Ion Cyclotron Resonance Frequency Waves Research Articles

Wave coupling simulation of ICRF antenna in EAST relying on the perfectly matched layer technique

A coupling model for the waves in ion cyclotron resonance frequency (ICRF) in tokamak plasmas is described. The simulation is performed based on the finite element method. A cold plasma dielectric tensor is used. A perfectly matched layer (PML) is applied to describe the ICRF wave single-pass absorption in the core plasma. The selection of PML characteristic parameters is optimized in a wide wave frequency range and validated for plane waves and ICRF waves. The dependencies of the coupling resistance on the plasma-antenna gap, average plasma density, and plasma profiles are given. The simulation results are in good agreement with the experimental results.

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.

Features of the repetition frequency of edge localized modes in EAST

This paper presents the features of the edge localized modes (ELMs) observed in the 2010 experimental campaign on the Experimental Advanced Superconducting Tokamak (EAST). The first high-confinement mode (H-mode) at an H-factor of HIPB98(y, 2)∼ 1 has been obtained with about 1 MW lower hybrid wave (LHW) heating power on the EAST tokamak. The ELMs in EAST are Type III ELMs with high frequency (several hundred Hertz) and low amplitude. ELM features for the following two types of EAST discharges are investigated: discharges with only LHW and those with both LHW and ion cyclotron resonance frequency (ICRF) wave. The ELMs are observed in a wide q 95 window ranging from 2.6 to 5.0, and the ELM frequencies decrease with the safety factor, q 95. In contrast to most observations, the ELM frequency in EAST appears to be weakly proportional to the power through the separatrix. When the ICRF wave is injected, the ELM frequency increases. The control of the ELM frequency and amplitude has been achieved by puffing impure argon gas into the EAST discharge.

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.

Heat loads on JET plasma facing components from ICRF and LH wave absorption in the SOL

In JET, lower hybrid (LH) and ion cyclotron resonance frequency (ICRF) wave absorption in the scrape-off layer can lead to enhanced heat fluxes on some plasma facing components (PFCs). Experiments have been carried out to characterize these heat loads in order to: (i) prepare JET operation with the Be wall which has a reduced power handling capability as compared with the carbon wall and (ii) better understand the physics driving these wave absorption phenomena and propose solutions for next generation systems to reduce them. When using ICRF, hot spots are observed on the antenna structures and on limiters close to the powered antennas and are explained by acceleration of ions in RF-rectified sheath potentials. High temperatures up to 800 °C can be reached on locations where a deposit has built up on tile surfaces. Modelling which takes into account the fast thermal response of surface layers can reproduce well the surface temperature measurements via infrared (IR) imaging, and allow evaluation of the heat fluxes local to active ICRF antennas. The flux scales linearly with the density at the antenna radius and with the antenna voltage. Strap phasing corresponding to wave spectra with lower k∥ values can lead to a significant increase in hot spot intensity in agreement with antenna modelling that predicts, in that case, an increase in RF sheath rectification. LH absorption in front of the antenna through electron Landau damping of the wave with high N∥ components generates hot spots precisely located on PFCs magnetically connected to the launcher. Analysis of the LH hot spot surface temperature from IR measurements allows a quantification of the power flux along the field lines: in the worst case scenario it is in the range 15–30 MW m−2. The main driving parameter is the LH power density along the horizontal rows of the launcher, the heat fluxes scaling roughly with the square of the LH power density. The local electron density in front of the grill increases with the LH launched power; this also enhances the intensity of the LH hot spots.

Feedback control of the sawtooth period through real time control of the ion cyclotron resonance frequency

Modification of the sawtooth period through ion cyclotron resonance frequency (ICRF) heating and current drive has been demonstrated in a number of experiments. The effect has been seen to depend critically on the location of the ICRF absorption region with respect to the q = 1 flux surface. Consequently, for ICRF to be a viable tool for sawtooth control, one must be able to control the ICRF absorption location in real time so as to follow variations in the location of the q = 1 surface. To achieve this, the JET ICRF system has been modified to allow the JET real time central controller to control the frequency of the ICRF generators. An algorithm for real time determination of the sawtooth period has been developed and a closed loop controller, which modifies the frequency of the ICRF generators to bring the measured sawtooth period to the desired reference value, has been implemented. This paper shows the first experimental demonstration of closed loop sawtooth period control by real time variation of the ICRF wave frequency.

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.

ICRF wall conditioning and plasma performance on EAST

ICRF (ion cyclotron resonance frequency) wall conditioning was performed on EAST superconducting tokamak. It included ICRF cleaning in-between shots, boronization associated with ICRF plasmas, and ICRF oxidation with an ITER-relevant full metallic wall material. A dedicated ICRF antenna was developed for wall conditioning on EAST. The RF frequency is 30 MHz, and maximum output power of RF system is 300 kW. The duty time of pulsed ICRF wave was set at 0.3 s on and 1.2 s off in presence of the toroidal magnetic field of 1–2 T. Optimization of start-up, ramp-up and plasma shaping for single and double-null configurations were obtained following the ICRF wall conditioning. High density, high plasma current and long pulse discharges were achieved on EAST.

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.

Summary of the Europhysics Topical Conference on Radiofrequency Heating and Current Drive of Fusion Devices

In recent years, impressive progress has been made both on the physics understanding and on the power handling of the main RF systems: Ion Cyclotron Resonance Frequency (ICRF), Lower Hybrid Waves (LH), Electron Cyclotron Resonance Frequency (ECRF). In many large experiments, RF techniques are now being considered as a tool to improve plasma performances and to understand the physics of tokamaks or stellarators. For instance, H-modes have been achieved with all methods, with low levels of impurities, ICRF has allowed to heat and to produce plasmas at very high densities with high n tau T, as well as plasmas where most of the current is bootstrap driven and off-axis ICRF and ECRH have been used to understand energy radial transport. Large plasma currents have been driven by LH waves, up to 2 MA in conjunction with ICRF or Neutral beams. Fast Wave Ion current drive has been shown to have large efffects on MHD activity. Use of synergistic effects between LH and ICRF waves appear to be a possible route to increase current drive efficiency.More than 200 MJoules of ICRF and 100 MJoules of LH power have been launched recently on one minute long plasma discharges with plasma current up to 2 MA. The development of powerful ECRH sources at high frequency is steadily progressing. Steady state ignited plasmas require current drive efficiencies which are still exceeding present achievements, but a few approaches to the problem have been discussed. Heating to ignition with ICRF is very promising both on the physics and on the technical side since present power and energy handling capability are comparable to those needed for a reactor. In addition to reaching ignition in fusion reactors, other roles for RF waves, such as burn control and profile control, have been extensively discussed.

Ion-cyclotron-frequency stabilization of internal kink mode and sawtooth oscillations in tokamaks.

It is proposed that the ponderomotive force due to applied ion-cyclotron resonance-frequency waves can stabilize the internal kink mode in tokamaks. The sufficient stability criterion is derived and the necessary power estimated. It is concluded that at the rf power level, present in the Joint European Torus experiment, the ponderomotive force effects are significant and may be responsible for the modification of the sawtooth activity observed in recent experiments.

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

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