Category Archives: Microwave Technology

Wave Guides for Micromagnetic Resonance

Yilmaz, Ali, and Marcel Utz. “Wave Guides for Micromagnetic Resonance.” In Micro and Nano Scale NMR, by Jens Anders and Jan G. Korvink, 75–108. Advanced Micro and Nanosystems. Weinheim, Germany: Wiley-VCH Verlag GmbH & Co. KGaA, 2018.

https://doi.org/10.1002/9783527697281.ch4.

In nuclear magnetic resonance, a system of nuclear spins exposed to a static magnetic interacts with an oscillatory field, usually in the radio frequency range. In most NMR setups, including all commercially available NMR spectrometers, coherent transitions between spin states are detected by a voltage induced into a conductor surrounding the sample. Whereas other detection techniques have their advantages in certain cases, inductive detection has proven to be both robust and easy to implement.

Millimeter-wave Sources for DNP-NMR #DNPNMR

Blank, Monica, and Kevin L Felch. “Millimeter-Wave Sources for DNP-NMR” 7 (2018): 12.

https://doi.org/10.1002/9780470034590.emrstm1582

Advances in both solid-state and vacuum-electron-based sources at frequencies greater than 200 GHz have been a key factor in the recent improvements in solid-state dynamic nuclear polarization (DNP) nuclear magnetic resonance (NMR) instrumentation. The current state of the art in solid-state sources and vacuum-electron devices (VEDs), such as extended interaction oscillators (EIOs), extended interaction klystrons (EIKs), and gyrotrons for DNP applications are described. The key features and design aspects of gyrotrons, which are presently the most promising DNP sources for high-field NMR systems, are detailed. In addition, the current capabilities of high-performance DNP gyrotron sources are illustrated. The status of ongoing research efforts in DNP gyrotrons and future directions are discussed.

Second Harmonic 527-GHz Gyrotron for DNP-NMR: Design and Experimental Results #DNPNMR

Jawla, Sudheer K., Robert G. Griffin, Ivan A. Mastovsky, Michael A. Shapiro, and Richard J. Temkin. “Second Harmonic 527-GHz Gyrotron for DNP-NMR: Design and Experimental Results.” IEEE Transactions on Electron Devices 67, no. 1 (January 2020): 328–34.

https://doi.org/10.1109/TED.2019.2953658

We report the design and experimental demonstration of a frequency tunable terahertz gyrotron at 527 GHz built for an 800-MHz dynamic nuclear polarization enhanced nuclear magnetic resonance (DNP-NMR) spectrometer. The gyrotron is designed at the second harmonic (ω = 2ωc) of the electron cyclotron frequency. It produces up to 9.3-W continuous microwave (CW) power at 527.2-GHz frequency using a diode type electron gun operating at V = 16.65 kV, Ib = 110 mA in a TE11,2,1 mode, corresponding to an efficiency of ∼0.5%. The gyrotron is tunable within ∼0.4 GHz by combining voltage and magnetic field tuning. The gyrotron has an internal mode converter that produces a Gaussian-like beam that couples to the HE11 mode of an internal 12-mm i.d. corrugated waveguide periscope assembly leading up to the output window. An external corrugated waveguide transmission line system is built including a corrugated taper from 12- to 16-mm i.d. waveguide followed by 3 m of the 16-mm i.d. waveguide The microwave beam profile is measured using a pyroelectric camera showing ∼84% HE11 mode content.

State-of-the-Art of High-Power Gyro-Devices and Free Electron Masers

Everything you ever wanted to know about gyrotrons, their applications and their design is summarized in this article. With a whooping 1528 references this article will be the encyclopedia of gyrotrons for many years to come.

