Cyclotron Harmonics Research Articles

Efficiency of the gyrotron with single and double confocal resonators

Low-voltage gyrotrons operating at near-terahertz frequencies are of great interest for numerous applications. Development of such gyrotrons in conventional configurations faces significant difficulties, especially in the case of operation at cyclotron harmonics. Some of these problems can be mitigated by using confocal resonators formed either by one pair of mirrors (a single configuration) or two pairs of mirrors (a double configuration). This paper describes the study of the effect of transverse nonuniformity of the resonator field on the gyrotron efficiency in both configurations. The study includes the analysis of operation in soft and hard self-excitation regimes.

Terahertz Large-Orbit High-Harmonic Gyrotrons at IAP RAS: Recent Experiments and New Designs

We describe new versions of high-harmonic gyrotrons with axis-encircling electron beams developed on the basis of two (80-keV pulsed and 30-keV continuous wave) experimental setups. Selective operation at the second (0.267 THz) and at the third (0.394 THz) cyclotron harmonics was observed in the 30-keV gyrotron. Cavities with periodic phase correctors, where a far-from-cutoff axial mode with a decreased diffraction Q-factor is excited in a gyrotron-like regime, are designed to improve the operation at the third harmonic, as well as to achieve the fourth-harmonic operation at frequencies up to 0.65 THz. At the pulsed gyrotron setup, a sectioned cavity with a decreased diffraction Q-factor was experimentally tested.

Improvement of Mode Selectivity of High-Harmonic Gyrotrons by Using Operating Cavities with Short Output Reflectors

An important problem in realization of gyrotrons operating at high cyclotron harmonics is discrimination of parasitic oscillations excited at lower harmonics. In this paper, we propose to use an operating cavity with a short output irregularity providing a partial reflection of the operating wave. This allows decreasing the length of the cavity with no considerable change in both the operating electron current and the efficiency of the gyrotron operation. At the same time, since the output irregularity weakly reflects parasitic low-frequency waves, the reduction in the length of the cavity leads to a significant increase in the starting currents of the parasitic near-cutoff waves.

Analysis of possible improvement of the plasma performance in JET due to the inward spatial channelling of fast-ion energy

Effects of the spatial chanelling (SC) of the energy of fusion-produced alpha particles—the spatial transfer of the energy of fast ions by destabilized eigenmodes and delivering this energy to bulk plasma particles (Kolesnichenko et al 2010 Phys. Rev. Lett. 104 075001)—on the plasma performance is studied. Analysis is carried out in the assumption that alpha particles located in the peripheral region of the plasma destabilize multiple fast magnetoacoustic modes (FMM) having global radial structure. The FMM with the frequencies close to cyclotron harmonics of alpha particles are considered. It is found that these FMM can be in resonance with the bulk plasma ions and electrons located in the central region of the plasma, delivering the alpha energy to this region. This improves the overall plasma confinement. In addition, it leads to anomalous ion heating when the ion damping of FMM exceeds the electron one. The damping rates of the considered waves are calculated. It is shown that reasonably small amplitude waves can receive and transfer across the flux surfaces as large power density as that required for spatial channelling of a considerable part of fusion energy. The developed theory of the inward spatial channelling is applied to JET experiments carried out during the deuterium–tritium-experiment campaign (DTE1), where presumably anomalous ion heating and improvement of the plasma confinement took place.

AORSA full wave calculations of helicon waves in DIII-D and ITER

Helicon waves have been recently proposed as an off-axis current drive actuator for DIII-D, FNSF, and DEMO tokamaks. Previous ray tracing modeling using GENRAY predicts strong single pass absorption and current drive in the mid-radius region on DIII-D in high beta tokamak discharges. The full wave code AORSA, which is valid to all order of Larmor radius and can resolve arbitrary ion cyclotron harmonics, has been used to validate the ray tracing technique. If the scrape-off-layer (SOL) is ignored in the modeling, AORSA agrees with GENRAY in both the amplitude and location of driven current for DIII-D and ITER cases. These models also show that helicon current drive can possibly be an efficient current drive actuator for ITER. Previous GENRAY analysis did not include the SOL. AORSA has also been used to extend the simulations to include the SOL and to estimate possible power losses of helicon waves in the SOL. AORSA calculations show that another mode can propagate in the SOL and lead to significant (~10%–20%) SOL losses at high SOL densities. Optimizing the SOL density profile can reduce these SOL losses to a few percent.

