ICRF Waves Research Articles

A full wave solver integrated with a Fokker–Planck code for optimizing ion heating with ICRF waves for the ITER deuterium–tritium plasma

Efficient ion heating is crucial for future fusion devices, and the only way to heat ions directly is ion cyclotron resonance heating. Reported here is a full wave solver integrated with a Fokker–Planck code for optimizing ion heating with ion cyclotron range of frequency waves for the International Thermonuclear Experimental Reactor deuterium–tritium plasma. Both the direct absorption of minority ions and the power transfer to bulk ions via collisions are considered, while also accounting for the edge effects on ion absorption near the core. The simulation results show that the appropriate scrape-off layer density profile and parallel wave number lead to enhanced edge coupling and broaden the absorption region with moderate absorption intensity of the minority ions, which is very important for ion heating. More power from the heated ions is transferred to bulk ions than to electrons through collisions in our simulation via optimization, and reducing the total RF power results in a significant increase of the absorbed fraction of bulk ions.

High-Energy Ion Generation During Difference-Frequency ICRF Wave Heating in GAMMA 10/PDX

High-Energy Ion Generation During Difference-Frequency ICRF Wave Heating in GAMMA 10/PDX

Active control of amplitude and phase of high-power RF systems in EAST ICRF heating experiments

The EAST ICRF system operating space has been extended in power and phase control with a low-level RF system for the new double-strap antenna. Then the multi-step power and periodic phase scanning experiment were conducted in L-mode plasma, respectively. In the power scanning experiment, the stored energy, radiation power, plasma impedance and the antenna's temperature all have positive responses during the short ramp-ups of PL,ICRF. The core ion temperature increased from 1 keV to 1.5 keV and the core heating area expanded from |Z|≤5cm to |Z|≤10cm during the injection of ICRF waves. In the phasing scanning experiment, in addition to the same conclusions as the previous relatively phasing scanning experiment, the superposition effect of the fluctuation of stored energy, radiation power and neutron yield caused by phasing change with dual antenna, resulting in the amplitude and phase shift, was also observed. The active control of RF output facilitates the precise control of plasma profiles and greatly benefits future experimental exploration.

Argon pumpout by ICRF waves in C-Mod L- and I-mode plasmas

Pumpout of argon ions by ICRF waves has been observed in C-Mod deuterium L- and I-mode plasmas that had a substantial hydrogen fraction. The effect is manifested by a reduction of core argon x-ray brightness up to a factor of 90% on time scales of tens of milliseconds following injection of ICRF power. For Ar16+, the pumpout is strongest for hydrogen minority concentrations between 0.25 and 0.4, when the ICRF waves are not expected to result in minority heating. Modeling with the TORIC code suggests that the pumpout process occurs when the H/D mode conversion layer overlaps with the 2nd harmonic impurity resonance layer. The magnitude of the argon pumpout is independent of ICRF power above an apparent threshold of ∼500 kW, independent of electron density and appears to decrease as the plasma current is increased. Potential application as a heavy impurity control tool in reactors is discussed.

Finite elements method based ICRF waves heating simulation integrating with SOL plasma for EAST tokamak

Abstract Ion cyclotron range of frequency (ICRF) waves heating simulation is often carried out in the core plasma region. However, the inclusion of scrape-off layer (SOL) plasma in the simulation model may lead to new physical phenomenon and needs to be studied. In this paper, we apply finite elements method (FEM) based on the approach of [Vallejos 2019], to simulate ICRF waves heating in account of the realistic SOL plasma for EAST tokamak. In the presence of density pedestal near last closed flux surface (LCFS), a kind of cavity mode is observed for the case of low parallel wave number. Near ion-ion hybrid resonance (IIR) layer in SOL region, mode conversion from fast waves to slow waves takes place. ICRF waves coupling characteristics are roughly consistent with the prediction of the dispersion relation except for some small deviations, which may be caused by fast waves reflection in the high field side. Approximate on axis heating of minority H is observed and power deposition zone broadens with parallel wave number increasing. Waves energy dissipation in SOL plasma is less than 7% and localized near IIR region. Furthermore, the comparison between D(H) and D(He-3) minority heating scenarios is also carried out. The results and conclusions in this paper can provide theoretical reference to ICRF heating experiments and may supply a new insight in ICRF waves form in the plasma edge.

