Showing papers by "Princeton Plasma Physics Laboratory published in 2019"

Predicting disruptive instabilities in controlled fusion plasmas through deep learning

Abstract: Nuclear fusion power delivered by magnetic-confinement tokamak reactors holds the promise of sustainable and clean energy1. The avoidance of large-scale plasma instabilities called disruptions within these reactors2,3 is one of the most pressing challenges4,5, because disruptions can halt power production and damage key components. Disruptions are particularly harmful for large burning-plasma systems such as the multibillion-dollar International Thermonuclear Experimental Reactor (ITER) project6 currently under construction, which aims to be the first reactor that produces more power from fusion than is injected to heat the plasma. Here we present a method based on deep learning for forecasting disruptions. Our method extends considerably the capabilities of previous strategies such as first-principles-based5 and classical machine-learning7–11 approaches. In particular, it delivers reliable predictions for machines other than the one on which it was trained—a crucial requirement for future large reactors that cannot afford training disruptions. Our approach takes advantage of high-dimensional training data to boost predictive performance while also engaging supercomputing resources at the largest scale to improve accuracy and speed. Trained on experimental data from the largest tokamaks in the United States (DIII-D12) and the world (Joint European Torus, JET13), our method can also be applied to specific tasks such as prediction with long warning times: this opens up the possibility of moving from passive disruption prediction to active reactor control and optimization. These initial results illustrate the potential for deep learning to accelerate progress in fusion-energy science and, more generally, in the understanding and prediction of complex physical systems. Using data from plasma-based tokamak nuclear reactors in the US and Europe, a machine-learning approach based on deep neural networks is taught to forecast disruptions, even those in machines on which the algorithm was not trained.

Overview of first Wendelstein 7-X high-performance operation

Abstract: The optimized superconducting stellarator device Wendelstein 7-X (with major radius $R=5.5\,\mathrm{m}$, minor radius $a=0.5\,\mathrm{m}$, and $30\,\mathrm{m}^3$ plasma volume) restarted operation after the assembly of a graphite heat shield and 10 inertially cooled island divertor modules. This paper reports on the results from the first high-performance plasma operation. Glow discharge conditioning and ECRH conditioning discharges in helium turned out to be important for density and edge radiation control. Plasma densities of $1-4.5\cdot 10^{19}\,\mathrm{m}^{-3}$ with central electron temperatures $5-10\,\mathrm{keV}$ were routinely achieved with hydrogen gas fueling, frequently terminated by a radiative collapse. Plasma densities up to $1.4\cdot 10^{20}\,\mathrm{m}^{-3}$were reached with hydrogen pellet injection and helium gas fueling. Here, the ions are indirectly heated, and at a central density of $8\cdot 10^{19}\,\mathrm{m}^{-3}$ a temperature of $3.4\,\mathrm{keV}$ with $T_e/T_i=1$ was accomplished, which corresponds to $nT_i(0)\tau_E=6.4\cdot 10^{19}\,\mathrm{keVs}/\mathrm{m}^3$ with a peak diamagnetic energy of $1.1\,\mathrm{MJ}$. The discharge behaviour has further improved with boronization of the wall. After boronization, the oxygen impurity content was reduced by a factor of 10, the carbon impurity content by a factor of 5. The reduced (edge) plasma radiation level gives routinely access to higher densities without radiation collapse, e.g. well above $1\cdot 10^{20}\,\mathrm{m}^{-2}$ line integrated density and $T_e=T_i=2\,\mathrm{keV}$ central temperatures at moderate ECRH power. Both X2 and O2 mode ECRH schemes were successfully applied. Core turbulence was measured with a phase contrast imaging diagnostic and suppression of turbulence during pellet injection was observed.

Neutrino physics with the PTOLEMY project: Active neutrino properties and the light sterile case

Abstract: The PTOLEMY project aims to develop a scalable design for a Cosmic Neutrino Background (CNB) detector, the first of its kind and the only one conceived that can look directly at the image of the Un ...

