Modulated Electron Cyclotron Heating Research Articles

Investigation of core transport changes in DIII-D discharges with off-axis T e profile peaks

DIII-D discharges that transition to H-mode solely with off-axis electron cyclotron heating (ECH) often exhibit strong off-axis peaking of electron temperature profiles at the heating location. Electron heat transport properties near these off-axis temperature peaks have been studied using modulated ECH. The Fourier analyzed electron temperature data have been used to infer electron thermal diffusivity. Comparisons with numerical solutions of the time-dependent electron thermal equation find that the data are consistent with a narrow region with electron diffusivity χ e an order of magnitude lower than the average value across the plasma, suggesting an electron internal transport barrier (ITB) near the ECH heating location. Detailed profile analysis and equilibrium reconstructions suggest that the formation of these ITBs are correlated with off-axis values of the safety factor q being near 1. Furthermore, the ECH driven H-mode discharges demonstrate more rapid electron heating rate near the ECH deposition location than L-mode discharges with higher auxiliary ECH heating power. Additional modeling attributes this difference to the modification of electron heat transport in the core at the L-H transition, which also sustains the off-axis electron temperature peaks.

Validation study of GENE on ASDEX Upgrade using perturbative thermal diffusivity measured with partial sawtooth heat pulses

Perturbative thermal diffusivity is measured with partial sawtooth-generated heat pulses on ASDEX Upgrade for the first time, and these measurements are used to validate the first nonlinear ion-scale gyrokinetic simulation that agrees with experimentally measured perturbative diffusivity. Recent work on Alcator C-Mod (Howard et al 2016 Phys. Plasmas 23 056109) and DIII-D (Holland et al 2017 Nucl. Fusion 57 066043) has shown that ion-scale gyrokinetics cannot adequately describe certain plasma conditions, and that in these instances multi-scale simulations may resolve the observed discrepancies. This work presents the first measurements of electron perturbative thermal diffusivity with partial sawteeth on ASDEX Upgrade (measuring values between 2 and 9 m2 s−1), and compares these measurements to those made with more established modulated electron cyclotron heating measurements, finding good agreement within experimental uncertainty. Perturbative diffusivity is found to scale inversely with collisionality across ASDEX Upgrade and Alcator C-Mod plasmas. Finally, perturbative diffusivity is used as a validation constraint in a study with the gyrokinetic code GENE, showing the first instance where an ion-scale gyrokinetic simulation can simultaneously match the experimental ion and electron heat fluxes and the perturbative thermal diffusivity. These results indicate that multi-scale effects may only be important in some plasmas, and the collisionality and ratio of high to low wavenumber linear growth rates may distinguish these cases.

Observation of the inward propagation of spontaneous toroidal flow from the plasma boundary in LHD

Spontaneous generation of toroidal flow from the separatrix and its inward radial propagation in association with the change in the electron temperature gradient have been observed near the plasma boundary in the modulated electron cyclotron heating (MECH) experiment in the Large Helical Device. The observations presented in this paper provide strongly the supports of the hypothesis for the conversion of the poloidal flow to the toroidal one at the plasma peripheral region, exhibiting a clear delay response on MECH in the time derivation of the toroidal flow (V̇ϕ≡∂Vϕ/∂t) to the poloidal one (V̇θ≡∂Vθ/∂t) at a normalized radius of ρ≈ 0.97. The ratio of order unity for |V̇ϕ/V̇θ| during a conversion phase from poloidal flow into toroidal one is consistent with that predicted by a quasi-stationary theoretical model based on the turbulent instability, regardless of its sign and spatial structure. The present work demonstrates a new dynamic response in the plasma momentum transport and represents a significant confirmation of its non-local nature.

Topology bifurcation of a magnetic flux surface in toroidal plasmas

The bifurcation of magnetic topology is identified by a significant change in the heat pulse propagation properties in magnetized plasmas. Clear evidence of stochastization of the magnetic surfaces near a rational surface is observed in the core plasma with weak magnetic shear in the Large Helical Device by slowly decreasing the magnetic shear and measured by applying heat pulses driven by modulated electron cyclotron heating (MECH). Three topologies of the magnetic flux surfaces (a nested magnetic island and partial and full stochastization) are identified by the patterns of heat pulse propagation observed in the flat temperature region in the plasma. Slow heat pulse propagation exhibiting a non-monotonically increasing delay time is evidence of a magnetic island, while the fast heat pulse propagation observed in the plasma with medium magnetic shear is evidence of the stochastization of the magnetic surfaces. The region with the fast heat pulse propagation varies with a slight change of magnetic shear. There are two types of stochastization of the magnetic surfaces. In one, the stochastization region is localized near the rational surface (partial stochastization) and in the other, the region is extended to the magnetic axis (full stochastization). The appearance of a stochastic magnetic field is not caused by MHD instability. The significant increase of the ratio of electron thermal diffusivity to ion thermal diffusivity is consistent with that expected by stochastization of the magnetic field.

