Dynamic nuclear polarization for sensitivity enhancement in modern solid-state NMR.

This paper presents a review of the experimental achievements related to the development of high-power gyrotron oscillators for long-pulse or CW operation 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, coaxial-cavity multi-megawatt gyrotrons, gyrotrons for technological and spectroscopy applications, relativistic gyrotrons, large orbit gyrotrons (LOGs), quasi-optical gyrotrons, fast- and slow-wave cyclotron autoresonance masers (CARMs), gyroklystrons, gyro-TWT amplifiers, gyrotwystron amplifiers, gyro-BWOs, gyro-harmonic converters, gyro-peniotrons, 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, megawatt-class 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 MW in 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 36 W (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).

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

We report the experimental demonstration of a gyrotron traveling-wave-tube amplifier at 250 GHz that uses a photonic band gap (PBG) interaction circuit. The gyrotron amplifier achieved a peak small signal gain of 38 dB and 45 W output power at 247.7 GHz with an instantaneous $\ensuremath{-}3\text{ }\text{ }\mathrm{dB}$ bandwidth of 0.4 GHz. The amplifier can be tuned for operation from 245--256 GHz. The widest instantaneous $\ensuremath{-}3\text{ }\text{ }\mathrm{dB}$ bandwidth of 4.5 GHz centered at 253.25 GHz was observed with a gain of 24 dB. The PBG circuit provides stability from oscillations by supporting the propagation of transverse electric (TE) modes in a narrow range of frequencies, allowing for the confinement of the operating ${\mathrm{TE}}_{03}$-like mode while rejecting the excitation of oscillations at nearby frequencies. This experiment achieved the highest frequency of operation for a gyrotron amplifier; at present, there are no other amplifiers in this frequency range that are capable of producing either high gain or high output power. This result represents the highest gain observed above 94 GHz and the highest output power achieved above 140 GHz by any conventional-voltage vacuum electron device based amplifier.

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Photonic-band-gap traveling-wave gyrotron amplifier.

We present results of a pulsed dynamic nuclear polarization (DNP) study at 035 T (97 GHz/147 MHz for electron/(1)H Larmor frequency) using a lab frame-rotating frame cross polarization experiment that employs electron spin locking fields that match the (1)H nuclear Larmor frequency, the so called NOVEL (nuclear orientation via electron spin locking) condition We apply the method to a series of DNP samples including a single crystal of diphenyl nitroxide (DPNO) doped benzophenone (BzP), 1,3-bisdiphenylene-2-phenylallyl (BDPA) doped polystyrene (PS), and sulfonated-BDPA (SA-BDPA) doped glycerol/water glassy matrices The optimal Hartman-Hahn matching condition is achieved when the nutation frequency of the electron matches the Larmor frequency of the proton, ω(1S) = ω(0I), together with possible higher order matching conditions at lower efficiencies The magnetization transfer from electron to protons occurs on the time scale of ∼100 ns, consistent with the electron-proton couplings on the order of 1-10 MHz in these samples In a fully protonated single crystal DPNO/BzP, at 270 K, we obtained a maximum signal enhancement of e = 165 and the corresponding gain in sensitivity of e(T1/T(B))(1/2)=230 due to the reduction in the buildup time under DNP In a sample of partially deuterated PS doped with BDPA, we obtained an enhancement of 323 which is a factor of ∼32 higher compared to the protonated version of the same sample and accounts for 49% of the theoretical limit For the SA-BDPA doped glycerol/water glassy matrix at 80 K, the sample condition used in most applications of DNP in nuclear magnetic resonance, we also observed a significant enhancement Our findings demonstrate that pulsed DNP via the NOVEL sequence is highly efficient and can potentially surpass continuous wave DNP mechanisms such as the solid effect and cross effect which scale unfavorably with increasing magnetic field Furthermore, pulsed DNP is also a promising avenue for DNP at high temperature

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Time domain DNP with the NOVEL sequence

Solid-state NMR spectroscopy has become a valuable tool for the characterization of both pure and formulated active pharmaceutical ingredients (APIs). However, NMR generally suffers from poor sensitivity that often restricts NMR experiments to nuclei with favorable properties, concentrated samples, and acquisition of one-dimensional (1D) NMR spectra. Here, we review how dynamic nuclear polarization (DNP) can be applied to routinely enhance the sensitivity of solid-state NMR experiments by one to two orders of magnitude for both pure and formulated APIs. Sample preparation protocols for relayed DNP experiments and experiments on directly doped APIs are detailed. Numerical spin diffusion models illustrate the dependence of relayed DNP enhancements on the relaxation properties and particle size of the solids and can be used for particle size determination when the other factors are known. We then describe the advanced solid-state NMR experiments that have been enabled by DNP and how they provide unique insight into the molecular and macroscopic structure of APIs. For example, with large sensitivity gains provided by DNP, natural isotopic abundance, 13 C-13 C double-quantum single-quantum homonuclear correlation NMR spectra of pure APIs can be routinely acquired. DNP also enables solid-state NMR experiments with unreceptive quadrupolar nuclei such as 2 H, 14 N, and 35 Cl that are commonly found in APIs. Applications of DNP-enhanced solid-state NMR spectroscopy for the molecular level characterization of low API load formulations such as commercial tablets and amorphous solid dispersions are described. Future perspectives for DNP-enhanced solid-state NMR experiments on APIs are briefly discussed.

