Cyclotron Resonance Frequency Waves Research Articles

Observation of a quasi-coherent high-frequency electromagnetic mode at the pedestal region in EAST RF-dominant H-modes

A quasi-coherent high-frequency mode (HFM), with frequency ∼300 kHz, has been observed in EAST H-modes obtained by lower hybrid current drive or combined heating of lower hybrid wave and ion cyclotron resonance frequency wave after lithium wall conditioning. The HFM with an initial growing phase in frequency and amplitude appears both following L–H transition and between edge-localized modes at a relatively high-collisionality . Detailed analysis from edge Langmuir probe has revealed that the HFM propagates in the electron diamagnetic direction in both the lab frame and the plasma frame. The decorrelation rate of HFM is smaller than the maximum E × B shearing rate at the steep gradient region by an order of magnitude. Furthermore, the growth, saturation and oscillations of HFM, including both frequency and amplitude, are strongly correlated with the evolution of pressure gradient. Thus, the limit-cycle-like oscillations between the amplitude and the frequency of HFM may play an important role in the formation and saturation of pedestal. During the saturation phase of pedestal pressure gradient, the HFM, sometimes accompanied by a mid-frequency (∼80 kHz) quasi-coherent mode, drives strong cross-field transport resulting in small crashes of pressure gradient and finally limits the pressure gradient.

Characteristics of edge-localized modes in the experimental advanced superconducting tokamak (EAST)

Edge-localized modes (ELMs) are the focus of tokamak edge physics studies because the large heat loads associated with ELMs have great impact on the divertor design of future reactor-grade tokamaks such as ITER. In the experimental advanced superconducting tokamak (EAST), the first ELMy high confinement modes (H-modes) were obtained with 1 MW lower hybrid wave power in conjunction with wall conditioning by lithium (Li) evaporation and real-time Li powder injection. The ELMs in EAST at this heating power are mostly type-III ELMs. They were observed close to the H-mode threshold power and produced small energy dumps (1–2% of the stored energy). Type-III ELMs produced a time-averaged peak heat flux of about 2 MW m−2 on the target plate, a value which is ∼10 times larger than that of ELM-free phases. A few isolated and large type-I-like ELM events were also observed in EAST with an energy loss of up to 5% of the stored energy. Statistically, the ELM frequencies are several hundred hertz and the frequency appears to decrease with q95, the safety factor at 95% of the flux surface. When an ion cyclotron resonance frequency wave was injected during the H-mode phases, the ELM repetition frequency increased immediately. The frequency and amplitude of type-III ELMs can be effectively influenced by puffing impurity argon gas.

Kinetic Theory of Plasma Waves

In the present paper a very brief introduction is provided to the theory of kinetic waves relevant to the description of wave heating in fusion machines and focussing mostly on radio frequency or ion cyclotron resonance frequency waves in tokamaks. The text starts by sketching the basic philosophy underlying the standardly adopted methods, describing the interaction of a single particle with a given wave and the assumptions typically made to arrive at a trustworthy description of the energy exchange, and ends by discussing some of the subtleties of the modeling of wave-particle interaction in inhomogeneous magnetized plasmas. None of the topics will be treated in full detail. Hence, by no means, this text is meant to be all-inclusive. Rather, it aims at providing a framework that should allow understanding what are the difficulties involved, leaving out the detailed derivation of the expressions as well as subtleties such as relativistic corrections. The interested reader is referred to the provided references - and the references given therein - for more in depth information.

Investigation of the Helical Divertor Function and the Future Plan of a Closed Divertor for Efficient Particle Control in the LHD Plasma Periphery

The function of the divertor plasmas on the particle control in the plasma periphery is investigated from viewpoints of magnetic field line structures and neutral particle transport in the Large Helical Device (LHD). It shows that the particle and heat deposition on the divertor plate arrays are qualitatively explained by the distribution of strike points calculated by magnetic field line tracing including a particle diffusion effect. Control of neutral particle fueling from the divertor plates is a critical issue for sustaining long-pulse discharges and achieving superdense core plasmas. The behavior of neutral particles in the plasma periphery has been investigated by Hα emission measurements and a neutral particle transport simulation. It reveals that gas fueling from the toroidally distributed divertor plates heated by protons accelerated by ion cyclotron resonance frequency wave is necessary for explaining measurements in a long-pulse discharge, and the spatial profile of the neutral particle density in the plasma periphery in various magnetic configurations is explained by the strike point distribution. Based on these analyses, a closed helical divertor configuration optimized for the intrinsic magnetic field line structure in the plasma periphery is proposed for efficient particle control and heat load reduction on the divertor plates.

A new sawtooth control mechanism relying on toroidally propagating ion cyclotron resonance frequency waves: Theory and Joint European Torus tokamak experimental evidence

The sawtooth control mechanism in plasmas employing toroidally propagating ion cyclotron resonance waves is extended. The asymmetrically distributed energetic passing ions are shown to modify the ideal internal kink mode when the position of the minority ion cyclotron resonance resides within a narrow region close to the q=1 surface. An analytical treatment of the internal kink mode in the presence of model distribution function with parallel velocity asymmetry is developed. The fast ion mechanism explains the strong sensitivity of sawteeth to resonance position, and moreover is consistent with dedicated Joint European Torus [F. Romanelli, Nucl. Fusion 49, 104006 (2009)] experiments which controlled sawteeth despite negligible current drive.