Thumm, Manfred. “State-of-the-Art of High-Power Gyro-Devices and Free Electron Masers.” Journal of Infrared, Millimeter, and Terahertz Waves, January 3, 2020.

https://doi.org/10.1007/s10762-019-00631-y

This paper presents a review of the experimental achievements related to the development of high-power gyrotron oscillators for long-pulse or CWoperation and pulsed gyrotrons for many applications. In addition, this work gives a short overview on the present development status of frequency step-tunable and multi-frequency gyrotrons, coaxialcavity multi-megawatt gyrotrons, gyrotrons for technological and spectroscopy applications, relativistic gyrotrons, large orbit gyrotrons (LOGs), quasi-optical gyrotrons, fastand slow-wave cyclotron autoresonance masers (CARMs), gyroklystrons, gyro-TWT amplifiers, gyrotwystron amplifiers, gyro-BWOs, gyro-harmonic converters, gyropeniotrons, magnicons, free electron masers (FEMs), and dielectric vacuum windows for such high-power mm-wave sources. Gyrotron oscillators (gyromonotrons) are mainly used as high-power millimeter wave sources for electron cyclotron resonance heating (ECRH), electron cyclotron current drive (ECCD), stability control, and diagnostics of magnetically confined plasmas for clean generation of energy by controlled thermonuclear fusion. The maximum pulse length of commercially available 140 GHz, megawattclass gyrotrons employing synthetic diamond output windows is 30 min (CPI and European KIT-SPC-THALES collaboration). The world record parameters of the European tube are as follows: 0.92 MW output power at 30-min pulse duration, 97.5% Gaussian mode purity, and 44% efficiency, employing a single-stage depressed collector (SDC) for energy recovery. A maximum output power of 1.5 MWin 4.0-s pulses at 45% efficiency was generated with the QST-TOSHIBA (now CANON) 110-GHz gyrotron. The Japan 170-GHz ITER gyrotron achieved 1 MW, 800 s at 55% efficiency and holds the energy world record of 2.88 GJ (0.8 MW, 60 min) and the efficiency record of 57% for tubes with an output power of more than 0.5 MW. The Russian 170-GHz ITER gyrotron obtained 0.99 (1.2) MW with a pulse duration of 1000 (100) s and 53% efficiency. The prototype tube of the European 2-MW, 170-GHz coaxial-cavity gyrotron achieved in short pulses the record power of 2.2 MW at 48% efficiency and 96% Gaussian mode purity. Gyrotrons with pulsed magnet for various short-pulse applications deliver Pout = 210 kW with τ = 20 μs at frequencies up to 670 GHz (η ≅ 20%), Pout = 5.3 kW at 1 THz (η = 6.1%), and Pout = 0.5 kW at 1.3 THz (η = 0.6%). Gyrotron oscillators have also been successfully used in materials processing. Such technological applications require tubes with the following parameters: f > 24 GHz, Pout = 4–50 kW, CW, η > 30%. The CW powers produced by gyroklystrons and FEMs are 10 kW (94 GHz) and 36W(15 GHz), respectively. The IR FEL at the Thomas Jefferson National Accelerator Facility in the USA obtained a record average power of 14.2 kW at a wavelength of 1.6 μm. The THz FEL (NOVEL) at the Budker Institute of Nuclear Physics in Russia achieved a maximum average power of 0.5 kW at wavelengths 50– 240 μm (6.00–1.25 THz).

Frequency-chirped dynamic nuclear polarization with magic angle spinning using a frequency-agile gyrotron #DNPNMR

Gao, Chukun, Nicholas Alaniva, Edward P. Saliba, Erika L. Sesti, Patrick T. Judge, Faith J. Scott, Thomas Halbritter, Snorri Th. Sigurdsson, and Alexander B. Barnes. “Frequency-Chirped Dynamic Nuclear Polarization with Magic Angle Spinning Using a Frequency-Agile Gyrotron.” Journal of Magnetic Resonance 308 (November 2019): 106586.

https://doi.org/10.1016/j.jmr.2019.106586

We demonstrate that frequency-chirped dynamic nuclear polarization (DNP) with magic angle spinning (MAS) improves the enhancement of nuclear magnetic resonance (NMR) signal beyond that of continuous-wave (CW) DNP. Using a custom, frequency-agile gyrotron we implemented frequencychirped DNP using the TEMTriPol-1 biradical, with MAS NMR at 7 Tesla. Frequency-chirped microwaves yielded a DNP enhancement of 137, an increase of 19% compared to 115 recorded with CW. The chirps were 120 MHz-wide and centered over the trityl resonance, with 7 W microwave power incident on the sample (estimated 0.4 MHz electron spin Rabi frequency). We describe in detail the design and fabrication of the frequency-agile gyrotron used for frequency-chirped MAS DNP. Improvements to the interaction cavity and internal mode converter yielded efficient microwave generation and mode conversion, achieving >10 W output power over a 335 MHz bandwidth with >110 W peak power. Frequency-chirped DNP with MAS is expected to have a significant impact on the future of magnetic resonance.