DNP-enhanced solid-state NMR spectroscopy of active pharmaceutical ingredients.

An experimental study of picosecond pulse amplification in a gyrotron-traveling wave tube (gyro-TWT) has been carried out. The gyro-TWT operates with 30 dB of small signal gain near 140 GHz in the HE 06 mode of a confocal waveguide. Picosecond pulses show broadening and transit time delay due to two distinct effects: the frequency dependence of the group velocity near cutoff and gain narrowing by the finite gain bandwidth of 1.2 GHz. Experimental results taken over a wide range of parameters show good agreement with a theoretical model in the small signal gain regime. These results show that in order to limit the pulse broadening effect in gyrotron amplifiers, it is crucial to both choose an operating frequency at least several percent above the cutoff of the waveguide circuit and operate at the center of the gain spectrum with sufficient gain bandwidth.

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Amplification of Picosecond Pulses in a 140-GHz Gyrotron-Traveling Wave Tube

We report the first experimental studies of picosecond pulse amplification and broadening in a gyrotron amplifier. These studies allow investigation of the amplification process in a new physical regime, where wave dispersion and finite gain bandwidth play a major role. The experiments were performed on a 140 GHz gyro-traveling wave tube (Gyro-TWT) operating in a higher order mode of a confocal waveguide. We observed significant pulse broadening for pulses shorter than 1 ns when the operating point of the amplifier was close to the waveguide cut-off. For example, a 580 ps pulse at 137.30 GHz, a frequency near cutoff, was measured to be broadened by 100 %, while pulses at higher frequency, further from cutoff, showed much smaller broadening. The results of these experimental studies show that for the picosecond pulse amplification necessary in applications such as Electron Paramagnetic Resonance (EPR), the operating point of the gyro-TWT should be chosen to be at least a few percent above the cut-off to avoid pulse distortion.

10.3: Experimental measurement of picosecond pulse amplification in a 140 GHz Gyro-TWT

The design and experimental study of a 140 GHz, 1 kW gyro-traveling wave tube (Gyro-TWT) operating in the HE 06 mode of a confocal waveguide is presented. A combination of quasioptical gain sections with dielectric loaded severs enables high gain operation with mode selectivity. A nanosecond-scale 120 mW pulse driver at 140 GHz was demonstrated. This input driver pulse will be amplified by the Gyro-TWT and the resulting output high-power short pulse will be transmitted to an EPR spectrometer probe. A quasioptical mode converter utilized to transform the higher order operating confocal mode into a Gaussian-like beam was designed and cold tested. The simulated and measured Gaussian beam patterns of the output mode converter are in good agreement.

An overmoded 140 GHz, 1 kW quasioptical gyro-twt with an internal mode converter

We report on the progress of a 140 GHz pulsed gyro-amplifier system being developed at MIT for enhanced Dynamic Nuclear Polarization for NMR (DNP-NMR). A confocal geometry configuration is being studied as an approach to an overmoded interaction waveguide. The new amplifier differs from a previous version by incorporating improvements to structural construction, optimized input coupling, and sever design1. The confocal geometry consists of two curved mirrors spaced equal to their radii of curvature (R = L = 6.8 mm, width = 5 mm). The operating frequency of 140 GHz is supported by the higher-order HE06 mode. In order to suppress lower-order modes, the confocal geometry has open sides to allow radiation to leak out of the confined structure. By an appropriate design of the confocal parameters, these lower-order modes should have a greater loss per unit length than the mode of operation. The two severs (length 2.51cm, minimum width 0.74 mm) help to prevent oscillations by providing high attenuation within the waveguide.

Progress of 140 GHz gyro-amplifier using confocal waveguide

We report results on the design, fabrication, and test of an improved circuit for a pulsed, 140 GHz, 1 kW gyro-amplifier. The new confocal waveguide circuit features improved mechanical construction, input coupling into the higher-order operational mode, and sever design. The goal is increased gain, bandwidth, and output power. Numerical simulations of the gain and preliminary experimental results are presented.

Haejin Kim

Papers

Amplification of Picosecond Pulses in a 140-GHz Gyrotron-Traveling Wave Tube

10.3: Experimental measurement of picosecond pulse amplification in a 140 GHz Gyro-TWT

An overmoded 140 GHz, 1 kW quasioptical gyro-twt with an internal mode converter

Progress of 140 GHz gyro-amplifier using confocal waveguide

Operation of a 140GHz gyro-amplifier using a confocal waveguide