Category Archives: THz

High Power Wideband Gyrotron Backward Wave Oscillator Operating towards the Terahertz Region

He, W., et al., High Power Wideband Gyrotron Backward Wave Oscillator Operating towards the Terahertz Region. Phys. Rev. Lett., 2013. 110(16): p. 165101.

http://www.ncbi.nlm.nih.gov/pubmed/23679610

Experimental results are presented of the first successful gyrotron backward wave oscillator (gyro-BWO) with continuous frequency tuning near the low-terahertz region. A helically corrugated interaction region was used to allow efficient interaction over a wide frequency band at the second harmonic of the electron cyclotron frequency without parasitic output. The gyro-BWO generated a maximum output power of 12 kW when driven by a 40 kV, 1.5 A, annular-shaped large-orbit electron beam and achieved a frequency tuning band of 88-102.5 GHz by adjusting the cavity magnetic field. The performance of the gyro-BWO is consistent with 3D particle-in-cell numerical simulations.

High power THz technologies using high frequency gyrotrons

Idehara, T., et al. High power THz technologies using high frequency gyrotrons. in Infrared, Millimeter, and Terahertz Waves (IRMMW-THz), 2012 37th International Conference on. 2012.

http://dx.doi.org/10.1109/IRMMW-THz.2012.6380408

Gyrotrons are extremely high power radiation sources even in THz frequency region comparing conventional radiation sources. We have developed gyrotron FU CW series for application to many high power THz technologies. Up to the present, nine CW gyrotrons were developed. Each gyrotron has its own objective and was optimized for respective application subject. Now, the applications are extended to wide fields including DNP-NMR measurement, ESR echo experiment, direct and precise measurement on hyperfine structure of positronium, experiment on effective operation of Bloch oscillator using semiconductor super lattice, etc. In this paper, we will introduce such high power THz technologies opened by high power THz radiation sources. Gyrotrons.

High power THz technologies using high frequency gyrotrons

Idehara, T., et al. High power THz technologies using high frequency gyrotrons. in Infrared, Millimeter, and Terahertz Waves (IRMMW-THz), 2012 37th International Conference on. 2012.

http://dx.doi.org/10.1109/IRMMW-THz.2012.6380408

Gyrotrons are extremely high power radiation sources even in THz frequency region comparing conventional radiation sources. We have developed gyrotron FU CW series for application to many high power THz technologies. Up to the present, nine CW gyrotrons were developed. Each gyrotron has its own objective and was optimized for respective application subject. Now, the applications are extended to wide fields including DNP-NMR measurement, ESR echo experiment, direct and precise measurement on hyperfine structure of positronium, experiment on effective operation of Bloch oscillator using semiconductor super lattice, etc. In this paper, we will introduce such high power THz technologies opened by high power THz radiation sources. Gyrotrons.

Phase cycling with a 240 GHz, free electron laser-powered electron paramagnetic resonance spectrometer

This is not an article directly related to DNP spectroscopy. However, it shows the tremendous progress made in the development of high-frequency, high-power sources that can be utilized for high-field EPR and eventually DNP experiments.

Edwards, D.T., et al., Phase cycling with a 240 GHz, free electron laser-powered electron paramagnetic resonance spectrometer. Phys. Chem. Chem. Phys., 2013.

http://dx.doi.org/10.1039/C3CP44492A

Electron paramagnetic resonance (EPR) powered by a free electron laser (FEL) has been shown to dramatically expand the capabilities of EPR at frequencies above [similar]100 GHz, where other high-power sources are unavailable. High-power pulses are necessary to achieve fast (<10 ns) spin rotations in order to alleviate the limited excitation bandwidth and time resolution that typically hamper pulsed EPR at these high frequencies. While at these frequencies, an FEL is the only source that provides [similar]1 kW of power and can be tuned continuously up to frequencies above 1 THz, it has only recently been implemented for one- and two-pulse EPR, and the capabilities of the FEL as an EPR source are still being expanded. This manuscript presents phase cycling of two pulses in an FEL-EPR spectrometer operating at 240 GHz. Given that the FEL, unlike amplifiers, cannot be easily phase-locked to a reference source, we instead apply retrospective data processing to measure the relative phase of each FEL pulse in order to correct the signal phase accordingly. This allows the measured signal to be averaged coherently, and the randomly changing phase of the FEL pulse results in a stochastic phase cycle, which, in the limit of many pulses, efficiently cancels artifacts and improves sensitivity. Further, the relative phase between the first and second pulse, which originates from the difference in path length traversed by each pulse, can be experimentally measured without phase-sensitive detection. We show that the relative phase of the two pulses can be precisely tuned, as well as distinctly switched by a fixed amount, with the insertion of a dielectric material into the quasi-optical path of one of the pulses. Taken together, these techniques offer many of the advantages of arbitrary phase control, and allow application of phase cycling to dramatically enhance signal quality in pulsed EPR experiments utilizing high-power sources that cannot be phase-locked.