Rutile dielectric loop-gap resonator for X-band EPR spectroscopy of small aqueous samples

Mett, Richard R., Jason W. Sidabras, James R. Anderson, Candice S. Klug, and James S. Hyde. “Rutile Dielectric Loop-Gap Resonator for X-Band EPR Spectroscopy of Small Aqueous Samples.” Journal of Magnetic Resonance 307 (October 2019): 106585.

https://doi.org/10.1016/j.jmr.2019.106585

The performance of a metallic microwave resonator that contains a dielectric depends on the separation between metallic and dielectric surfaces, which affects radio frequency currents, evanescent waves, and polarization charges. The problem has previously been discussed for an X-band TE011 cylindrical cavity resonator that contains an axial dielectric tube (Hyde and Mett, 2017). Here, a short rutile dielectric tube inserted into a loop-gap resonator (LGR) at X-band, which is called a dielectric LGR (dLGR), is considered. The theory is developed and experimental results are presented. It was found that a central sample loop surrounded by four ‘‘flux-return” loops (i.e., 5-loop–4-gap) is preferable to a 3-loop–2-gap configuration. For sufficiently small samples (less than 1 mL), a rutile dLGR is preferred relative to an LGR both at constant K (B1= Pl) and at constant incident power. Introduction of LGR technology to X-band EPR was a significant advance for site-directed spin labeling because of small sample size and high K. The rutile dLGR introduced in this work offers further extension to samples that can be as small as 50 nL when using typical EPR acquisition times.

Perspectives on microwave coupling into cylindrical and spherical rotors with dielectric lenses for magic angle spinning dynamic nuclear polarization #DNPNMR

Chen, Pin-Hui, Chukun Gao, and Alexander B. Barnes. “Perspectives on Microwave Coupling into Cylindrical and Spherical Rotors with Dielectric Lenses for Magic Angle Spinning Dynamic Nuclear Polarization.” Journal of Magnetic Resonance, July 2019, 106518.

https://doi.org/10.1016/j.jmr.2019.07.005

Continuous wave dynamic nuclear polarization (DNP) increases the sensitivity of NMR, yet intense microwave fields are required to transition magic angle spinning (MAS) DNP to the time domain. Here we describe and analyze Teflon lenses for cylindrical and spherical MAS rotors that focus microwave power and increase the electron Rabi frequency, m1s. Using a commercial simulation package, we solve the Maxwell equations and determine the propagation and focusing of millimeter waves (198 GHz). We then calculate the microwave intensity in a time-independent fashion to compute the m1s. With a nominal microwave power input of 5 W, the average m1s is 0.38 MHz within a 22 lL sample volume in a 3.2 mm outer diameter (OD) cylindrical rotor without a Teflon lens. Decreasing the sample volume to 3 lL and focusing the microwave beam with a Teflon lens increases the m1s to 1.5 MHz. Microwave polarization and intensity perturbations associated with diffraction through the radiofrequency coil, losses from penetration through the rotor wall, and mechanical limitations of the separation between the lens and sample are significant challenges to improving microwave coupling in MAS DNP instrumentation. To overcome these issues, we introduce a novel focusing strategy using dielectric microwave lenses installed within spinning rotors. One such 9.5 mm OD cylindrical rotor assembly implements a Teflon focusing lens to increase the m1s to 2.7 MHz within a 2 lL sample. Further, to access high spinning frequencies while also increasing m1s, we analyze microwave coupling into MAS spheres. For 9.5 mm OD spherical rotors, we compute a m1s of 0.36 MHz within a sample volume of 161 lL, and 2.5 MHz within a 3 lL sample placed at the focal point of a novel double lens insert. We conclude with an analysis and discussion of sub-millimeter diamond spherical rotors for time domain DNP at spinning frequencies >100 kHz. Submillimeter spherical rotors better overlap a tightly focused microwave beam, resulting in a m1s of 2.2 MHz. Lastly, we propose that sub-millimeter dielectric spherical microwave resonators will provide a means to substantially improve electron spin control in the future.