Higher Harmonics in the Perturbative Transport Study in TJ-II ECH Plasma

We analyzed the higher harmonics of temperature perturbation in the modulated electron cyclotron heating experiment on TJ-II plasma. The higher harmonics (e.g., 5th harmonic) exhibited significantly weaker decay in amplitude as they propagated in radius as compared with the prediction by diffusive model. The change in the time derivative of temperature at the onset (and turning-off) of the heating power propagates in radius with very little temporal smoothening.

Topology bifurcation of a magnetic flux surface in magnetized plasmas

Bifurcation phenomena in the radial propagation of a heat pulse driven by modulated electron cyclotron heating are observed in the Large Helical Device. There are two patterns of heat pulse propagation observed in the flat temperature region. One is a bi-directional slow heat pulse propagation observed in the fast magnetic shear drop and the other is a fast heat pulse propagation observed in the slow magnetic shear drop. This bifurcation in the heat pulse propagation suggests the topology bifurcation of a magnetic flux surface in the plasma.

Testing gyrokinetic simulations of electron turbulence

An extensive set of tests comparing gyrokinetic predictions of temperature-gradient driven electron turbulence to power balance transport analyses and fluctuation measurements are presented. These tests use data from an L-mode validation study on the DIII-D tokamak (Luxon 2002 Nucl. Fusion 42 614) in which the local value of is varied by modulated electron cyclotron heating; the GYRO code (Candy and Waltz 2003 J. Comput. Phys. 186 545) is used to make the gyrokinetic predictions. Using a variety of novel measures, both local and global nonlinear simulations are shown to predict key characteristics of the electron energy flux Qe and long-wavelength (low-k) Te fluctuations, but systematically underpredict (by roughly a factor of two) the ion energy flux Qi. A new synthetic diagnostic for comparison to intermediate wavelength Doppler backscattering measurements is presented, and used to compare simulation predictions against experiment. In contrast to the agreement observed in the low-k Te fluctuation comparisons, little agreement is found between the predicted and measured intermediate-k density fluctuation responses. The results presented in this paper significantly expand upon those previously reported in DeBoo et al (2010 Phys. Plasmas 17 056105), comparing transport and multiple turbulence predictions from numerically converged local and global simulations for all four experimental heating configurations (instead of only fluxes and low-k Te fluctuations for one condition) to measurements and power balance analyses.

Dynamic Transport Study of Electron Thermal Energy in Nonlinear Fusion Plasma

In nuclear fusion plasmas, both thermal energy and particle transports governed by plasma turbulence are anomalously enhanced above neoclassical levels. Plasma turbulence induces various complex phenomena in transport processes, such as nonlinearity and nonlocality. Therefore, it is very important to clarify the relationship between plasma turbulence and anomalous transports. We have approached these complicated problems by analyzing the dynamics, which are recognized as temporal trajectories in a flux-gradient space, rather than using conventional power balance. In particular, in fusion research, it is critical to elucidate the mechanism of electron thermal energy transport, because the incoming burning plasmas are sustained by the heating of alpha particles. In Large Helical Device (LHD), the dynamic relationships between electron thermal fluxes and electron temperature gradients are investigated using modulated electron cyclotron heating and modern electron cyclotron emission diagnostic systems. Some trajectories, such as a hysteresis loop and a line segment with a steep slope, are observed in high-temperature LHD plasmas. Strong nonlinear properties in the transport are revealed by studying the dynamics.

Electron energy transport inferences from modulated electron cyclotron heating in DIII-D

Electron thermal energy transport in high-temperature plasmas is poorly understood. A useful characterization of the transport as energy flux versus temperature gradient may be obtained for a modest range of fluxes and gradients about an equilibrium by using modulated electron cyclotron heating. The transport is rapid and stiff (high χinc) at the outside, becoming slower and more diffusive toward the center.

Heat Pinches in Electron-Heated Tokamak Plasmas: Theoretical Turbulence Models versus Experiments

Two fluid turbulence models, the drift wave based quasilinear 1.5D Weiland model and the electromagnetic global 3D nonlinear model CUTIE, have been used to account for heat pinch evidence in off-axis modulated electron cyclotron heating experiments in the Rijnhuizen Tokamak Project. Both models reproduce the main features indicating inward heat convection in mildly off-axis cases. In far-off-axis cases with hollow electron temperature profiles, the existence of outward convection was reproduced only by CUTIE. Turbulence mechanisms driving heat convection in the two models are discussed.