Explosive Magnetotail Activity

Abstract: Modes and manifestations of the explosive activity in the Earth’s magnetotail, as well as its onset mechanisms and key pre-onset conditions are reviewed. Two mechanisms for the generation of the pre-onset current sheet are discussed, namely magnetic flux addition to the tail lobes, or other high-latitude perturbations, and magnetic flux evacuation from the near-Earth tail associated with dayside reconnection. Reconnection onset may require stretching and thinning of the sheet down to electron scales. It may also start in thicker sheets in regions with a tailward gradient of the equatorial magnetic field $B_{z}$ ; in this case it begins as an ideal-MHD instability followed by the generation of bursty bulk flows and dipolarization fronts. Indeed, remote sensing and global MHD modeling show the formation of tail regions with increased $B_{z}$ , prone to magnetic reconnection, ballooning/interchange and flapping instabilities. While interchange instability may also develop in such thicker sheets, it may grow more slowly compared to tearing and cause secondary reconnection locally in the dawn-dusk direction. Post-onset transients include bursty flows and dipolarization fronts, micro-instabilities of lower-hybrid-drift and whistler waves, as well as damped global flux tube oscillations in the near-Earth region. They convert the stretched tail magnetic field energy into bulk plasma acceleration and collisionless heating, excitation of a broad spectrum of plasma waves, and collisional dissipation in the ionosphere. Collisionless heating involves ion reflection from fronts, Fermi, betatron as well as other, non-adiabatic, mechanisms. Ionospheric manifestations of some of these magnetotail phenomena are discussed. Explosive plasma phenomena observed in the laboratory, the solar corona and solar wind are also discussed.

Performance of Wendelstein 7-X stellarator plasmas during the first divertor operation phase

Abstract: Wendelstein 7-X is the first comprehensively optimized stellarator aiming at good confinement with plasma parameters relevant to a future stellarator power plant. Plasma operation started in 2015 using a limiter configuration. After installing an uncooled magnetic island divertor, extending the energy limit from 4 to 80 MJ, operation continued in 2017. For this phase, the electron cyclotron resonance heating (ECRH) capability was extended to 7 MW, and hydrogen pellet injection was implemented. The enhancements resulted in the highest triple product (6.5 × 1019 keV m−3 s) achieved in a stellarator until now. Plasma conditions [Te(0) ≈ Ti(0) ≈ 3.8 keV, τE > 200 ms] already were in the stellarator reactor-relevant ion-root plasma transport regime. Stable operation above the 2nd harmonic ECRH X-mode cutoff was demonstrated, which is instrumental for achieving high plasma densities in Wendelstein 7-X. Further important developments include the confirmation of low intrinsic error fields, the observation of current-drive induced instabilities, and first fast ion heating and confinement experiments. The efficacy of the magnetic island divertor was instrumental in achieving high performance in Wendelstein 7-X. Symmetrization of the heat loads between the ten divertor modules could be achieved by external resonant magnetic fields. Full divertor power detachment facilitated the extension of high power plasmas significantly beyond the energy limit of 80 MJ.

Overview of the JET preparation for deuterium–tritium operation with the ITER like-wall

Abstract: For the past several years, the JET scientific programme (Pamela et al 2007 Fusion Eng. Des. 82 590) has been engaged in a multi-campaign effort, including experiments in D, H and T, leading up to 2020 and the first experiments with 50%/50% D–T mixtures since 1997 and the first ever D–T plasmas with the ITER mix of plasma-facing component materials. For this purpose, a concerted physics and technology programme was launched with a view to prepare the D–T campaign (DTE2). This paper addresses the key elements developed by the JET programme directly contributing to the D–T preparation. This intense preparation includes the review of the physics basis for the D–T operational scenarios, including the fusion power predictions through first principle and integrated modelling, and the impact of isotopes in the operation and physics of D–T plasmas (thermal and particle transport, high confinement mode (H-mode) access, Be and W erosion, fuel recovery, etc). This effort also requires improving several aspects of plasma operation for DTE2, such as real time control schemes, heat load control, disruption avoidance and a mitigation system (including the installation of a new shattered pellet injector), novel ion cyclotron resonance heating schemes (such as the threeions scheme), new diagnostics (neutron camera and spectrometer, active Alfven eigenmode antennas, neutral gauges, radiation hard imaging systems…) and the calibration of the JET neutron diagnostics at 14 MeV for accurate fusion power measurement. The active preparation of JET for the 2020 D–T campaign provides an incomparable source of information and a basis for the future D–T operation of ITER, and it is also foreseen that a large number of key physics issues will be addressed in support of burning plasmas.