Optimized microwave delivery in dDNP #DNPNMR

Albannay, Mohammed M., Joachim M.O. Vinther, Andrea Capozzi, Vitaliy Zhurbenko, and Jan Henrik Ardenkjaer-Larsen. “Optimized Microwave Delivery in DDNP.” Journal of Magnetic Resonance 305 (August 2019): 58–65.

https://doi.org/10.1016/j.jmr.2019.06.004

Dissolution dynamic nuclear polarization (dDNP) has permitted the production of highly polarized liquid-state samples, enabling real-time imaging of metabolic processes non-invasively in vivo. The desire for higher magnetic resonance sensitivity has led to the development of multiple home-built and commercial dDNP polarizers employing solid-state microwave sources. Providing efficient microwave delivery that avoids unwanted heating of the sample is a crucial step to achieve high nuclear polarization. Consequently, a process is described to reduce waveguide attenuation due to resistive loss thereby doubling the delivered power. A mirror and reflector are designed and tested to increase the microwave field density across the sample volume resulting in a 2.3 dB increase of delivered power. Thermal considerations with regards to waveguide geometry and dDNP probe design are discussed. A thermal model of the dDNP probe is computed and experimentally verified.

Improved waveguide coupling for 1.3 mm MAS DNP probes at 263 GHz #DNPNMR

Purea, Armin, Christian Reiter, Alexandros I. Dimitriadis, Emile de Rijk, Fabien Aussenac, Ivan Sergeyev, Melanie Rosay, and Frank Engelke. “Improved Waveguide Coupling for 1.3 Mm MAS DNP Probes at 263 GHz.” Journal of Magnetic Resonance 302 (May 2019): 43–49.

https://doi.org/10.1016/j.jmr.2019.03.009

We consider the geometry of a radially irradiated microwave beam in MAS DNP NMR probes and its impact on DNP enhancement. Two related characteristic features are found to be relevant: (i) the focus of the microwave beam on the DNP MAS sample and (ii) the microwave magnetic field magnitude in the sample. We present a waveguide coupler setup that enables us to significantly improve beam focus and field magnitude in 1.3 mm MAS DNP probes at a microwave frequency of 263 GHz, which results in an increase of the DNP enhancement by a factor of 2 compared to previous standard hardware setups. We discuss the implications of improved coupling and its potential to enable cutting-edge applications, such as pulsed high-field DNP and the use of low-power solid-state microwave sources.

Laser-driven semiconductor switch for generating nanosecond pulses from a megawatt gyrotron

Picard, Julian F., Samuel C. Schaub, Guy Rosenzweig, Jacob C. Stephens, Michael A. Shapiro, and Richard J. Temkin. “Laser-Driven Semiconductor Switch for Generating Nanosecond Pulses from a Megawatt Gyrotron.” Applied Physics Letters 114, no. 16 (April 22, 2019): 164102. 

https://doi.org/10.1063/1.5093639

A laser-driven semiconductor switch (LDSS) employing silicon (Si) and gallium arsenide (GaAs) wafers has been used to produce nanosecond-scale pulses from a 3 ls, 110 GHz gyrotron at the megawatt power level. Photoconductivity was induced in the wafers using a 532 nm laser, which produced 6 ns, 230 mJ pulses. Irradiation of a single Si wafer by the laser produced 110 GHz RF pulses with a 9 ns width and >70% reflectance. Under the same conditions, a single GaAs wafer yielded 24 ns 110 GHz RF pulses with >78% reflectance. For both semiconductor materials, a higher value of reflectance was observed with increasing 110 GHz beam intensity. Using two active wafers, pulses of variable length down to 3 ns duration were created. The switch was tested at incident 110 GHz RF power levels up to 600 kW. A 1-D model is presented that agrees well with the experimentally observed temporal pulse shapes obtained with a single Si wafer. The LDSS has many potential uses in high power millimeter-wave research, including testing of high-gradient accelerator structures.

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