Smooth Frequency Tuning Research Articles

Frequency Tuning Characteristics of a High-Power Sub-THz Gyrotron with Quasi-Optical Cavity

Motivated by some emerging high-frequency applications, a high-power frequency-tunable sub-THz quasi-optical gyrotron cavity based on a confocal waveguide is designed in this paper. The frequency tuning characteristics of different approaches, including magnetic field tuning, mirror separation adjustment, and hybrid tuning, have been investigated by particle-in-cell (PIC) simulation. Results predict that it is possible to realize a smooth continuous frequency tuning band with an extraordinarily broad bandwidth of 41.55 GHz, corresponding to a relative bandwidth of 18.7% to the center frequency of 0.22 THz. The frequency tunability is provided by varying the separation distance between two mirrors and correspondingly adjusting the external magnetic field. During the frequency tuning, the output power remains higher than 20 kW, which corresponds to an interaction efficiency of 10%. Providing great advantages in terms of broad bandwidth, smooth tuning, and high power, this research may be conducive to the development of high-power frequency-tunable THz gyrotron oscillators.

Theoretical and Experimental Investigations of Terahertz-Range Gyrotrons with Frequency and Spectrum Control

We present the results of recent IAP RAS investigations, which were aimed at controlling the radiation frequency and spectrum of THz-range medium-power gyrotrons. Problems of extending the smooth frequency tuning band and providing high-frequency stability in CW regimes are discussed. The development of gyrotrons with stated parameters are of interest for many modern applications, including DNP/NMR and RAD spectroscopy, direct measurements of positronium hyperfine structure, and diagnostic of various media.

Analysis of the Methods of Discrete and Smooth Frequency Tuning in Gyrotrons for Spectroscopy, on the Example of a Generator Operated in the 0.20–0.27 THz Frequency Range

We consider the main features of a low-power frequency-tunable gyrotron with an oversized cavity, which is designed for the purposes of nuclear magnetic resonance spectroscopy and other applications and operates in the 0.20–0.27 frequency range producing an output power of 200 W. We study the possibilities of wideband output frequency tuning by exciting a sequence of modes with similar caustics using magnetic-field variations and smooth tuning due to the excitation of modes with a great number of longitudinal variations. Aiming at widening the frequency tuning range, we also analyzed the possibility of smooth frequency tuning determined by controlled variations of the cavity temperature. Specific features of the electron-optical system of such a gyrotron is discussed, along with the possibility of increasing its efficiency by means of single-stage recovery of the residual energy of the electron beam.

Frequency Tunable sub-THz Gyrotron for Direct Measurements of Positronium Hyperfine Structure

The feasibility of a high-power sub-THz gyrotron with smooth wideband frequency tuning suitable for direct measurement of the positronium hyperfine structure is demonstrated numerically using both averaged equations and PIC-code simulations. Analytical estimates show that the frequency-tunable powerful sub-THz radiation can be generated through the excitation of high-order axial modes in a gyrotron with short cavity driven by an electron beam with high current. Simulations show that an output power of 0.5–1 kW can be obtained at a frequency of about 0.2 THz within a 10-GHz band which are the parameters needed for testing of quantum electrodynamics predictions through the spectroscopy of positronium.

Smooth Wideband Frequency Tuning in Low-Voltage Gyrotron With Cathode-End Power Output

The possibility of a smooth electronic frequency tuning for medium-power gyrotrons operated at the fundamental cyclotron resonance has been demonstrated in the numerical simulations. The use of a low operating voltage allows a drastic increase in the gyrotron operation efficiency at the higher axial modes and, hence, in the bandwidth of continuous frequency tuning. A gyrotron cavity with power output in the direction of the cathode end can provide a smoother power versus frequency dependence in the gyro-backward wave oscillator regime than a cavity with the conventional output in the direction of the collector. Simulations for a 2-kV gyrotron with a frequency of about 264-GHz demonstrate 3-GHz full-width at half-maximum frequency tuning with a power level of 15 to 30 W, which are attractive parameters for spectroscopic applications.

Plasma relativistic microwave amplifier with smooth frequency tuning from 2.4 to 3.2 GHz

Earlier, it was shown that the plasma relativistic microwave amplifier can operate at two frequencies, 2 and 3.2 GHz. In the present work, it is shown that, by varying the plasma density from one microwave pulse to another, it is possible to amplify the input signals to a power of 50–80 MW at any frequency in the range 2.4–3.2 GHz.

Multimirror quasi-cylindrical cavity resonators for frequency-tunable gyrotrons

We consider the possibility to create a gyrotron with smooth mechanical frequency tuning on the basis of multimirror quasi-optical resonators. The two-dimensional problem of finding igenmodes at the open quasi-cylindrical resonator of such a type is solved. Parameters of the gyrotron based on such a device, namely, its efficiency and starting current, are estimated.

Continuous-Wave Operation of a Frequency-Tunable 460-GHz Second-Harmonic Gyrotron for Enhanced Nuclear Magnetic Resonance

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%.

Solid-State Laser at 914 nm With Narrow Linewidth for Frequency Standard Based on Cool Magnesium Atoms

In this paper, we present the results of investigations aimed at the creation of a single-frequency Nd:YVO4 laser at 914 nm with smooth frequency tuning. An 8W diode laser is used for pumping. The output power of the laser in the cw mode is 300 mW, and the radiation linewidth is less than 250 kHz. The use of this laser as a source of probe radiation at 457 nm for the optical frequency standard based on the 1 S03P1 intercombination line of magnesium atoms cooled and captured in a magneto-optical trap holds much promise.