Development of Terahertz-Range Planar Gyrotrons with Transverse Energy Extraction Operating at Cyclotron Harmonics

Development of Terahertz-Range Planar Gyrotrons with Transverse Energy Extraction Operating at Cyclotron Harmonics

MASTER OT J132104.04+560957.8: A Polar with Absorption–Emission Line Reversals

Abstract We present time-resolved photometry and spectroscopy of the recently classified polar MASTER OT J132104.04+560957.8. The spectrum shows a smooth, nonthermal continuum at the time of maximum light, without any individually discernible cyclotron harmonics. Using homogenous cyclotron modeling, we interpret this as cyclotron radiation whose individual harmonics have blended together, and on this basis, we loosely constrain the magnetic-field strength to be less than ∼30 MG. In addition, for about one-tenth of the orbital period, the Balmer and He i emission lines transition into absorption features, with He ii developing an absorption core. We use our observations of this phenomenon to test theoretical models of the accretion curtain and conclude that the H and He i lines are produced throughout the curtain, in contravention of theoretical predictions of separate H and He i line-forming regions. Moreover, a significant amount of He ii emission originates within the accretion curtain, implying that the curtain is significantly hotter than expected from theory. Finally, we comment on the object’s long-term photometry, including evidence that it recently transitioned into a prolonged, exceptionally stable high state following a potentially decades-long low state.

Design and RF test of a prototype traveling wave antenna for helicon current drive in KSTAR

Non-inductive current drive by fast wave at very high ion cyclotron harmonics, known as ‘helicons’ has the potential for high off-axis current drive efficiency compared with the other known non-inductive current drive techniques. However, non-inductive current drive by helicon wave has not been validated experimentally. To validate its anticipated performance experimentally, an antenna design is one of the most important issues. A Traveling wave antenna has particularly valuable features for launching the fast wave such as load resiliency, narrow n|| spectrum, and simple RF circuits without additional-external matching systems. Low power level helicon wave coupling experiments has been conducted successfully using a mock-up TWA in KSTAR. In the next step, in order to investigate a high power performance, a new prototype TWA based on the mock-up TWA has been designed, fabricated and measured for a medium power (100–300kW) RF system. The prototype TWA having a Faraday shield was made of copper and consists of 10 current straps with 5inch coaxial feeding lines as input and output ports. The detailed design parameters and electromagnetic characteristics of prototype TWA are discussed.

Broadband Amplification of Low-Terahertz Signals Using Axis-Encircling Electrons in a Helically Corrugated Interaction Region.

Experimental results are presented of a broadband, high power, gyrotron traveling wave amplifier (gyro-TWA) operating in the (75-110)-GHz frequency band and based on a helically corrugated interaction region. The second harmonic cyclotron mode of a 55-keV, 1.5-A, axis-encircling electron beam is used to resonantly interact with a traveling TE_{21}-like eigenwave achieving broadband amplification. The gyro-TWA demonstrates a 3-dB gain bandwidth of at least 5.5GHz in the experimental measurement with 9GHz predicted for a wideband drive source with a measured unsaturated output power of 3.4kW and gain of 36-38dB. The approach may allow a gyro-TWA to operate at 1THz.

Whistler Mode Waves Below Lower Hybrid Resonance Frequency: Generation and Spectral Features

AbstractEquatorial noise in the frequency range below the lower hybrid resonance frequency, whose structure is shaped by high proton cyclotron harmonics, has been observed by the Cluster spacecraft. We develop a model of this wave phenomenon which assumes (as, in general, has been suggested long ago) that the observed spectrum is excited due to loss cone instability of energetic ions in the equatorial region of the magnetosphere. The wavefield is represented as a sum of constant frequency wave packets which cross a number of cyclotron resonances while propagating in a highly oblique mode along quite specific trajectories. The growth (damping) rate of these wave packets varies both in sign and magnitude along the raypath, making the wave net amplification, but not the growth rate, the main characteristic of the wave generation process. The growth rates and the wave amplitudes along the ray paths, determined by the equations of geometrical optics, have been calculated for a 3‐D set of wave packets with various frequencies, initial L shells, and initial wave normal angles at the equator. It is shown that the dynamical spectrum resulting from the proposed model qualitatively matches observations.

Method of Providing the High Cyclotron Harmonic Operation Selectivity in a Gyrotron With a Spatially Developed Operating Mode

An electrodynamic method for suppressing low-frequency oscillations at the fundamental cyclotron resonance in a gyrotron at a high cyclotron harmonic is proposed and theoretically investigated. According to simulations, the use of this method in the terahertz gyrotron at the second cyclotron harmonic operating at the spatially developed mode TE63,15 can provide a multiple excess of the starting currents of the parasitic modes over the starting current of the desired oscillations.