Hybrid-kinetic Simulations of Ion Heating in Alfvénic Turbulence

Abstract: We present three-dimensional, hybrid-kinetic numerical simulations of driven Alfven-wave turbulence of relevance to the collisionless near-Earth solar wind. Special attention is paid to the spectral transition that occurs near the ion-Larmor scale and to the origins of preferential perpendicular ion heating and of non-thermal wings in the parallel distribution function. Several novel diagnostics are used to show that the ion heating rate increases as the kinetic-Alfven-wave fluctuations, which comprise the majority of the sub-ion-Larmor turbulent cascade, attain near-ion-cyclotron frequencies. We find that ${\\approx}75$-$80\\%$ of the cascade energy goes into heating the ions, broadly consistent with the near-Earth solar wind. This heating is accompanied by clear velocity-space signatures in the particle energization rates and the distribution functions, including a flattened core in the perpendicular-velocity distribution and non-Maxwellian wings in the parallel-velocity distribution. The latter are attributed to transit-time damping and the pitch-angle scattering of perpendicularly heated particles into the parallel direction. Accompanying these features is a steepening of the spectral index of sub-ion-Larmor magnetic-field fluctuations beyond the canonical $-2.8$, as field energy is transferred to thermal energy. These predictions may be tested by measurements in the near-Earth solar wind.

Overview of the recent experimental research on the J-TEXT tokamak

Abstract: Recent J-TEXT research has highlighted the significance of the role that non-axisymmetric magnetic perturbations, so called 3D magnetic perturbation (MP) fields, play in fundamentally 2D concept, i.e. tokamak. This paper presents the J-TEXT results achieved over the last two years, especially on the impacts of 3D MP fields on MHD instabilities, plasma disruptions and plasma turbulence transport. On J-TEXT, the resonant MPs (RMPs) system, capable of providing either a static or a high frequency (up to 8 kHz) rotating RMP field, has been upgraded by adding a new set of 12 in-vessel saddle coils. The shattered pellet injection (SPI) system has been built in J-TEXT in the spring of 2018. The new capabilities advance J-TEXT to be a forefront of international magnetic fusion facilities, allow a flexible study of 3D effects and disruption mitigation in a tokamak. The fast rotating RMP field has been successfully applied for avoidance of mode locking and the prevention of plasma disruption. A new control strategy, which applies pulsed RMP to the tearing mode only during the accelerating phase region, was proved by nonlinear numerical modelling to be efficient in accelerating mode rotation and even completely suppresses the mode. Remarkably, the rotating tearing mode was completely suppressed by the electrode biasing. The impacts of 3D magnetic topology on the turbulences have been investigated on J-TEXT. It is found that the fluctuations of electron density, electron temperature, and plasma potential can be significantly modulated by the island structure, and a larger fluctuation level appears at the X-point of islands. The suppression of runaway electrons during disruptions is essential to the operation of ITER, and it has been reached by utilizing the 3D magnetic perturbations on J-TEXT. This may provide an alternative mechanism of runaway suppression for large-scale tokamak and ITER.

2D axial-azimuthal particle-in-cell benchmark for low-temperature partially magnetized plasmas

Abstract: The increasing need to demonstrate the correctness of computer simulations has highlighted the importance of benchmarks. We define in this paper a representative simulation case to study low-temperature partially-magnetized plasmas. Seven independently developed Particle-In-Cell codes have simulated this benchmark case, with the same specified conditions. The characteristics of the codes used, such as implementation details or computing times and resources, are given. First, we compare at steady-state the time-averaged axial profiles of three main discharge parameters (axial electric field, ion density and electron temperature). We show that the results obtained exhibit a very good agreement within 5% between all the codes. As ExB discharges are known to cause instabilities propagating in the direction of electron drift, an analysis of these instabilities is then performed and a similar behaviour is retrieved between all the codes. A particular attention has been paid to the numerical convergence by varying the number of macroparticles per cell and we show that the chosen benchmark case displays a good convergence. Detailed outputs are given in the supplementary data, to be used by other similar codes in the perspective of code verification. 2D axial-azimuthal Particle-In-Cell benchmark for low-temperature partially ..

