Optimization of simulated high-field side lower hybrid current drive coupling using machine learning predictions of scrape-off layer density

Significant progress on both L- and H-mode long-pulse discharges has been made recently in Experimental Advanced Superconducting Tokamak (EAST) with lower hybrid current drive (LHCD) [J. Li et al., Nature Phys. 9, 817 (2013) And B. N. Wan et al., Nucl. Fusion 53, 104006 (2013).]. In this paper, LHCD experiments at high density in L-mode plasmas have been investigated in order to explore possible methods of improving current drive (CD) efficiency, thus to extend the operational space in long-pulse and high performance plasma regime. It is observed that the normalized bremsstrahlung emission falls much more steeply than 1/ne_av (line-averaged density) above ne_av = 2.2 × 1019 m−3 indicating anomalous loss of CD efficiency. A large broadening of the operating line frequency (f = 2.45 GHz), measured by a radio frequency (RF) probe located outside the EAST vacuum vessel, is generally observed during high density cases, which is found to be one of the physical mechanisms resulting in the unfavorable CD efficiency. Collisional absorption of lower hybrid wave in the scrape off layer (SOL) may be another cause, but this assertion needs more experimental evidence and numerical analysis. It is found that plasmas with strong lithiation can improve CD efficiency largely, which should be benefited from the changes of edge parameters. In addition, several possible methods are proposed to recover good efficiency in future experiments for EAST.

Assessing the performance of lower hybrid current drive (LHCD) at high density is critical for developing non-inductive current drive systems on future steady-state experiments. Excellent LHCD efficiency has been observed during fully non-inductive operation (η=2.0−2.5×1019 AW–1m–2 at n¯e=0.5×1020 m–3) on Alcator C-Mod [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)] under conditions (ne, magnetic field and topology, and LHCD frequency) relevant to ITER [S. Shiraiwa et al., Nucl. Fusion 51, 103024 (2011)]. To extend these results to advanced tokamak regimes with higher bootstrap current fractions on C-Mod, it is necessary to increase n¯e to 1.0−1.5×1020 m−3. However, the number of current-carrying, non-thermal electrons generated by LHCD drops sharply in diverted configurations at densities that are well below the density limit previously observed on limited tokamaks. In these cases, changes in scrape off layer (SOL) ionization and density profiles are observed during LHCD, indicating that significant power is transferred from the LH waves to the SOL. Fokker-Planck simulations of these discharges utilizing ray tracing and full wave propagation codes indicate that LH waves in the high density, multi-pass absorption regime linger in the plasma edge, and SOL region, where absorption near or outside the LCFS results in the loss of current drive efficiency. Modeling predicts that non-thermal emission increases with stronger single-pass absorption. Experimental data show that increasing Te in high density LH discharges results in higher non-thermal electron emission, as predicted by the models.

The effect on the scrape-off layer (SOL) of changing the position of a tokamak limiter from the low field side (LFS) to the high field side (HFS) of the plasma is considered. Conservation of magnetic flux in the SOL requires that the area, A||, for flow of particles and energy parallel to the magnetic field, be smaller on the HFS. The effect that this reduction in A|| has on the SOL is that, when the limiter is on the HFS, as compared with the LFS, then the characteristic scrape-off thickness is increased and the plasma particle and energy densities are higher. Such increases are described. They were observed in an experiment in JET in which the plasma was limited first on the LFS and then, during the same discharge, on the HFS. The magnitude of the effect was larger than expected, with the line integrated density of the SOL, nλn, having an average increase by a factor of 5.8 ± 1.1 and an increase by a factor of 7 at the respective limiter. Moreover, there was an increase in the characteristic scrape-off thickness for power flow, λp, by a factor of 2.5 ± 0.4 on average, and a corresponding increase by a factor of 3.0 at the respective limiter. Such an increase in n λn can substantially improve the screening of recycled particles and can increase wall pumping, although not strongly in the experiments described, which were in helium to avoid these effects. The large increase in λp measured in JET is useful in spreading power over a larger area of a limiter on the HFS. Evidence is presented which suggests that either the observed enhancement of the effect on the SOL above expectations is due to a reduction in the Mach number at the limiter sheath when on the HFS or there are fundamental problems with the modelling procedure usually used for the SOL. Related experiments are discussed