Phase-space resolved measurement of 2nd harmonic ion cyclotron heating using FIDA tomography at the ASDEX Upgrade tokamak

Recent upgrades to the FIDA (fast-ion D-alpha) diagnostic at ASDEX Upgrade allow to reconstruct the fast-ion phase space at several radial positions with decent energy and pitch resolution. These new diagnostic capabilities are applied to study the physics of 2nd harmonic ion cyclotron heating, which is a foreseen heating scenario for ITER. In particular, the acceleration of deuterium beam ions above the injection energy by absorption of ion cyclotron waves at the 2nd harmonic is investigated and compared to theoretical predictions by the TORIC-SSFPQL and TORIC-NUBEAM code packages. Furthermore, comparisons to other fast-ion diagnostics (neutron yield and neutral particle analyzers) are discussed.

Gyrokinetic electron and fully kinetic ion simulations of fast magnetosonic waves in the magnetosphere

Two-dimensional simulations using a gyrokinetic electron and fully kinetic ion (GeFi) scheme are preformed to study the excitation of fast magnetosonic waves in the terrestrial magnetosphere, which arise from the ion Bernstein instability driven by proton velocity distributions with a positive slope with respect to the perpendicular velocity. Since both ion and electron kinetics are relevant, particle-in-cell (PIC) simulations have often been employed to study the wave excitation. However, the full particle-in-cell scheme is computationally expensive for simulating waves in the ion scale because the electron scale must be fully resolved. Therefore, such simulations are limited to reduced proton-to-electron mass ratio (mp/me) and light-to-Alfvén speed ratio (c/vA). The present study exploits the GeFi scheme that can break through these limitations to some extent, so larger mp/me and c/vA can be used. In the simulations presented, the ion Bernstein instability is driven by a proton velocity distribution composed of 10% energetic protons with a shell distribution and 90% relatively cool, background protons with a Maxwellian distribution. The capability of the GeFi code in simulating the ion Bernstein instability is first demonstrated by comparing a GeFi simulation using reduced mass ratio (mp/me=100) and speed ratio (c/vA=15) to a corresponding PIC simulation as well as linear dispersion analysis. A realistic speed ratio (c/vA=400) and a larger mass ratio (mp/me=400) are then adopted in the GeFi code to explore how the results vary. It is shown that, as the increased mp/me and c/vA lead to a larger lower hybrid frequency, ion Bernstein waves are excited at more ion cyclotron harmonics, consistent with the general prediction of linear dispersion theory. On the other hand, the GeFi simulations also revealed some interesting features after the instability saturation, which are likely related to nonlinear wave-wave interactions.

Oxygen cyclotron harmonic waves observed using Van Allen Probes

Fine structured multiple-harmonic electromagnetic emissions at frequencies around the equatorial oxygen cyclotron harmonics are observed by Van Allen Probe A outside the core plasmasphere (L~5) off the magnetic equator (MLAT~−7.5°) during a geomagnetic storm. We find that the multiple-harmonic emissions have power spectrum density (PSD) peaks during 2–8 equatorial oxygen gyroharmonics (f ~ n \(f_{O^{+}}\), n=2–8), while the fundamental mode (n=1) is absent, implying that the harmonic waves are generated near the equator and propagate into the observation region. Additionally, these electromagnetic emissions are linearly polarized. Different from the equatorial noise emission that propagates considerably obliquely, these emissions have moderate wave normal angles (approximately 40°–60°), which predominately increase as the harmonic number increases. Considering their frequency and wave normal angle characteristics, it is suggested that these multiple-harmonic emissions play an important role in the dynamic variation of radiation belt electrons.

MeV-range velocity-space tomography from gamma-ray and neutron emission spectrometry measurements at JET

We demonstrate the measurement of a 2D MeV-range ion velocity distribution function by velocity-space tomography at JET. Deuterium ions were accelerated into the MeV-range by third harmonic ion cyclotron resonance heating. We made measurements with three neutron emission spectrometers and a high-resolution γ-ray spectrometer detecting the γ-rays released in two reactions. The tomographic inversion based on these five spectra is in excellent agreement with numerical simulations with the ASCOT–RFOF and the SPOT–RFOF codes. The length of the measured fast-ion tail corroborates the prediction that very few particles are accelerated above 2 MeV due to the weak wave-particle interaction at higher energies.