Machine learning for disruption warnings on Alcator C-Mod, DIII-D, and EAST

Abstract: This paper reports on disruption prediction using a shallow machine learning method known as a random forest, trained on large databases containing only plasma parameters that are available in real-time on Alcator C-Mod, DIII-D, and EAST. The database for each tokamak contains parameters sampled ~106 times throughout ~104 discharges (disruptive and non-disruptive) over the last four years of operation. It is found that a number of parameters (e.g. , , , ) exhibit changes in aggregate as a disruption is approached on one or more of these tokamaks. However, for each machine, the most useful parameters, as well as the details of their precursor behaviors, are markedly different. When the prediction problem is framed using a binary classification scheme to discriminate between time slices 'close to disruption' and 'far from disruption', it is found that the prediction algorithms differ substantially in performance among the three machines on a time slice-by-time slice basis, but have similar disruption detection rates (~80%–90%) on a shot-by-shot basis after appropriate optimisation. This could have important implications for disruption prediction and avoidance on ITER, for which development of a training database of disruptions may be infeasible. The algorithm's output is interpretable using a method that identifies the most strongly contributing input signals, which may have implications for avoiding disruptive scenarios. To further support its real-time capability, successful applications in inter-shot and real-time environments on EAST and DIII-D are also discussed.

Integrated operation of steady-state long-pulse H-mode in Experimental Advanced Superconducting Tokamak

Abstract: Recent Experimental Advanced Superconducting Tokamak (EAST) experiments have successfully demonstrated a long-pulse steady-state scenario with improved plasma performance through integrated operation since the last IAEA FEC in 2016. A discharge with a duration over 100 s using pure radio frequency (RF) power heating and current drive has been obtained with the required characteristics for future long-pulse tokamak reactors such as good energy confinement quality (H98y2 ~ 1.1) with electron internal transport barrier inside ρ < 0.4, small ELMs (frequency ~100–200 Hz), and good control of impurity and heat exhaust with the tungsten divertor. The optimization of X-point, plasma shape, the outer gap and local gas puffing near the low hybrid wave (LHW) antenna were integrated with global parameters of B T and line-averaged electron density for higher current drive efficiency of LHW and on-axis deposition of electron cyclotron heating in the long-pulse operation. More recently, a high β P RF-only discharge (β P ~ 1.9 and β N ~ 1.5, /n GW ~ 0.80, f bs ~ 45% at q 95 ~ 6.8) was successfully maintained over 24 s with improved hardware capabilities, demonstrating performance levels needed for the China Fusion Engineering Test Reactor steady-state operation. A higher energy confinement is observed at higher β P and with favorable toroidal field direction. Towards the next goal (≥400 s long-pulse H-mode operations with ~50% bootstrap current fraction) on EAST, an integrated control of the current density profile, pressure profile and radiated divertor will be addressed in the near future.

Progress in disruption prevention for ITER

Abstract: E.J. Strait, J.L. Barr, M. Baruzzo, J.W. Berkery, R.J. Buttery, P.C. de Vries, N.W. Eidietis, R.S. Granetz, J.M. Hanson, C.T. Holcomb, D.A. Humphreys, J.H. Kim, E. Kolemen, M. Kong, M.J. Lanctot, M. Lehnen, E. Lerche, N.C. Logan, M. Maraschek, M. Okabayashi, J.K. Park, A. Pau, G. Pautasso, F.M. Poli, C. Rea, S.A. Sabbagh, O. Sauter, E. Schuster, U.A. Sheikh, C. Sozzi, F. Turco, A.D. Turnbull, Z.R. Wang, W.P. Wehner, L. Zeng

Real-time capable modeling of neutral beam injection on NSTX-U using neural networks

Abstract: A new model of heating, current drive, torque and other effects of neutral beam injection on NSTX-U that uses neural networks has been developed. The model has been trained and tested on the results of the Monte Carlo code NUBEAM for the database of experimental discharges taken during the first operational campaign of NSTX-U. By projecting flux surface quantities onto empirically derived basis functions, the model is able to efficiently and accurately reproduce the behavior of both scalars, like the total neutron rate and shine through, and profiles, like beam current drive and heating. The model has been tested on the NSTX-U real-time computer, demonstrating a rapid execution time orders of magnitude faster than the Monte Carlo code that is well suited for the iterative calculations needed to interpret experimental results, optimization during scenario development activities, and real-time plasma control applications. Simulation results of a proposed design for a nonlinear observer that embeds the neural network calculations to estimate the poloidal flux profile evolution, as well as and fast ion diffusivity, are presented.