RF wave propagation simulation for ICRF antenna in EAST

ABSTRACT The propagation performance of radiofrequency (RF) waves excited by ICRF antenna in plasma has a great influence on the coupling effect. In this paper, the propagation characteristics of fast wave (FW) and slow wave (SW) under tokamak experimental condition are calculated, and the influence of angular frequency w 0, antenna power spectrum k// and tokamak total magnetic field B 0 is illustrated based on the typical operating parameters in EAST. By applying perfect matched layer (PML) boundary processing method to simulate single-pass absorption and using finite element method (FEM) to solve Maxwell equations in frequency domain, the propagation and coupling simulation of RF waves in cold plasma for EAST ICRF antenna in B-window were carried out to obtain the polarization characteristics of fields and the relationship between the coupling impedance and the gap length from antenna to the FW cut-off layer. The analysis results in this paper provide useful reference and guidance for the study of propagation coupling of ICRF waves in EAST and other magnetic confined fusion devices.

Direct toroidal torque driven by ICRF heating and its dependence on the plasma rotation

The contribution of ICRF waves as source of direct external toroidal torque is investigated and shown to be too small to be the main mechanism behind the common experimental observations of plasma rotation in the presence of ICRF-heating. In particular, the impact of plasma rotation on the direct ICRF torque is addressed with the outcome that plasma rotation mainly influences the profiles of the ICRF torque and not its total value.

The Role of Combined ICRF and NBI Heating in JET Hybrid Plasmas in Quest for High D-T Fusion Yield

Combined ICRF and NBI heating played a key role in achieving the world-record fusion yield in the first deuterium-tritium campaign at the JET tokamak in 1997. The current plans for JET include new experiments with deuterium-tritium (D-T) plasmas with more ITER-like conditions given the recently installed ITER-like wall (ILW). In the 2015-2016 campaigns, significant efforts have been devoted to the development of high-performance plasma scenarios compatible with ILW in preparation of the forthcoming D-T campaign. Good progress was made in both the inductive (baseline) and the hybrid scenario: a new record JET ILW fusion yield with a significantly extended duration of the high-performance phase was achieved. This paper reports on the progress with the hybrid scenario which is a candidate for ITER longpulse operation (∼1000 s) thanks to its improved normalized confinement, reduced plasma current and higher plasma beta with respect to the ITER reference baseline scenario. The combined NBI+ICRF power in the hybrid scenario was increased to 33 MW and the record fusion yield, averaged over 100 ms, to 2.9x1016 neutrons/s from the 2014 ILW fusion record of 2.3x1016 neutrons/s. Impurity control with ICRF waves was one of the key means for extending the duration of the high-performance phase. The main results are reviewed covering both key core and edge plasma issues.

Sequential modelling of ICRF wave near RF fields and asymptotic RF sheaths description for AUG ICRF antennas

A sequence of simulations is performed with RAPLICASOL and SSWICH to compare two AUG ICRF antennas. RAPLICASOL outputs have been used as input to SSWICH-SW for the AUG ICRF antennas. Using parallel electric field maps and the scattering matrix produced by RAPLICASOL, SSWICH-SW, reduced to its asymptotic part, is able to produce a 2D radial/poloidal map of the DC plasma potential accounting for the antenna input settings (total power, power balance, phasing). Two models of antennas are compared: 2-strap antenna vs 3-strap antenna. The 2D DC potential structures are correlated to structures of the parallel electric field map for different phasing and power balance. The overall DC plasma potential on the 3-strap antenna is lower due to better global RF currents compensation. Spatial proximity between regions of high RF electric field and regions where high DC plasma potentials are observed is an important factor for sheath rectification.