Diffusive scattering of electrons by electron holes around injection fronts

AbstractVan Allen Probes have detected nonlinear electrostatic spikes around injection fronts in the outer radiation belt. These spikes include electron holes (EH), double layers, and more complicated solitary waves. We show that EHs can efficiently scatter electrons due to their substantial transverse electric fields. Although the electron scattering driven by EHs is diffusive, it cannot be evaluated via the standard quasi‐linear theory. We derive analytical formulas describing local electron scattering by a single EH and verify them via test particle simulations. We show that the most efficiently scattered are gyroresonant electrons (crossing EH on a time scale comparable to the local electron gyroperiod). We compute bounce‐averaged diffusion coefficients and demonstrate their dependence on the EH spatial distribution (latitudinal extent and spatial filling factor) and individual EH parameters (amplitude of electrostatic potential, velocity, and spatial scales). We show that EHs can drive pitch angle scattering of 5 keV electrons at rates 10−2−10−4 s−1 and, hence, can contribute to electron losses and conjugated diffuse aurora brightenings. The momentum and pitch angle scattering rates can be comparable, so that EHs can also provide efficient electron heating. The scattering rates driven by EHs at L shells L ∼ 5–8 are comparable to those due to chorus waves and may exceed those due to electron cyclotron harmonics.

Nonlinear fundamental and harmonic cyclotron resonant scattering of radiation belt ultrarelativistic electrons by oblique monochromatic EMIC waves

AbstractCyclotron resonant scattering by electromagnetic ion cyclotron (EMIC) waves has been considered to be responsible for the rapid loss of radiation belt high‐energy electrons. For parallel‐propagating EMIC waves, the nonlinear character of cyclotron resonance has been revealed in recent studies. Here we present the first study on the nonlinear fundamental and harmonic cyclotron resonant scattering of radiation belt ultrarelativistic electrons by oblique EMIC waves on the basis of test particle simulations. Higher wave obliquity produces stronger nonlinearity of harmonic resonances but weaker nonlinearity of fundamental resonance. Compared to the quasi‐linear prediction, these nonlinear resonances yield a more rapid loss of electrons over a wider pitch angle range. In the quasi‐linear regime, the ultrarelativistic electrons are lost in the equatorial pitch angle range αeq<75°, nearly independent of wave normal angle ψ. In contrast, the upper pitch angle cutoff of nonlinear losses tends to increase with the wave normal angle increasing, which is about αeq=82° at ψ = 0° and αeq>87.5° at ψ = 20° and 40°. At the resonant pitch angles αeq<75°, the difference between quasi‐linear and nonlinear loss timescales tends to decrease with the wave normal angle increasing. At ψ = 0° and 20°, the nonlinear electron loss timescale is 10% shorter than the quasi‐linear prediction; at ψ = 40°, the difference in loss timescales is reduced to <5%.

Use of Quasiregular Resonator Cavities with Short Phase Correctors in Gyrotrons Operated at Higher Cyclotron Harmonics

We discuss the possibilities of using quasiregular resonator cavities with short irregularities, which ensure correction of the wave phase, in low-power gyrotrons operated at higher cyclotronfrequency harmonics. The use of such phase correctors can help with solving two problems, namely, increasing the selectivity of excitation of a higher cyclotron harmonic and decreasing the diffraction Q-factor of the gyrotron wave excited in an extended cavity.

On all-FLR order numerical modelling of RF wave propagation and damping

Determining the fate of waves in hot magnetized plasmas in magnetic confinement fusion machines close to arbitrary cyclotron harmonics retaining finite Larmor radius effects requires solving the relevant integro-differential wave equation for waves of arbitrary wavelength. Anticipating exploitation in as realistic geometry as possible this requires massive computer resources. Work is ongoing to attempt reducing the required CPU memory and time. As Morlet wavelets are localised in x-space as well as k-space, they potentially offer a means to solve the relevant wave equation at reduced CPU requirement cost. Encouraging first results are obtained.

A new quasilinear formulation for ICRF plasmas in a toroidal geometry

We present a new formulation for quasilinear velocity space diffusion for ICRF plasmas that considers two different aspects: (1) finite Larmor radius approximation and (2) includes the effect of toroidal geometry and constructs a positive definite form. In the first aspect, the Kennel-Engelmann (K-E) quasilinear diffusion coefficients are successfully approximated in a small Larmor radius limit and implemented for the numerical codes (TORIC-CQL3D). In the second aspect, the quasilinear diffusion is reformulated in a toroidal geometry in order to include the parallel dynamics in the inhomogeneous plasmas and magnetic fields. We use these two quasilinear formulations to simulate ITER plasmas with ICRF injection for minority fundamental heating and Tritium second harmonic cyclotron heating.