Electron Cyclotron Waves Research Articles

Real-time beam tracing for control of the deposition location of electron cyclotron waves

Plasma control techniques that use electron cyclotron (EC) resonance heating and current drive such as control of neoclassical tearing modes require accurate control of the deposition location of EC beams. ASDEX Upgrade has successfully implemented a real-time version of the beam-tracing code TORBEAM into its real-time diagnostic system to act as a globally available module that calculates current deposition location and its sensitivity from other real-time diagnostic measurements for all its moveable EC wave launchers. Based on a highly (100×) accelerated version of TORBEAM, the software implementation as a diagnostic process uses parallelization and achieves cycle times of 15–20ms for determining the radial deposition location of 12 beams in the plasma. This cycle time includes data input–output overhead arising from the use of available real-time signals. The system is by design portable to other machines such as ITER.

Oblique electromagnetic electron cyclotron waves for Kappa distribution with AC field in planetary magnetospheres

The dispersion relation for obliquely propagating relativistic electromagnetic electron cyclotron (EMEC) waves in collision-less magnetoplasma is obtained. Investigations for EMEC waves in magnetosphere of Jupiter, Saturn and Uranus have been done, in presence of perpendicular AC electric field for Kappa distribution function. The relativistic temporal growth rate is calculated using method of characteristic solution. Using the data provided by spacecrafts like Cassini, Voyager 1 and 2, while exploring the magnetosphere of Jupiter, Saturn and Uranus, is used to plot graphs showing growth rate being effected by various parameters. Comprehensive parametric analysis have been done at different radial distances of the planets. It is concluded that beside huge difference in magnetospheric configuration, temperature anisotropy remains the main source of energy in case of Jupiter and Uranus. While studying EMEC waves in magnetosphere of Saturn, it is inferred that growth rate attains maximum magnitude when angle of propagation increases. Also, the results and its interpretations explain how the growth of EMEC wave modifies in different magnetospheric conditions.

Theoretical study of electromagnetic electron cyclotron waves in the presence of AC field in Uranian magnetosphere

Electromagnetic electron cyclotron (EMEC) waves with temperature anisotropy in the magnetosphere of Uranus have been studied in present work. EMEC waves are investigated using method of characteristic solution by kinetic approach, in presence of AC field. In 1986, Voyager 2 encounter with Uranus revealed that magnetosphere of Uranus exhibit non-Maxwellian high-energy tail distribution. So, the dispersion relation, real frequency and growth rate are evaluated using Lorentzian Kappa distribution function. Effect of temperature anisotropy, AC frequency and number density of particles is found. The study is also extended to oblique propagation of EMEC waves in presence and absence of AC field. Through comprehensive mathematical analysis it is found that when EMEC wave propagates parallel to intrinsic magnetic field of Uranus, its growth is more enhanced than in case of oblique propagation. Results are also discussed in context to magnetosphere of Earth and also gives theoretical explanation to existence of high energetic particles observed by Voyager 2 in the magnetosphere of Uranus. The results can present a further insight into the nature of electron-cyclotron instability condition for the whistler mode waves in the outer radiation belts of Uranus or other space plasmas.

A study on the effects of magnetohydrodynamic perturbations on nonthermal beam formation during the current decay phase of disruptions in the T-10 tokamak

Repetitive bursts of nonthermal (Eγ ∼ 15 − 150 keV) x-ray radiation are observed during the plasma current decay phase of the density limit disruptions in the T-10 tokamak. The repetition rate of the nonthermal bursts correlates with the current decay rate. Analysis indicates that the phenomena can be connected with the nonthermal electrons initiated during the abrupt growth of the magnetohydrodynamic (MHD) modes in the series of minor disruptions. Application of the electron cyclotron waves in the post energy quench plasma allows control of the bursting MHD modes and prevention of nonthermal beam formation. Phenomenological modelling indicates that prevention of the nonthermal beams can be connected with elimination of the electric fields induced during magnetic reconnection in the series of minor disruptions, and reduction of the ‘equilibrium’ longitudinal electric field during auxiliary plasma heating.