The density dependence of edge-localized-mode suppression and pump-out by resonant magnetic perturbations in the DIII-D tokamak

Abstract: The density dependence of edge-localized-mode (ELM) suppression and density pump-out (density reduction) by n = 2 resonant magnetic perturbations (RMPs) is consistent with the effects of narrow well-separated magnetic islands at the top and bottom of the H-mode pedestal in DIII-D low-collisionality plasmas. Nonlinear two-fluid MHD simulations for DIII-D ITER similar shape discharges show that, at low collisionality (ν*e < 0.5), low pedestal density is required for resonant field penetration at the pedestal top (ne,ped ≈ 2.5 × 1019 m−3 at ψN ≈ 0.93), consistent with the ubiquitous low density requirement for ELM suppression in these DIII-D plasmas. The simulations predict a drop in the pedestal pressure due to parallel transport across these narrow width (ΔψN ≈ 0.02) magnetic islands at the top of the pedestal that is stabilizing to Peeling-Ballooning-Modes and comparable to the pedestal pressure reduction observed in experiment at the onset of ELM suppression. The simulations predict density pump-out at experimentally relevant levels (Δne/ne ≈ −20%) at low pedestal collisionality (ν*e ≈ 0.1) due to very narrow (ΔψN ≈ 0.01–0.02) RMP driven magnetic islands at the pedestal foot at ψN ≈ 0.99. The simulations show decreasing pump-out with increasing density, consistent with experiment, resulting from the inverse dependence of parallel particle transport on collisionality at the foot of the pedestal. The robust screening of resonant fields is predicted between the top and bottom of the pedestal during density pump-out and ELM suppression, consistent with the preservation of strong temperature gradients in the edge transport barrier as seen in experiment.The density dependence of edge-localized-mode (ELM) suppression and density pump-out (density reduction) by n = 2 resonant magnetic perturbations (RMPs) is consistent with the effects of narrow well-separated magnetic islands at the top and bottom of the H-mode pedestal in DIII-D low-collisionality plasmas. Nonlinear two-fluid MHD simulations for DIII-D ITER similar shape discharges show that, at low collisionality (ν*e < 0.5), low pedestal density is required for resonant field penetration at the pedestal top (ne,ped ≈ 2.5 × 1019 m−3 at ψN ≈ 0.93), consistent with the ubiquitous low density requirement for ELM suppression in these DIII-D plasmas. The simulations predict a drop in the pedestal pressure due to parallel transport across these narrow width (ΔψN ≈ 0.02) magnetic islands at the top of the pedestal that is stabilizing to Peeling-Ballooning-Modes and comparable to the pedestal ...

High fusion performance in Super H-mode experiments on Alcator C-Mod and DIII-D

Abstract: The 'Super H-Mode' regime is predicted to enable pedestal height and fusion performance substantially higher than standard H-Mode operation. This regime exists due to a bifurcation of the pedestal pressure, as a function of density, that is predicted by the EPED model to occur in strongly shaped plasmas above a critical pedestal density. Experiments on Alcator C-Mod and DIII-D have achieved access to the Super H-Mode (and Near Super H) regime, and obtained very high pedestal pressure, including the highest achieved on a tokamak (p ped ∼ 80 kPa) in C-Mod experiments operating near the ITER magnetic field. DIII-D Super H experiments have demonstrated strong performance, including the highest stored energy in the present configuration of DIII-D (W ∼ 2.2-3.2 MJ), while utilizing only about half of the available heating power (P heat ∼ 7-12 MW). These DIII-D experiments have obtained the highest value of peak fusion gain, Q DT,equiv ∼ 0.5, achieved on a medium scale (R < 2 m) tokamak. Sustained high performance operation (β N ∼ 2.9, H98 ∼ 1.6) has been achieved utilizing n = 3 magnetic perturbations for density and impurity control. Pedestal and global confinement has been maintained in the presence of deuterium and nitrogen gas puffing, which enables a more radiative divertor condition. A pair of simple performance metrics is developed to assess and compare regimes. Super H-Mode access is predicted for ITER and expected, based on both theoretical prediction and observed normalized performance, to allow ITER to achieve its goals (Q = 10) at I p < 15 MA, and to potentially enable more compact, cost effective pilot plant and reactor designs.