Transition phenomena and thermal transport properties in LHD plasmas with an electron internal transport barrier

Two types of improved core confinement were observed during centrally focused electron cyclotron heating (ECH) into plasmas sustained by counter (CNTR) and Co neutral beam injections (NBI) in the Large Helical Device. The CNTR NBI plasma displayed transition phenomena to the high-electron-temperature state and had a clear electron internal transport barrier, while the Co NBI plasma did not show a clear transition or an ECH power threshold but showed broad high temperature profiles with moderate temperature gradient. This indicated that the Co NBI plasma with additional ECH also had an improved core confinement. The electron heat transport characteristics of these plasmas were directly investigated using heat pulse propagation excited by modulated ECH. These effects appear to be related to the m/n = 2/1 rational surface or the island induced by NBI beam-driven current.

Experimental Study of Trapped-Electron-Mode Properties in Tokamaks: Threshold and Stabilization by Collisions

Trapped electron modes are one of the candidates to explain turbulence driven electron heat transport observed in tokamaks. This instability has two characteristics: a threshold in normalized gradient and stabilization by collisions. Experiments using modulated electron cyclotron heating in the ASDEX Upgrade tokamak demonstrate explicitly the existence of the threshold. The stabilization with increasing collisionality is evidenced by a strong decrease of the propagation of heat pulses, explained by a transition to ion temperature gradient driven transport. These results are supported by linear gyrokinetic calculations.

Nonlinear pertubative electron heat transport study in the ASDEX Upgrade tokamak

Electron heat transport properties have been investigated by means of localized steady-state and modulated electron cyclotron heating in the ASDEX Upgrade tokamak. The experimental findings have been successfully compared with the model assumption of a threshold gradient length, LTc, (1/LT = −∇Te0/Te0), in the electron temperature Te0 below which the electron thermal conductivity measured under steady-state conditions, χePB, shows a sudden increase and the heat conduction ‘seen’ by a temperature perturbation, χeHP, switches from low to high values.In this work electron cyclotron heating (ECH) power is used both to modify the steady-state gradient length and to induce a nonlinear transition from low to high electron heat transport at the position where the temperature gradient crosses its critical (or threshold) value. Steady-state ECH has been injected at different radial positions and the changes induced in electron heat transport have been studied by means of cold pulses launched from the plasma boundary. Cold pulses, obtained by injecting radiating impurities by means of laser blow off of silicon, have proved to be a flexible tool for investigating both the radial position of the step in χeHP and the dependence on local plasma parameters of the underlying physical mechanism.

ITER-relevant H-mode physics at ASDEX Upgrade

A variety of techniques are being tested on ASDEX Upgrade for avoiding or reducing the divertor energy load caused by ELMs. Quiescent H-mode operation has been improved by operation with recently boronized vessel walls. Zeff values in QH-modes are now ∼2.5, comparable with those found in ELMy H-modes of similar density. Type II ELM operation has been generated in discharges with ICRF additional heating alone, thus demonstrating its compatibility with reactor-relevant low core momentum and particle input. Furthermore, Type II ELM operation at low (<1) values of edge collisionality has been achieved using strong plasma shaping and high input power. Modification of the ELM frequency and of the ELM energy has been demonstrated using pellet injection, supersonic pulsed gas injection, magnetic triggering via vertical position oscillations and modulated edge electron cyclotron heating and current drive. ELM control using pellet pace making is now being incorporated into integrated scenarios that aim to provide energy and particle exhaust compatible with ITER requirements while maximizing the core confinement.

Signal processing techniques based on singular value decomposition applied to modulated ECH experiments

The analysis and interpretation of perturbative modulated electron cyclotron heating (MECH) discharges for power deposition localization using different diagnostics, such as electron cyclotron emission (ECE) and soft x-ray (SXR) measurements, are presented. In particular, the experimental determination of the MECH power deposition is investigated, which is a fundamental requirement, for example, for heat transport studies. The most important problem is related to the coupling between the MECH and the sawtooth activity of the plasma, which disturbs both ECE and SXR measurements. Several techniques have been adopted to circumvent this difficulty. In particular, the singular value decomposition (SVD) and the generalized singular value decomposition (GSVD) have been tested. However, both methods are incapable of treating the problem correctly, which leads to potential misinterpretation of the results. A new method based on system identification using the SVD (SI-SVD) is developed and applied. This method, within reasonable limits induced by the assumption of linearity, is capable of simultaneously separating the MECH from the sawtooth contributions to both ECE electron temperature measurements and SXR emission measurements. Such a method is in particular applied to a neutral beam injection (NBI) heated ASDEX Upgrade discharge in which MECH is added in order to analyse electron heat transport in a mostly ion-heated plasma. Since the NBI heating is also partly modulated with short pulses which coincide with the sawtooth crashes to improve their stability, both the MECH and the NBI deposition profiles are determined. Moreover, treating the signals with the SI-SVD procedure enables a study of the plasma dynamic response also at higher MECH harmonic numbers.