Conceptual design of CFETR electron cyclotron wave system

China Fusion Engineering Test Reactor (CFETR) is a test tokamak which is built for magnetically confined fusion plasma experiments. The electron cyclotron (EC) wave system of CFETR is designed to inject 20MW RF power into the plasma for heating and current drive (H&CD) applications. The EC wave system consists of RF sources, twenty transmission lines (TLs) and one equatorial launcher. RF sources contain twenty gyrotrons with the output power 1MW. There are series of microwave components distributed along the TL and the percentage of power losses of each TL is about 8.7%. In the equatorial launcher, five RF beams are injected into one focusing mirror and then reflected to the plasma via one injection mirror. The focusing mirror is spherical to focus Gaussian beam and the injection mirror which is flat can steer in the toroidal direction. After optic calculation and optimization, all the quasi-optical parameters for launcher design are given. Combining with the thermal stress analysis, the chosen inner diameter of water channel of injection mirror is 12mm and the suggested water velocity is 3m/s.

COLD BEAM INJECTION IN RELATIVISTIC EMEC WAVE FOR KAPPA DISTRIBUTION FUNCTION WITH AC FIELD FOR MAGNETO-PLASMA

The effect of cold electron beam on electromagnetic electron cyclotron (EMEC) wave has been studied by using the unperturbed Lorentzian (Kappa) distribution in the magnetosphere for relativistic plasma. The dispersion relation is obtained by using the method of characteristic solutions and kinetic approach. An expression for the growth rate of a system has been calculated. It is inferred that in addition to the relativistic plasma obliquity and effect of cold electron beam modifies the growth rate and it also shifts the wave band significantly. The relativistic electrons by increasing the growth rate and widening the bandwidth may explain a wide frequency range of EMEC wave emissions in the magnetosphere.

EC Radiative Transport and Losses in DEMO-like High-Temperature Plasmas

Electron cyclotron (EC) wave power losses tend to play a significant role in the local electron power balance of a tokamak fusion reactor for electron temperatures above 35 keV as considered for steady-state operation. Using the RAYTEC code a comparative study of EC wave and bremsstrahlung contributions to the local and total power balance in relation to the α -particle heating power was carried out for a range of plasma conditions, wall reflection coefficients of the EC waves, and magnetic fields.

Effect of turbulence on electron cyclotron current drive and heating in ITER

Non-linear local electromagnetic gyrokinetic turbulence simulations of the ITER standard scenario H-mode are presented for the q = 3/2 and q = 2 surfaces. The turbulent transport is examined in regions of velocity space characteristic of electrons heated by electron-cyclotron waves. Electromagnetic fluctuations and sub-dominant micro-tearing modes are found to contribute significantly to the transport of the accelerated electrons, even though they have only a small impact on the transport of the bulk species. The particle diffusivity for resonant passing electrons is found to be less than 0.15 m2 s−1, and their heat conductivity is found to be less than 2 m2 s−1. Implications for the broadening of the current drive and energy deposition in ITER are discussed.

On the criteria guiding the design of the upper electron-cyclotron launcher for ITER

Electron cyclotron waves injected from an antenna located in the upper part of the vessel will be employed in ITER to control MHD instabilities, particularly neoclassical tearingmodes (NTMs). The derivation of the NTM stabilization criteria used up to now to guide the optimization of the launcher is reviewed in this paper and their range of validity elucidated. Possible effects leading to a deterioration of the predicted performance through a broadening of the EC deposition profile are discussed. The most detrimental effect will likely be the scattering of the EC beams from density fluctuations, resulting in a beam broadening in the 100% range. The combined impact of these effects with that of beam misalignment (with respect to the targeted surface) is discussed for a time slice of the standard Q = 10 H-mode scenario.

Scattering of ECRF waves by edge density fluctuations and blobs

The scattering of electron cyclotron waves by density blobs embedded in the edge region of a fusion plasma is studied using a full-wave model. The full-wave theory is a generalization of the usual approach of geometric optics ray scattering by blobs. While the latter allows for only refraction of waves, the former, more general formulation, includes refraction, reflection, and diffraction of waves. Furthermore, the geometric optics, ray tracing, model is limited to blob densities that are slightly different from the background plasma density. Observations in tokamak experiments show that the fluctuating density differs from the background plasma density by 20% or more. Thus, the geometric optics model is not a physically realistic model of scattering of electron cyclotron waves by plasma blobs. The differences between the ray tracing approach and the full-wave approach to scattering are illustrated in this paper.

