In this paper, we propose to use high-temperature superconducting REBCO magnet technology in the magnet design, with a focus on managing stress, and implementing highly reliable winding and quench protection methods. We also propose to use an actively shielded low-temperature superconducting NbTi magnet in the lower field region to cost-effectively contribute a ∼5 T field and reduce the fringe field.
Dynamic nuclear polarization (DNP) is a method that permits NMR signal intensities of solids and liquids to be enhanced significantly, and is therefore potentially an important tool in structural and ...mechanistic studies of biologically relevant molecules. During a DNP experiment, the large polarization of an exogeneous or endogeneous unpaired electron is transferred to the nuclei of interest (I) by microwave (microw) irradiation of the sample. The maximum theoretical enhancement achievable is given by the gyromagnetic ratios (gamma(e)gamma(l)), being approximately 660 for protons. In the early 1950s, the DNP phenomenon was demonstrated experimentally, and intensively investigated in the following four decades, primarily at low magnetic fields. This review focuses on recent developments in the field of DNP with a special emphasis on work done at high magnetic fields (> or =5 T), the regime where contemporary NMR experiments are performed. After a brief historical survey, we present a review of the classical continuous wave (cw) DNP mechanisms-the Overhauser effect, the solid effect, the cross effect, and thermal mixing. A special section is devoted to the theory of coherent polarization transfer mechanisms, since they are potentially more efficient at high fields than classical polarization schemes. The implementation of DNP at high magnetic fields has required the development and improvement of new and existing instrumentation. Therefore, we also review some recent developments in microw and probe technology, followed by an overview of DNP applications in biological solids and liquids. Finally, we outline some possible areas for future developments.
The design, operation, and characterization of a continuous-wave (CW) tunable second-harmonic 460-GHz gyrotron are reported. The gyrotron is intended to be used as a submillimeter-wave source for ...700-MHz nuclear magnetic resonance experiments with sensitivity enhanced by dynamic nuclear polarization. The gyrotron operates in the whispering-gallery mode TE 11, 2 and has generated 16 W of output power with a 13-kV 100-mA electron beam. The start oscillation current measured over a range of magnetic field values is in good agreement with theoretical start currents obtained from linear theory for successive high-order axial modes TE 11, 2, q . The minimum start current is 27 mA. Power and frequency tuning measurements as a function of the electron cyclotron frequency have also been carried out. A smooth frequency tuning range of 1 GHz was obtained for the operating second-harmonic mode either by magnetic field tuning or beam voltage tuning. Long-term CW operation was evaluated during an uninterrupted period of 48 h, where the gyrotron output power and frequency were kept stable to within ±0.7% and ± 6 ppm, respectively, by a computerized control system. Proper operation of an internal quasi-optical mode converter implemented to transform the operating whispering-gallery mode to a Gaussian-like beam was also verified. Based on the images of the gyrotron output beam taken with a pyroelectric camera, the Gaussian-like mode content of the output beam was computed to be 92% with an ellipticity of 12%.
A linearly polarized (LP mn ) mode basis set for oversized, corrugated, metallic waveguides is derived for the special case of quarter-wavelength-depth circumferential corrugations. The relationship ...between the LP mn modes and the conventional modes (HE mn , EH mn , TE 0n , TM 0n ) of the corrugated guide is shown. The loss in a gap or equivalent miter bend in the waveguide is calculated for single-mode and multimode propagation on the line. In the latter case, it is shown that modes of the same symmetry interfere with one another, causing enhanced or reduced loss, depending on the relative phase of the modes. If two modes with azimuthal (m) indexes that differ by one propagate in the waveguide, the resultant centroid and the tilt angle of radiation at the guide end are shown to be related through a constant of the motion. These results describe the propagation of high-power linearly polarized radiation in overmoded corrugated waveguides.
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
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
1
H-
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.
1 ppm membrane protein
13
C linewidths and short proton T
1
's (1.3 s) are demonstrated in cryogenic MAS DNP experiments.
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.
We report the short-pulse operation of a 460 GHz gyrotron oscillator both at the fundamental (near 230 GHz) and second harmonic (near 460 GHz) of electron cyclotron resonance. During operation in a ...microsecond pulse length regime with 13-kV beam voltage and 110-mA beam current, the instrument generates several watts of power in two second harmonic modes, the TE/sub 2,6,1/ at 456.15 GHz and the TE/sub 0,6,1/ at 458.56 GHz. Operation in the fundamental modes, including the TE/sub 0,3,1/ mode at 237.91 GHz and the TE/sub 2,3,1/ at 233.15 GHz, is observed at output powers up to 70 W. Further, we demonstrate broadband continuous frequency tuning of the fundamental modes of the oscillator over a range of more than 2 GHz through variation of the magnetic field alone. We interpret these results in terms of smooth transitions between higher order axial modes of the resonator. The 460 GHz gyrotron is currently being processed for continuous duty operation, where it will serve as a microwave source for sensitivity-enhanced nuclear magnetic resonance (dynamic nuclear polarization) studies at 16 T (700 MHz /sup 1/H), a field strength which is two-fold higher than has been accessible with previous technology.
We introduce a novel design for millimeter wave electromagnetic structures within magic angle spinning (MAS) rotors. In this demonstration, a copper coating is vacuum deposited onto the outside ...surface of a sapphire rotor at a thickness of 50 nm. This thickness is sufficient to reflect 197‐GHz microwaves, yet not too thick as to interfere with radiofrequency fields at 300 MHz or prevent sample spinning due to eddy currents. Electromagnetic simulations of an idealized rotor geometry show a microwave quality factor of 148. MAS experiments with sample rotation frequencies of ωr/2π = 5.4 kHz demonstrate that the drag force due to eddy currents within the copper does not prevent sample spinning. Spectra of sodium acetate show resolved 13C J‐couplings of 60 Hz and no appreciable broadening between coated and uncoated sapphire rotors, demonstrating that the copper coating does not prevent shimming and high‐resolution nuclear magnetic resonance spectroscopy. Additionally, 13C Rabi nutation curves of ω1/2π = 103 kHz for both coated and uncoated rotors indicate no detrimental impact of the copper coating on radio frequency coupling of the nuclear spins to the sample coil. We present this metal coated rotor as a first step towards an MAS resonator. MAS resonators are expected to have a significant impact on developments in electron decoupling, pulsed dynamic nuclear polarization (DNP), room temperature DNP, DNP with low‐power microwave sources, and electron paramagnetic resonance detection.
Thin metallic boundary layers in magic angle spinning rotors can establish microwave cavities yet still permit spinning and high‐resolution spectra. A Goldilocks metallic layer is employed, which is thick enough to reflect microwaves yet sufficiently thin to pass nuclear magnetic resonance radio frequencies. We show that 50 nM is just right.
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