Overview of physics studies on ASDEX upgrade

Abstract: The ASDEX Upgrade (AUG) programme, jointly run with the EUROfusion MST1 task force, continues to significantly enhance the physics base of ITER and DEMO. Here, the full tungsten wall is a key asset for extrapolating to future devices. The high overall heating power, flexible heating mix and comprehensive diagnostic set allows studies ranging from mimicking the scrape-off-layer and divertor conditions of ITER and DEMO at high density to fully non-inductive operation (q 95 = 5.5, ) at low density. Higher installed electron cyclotron resonance heating power 6 MW, new diagnostics and improved analysis techniques have further enhanced the capabilities of AUG. Stable high-density H-modes with MW m-1 with fully detached strike-points have been demonstrated. The ballooning instability close to the separatrix has been identified as a potential cause leading to the H-mode density limit and is also found to play an important role for the access to small edge-localized modes (ELMs). Density limit disruptions have been successfully avoided using a path-oriented approach to disruption handling and progress has been made in understanding the dissipation and avoidance of runaway electron beams. ELM suppression with resonant magnetic perturbations is now routinely achieved reaching transiently . This gives new insight into the field penetration physics, in particular with respect to plasma flows. Modelling agrees well with plasma response measurements and a helically localised ballooning structure observed prior to the ELM is evidence for the changed edge stability due to the magnetic perturbations. The impact of 3D perturbations on heat load patterns and fast-ion losses have been further elaborated. Progress has also been made in understanding the ELM cycle itself. Here, new fast measurements of and E r allow for inter ELM transport analysis confirming that E r is dominated by the diamagnetic term even for fast timescales. New analysis techniques allow detailed comparison of the ELM crash and are in good agreement with nonlinear MHD modelling. The observation of accelerated ions during the ELM crash can be seen as evidence for the reconnection during the ELM. As type-I ELMs (even mitigated) are likely not a viable operational regime in DEMO studies of 'natural' no ELM regimes have been extended. Stable I-modes up to have been characterised using -feedback. Core physics has been advanced by more detailed characterisation of the turbulence with new measurements such as the eddy tilt angle - measured for the first time - or the cross-phase angle of and fluctuations. These new data put strong constraints on gyro-kinetic turbulence modelling. In addition, carefully executed studies in different main species (H, D and He) and with different heating mixes highlight the importance of the collisional energy exchange for interpreting energy confinement. A new regime with a hollow profile now gives access to regimes mimicking aspects of burning plasma conditions and lead to nonlinear interactions of energetic particle modes despite the sub-Alfvenic beam energy. This will help to validate the fast-ion codes for predicting ITER and DEMO.

Promising High-Confinement Regime for Steady-State Fusion.

Abstract: A reproducible stationary high-confinement regime with small ``edge-localized modes'' (ELMs) has been achieved recently in the Experimental Advanced Superconducting Tokamak, which has a metal wall and low plasma rotation as projected for a fusion reactor. We have uncovered that this small ELM regime is enabled by a wide edge transport barrier (pedestal) with a low density gradient and a high density ratio between the pedestal foot and top. Nonlinear simulations reveal, for the first time, that the underlying mechanism for the observed small ELM crashes is the upper movement of the peeling boundary induced by an initial radially localized collapse in the pedestal, which stops the growth of instabilities and further collapse of the pedestal, thus providing a physics basis for mitigating ELMs in future steady-state fusion reactors.

Pressure Tensor Elements Breaking the Frozen-In Law During Reconnection in Earth's Magnetotail.

Abstract: Aided by fully kinetic simulations, spacecraft observations of magnetic reconnection in Earth's magnetotail are analyzed. The structure of the electron diffusion region is in quantitative agreement with the numerical model. Of special interest, the spacecraft data reveal how reconnection is mediated by off-diagonal stress in the electron pressure tensor breaking the frozen-in law of the electron fluid.

Verification and validation of integrated simulation of energetic particles in fusion plasmas

Abstract: This paper reports verification and validation of linear simulations of Alfvén eigenmodes in the current ramp phase of DIII-D L-mode discharge #159243 using gyrokinetic, gyrokineticMHD hybrid, and eigenvalue codes. Using a classical fast ion profile, all simulation codes find that reversed shear Alfvén eigenmodes (RSAE) are the dominant instability. The real frequencies from all codes have a coefficient of variation of less than 5% for the most unstable modes with toroidal mode number n = 4 and 5. The simulated RSAE frequencies agree with experimental measurements if the minimum safety factor qmin is adjusted, within experimental errors. The simulated growth rates exhibit greater variation, and simulations find that pressure gradients of thermal plasmas make a significant contribution to the growth rates. Mode structures of the dominant modes agree well among all codes. Moreover, using a calculated fast ion profile that takes into account the diffusion by multiple unstable modes, a toroidal Alfvén eigenmode (TAE) with n = 6 is found to be unstable in the outer edge, consistent with the experimental observations. Variations of the real frequencies and growth rates of the TAE are slightly larger than those of the RSAE. Finally, electron temperature fluctuations and radial phase shifts from simulations show no significant differences with the experimental data for the strong n = 4 RSAE, but significant differences for the weak n = 6 TAE. The verification and validation for the linear Alfvén eigenmodes is the first step to develop an integrated simulation of energetic particles confinement in burning plasmas incorporating multiple physical processes. Nuclear Fusion Verification and validation of integrated simulation of energetic particles in fusion plasmas S. Taimourzadeh et al Verification and validation of integrated simulation of energetic particles in fusion plasmas Printed in the UK 066006 NUFUAU © EURATOM 2019 59 Nucl. Fusion