Gradient length driven electron heat transport study in modulated electron cyclotron heating FTU tokamak

Perturbative and steady-state heat transport of FTU tokamak in current ramp-up discharges are investigated by means of modulated electron cyclotron heating (MECH). Perturbative and steady-state transport experiments are coherent with an electron heat transport which switches from low to high values when electron temperature gradient length reaches a threshold value 1/LTc. The threshold value 1/LTcis shown to be proportional to the ratio s/q.The experimental findings are compared to predictions of an empirical model based on the assumption of a threshold gradient length, LTc (1/LT = |∇Te/Te|), in the electron temperature Te below which electron thermal diffusivity, χe, switches from low to high values. Plasma responses to steady state and MECH are modelled assuming the electron diffusivity as χPB = χ0 + αTe3/2(1/LT-1/LTc)1/2; here Te3/2 reflects the gyro-Bohm assumption, χ0 represents the heat transport for 1/LT<1/LTc and the term (1/LT-1/LTc)1/2, which sets in for 1/LT>1/LTc, mimics an extra transport possibly due to electron temperature gradient (ETG) modes. In agreement with ETG threshold 1/LTc is shown to be correlated with the magnetic shear s.

Modulated ECH power absorption measurements usinga diamagnetic loop in the TCV tokamak

The additional power absorbed by the plasma can be determined from the time derivative of the total plasma energy, which can be estimated from the diamagnetic flux of the plasma using a diamagnetic loop. The main difficulty in using diamagnetic measurements to estimate the kinetic energy is the compensation of the flux measurement sensitivity to poloidal magnetic fields, which is not always easy to adjust. A method based on the temporal variations of the diamagnetic flux of the plasma during modulated electron cyclotron heating (MECH) has been developed. Using MECH has the advantage that these poloidal fields are not significantly modulated and a good compensation of these fields is not necessary. However, a good compensation of the vessel poloidal image current is crucial to ensure a sufficiently large bandwidth. The application of this diagnostic to studies of the extraordinary mode (X-mode) absorption at the third electron cyclotron harmonic frequency (X3) has been performed on the TCV tokamak in plasmas pre-heated by the X-mode at the second harmonic (X2). A MECH frequency scan has allowed the determination of an optimum modulation frequency, situated at about 200-250 Hz. Based on this diagnostic, full single-pass absorption of the injected X3 power was measured with the X2 pre-heating in co-current drive. This high absorption is more than a factor of 2 higher than that predicted by the linear ray tracing code TORAY. Experimental evidence indicates that a large fraction of the X3 power is absorbed by electrons in an energetic tail created by the X2 pre-heating.

Experimental tests of transport models using modulated electron cyclotron heating

Both the dynamic and equilibrium thermal responses of an L mode plasma torepetitive ECH heat pulses were measured and compared with predictions from severalthermal transport models. While no model consistently agreed with all observations,the GLF23 model was most consistent with the perturbed electron and ion temperatureresponses for one of the cases studied, which may indicate a key role played by electronmodes in the core of these discharges. Generally, the IIF and MM models performed wellfor the perturbed electron response while the GLF23 and IFS/PPPL models agreed with theperturbed ion response for all three cases studied. No single model agreed well withthe equilibrium temperature profiles measured.

`Non-local' response of RTP ohmic plasmas to peripheral perturbations

A `non-local' response of the plasma core triggered by peripheral plasma perturbations other than laser ablation is found in the RTP tokamak. Oblique pellet injection (OPI) has been used to induce fast cooling of the peripheral plasma. In response, an inward cold pulse (Te drop) and a slightly delayed core Te rise are observed. A somewhat similar `non-local' response is observed when the peripheral plasma is heated by modulated electron cyclotron heating or by fast current ramps, i.e. the core temperature drops in response to the peripheral heating. The plasma conditions for the occurrence of the `non-local' response have been investigated. The core Te rise following OPI is associated with the formation of a large temperature gradient in the region 1 < q < 2. The Te rise is largest at low electron density and for large pellet deposition radii. Above a critical density the Te rise disappears and only the (weaker) drop in core Te is observed. Time dependent transport simulations show that the propagation of the inward cold pulse is consistent with local transport, while the core Te rise is a slower phenomenon requiring a large transient drop of χe in the region 1 < q < 2.

Nonlocal Transient Transport and Thermal Barriers in Rijnhuizen Tokamak Project Plasmas

In the Rijnhuizen Tokamak Project plasmas, a transient rise of the core electron temperature is observed when hydrogen pellets are injected tangentially to induce fast cooling of the peripheral region. High-resolution Thomson scattering measurements show that the ${T}_{e}$ rise is associated with large temperature gradients in the region $1&lt;q&lt;2$. This region acts as a layer of transiently increased thermal resistivity (transport barrier) when probed by fast heat pulses from modulated electron cyclotron heating.