Review of Subterahertz and Terahertz Gyrodevices at IAP RAS and FIR FU

The maximal frequency of radiation higher than 1 THz has been recently obtained in pulse gyrotrons both at IAP (Nizhny Novgorod, Russia) and FIR (Fukui, Japan). CW generation at a 2.2-kW power level is radiated from a 300-GHz gyrotron and used for technological applications. New gyrotrons demonstrate single-mode operation at the second cyclotron harmonic with a frequency of 395 GHz in the CW regime with a power of 100 W and at the third harmonic with frequencies of 371-414 GHz in 10-mus pulses with power of 10-20 kW. Methods of selective excitation of higher cyclotron harmonics, frequency multiplication, and smooth frequency tuning in terahertz gyrotrons are also discussed in the review.

The Design of the 394.6 Ghz Continuously Tunable Coaxial Gyrotron for DNP Spectroscopy

Results of the numerical investigations of the coaxial-cavity gyrotron with smooth frequency tuning are presented. The tuning is achieved by moving an internal cone-shaped rod along the device axis, which ensures smooth variations in eigenfrequencies of the cavity. The application of an inner conductor with an impedance surface reduces the thermal load on the internal road. The calculation results demonstrate the possibility of tuning the oscillator frequency by 8 GHz for the 394.6 GHz central frequency (within a frequency band of about 2%) with an output power of about several hundreds watts in the CW regime.

High-frequency properties of double-well nanostructures

On the basis of analytical and numerical solutions of the Schrodinger equation, the active polarization current and emission efficiency of double-well nanostructures were calculated in a wide range of amplitudes of alternating electromagnetic field. It is shown that generation in the important terahertz region with smooth frequency tuning and the highest efficiency are possible. The behavior of the coefficient of reflection of electrons from the structure is studied; this coefficient goes to zero under the conditions of maximum efficiency.

Frequency-Tunable CW Gyro-BWO With a Helically Rippled Operating Waveguide

Operation of a continuous wave gyrotron backward-wave oscillator (gyro-BWO) with a helically rippled operating waveguide has been experimentally studied. The gyro-BWO exploits a dc oil-cooled magnet with magnetic field up to 0.5 T and utilizes a weakly relativistic (20 keV) electron beam produced by a magnetron injection gun. Stable generation at the second cyclotron harmonic with a maximum power of 7 kW and an efficiency of 15% at a frequency of 24.7 GHz was achieved. Smooth oscillation frequency tuning by varying the magnetic field was measured to be as wide as 5% at the half-power level. The first gyro-BWO operation with a single-stage energy recovery system was realized. The use of a depressed collector provided an efficiency increase up to 23% and an opportunity for reduction of the main power supply voltage down to 10 kV.

Calcium activation of BK(Ca) potassium channels lacking the calcium bowl and RCK domains.

In many physiological systems such as neurotransmitter release, smooth muscle relaxation and frequency tuning of auditory hair cells, large-conductance calcium-activated potassium (BK(Ca)) channels create a connection between calcium signalling pathways and membrane excitability. BK(Ca) channels are activated by voltage and by micromolar concentrations of intracellular calcium. Although it is possible to open BK(Ca) channels in the absence of calcium, calcium binding is essential for their activation under physiological conditions. In the presence of intracellular calcium, BK(Ca) channels open at more negative membrane potentials. Many experiments investigating the molecular mechanism of calcium activation of the BK(Ca) channel have focused on the large intracellular carboxy terminus, and much evidence supports the hypothesis that calcium-binding sites are located in this region of the channel. Here we show that BK(Ca) channels that lack the whole intracellular C terminus retain wild-type calcium sensitivity. These results show that the intracellular C terminus, including the 'calcium bowl' and the RCK domain, is not necessary for the calcium-activated opening of these channels.

Dependence of volume FEL (VFEL) threshold conditions on undulator parameters

The volume free electron laser uses a volume distributed feedback (VDFB), which can essentially reduce the threshold current of generation providing the possibility of a smooth frequency tuning. An undulator VFEL is considered.

A Tunable Reentrant Resonator with Transverse Orientation of Electric Field forin VivoEPR Spectroscopy

There has been a need for development of microwave resonator designs optimized to provide high sensitivity and high stability for EPR spectroscopy and imaging measurements ofin vivosystems. The design and construction of a novel reentrant resonator with transversely oriented electric field (TERR) and rectangular sample opening cross section for EPR spectroscopy and imaging ofin vivobiological samples, such as the whole body of mice and rats, is described. This design with its transversely oriented capacitive element enables wide and simple setting of the center frequency by trimming the dimensions of the capacitive plate over the range 100–900 MHz with unloadedQvalues of approximately 1100 at 750 MHz, while the mechanical adjustment mechanism allows smooth continuous frequency tuning in the range ±50 MHz. This orientation of the capacitive element limits the electric field based loss of resonatorQobserved with large lossy samples, and it facilitates the use of capacitive coupling. Both microwave performance data and EPR measurements of aqueous samples demonstrate high sensitivity and stability of the design, which make it well suited forin vivoapplications.

Application of a continuously tunable, cw optical parametric oscillator for high-resolution spectroscopy

We report the use of a smoothly tunable, single-frequency continuous-wave optical parametric oscillator (OPO) for high-resolution spectroscopy. The OPO is based on potassium titanyl phosphate and is resonant for both signal and idler fields, resulting in a device with a very low pump power threshold of 30 mW. The frequency-selective nature of the doubly resonant oscillator ensures that the signal and idler modes can be tuned across the entire phase-match bandwidth without the need for additional intracavity frequency-selective components. Smooth frequency tuning of the output of the OPO is obtained by tuning of the pump laser. To demonstrate the practicality of our OPO we recorded the absorption spectrum of cesium vapor in the 1-microm spectral region.