Phase cycling with a 240 GHz, free electron laser-powered electron paramagnetic resonance spectrometer

This is not an article directly related to DNP spectroscopy. However, it shows the tremendous progress made in the development of high-frequency, high-power sources that can be utilized for high-field EPR and eventually DNP experiments.

Edwards, D.T., et al., Phase cycling with a 240 GHz, free electron laser-powered electron paramagnetic resonance spectrometer. Phys. Chem. Chem. Phys., 2013.

http://dx.doi.org/10.1039/C3CP44492A

Electron paramagnetic resonance (EPR) powered by a free electron laser (FEL) has been shown to dramatically expand the capabilities of EPR at frequencies above [similar]100 GHz, where other high-power sources are unavailable. High-power pulses are necessary to achieve fast (<10 ns) spin rotations in order to alleviate the limited excitation bandwidth and time resolution that typically hamper pulsed EPR at these high frequencies. While at these frequencies, an FEL is the only source that provides [similar]1 kW of power and can be tuned continuously up to frequencies above 1 THz, it has only recently been implemented for one- and two-pulse EPR, and the capabilities of the FEL as an EPR source are still being expanded. This manuscript presents phase cycling of two pulses in an FEL-EPR spectrometer operating at 240 GHz. Given that the FEL, unlike amplifiers, cannot be easily phase-locked to a reference source, we instead apply retrospective data processing to measure the relative phase of each FEL pulse in order to correct the signal phase accordingly. This allows the measured signal to be averaged coherently, and the randomly changing phase of the FEL pulse results in a stochastic phase cycle, which, in the limit of many pulses, efficiently cancels artifacts and improves sensitivity. Further, the relative phase between the first and second pulse, which originates from the difference in path length traversed by each pulse, can be experimentally measured without phase-sensitive detection. We show that the relative phase of the two pulses can be precisely tuned, as well as distinctly switched by a fixed amount, with the insertion of a dielectric material into the quasi-optical path of one of the pulses. Taken together, these techniques offer many of the advantages of arbitrary phase control, and allow application of phase cycling to dramatically enhance signal quality in pulsed EPR experiments utilizing high-power sources that cannot be phase-locked.

THz Transmission Lines for DNP-NMR

A transmission line, linking the gyrotron (or solid-state) source to the NMR probe is an essential piece of THz instrumentation for DNP-NMR spectrometers. While quasi-optical transmission system have been used for setups using low-power solid-state sources, far more typical is the use of circular (corrugated) waveguides to deliver the THz power to the sample. Below is a list of two articles that have been published recently covering the topic of low-loss THz transmission lines for application in DNP-NMR spectroscopy:

Nanni, E., et al., Low-loss Transmission Lines for High-power Terahertz Radiation. J. Infrared Millim. Te., 2012: p. 1-20.

http://dx.doi.org/10.1007/s10762-012-9870-5

Bogdashov, A., et al., Transmission Line for 258 GHz Gyrotron DNP Spectrometry. J. Infrared Millim. Te., 2011. 32(6): p. 823-837.

http://dx.doi.org/10.1007/s10762-011-9787-4

THz Transmission Lines for DNP-NMR

A transmission line, linking the gyrotron (or solid-state) source to the NMR probe is an essential piece of THz instrumentation for DNP-NMR spectrometers. While quasi-optical transmission system have been used for setups using low-power solid-state sources, far more typical is the use of circular (corrugated) waveguides to deliver the THz power to the sample. Below is a list of two articles that have been published recently covering the topic of low-loss THz transmission lines for application in DNP-NMR spectroscopy:

Nanni, E., et al., Low-loss Transmission Lines for High-power Terahertz Radiation. J. Infrared Millim. Te., 2012: p. 1-20.

http://dx.doi.org/10.1007/s10762-012-9870-5

Bogdashov, A., et al., Transmission Line for 258 GHz Gyrotron DNP Spectrometry. J. Infrared Millim. Te., 2011. 32(6): p. 823-837.

http://dx.doi.org/10.1007/s10762-011-9787-4

Development of DNP-Enhanced High-Resolution Solid-State NMR System for the Characterization of the Surface Structure of Polymer Materials

Horii, F., et al., Development of DNP-Enhanced High-Resolution Solid-State NMR System for the Characterization of the Surface Structure of Polymer Materials. J. Infrared Millim. Te., 2012: p. 1-10.