Neoclassical tearing modes (NTM) are a class of resistive instabilities that arise in tokamaks at rational surfaces and form magnetic islands. These islands flatten the pressure gradient, reducing plasma performance and may lead to disruptions should they grow large enough. Driving current within the island can stabilize an NTM, which has been achieved with electron cyclotron current drive (ECCD) on multiple devices. An alternative to ECCD is lower hybrid current drive (LHCD), which offers the advantages of increased current drive efficiency and reduced system cost. LHCD has been viewed as poorly suited for NTM suppression due to the large spatial extent of the driven current when in the multi-pass absorption regime (as has been the case in all past LHCD experiments). However, the driven current is more localized when in the single pass absorption regime, as is predicted for the DIII-D high field side (HFS) LHCD experiment. This work evaluates the feasibility of NTM suppression with HFS LHCD on DIII-D by predicting the island growth rate for a set of representative DIII-D plasmas via the modified Rutherford equation with and without the application of LHCD. In these plasmas, NTM suppression is achieved at reasonable power levels, even with finite misalignment between LH current and the island. The effect of current condensation was included and found to be most significant at smaller island sizes, assuming an experimentally typical temperature perturbation amplitude of 10%.

In the first experimental campaign of Wendelstein 7-X (W7-X), a combined probe head mounted on the multi-purpose manipulator has been used to measure the scrape-off layer (SOL) turbulence characteristics. The preliminary experimental results are summarized to illustrate the SOL turbulence properties in the limiter configuration on W7-X. In a standard limiter configuration, significant electrostatic fluctuations can be found in the near SOL, and the dominant frequency of fluctuation power is below 100 kHz. The auto-correlation spectrum power law decay factor is α ≈ −1 below 40 kHz and α ≈ –2 between 50 and 200 kHz. A broadband spectrum appears between 240 and 380 kHz with a low spectral power density, but a high cross-correlation coefficient. The statistical characteristics of turbulence are calculated by the two-point cross-correlation technique. A clear poloidal dispersion relation is found in the spectrum S(kθ, f), propagating along the ion diamagnetic drift direction with a group velocity (below 100 kHz) about 0.56 km/s in the near SOL in the laboratory frame. The poloidal correlation length is around 5–10 mm in SOL. The turbulence phase velocity is about 0.5–1 km/s when close to the last closed flux surface, which is comparable with the poloidal E × B drift speed.

High field side LHCD launcher study for KSTAR

Cross-field particle transport in the scrape-off layer (SOL) of Alcator C-Mod [Phys. Plasmas 1, 1511 (1994)] can be characterized by an effective particle diffusivity (Deff) that increases markedly with distance from the separatrix. As a consequence, recycling onto the main-chamber walls is large compared to plasma flows into the divertor volume. The SOL exhibits a two-layer structure: Steep gradients and moderate fluctuation levels are typically found in a ∼5 mm region near the separatrix (near SOL) where parallel electron conduction typically dominates energy losses. Small gradients and larger fluctuation levels with longer correlation times are found outside this region (far SOL). Deff in the near SOL increases strongly with local plasma collisionality normalized to the magnetic connection length. As the discharge density limit is approached, Deff and associated fluctuation levels become large across the entire SOL and cross-field heat convection everywhere exceeds parallel conduction losses, impacting the power balance of the discharge.