3-Dimensional density profiles in edge plasma simulations for ICRF heating

In this paper we discuss improvements made to two codes for the simulation of ICRF waves in edge plasmas: SSWICH-SW, which self-consistently models the interplay between sheath physics and radiofrequency waves (the slow wave), and RAPLICASOL, a Finite Element solver for Maxwell's equations in the cold plasma approximation. We have extended both to be able to handle 3D plasma density profiles. A comparison between a 1D and a 3D simulation reveals that the density profile dimensionality has a relatively small effect on E || at the aperture, but a large effect on the sheath potential at the antenna limiters

Characterization of the mutual influence of Ion Cyclotron and Lower Hybrid Range of frequencies systems on EAST

Waves in the Ion Cyclotron (ICRF) and Lower Hybrid (LH) Range of Frequencies are efficient techniques respectively to heat the plasma and drive current. Main difficulties come from a trade-off between good RF coupling and acceptable level of impurities release. The mutual influence of both systems makes such equilibrium often hard to reach [1]. In order to investigate those interactions based on Scrape-Off Layer (SOL) plasma parameters, a new reciprocating probe was designed allying a three tips Langmuir probe with an emissive wire. The emissive filament provides a precise measure of plasma potential [2], which can be used to calibrate Langmuir probe's results. This paper reports on experimental results obtained on EAST, where there are two ICRF antennas and two LH launchers. Among others diagnostics, the new reciprocating probe enabled to evidence the deleterious influence of ICRF power on LHWs coupling in L-mode plasmas. In areas connected with an active ICRF antenna, SOL potentials increase while densities tend to decrease, respectively enhancing impurities release and deteriorating LHWs coupling. This phenomenon has mostly been attributed to RF sheath; the one that forms on top of Plasma Facing Components (PFCs) and causes ExB density convections [3]. From those experiments it seems ICRF has a strong influence on magnetically connected areas, both in the near field – influencing ICRF waves coupling – and in farther locations such as in front of LH grills. Moreover, influence of ICRF on LH system was observed both in L and H modes. Those results are consistent with RF sheath rectification process. Concerning the influence of LHWs on ICRF coupling, nothing was observed in L-mode. Besides during H-mode experiments, LHWs have been identified as having a mitigating effect on ELMs [4], which on average lowers the pedestal, increasing edge densities to the profit of ICRF waves coupling.

Antenna to Core: A New Approach to RF Modelling

A new technique for the calculation of RF waves in toroidal geometry enables the simultaneous incorporation of antenna geometry, plasma facing components (PFCs), the scrape off-layer (SOL) and core propagation [Shiraiwa et al, “HIS-TORIC: Extending core ICRF wave simulation to include realistic SOL plasmas”, Nucl. Fusion in press 10 (2017)]. The calculation of antenna coupling has been traditionally performed separately from those of core propagation whereas this method allows them to be combined permitting the modelling of such phenomena as waves reflected from plasma-vacuum boundary or cyclotron resonances back to the antenna. Calculations with this technique also naturally capture wave propagation in the SOL and its interactions with non-conforming PFCs permitting self-consistent calculation of core absorption and edge power loss.

Measurements of ICRF wave-induced density fluctuations in LHD by a microwave reflectometer

An O-mode microwave reflectometer has been developed to measure ICRF wave induced electron density fluctuations in LHD plasmas. The system has two probing frequencies (28.8 and 30.1 GHz) to measure two spatial points simultaneously. The rms density fluctuation levels are typically 0.01%. The linearity between the measured density fluctuation amplitude and the square root of the RF power is discussed. The decay length of the RF field was estimated to be 1 to 7 m under the operational condition investigated. A typical spatial distance between the two measurement points corresponding to the two probing frequencies is a few centimeters, and the fluctuation amplitudes at the two points are similar in amplitude. The phase difference between the two fluctuations show in-phase relationship on average. Out-of phase relationships, which implies a standing wave structure, are often observed when the wave absorption is expected to be poor.

Time Evolving Stucture of Alfvén Ion-Cyclotron Waves in GAMMA 10

A two-channel reflectometer has been developed and successfully applied to the GAMMA10 central cell to investigate the spatial structure of spontaneously excited Alfven ion-cyclotron (AIC) waves. At the frequencies of externally applied ICRF waves and AIC waves, good correlations of well above the statistical noise level are obtained between two density fluctuations at axially separated positions in the central cell. Density fluctuations at various radial positions and two axially separated positions show that the power distribution among the AIC waves changes much both in radial and axial directions especially in the earlier period just after the excitation and in the core region of r/a < 0.3. Phase differences of the AIC waves at axially separated positions show features of the transformation of the AIC waves from propagating to standing waves. The pass of the node of the standing wave is also observed. The time of the pass is different by AIC waves, which clearly indicates difference of the axial wavelengths of the simultaneously excited AIC waves.