http://dx.doi.org/10.1007/s10762-012-9874-1

A dynamic nuclear polarization (DNP)-enhanced cross-polarization/magic-angle spinning (DNP/CP/MAS) NMR system has been developed by combining a 200 MHz Chemagnetics CMX-200 spectrometer operating at 4.7 T with a high-power 131.5 GHz Gyrotron FU CW IV. The 30 W sub-THz wave generated in a long pulse TE41 mode with a frequency of 5 Hz was successfully transmitted to the modified Doty Scientific low-temperature CP/MAS probe through copper smooth-wall circular waveguides. Since serious RF noises on NMR signals by arcing in the electric circuit of the probe and undesired sample heating were induced by the continuous sub-THz wave pulse irradiation with higher powers, the on-off sub-THz wave pulse irradiation synchronized with the NMR detection was developed and the appropriate setting of the irradiation time and the cooling time corresponding to the non-irradiation time was found to be very effective for the suppression of the arcing and the sample heating. The attainable maximum DNP enhancement was more than 30 folds for C1 13 C-enriched D -glucose dissolved in the frozen medium containing mono-radical 4-amino-TEMPO. The first DNP/CP/MAS 13 C NMR spectra of poly(methyl methacrylate) (PMMA) sub-micron particles were obtained at the dispersed state in the same frozen medium, indicating that DNP-enhanced 1 H spins effectively diffuse from the medium to the PMMA particles through their surface and are detected as high-resolution 13 C spectra in the surficial region to which the 1 H spins reach. On the basis of these results, the possibility of the DNP/CP/MAS NMR characterization of the surface structure of nanomaterials including polymer materials was discussed.

Development of DNP-Enhanced High-Resolution Solid-State NMR System for the Characterization of the Surface Structure of Polymer Materials

Horii, F., et al., Development of DNP-Enhanced High-Resolution Solid-State NMR System for the Characterization of the Surface Structure of Polymer Materials. J. Infrared Millim. Te., 2012: p. 1-10.

http://dx.doi.org/10.1007/s10762-012-9874-1

A dynamic nuclear polarization (DNP)-enhanced cross-polarization/magic-angle spinning (DNP/CP/MAS) NMR system has been developed by combining a 200 MHz Chemagnetics CMX-200 spectrometer operating at 4.7 T with a high-power 131.5 GHz Gyrotron FU CW IV. The 30 W sub-THz wave generated in a long pulse TE41 mode with a frequency of 5 Hz was successfully transmitted to the modified Doty Scientific low-temperature CP/MAS probe through copper smooth-wall circular waveguides. Since serious RF noises on NMR signals by arcing in the electric circuit of the probe and undesired sample heating were induced by the continuous sub-THz wave pulse irradiation with higher powers, the on-off sub-THz wave pulse irradiation synchronized with the NMR detection was developed and the appropriate setting of the irradiation time and the cooling time corresponding to the non-irradiation time was found to be very effective for the suppression of the arcing and the sample heating. The attainable maximum DNP enhancement was more than 30 folds for C1 13 C-enriched D -glucose dissolved in the frozen medium containing mono-radical 4-amino-TEMPO. The first DNP/CP/MAS 13 C NMR spectra of poly(methyl methacrylate) (PMMA) sub-micron particles were obtained at the dispersed state in the same frozen medium, indicating that DNP-enhanced 1 H spins effectively diffuse from the medium to the PMMA particles through their surface and are detected as high-resolution 13 C spectra in the surficial region to which the 1 H spins reach. On the basis of these results, the possibility of the DNP/CP/MAS NMR characterization of the surface structure of nanomaterials including polymer materials was discussed.

Low-loss Transmission Lines for High-power Terahertz Radiation

Nanni, E., et al., Low-loss Transmission Lines for High-power Terahertz Radiation. J. Infrared Millim. Te., 2012: p. 1-20.

http://dx.doi.org/10.1007/s10762-012-9870-5

Applications of high-power Terahertz (THz) sources require low-loss transmission lines to minimize loss, prevent overheating and preserve the purity of the transmission mode. Concepts for THz transmission lines are reviewed with special emphasis on overmoded, metallic, corrugated transmission lines. Using the fundamental HE 11 mode, these transmission lines have been successfully implemented with very low-loss at high average power levels on plasma heating experiments and THz dynamic nuclear polarization (DNP) nuclear magnetic resonance (NMR) experiments. Loss in these lines occurs directly, due to ohmic loss in the fundamental mode, and indirectly, due to mode conversion into high order modes whose ohmic loss increases as the square of the mode index. An analytic expression is derived for ohmic loss in the modes of a corrugated, metallic waveguide, including loss on both the waveguide inner surfaces and grooves. Simulations of loss with the numerical code HFSS are in good agreement with the analytic expression. Experimental tests were conducted to determine the loss of the HE 11 mode in a 19 mm diameter, helically-tapped, three meter long brass waveguide with a design frequency of 330 GHz. The measured loss at 250 GHz was 0.029 ± 0.009 dB/m using a vector network analyzer approach and 0.047 ± 0.01 dB/m using a radiometer. The experimental results are in reasonable agreement with theory. These values of loss, amounting to about 1% or less per meter, are acceptable for the DNP NMR application. Loss in a practical transmission line may be much higher than the loss calculated for the HE 11 mode due to mode conversion to higher order modes caused by waveguide imperfections or miter bends.

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