Scattering of diffracting beams of electron cyclotron waves by random density fluctuations in inhomogeneous plasmas

The physics and first results of the new WKBeam code for electron cyclotron beams in tokamak plasmas are presented. This code is developed on the basis of a kinetic radiative transfer model which is general enough to account for the effects of diffraction and density fluctuations on the beam. Our preliminary numerical results show a significant broadening of the power deposition profile in ITER due to scattering from random density fluctuations at the plasma edge, while such scattering effects are found to be negligible in medium-size tokamaks like ASDEX upgrade.

Summary for the Theory Session at EC-18

A brief review of the theory contributions presented in the EC-18 Workshop is given. The covered spectrum of topics is quite broad including the electron cyclotron wave propagation physics as well the results of numerical modelling of transport and current drive.

On recent results in the modelling of neoclassical-tearing-mode stabilization via electron cyclotron current drive and their impact on the design of the upper EC launcher for ITER

Electron cyclotron wave beams injected from a launcher placed in the upper part of the vessel will be used in ITER to control MHD instabilities, in particular neoclassical tearing modes (NTMs). Simplified NTM stabilization criteria have been used in the past to guide the optimization of the launcher. Their derivation is reviewed in this paper and their range of applicability clarified. Moreover, possible effects leading to a deterioration of the predicted performance are discussed. Particularly critical in this context is the broadening of the electron-cyclotron (EC) deposition profiles. It is argued that the most detrimental effect for ITER is likely to be the scattering of the EC beams from density fluctuations due to plasma turbulence, resulting in a beam broadening by about a factor of two. The combined impact of these effects with that of beam misalignment (with respect to the targeted surface) is investigated by solving the Rutherford equation in a form that retains the most relevant terms. The perspectives for NTM stabilization in the Q = 10 ITER scenario are discussed.

Dynamics of multiple flux tubes in sawtoothing KSTAR plasmas heated by electron cyclotron waves: II. Theoretical and numerical analysis

The dynamics of multiple closed flux tubes in the core of a sawtoothing tokamak plasma are studied using nonlinear simulations. This is motivated by recent observations of long-lived hot spots in the electron cyclotron emission (ECE) images of KSTAR plasmas with electron cyclotron heating (ECH) (Yun et al 2012 Phys. Rev. Lett. 109 145003). Using an empirical source term in a reduced set of MHD equations, it is shown that flux tubes with helicity h = 1 are easily produced and survive for the observed time intervals only if the safety factor is close to unity (|q − 1| ≪ 0.5%) and the magnetic shear is small (|s| ≪ 1). This suggests that sawteeth in KSTAR leave behind wide regions where q ≈ 1. On the basis of the relevant time scales, we discuss how this magnetic geometry and the spatial localization of the EC resonance may allow ECH to actively induce the formation of flux tubes. Using simulations with q profiles that possess a wide q = 1 region inside the sawtooth inversion radius, we examine how the flux tubes merge and annihilate, and how their dynamics depend on the strength of the drive. The phenomena seen in the simulations and experiments lead us to conclude that, during the sawtooth ramp phase, there is a dynamic competition between sources and sinks of thermal and magnetic energy, where the flux tubes may play an important role; both as carriers of and channels for energy. The development of self-consistent simulation models is motivated and directions for future experiments are given.