A dielectric response model for FEM solutions of ICRF wave fields

Modelling of fast wave ICRF heating in large machines with high density such as DEMO is challenging because of the short wave lengths. Therefore, fast, efficient global wave solvers are necessary. A major difficulty with calculating the wave field in a spatial dispersive medium is that the dielectric tensor becomes a function of the local wave vector, which in its turn depends on the solution. Furthermore, the solution may consist of several waves co-existing at the same location subjected to separate response functions. In order to model upshift of the parallel wave vector, higher order FLR-effects on the cyclotron absorption and TTMP damping for the electron absorption methods based on iteration, suitable for FEM codes, are proposed.

Feasibility of Reduced Tritium Circulation in the Heliotron Reactor by Enhancing Fusion Reactivity Using ICRF

A scheme for reducing the tritium fraction in DT fusion reactors is investigated by means of enhancing the fusion reactivity using high-power ICRF heating in heliotron reactors. We assume a situation that the density fraction of tritons is less than 10%, and the minority tritons are accelerated by ICRF waves. We then analyze the increase of fusion reactivity by assuming an effective temperature of high-energy tritons and examine the possibility of realizing a fusion reactor with this concept. The required ICRF power and the generated fusion power are also estimated.

ICRF Heating in Helical Reactor

ICRF heating is a favorable high-density plasma heating method, since the ICRF wave can propagate in high-density plasma. ICRF heating in a helical reactor was calculated by using an antenna code and a ray tracing code assuming high-density plasma without resonance layers of α-particles in the plasma core. Enough loading resistance and a low electric field inside the ICRF antenna was obtained. Strong heating of plasma is expected in high-density plasma by heating electron and tritium, especially with the configuration of the second harmonic resonance layer of tritium on the magnetic axis shifted onto the saddle point.

Effective Excitation of ICRF Waves by Use of Phased Antennas in GAMMA 10

A numerical analysis is performed with two-dimensional wave code for effective excitation of the m = + 1 fast Alfvén waves in an axisymmetric central cell of GAMMA 10. Plasma production with fast waves depends on the wave excitation in the plasma. Eigenmodes are strongly formed with large amplitude when the boundary conditions are satisfied. As an optimum density for each eigenmode exists discretely, the density is clamped at the value where the eigenmode is strongly formed. For higher density plasma production, formation of eigenmodes should be controlled as the density increases. In this study, pairs of phased antennas are adopted for the effective excitation of eigenmodes. The optimum configuration of antennas and their phase difference are investigated in the present geometry. It is found that the eigenmodes can be effectively excited by controlling the phase difference between a pair of antennas.

Edge plasma behavior in the GAMMA 10 tandem mirror based on gas puff imaging experiments with a high-speed camera

Detailed results of edge plasma and neutral particle behavior are described in the gas puff imaging experiments performed in the GAMMA 10 tandem mirror for the first time. Visible imaging measurement by using a high-speed camera (Ultima-SE, Photron Inc.) was recently performed in the central-cell midplane of GAMMA 10. In the standard plasma discharge heated by ICRF wave, a short gas puff of hydrogen (3ms) was carried out and the time evolution of light emission was visualized precisely. It was found that the time evolution of the emission cloud was similar to that of Hα line intensity measured nearby and was localized close to the gas puff port. Asymmetry in diffusion of the emission cloud is observed radial and axial direction of plasma column in the central-cell. This localization phenomenon of neutrals is qualitatively well explained by a fully 3-dimensional simulation using DEGAS neutral particle transport code.

RF Field Study near the Ion Cyclotron Frequency in the HANBIT Mirror Device

The purpose of this study is to understand the ICRF wave heating and the sideband coupling characteristics in the HANBIT mirror plasmas. The effects of the magnetic field strength on the RF field fluctuations have also been studied. Amplitudes and frequencies of the sideband field are found to depend on the applied magnetic field. When the magnetic field strength is over 0.229 T~0.233 T (ɷ/ɷci ~ 1), it is shown that the resonance of ICRF wave enhances the sideband coupling on the lower frequency side. The coupling of two split RF frequencies result in a beat wave signal. This phenomenon is shown to be particularly noticeable at a magnetic field of 0.229 T.