Iterated finite-orbit Monte Carlo simulations with full-wave fields for modeling tokamak ion cyclotron resonance frequency wave heating experiments

The five-dimensional finite-orbit Monte Carlo code ORBIT-RF [M. Choi et al., Phys. Plasmas 12, 1 (2005)] is successfully coupled with the two-dimensional full-wave code all-orders spectral algorithm (AORSA) [E. F. Jaeger et al., Phys. Plasmas 13, 056101 (2006)] in a self-consistent way to achieve improved predictive modeling for ion cyclotron resonance frequency (ICRF) wave heating experiments in present fusion devices and future ITER [R. Aymar et al., Nucl. Fusion 41, 1301 (2001)]. The ORBIT-RF/AORSA simulations reproduce fast-ion spectra and spatial profiles qualitatively consistent with fast ion D-alpha [W. W. Heidbrink et al., Plasma Phys. Controlled Fusion 49, 1457 (2007)] spectroscopic data in both DIII-D [J. L. Luxon, Nucl. Fusion 42, 614 (2002)] and National Spherical Torus Experiment [M. Ono et al., Nucl. Fusion 41, 1435 (2001)] high harmonic ICRF heating experiments. This work verifies that both finite-orbit width effect of fast-ion due to its drift motion along the torus and iterations between fast-ion distribution and wave fields are important in modeling ICRF heating experiments.

Monte Carlo orbit/full wave simulation of ion cyclotron resonance frequency wave damping on resonant ions in tokamaks

To investigate the experimentally observed interaction between beam ion species and fast Alfvén wave (FW), a Monte Carlo code, ORBIT-RF [V. S. Chan, S. C. Chiu, and Y. A. Omelchenko, Phys. Plasmas 9, 501 (2002)], which solves the time-dependent Hamiltonian guiding center drift equations, has been upgraded to incorporate a steady-state neutral beam ion slowing-down distribution, a quasilinear high harmonic radio frequency diffusion operator and the wave fields from the two-dimensional ion cyclotron resonance frequency full wave code (TORIC4) [M. Brambilla, Plasma Phys. Controlled Fusion 41, 1 (1999)]. Comparison of ORBIT-RF simulation of power absorption with fixed amplitudes of FW fields from TORIC4 power absorption calculation, which assumes Maxwellian plasma distributions, attains agreement within a factor of two. The experimentally measured enhanced neutron rate is reproduced to within 30% from ORBIT-RF simulation using a single dominant toroidal and poloidal wave number.

Control of sawteeth and triggering of NTMs with ion cyclotron resonance frequency waves in JET

A new scenario to delay or prevent neoclassical tearing mode (NTM) onset is presented. By active sawtooth destabilization, short period and low amplitude sawteeth are generated, such that the sawtooth produced NTM seed island is reduced and the threshold normalized plasma pressure for triggering of NTMs, βNonset, is increased. The scenario has been explored experimentally in the Joint European Torus (JET). Ion cyclotron resonance frequency (ICRF) waves tuned to the 2nd harmonic H-minority resonance have been used for sawtooth control. Whereas ICRF waves generally induce sawtooth stabilization, favouring the triggering of NTMs and reducing βNonset, the present experiments show that by toroidally directed waves, ion cyclotron current drive is produced, and that sawteeth can be destabilized by careful positioning of the 2nd harmonic H resonance layer with respect to the sawtooth inversion radius. As a result, NTM onset is delayed and βNonset is increased above its value obtained in discharges with additional heating from neutral beam injection alone.

On the orbit-averaged Monte Carlo operator describing ion cyclotron resonance frequency wave–particle interaction in a tokamak

In a toroidal plasma the distribution function of ions interacting resonantly with waves in the ion cyclotron range of frequencies (ICRF) can be described with a three-dimensional orbit-averaged Fokker–Planck equation. This equation can be solved with a Monte Carlo method. Explicit expressions for the Monte Carlo operator describing wave–particle interaction, within the framework of quasilinear theory, are given. Furthermore, properties of the operator are discussed.

Excitation of Ion Cyclotron Resonance Frequency Waves in Tokamaks by Waveguide Launcher

Excitation of Ion Cyclotron Resonance Frequency Waves in Tokamaks by Waveguide Launcher

Electron Cyclotron Heating in Weakly Relativistic Finite-Beta Tandem Mirror Plasmas

Electron cyclotron resonance frequency wave and absorption theory and ray tracing are reviewed and a numerical model for the study of electron heating in tandem mirror plugs and barriers is presented. It is found that substantial shifts in the spatial energy deposition profile occur from the cold plasma electron cyclotron resonance at elevated temperatures and that surface absorption can be substantial. The O-mode exhibits a ray trajectory that more easily penetrates to the plasma core and has an adequate single-pass absorption at temperatures >10 keV.