Electron-cyclotron-resonance heating in Wendelstein 7-X: A versatile heating and current-drive method and a tool for in-depth physics studies

Abstract: For stellarators, which need no or only small amounts of current drive, electron-cyclotron-resonance heating (ECRH) is a promising heating method even for the envisaged application in a fusion power plant. Wendelstein 7-X (W7-X) is equipped with a steady-state capable ECRH system, operating at 140 GHz, which corresponds to the 2nd cyclotron harmonic of the electrons at a magnetic field of 2.5 T. Ten gyrotrons are operational and already delivered 7 MW to W7-X plasmas. Combined with pellet injection, the highest triple product (0.68 ×1020 keV m-3 s), observed up to now in stellarators, was achieved (Sunn Pedersen et al 2018 Plasma Phys. Control. Fusion 61 014035). For the first time, W7-X plasmas were sustained by 2nd harmonic O-mode heating, approaching the collisionality regime for which W7-X was optimized. Power deposition scans did not show any indication of electron temperature profile resilience. In low-density, low-power plasmas a compensation of the bootstrap current with electron-cyclotron current drive (ECCD) was demonstrated. Sufficiently strong ECCD close to the plasma centre produced periodic internal plasma-crash events, which coincide with the appearance of low order rationals of the rotational transform.

A Unifying Framework for Spectrum-Preserving Graph Sparsification and Coarsening

Abstract: How might one ``reduce'' a graph? That is, generate a smaller graph that preserves the global structure at the expense of discarding local details? There has been extensive work on both graph sparsification (removing edges) and graph coarsening (merging nodes, often by edge contraction); however, these operations are currently treated separately. Interestingly, for a planar graph, edge deletion corresponds to edge contraction in its planar dual (and more generally, for a graphical matroid and its dual). Moreover, with respect to the dynamics induced by the graph Laplacian (e.g., diffusion), deletion and contraction are physical manifestations of two reciprocal limits: edge weights of $0$ and $\infty$, respectively. In this work, we provide a unifying framework that captures both of these operations, allowing one to simultaneously sparsify and coarsen a graph while preserving its large-scale structure. The limit of infinite edge weight is rarely considered, as many classical notions of graph similarity diverge. However, its algebraic, geometric, and physical interpretations are reflected in the Laplacian pseudoinverse $\mat{L}^\dagger$, which remains finite in this limit. Motivated by this insight, we provide a probabilistic algorithm that reduces graphs while preserving $\mat{L}^\dagger$, using an unbiased procedure that minimizes its variance. We compare our algorithm with several existing sparsification and coarsening algorithms using real-world datasets, and demonstrate that it more accurately preserves the large-scale structure.

Compact steady-state tokamak performance dependence on magnet and core physics limits

Abstract: Compact tokamak fusion reactors using advanced high-temperature superconducting magnets for the toroidal field coils have received considerable recent attention due to the promise of more compact d...

Gyrokinetic analysis and simulation of pedestals to identify the culprits for energy losses using 'fingerprints'

Abstract: Fusion performance in tokamaks hinges critically on the efficacy of the edge transport barrier (ETB) in suppressing energy losses. The new concept of 'fingerprints' is introduced to identify the in ...

First Divertor Physics Studies in Wendelstein 7-X

Abstract: This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014–2018 and 2019–2020 under grant agreement No. 633053.