An ad hoc calculation is presented to interpret the growth of sideband waves during lower hybrid current drive (LHCD) experiments on Experimental Advanced Superconducting Tokamak (EAST). We adopt the solution of the non-linear dispersion relation for parametric instabilities to describe the growth rate of ion cyclotron quasi-modes (ICQMs) in the peripheral plasma, and use the WKB solution of the propagation of electrostatic waves in two dimensions to model the variation of the electric field in the scrape-off-layer (SOL). We then calculate the growth rates and amplification factors of the sideband waves associated with ICQM in the parallel and perpendicular coupling limits. The onset and substantial growth of the first down-shifted sideband of lower hybrid waves (LHWs) are associated with elevation of the plasma density in the outer SOL where the growth rate of ICQM in the parallel coupling limit increases sharply after the density is elevated. The peak frequencies associated with the first harmonic of ICQM obtained from calculating the amplification factors in the parallel coupling limit ( A para ) agree very well with the observed frequencies of the first down-shifted sideband of 2.45 GHz LHWs in EAST pulse 43 772 and 86 521. The consistency between the calculation and the observation indicates that the sideband waves are associated with parametric decay instability of ICQM excited in the outer SOL through parallel coupling. The amplitude of A para obtained from a single pass of LHWs through the SOL of EAST is, however, one order of magnitude smaller than unity, meaning that it remains elusive whether the substantial growth of sideband waves mainly occurs in front of the antenna or requires multiple passes of LHWs across the SOL region.

The Advanced Divertor and RF tokamak eXperiment (ADX) is a compact, high-field device proposed by the MIT Plasma Science and Fusion Center and collaborators [1], which will address critical gaps in world fusion research on the pathway to fusion energy. In addition to developing and testing new divertor concepts at reactor level magnetic field strengths and power densities, ADX will test new antenna concepts for Lower Hybrid Current Drive (LHCD) and Ion Cyclotron Range of Frequency (ICRF) heating systems. In particular, ADX will be purpose-built to allow antennas to be positioned on the high magnetic field side of the torus, i.e., on the inner wall. With antennas placed at this location, plasma-wall interactions are greatly reduced and favorable RF wave physics projects to dramatic improvements in current drive efficiency and current profile control as well as very effective scenarios for RF heating and flow drive [2][3][4]. Initial designs for a high field side LHCD and ICRF antennas have been completed and are analyzed to determine the loads induced during a full-current plasma disruption. While locating antennas at the inner wall is beneficial from an RF standpoint, it exposes them to a higher toroidal field which, when combined with the eddy currents caused by a disrupting plasma, will lead to higher loads. Using COMSOL Multiphysics [5], a model of the ADX vessel and coils is created to predict the magnetic fields, eddy currents and loads acting on the antennas during a disruption. Structural models are then run to predict the stresses and to provide guidance for design improvement, such as determining where structural reinforcements may be necessary.

Modeling of carbon transport in the divertor and SOL of DIII-D during high performance plasma operation

Efficient coupling of lower hybrid (LH) waves in conditions close to those foreseen in ITER has been obtained in advanced scenario plasmas in the JET tokamak. Up to 3 MW of lower hybrid current drive (LHCD) power has been coupled at a distance between the separatrix and the launcher of 11 cm, in the presence of edge localized mode activity. The key to the improved LH wave coupling is local control of the Scrape-Off-Layer (SOL) density through gas injection in the region magnetically connected to the launcher. This increases the electron density in front of the launcher so as to improve the coupling of the LH waves, i.e. reduce the reflected power in the launcher. The average power reflection coefficient was 5.7% with gas injection, at 11 cm distance between the separatrix and the launcher. A change in the gas injection design has made the gas puffing more efficient, making the use of D2 injection possible as an alternative to CD4. At similar injected electrons/s rates, D2 gives higher electron density in the SOL than CD4, resulting in better LH coupling with D2. The possibility of using D2 instead of the earlier used CD4 is an encouraging result in view of ITER operation, as CD4 may not be compatible in ITER due to the problem of tritium retention in deposited carbon layers.

He 3 gas was puffed from the wall into Ohmic low confinement-mode discharges of the Alcator C-Mod [I. H. Hutchinson et al., Phys. Plasmas 1, 1551 (1994)] tokamak and the charged states were measured near the wall with the omegatron ion mass spectrometer. Analysis of the data shows that the concentrations of singly- and doubly-ionized helium near the wall are approximately equal. The electron temperature and density at the omegatron are too low to account for ionization of helium in the local flux tube, therefore the helium is ionized in a hotter region of the edge plasma and is transported to the omegatron. A simple one-dimensional radial transport model reproduces the observed values of charge state flux and density, but only if rapid cross-field transport is included, increasing with distance from the separatrix. A constant cross-field diffusion coefficient of order 2 m2/s and an outward convection velocity profile increasing to of order 100 m/s in the far scrape-off layer is implied.