Dynamics of multiple flux tubes in sawtoothing KSTAR plasmas heated by electron cyclotron waves: I. Experimental analysis of the tube structure

Multiple (two or more) flux tubes are commonly observed inside and/or near the q = 1 flux surface in KSTAR tokamak plasmas with localized electron cyclotron resonance heating and current drive (ECH/CD). Detailed 2D and quasi-3D images of the flux tubes obtained by an advanced imaging diagnostic system showed that the flux tubes are m/n = 1/1 field-aligned structures co-rotating around the magnetic axis. The flux tubes typically merge together and become like the internal kink mode of the usual sawtooth, which then collapses like a usual sawtooth crash. A systematic scan of ECH/CD beam position showed a strong correlation with the number of flux tubes. In the presence of multiple flux tubes close to the q = 1 surface, the radially outward heat transport was enhanced, which explains naturally temporal changes of electron temperature. We emphasize that the multiple flux tubes are a universal feature distinct from the internal kink instability and play a critical role in the control of sawteeth using ECH/CD.

Study of Electromagnetic Electron-Cyclotron Waves with Kappa Distribution Function in the Magnetosphere

Study of Electromagnetic Electron-Cyclotron Waves with Kappa Distribution Function in the Magnetosphere

Theory of the ordinary and extraordinary mode coupling in fluctuating plasmas

The theory of the linear coupling between the ordinary and extraordinary electron cyclotron waves in the vicinity of the plasma cut-off has been generalized for configurations with harmonic perturbations of plasma density, which is relevant, in particular, to the magnetic flux fluctuations in a toroidal fusion experiment. It is shown that for a large aspect ratio, tokamak fluctuations result in the amplitude and phase distortion of the quasi-optical beam on its way towards the coupling region located near the cut-off layer, but have no essential influence on the efficiency of the mode interaction inside the coupling region. However, this may not be valid in a spherical tokamak, where the perturbations may be significant inside the coupling region. We find that density fluctuations cannot be fully responsible for a low efficiency of the OXB heating of the overdense plasma observed in the present-day experiments.

ECCD performance analysis of future KSTAR ECH systems for extended applications

The ability of the KSTAR 170 GHz electron cyclotron (EC) wave launcher to drive a localized current is analyzed by means of the TORAY-GA ray-tracing code looking at extended physics application of the EC current drive (CD) in the KSTAR. Computation reveals that the EC-driven CD performance (J CD ), as compared to the plasma’s boot-strap current (J BS ), i.e., the ratio J CD /J BS , for the normalized minor radius, ρ > 0.5, is not good for the current design of the 1-MW 170-GHz launcher. Calculations at a lower frequency of 105 GHz have shown improved values for the J CD . Depending upon the launcher port’s availability, computations are carried out to find the pivot position for future proposed launchers by optimizing the figures of merit at respective locations. These computations show that Z = +30 cm gives the maximum figure of merit. Computations also show a possible synergy between the two launchers, one pivoted at Z = +30 cm and the other at Z = −25.2 cm. The main aim of this study is to provide guidance for the design of both launchers in order to optimize their performance according to the physics application.

ELECTROMAGNETIC ELECTRON-CYCLOTRON WAVES WITH AC FIELD IN THE MAGNETOSPHERE

In this paper the effect of externally injected beam of cold electrons on electromagnetic electron-cyclotron (EMEC) waves in the magnetosphere has been discussed. The investigation is conducted using the methodology of characteristic solution and considering kappa distribution function in the presence of AC field. The objective of present study is to examine the variation in growth rate of EMEC waves when temperature anisotropy, magnitude of AC field and number density of energetic particles varies. It is inferred that EMEC waves grow more significantly when propagating oblique to magnetic field direction rather than parallel to magnetic field direction. Also that as the temperature anisotropy and number density of background plasma increases, growth rate of EMEC waves increases.

Do nonlinear waves evolve in a universal manner in dusty and other plasma environments?

Using a fluid theory approach, this article provides a comparative study on the evolution of nonlinear waves in dusty plasmas, as well as other plasma environments, viz electron-ion, and electron-positron plasmas. Where applicable, relevance to satellite measurements is pointed out. A range of nonlinear waves from low frequency (ion acoustic and ion cyclotron waves), high frequency (electron acoustic and electron cyclotron waves) in electron-ion plasmas, ultra-low frequency (dust acoustic and dust cyclotron waves) in dusty plasmas and in electron-positron plasmas are discussed. Depending upon the plasma parameters, saw-tooth and bipolar structures are shown to evolve.