Scalings for Tokamak Energy Confinement

Abstract: On the basis of an analysis of the ITER L-mode energy confinement database, two new scaling expressions for tokamak L-mode energy confinement are proposed, namely a power law scaling and an offset-linear scaling. The analysis indicates that the present multiplicity of scaling expressions for the energy confinement time TE in tokamaks (Goldston, Kaye, Odajima-Shimomura, Rebut-Lallia, etc.) is due both to the lack of variation of a key parameter combination in the database, fs = 0.32 R a^.75 k^ 5 ~ A a^.25 k^.5, and to variations in the dependence of rE on the physical parameters among the different tokamaks in the database. By combining multiples of fs and another factor, fq = 1.56 a^2 kB/R Ip = qeng/3.2, which partially reflects the tokamak to tokamak variation of the dependence of TE on q and therefore implicitly the dependence of TE on Ip and n,., the two proposed confinement scaling expressions can be transformed to forms very close to most of the common scaling expressions. To reduce the multiplicity of the scalings for energy confinement, the database must be improved by adding new data with significant variations in fs, and the physical reasons for the tokamak to tokamak variation of some of the dependences of the energy confinement time on tokamak parameters must be clarified

Overview of KSTAR research progress and future plans toward ITER and K-DEMO

Abstract: A decade long operation of KSTAR has contributed significantly to operation of superconducting tokamak device and advancement of tokamak physics which will be beneficial for the ITER and K-DEMO programs. Even with limited heating capability, various conventional as well as new operating regimes have been explored and achieved great performance. As examples, a long pulse H-mode operation with and without the ELM-crash was well over 60 seconds and over 30 seconds, respectively. Unique capabilities of KSTAR allowed to improve control capability of harmful instabilities and have been instrumental for many exciting new physics. The intricate IVCC system has been a great perturbation tool in studying of threshold power of the L/H transition in the presence of non-axisymmetric fields, rotation physics due to NTV effects, heat dispersal in divertor system and predictive control of the ELM-crash with a priori modeling. The state-of-the-art 2D/3D microwave imaging systems uncovered many new exciting new physics in the MHD and turbulence transport physics. They are validation of q0agt;1 right after the sawtooth crash, self-consistent asymmetric distribution of plasma turbulence amplitude and flow in the presence of 2/1 island, and underlying physics of low frequency turbulence induced by 3D resonant fields in suppression of the ELM-crash through non-linear interaction with the ELMs. In the turbulence area, non-diffusive "avalanche" transport event and role of quiescent coherent mode in confinement are studied. To accommodate anticipating higher performance of the KSTAR plasmas with the increased heating powers (NBI and ECH), new divertor/internal interface with full active cooling system will be implemented after the increased heating power systems and new current drive systems are fully tested. Upgrade plan for the internal hardware and efficient current drive system may allow a long pulse operation of higher performance plasmas at βNagt;3.0 with fbs~0.5 and Tiagt;10keV. a#13; a#13;

Quasioptical modeling of wave beams with and without mode conversion. I. Basic theory

Abstract: This work opens a series of papers where we develop a general quasi-optical theory for mode-converting electromagnetic beams in plasma and implement it in a numerical algorithm. Here, the basic theory is introduced. We consider a general quasimonochromatic multicomponent wave in a weakly inhomogeneous linear medium with no sources. For any given dispersion operator that governs the wave field, we explicitly calculate the approximate operator that governs the wave envelope ψ to the second order in the geometrical-optics parameter. Then, we further simplify this envelope operator by assuming that the gradient of ψ transverse to the local group velocity is much larger than the corresponding parallel gradient. This leads to a parabolic differential equation for ψ (“quasioptical equation”) on the basis of the geometrical-optics polarization vectors. Scalar and mode-converting vector beams are described on the same footing. We also explain how to apply this model to electromagnetic waves in general. In the next papers of this series, we report successful quasioptical modeling of radio frequency wave beams in magnetized plasma based on this theory.This work opens a series of papers where we develop a general quasi-optical theory for mode-converting electromagnetic beams in plasma and implement it in a numerical algorithm. Here, the basic theory is introduced. We consider a general quasimonochromatic multicomponent wave in a weakly inhomogeneous linear medium with no sources. For any given dispersion operator that governs the wave field, we explicitly calculate the approximate operator that governs the wave envelope ψ to the second order in the geometrical-optics parameter. Then, we further simplify this envelope operator by assuming that the gradient of ψ transverse to the local group velocity is much larger than the corresponding parallel gradient. This leads to a parabolic differential equation for ψ (“quasioptical equation”) on the basis of the geometrical-optics polarization vectors. Scalar and mode-converting vector beams are described on the same footing. We also explain how to apply this model to electromagnetic waves in general. In the next...