Two aspects of edge turbulence and transport in Alcator C-Mod are explored. The quasi-coherent mode (QCM), an edge fluctuation present in Enhanced Dα H-mode plasmas, is examined with regard to its role in the enhanced particle transport found in these plasmas, its in/out asymmetry, its poloidal wave number and its radial width and location. It is shown to play a dominant role in the perpendicular particle transport. The QCM is not observed at the inboard midplane, indicating that its amplitude there is significantly smaller than on the outboard side. The peak amplitude of the QCM is found just inside the separatrix, with a radial width ≳5 mm, leading to a non-zero amplitude outside the separatrix and qualitatively consistent with its transport enhancement. Also examined are the characteristics of the intermittent convective transport, associated with the larger scale turbulent structures, also called blobs, and typically occurring in the scrape-off-layer (SOL). These turbulent structures are qualitatively similar in L- and H-mode. When their perpendicular extent, occurrence frequencies and magnitudes are compared, it is found that their size is somewhat smaller in ELMfree H-Mode, while their frequency is similar. A clear difference is seen in the magnitude of these turbulent fluctuations in the far SOL, with ELMfree H-mode showing a smaller perturbation there than L-mode. As the Greenwald density limit is approached (n/nGW ⩾ 0.7), blobs are seen inside the separatrix consistent with the observation that the high cross-field transport region, normally found in the far SOL, penetrates the closed flux surfaces at high n/nGW.

An extensive experimental survey of plasma potentials induced by ion cyclotron range-of frequency (ICRF) heating was carried out in the scrape-off layer (SOL) plasmas on the Alcator C-Mod tokamak. Enhanced plasma potentials >100 V are observed at locations where local magnetic fields map to active ICRF antennas. In these cases, the enhanced potential appears only when a local plasma density threshold is surpassed—a threshold that is quantitatively consistent with slow wave (SW) RF rectification theory. However, in many cases large potential enhancements are found in locations that do not map along magnetic field lines to active antennas without obstruction, i.e. locations that are inaccessible to SWs launched by the active antennas. Enhanced potentials in these ‘unmapped’ locations are correlated with local plasma parameters, ICRF electromagnetic fields associated with the fast wave (FW) and SW, launched wave spectra, and the boundary surface geometry. It is found that enhanced plasma potentials in unmapped locations correlate with the FW field strength. These observations are qualitatively consistent with a model that accounts for the conversion of FWs to SWs at conducting surfaces oriented at an oblique angle with respect to the magnetic field, with the SW leading to sheath rectification. In addition, enhanced plasma potentials are found far into the shadow of passive limiter structures. These are correlated with the magnitude of the local FW field strength, yet the effect does not follow any present model. Overall, ICRF-induced plasma potentials may appear in regions far removed from the active antennas, yet due to the complex response of the SOL potentials at a variety of boundary surfaces, it remains unclear what part of the plasma-facing wall should be targeted to mitigate ICRF-induced impurities. The results also suggest that operating active ICRF antennas in a high single pass absorption regime is crucial in minimizing the effects of the FW fields on plasma–material interactions.

At high plasma density, the lower hybrid current drive (LHCD) system in tokamaks suffers significant efficiency loss caused by parametric decay instability (PDI) induced in the scrape-off layer (SOL) plasma. There, the lower hybrid waves (LHWs) launched from the antenna deposit a large fraction of their power through PDI, leading to the phenomenon of nonlinear density limit of LHCD. In this paper, we present a self-consistent modelling of the power deposition of LHWs in the SOL plasma by coupling PDI to the propagation of LHWs, the power loss of LHCD is evaluated quantitively, and a theoretical scaling of the nonlinear density limit n PDI ∝ P 0 −2/3 L y -2/3 T e ω 0 2 B 4/3 0 is acquired.