Alcator C-Mod Research Articles

Cross machine investigation of magnetic tokamak dust: Morphological and elemental analysis

The presence of magnetic dust can be an important issue for future fusion reactors where plasma breakdown is critical. Magnetic dust has been collected from contemporary fusion devices (FTU, Alcator C-Mod, COMPASS and DIII-D) that feature different plasma facing components. The results of morphological and elemental analysis are presented. Magnetic dust is based on steel or nickel alloys and its magnetism is generated by intense plasma-material interactions. In spite of the strong similarities in terms of morphology and composition, X-ray diffraction analysis revealed differences in the structural evolution that leads to non-trivial magnetic responses.

Design of Cooling Water System (CWS) for CRAFT Lower Hybrid Current Driven (LHCD) System

Lower hybrid current driven (LHCD) system, as the most efficient non-inductive current drive method in the tokamak, is an integral part of the Comprehensive Research Facility for Fusion Technology (CRAFT). The cooling water system (CWS) is necessary to be designed to remove the thermal power generated by the clients of the LHCD system so that system could operate safely. Therefore, a thermal hydraulic model is developed by AFT Fathom aimed at investigating thermal hydraulic behavior of the system under the normal operation. According to the calculation results, the CWS can provide required pressure, temperature and flow rate to address the client requirements of LHCD system. The study could provide a design reference for the construction of cooling water system for LHCD system and other CRAFT subsystems.

New, improved analysis of correlation ECE data to accurately determine turbulent electron temperature spectra and magnitudes (invited).

Turbulent electron temperature fluctuation measurement using a correlation electron cyclotron emission (CECE) radiometer has become an important diagnostic for studying energy transport in fusion plasmas, and its use is widespread in tokamaks (DIII-D, ASDEX Upgrade, Alcator C-Mod, Tore Supra, EAST, TCV, HL-2A, etc.). The CECE diagnostic typically performs correlation analysis between two closely spaced (within the turbulent correlation length) ECE channels that are dominated by uncorrelated thermal noise emission. This allows electron temperature fluctuations embedded in the thermal noise to be revealed and fluctuation level and spectra determined. We have demonstrated a new, improved CECE coherency-based analysis for calculating the temperature fluctuation frequency spectrum and level, which has been verified both numerically through the simulation of synthetic ECE radiometer data and through analysis of experimental data from the CECE system on DIII-D. The new formulation places coherency-based analysis on a firm foundational footing and corrects some currently published methodologies. This new method accurately accounts for bias error in the coherence function and correctly calculates noise levels for a fixed data record length. It provides excellent accuracy in determining temperature fluctuation level (e.g., <10% error) even for a small realization number in the ensemble average. The method also has a smaller uncertainty (i.e., error bar) in the power spectrum when compared to the more standard cross-power method when evaluated at low coherency. Direct calculation of system noise level using correlation between randomized intermediate frequency signals is recommended.

The role of edge fueling in determining the pedestal density in high neutral opacity Alcator C-Mod experiments

SOLPS-ITER modeling of EDA H-mode experiments on Alcator C-Mod find that the electron density pedestal structure is unaffected by increased gas fueling when approaching ITER-like opaqueness conditions. SOLPS-ITER simulations show a decrease in the neutral penetration depth with increasing pedestal density, similar to prior experiments (Hughes et al., 2006). Neutral density and penetration depth vary poloidally, and we show that the highest neutral densities at the separatrix are found closest to the gas puff locations both on the high field side and the low field side. The decrease in the neutral penetration depth with increasing pedestal density, as well as the decrease in the neutral density at the separatrix, are not just limited to the midplane, but persist at every poloidal location. We find, however, that when gas puffs of similar magnitude as those employed in experiments are added, a much larger increase in the electron density is observed over the whole modeled plasma radius. This does suggest that changes in transport will need to be included self-consistently. • SOLPS-ITER provides insight on the dynamics of neutrals in high opacity H-modes. • SOL neutral opacity can be quantified by radial neutral density e-folding length. • Density pedestal formation is mostly unaffected as pedestal opacity increases. increases. • High gas-puffing only moderately increases neutral penetration of the pedestal. • At high pedestal opacity, main chamber fueling dominates fueling through the X-point.

Application of motional Stark effect in situ background correction to a superconducting tokamak

A polychrometer-type motional Stark effect (MSE) diagnostic technique, originally developed for the Alcator C-Mod tokamak, has been extended and applied to the Korea Superconducting Advanced Tokamak Research (KSTAR) device, the long-pulse superconducting tokamak, for the first time. It demonstrates a successful in situ subtraction of the polarized reflections off the vacuum vessel wall, sometimes up to half the total signal in some sightlines. To avoid the secondary neutral beam emission that may contaminate conventional beam-into-gas calibrations, a new approach, where the beam-into-gas measurements are made at various torus pressures with fixed vacuum fields, has been devised, which is possible with the stable superconducting coil systems of KSTAR. The validity of this new calibration scheme has been checked via plasma jog experiments. The experimental evidence of the polarized background light and the necessity of its correction in the MSE measurements made in KSTAR are presented as well.

Impact of lithium wall conditioning and wave-frequency on high density lower hybrid current drive experiment on EAST

Abstract A series of dedicated lower hybrid current drive (LHCD) experiments on EAST shows that lithium wall conditioning extends LH current drive and heating up to the line-averaged density of n - e ≈ 4x1019 m−3 for both 2.45 and 4.6 GHz. Current drive at such a high density is crucial for the development of long-pulse non-inductive scenarios on EAST. With lithiation, the LH power injection of 1.5 MW at 2.45 GHz resulted in a drop of loop voltage of ~ 0.3 V, which is a comparable loop voltage drop observed with 1.1 MW at 4.6 GHz. The observed decrease in loop voltage is attributed mostly to the RF heating effect. Another LHCD experiment suggests that lithium wall coating has a more significant impact on the scrape-off-layer (SOL) properties than changes in the Greenwald fraction. LHCD at 2.45 GHz still suffers from a loss of efficiency. Enhanced power ionization in front of the launcher may cause the onset of density-dependent wave instabilities. The rise in the midplane SOL density may also accelerate a transition in the divertor regime, leading to additional ionization and collisional losses in the X-point divertor plasma. Ray-tracing modeling supports that a lower wave frequency is more prone to collisional power loss. The experiments confirm that lithiation is a useful tool to control the SOL plasma, and suggest that density control in front of the launcher may be critical to mitigating power loss mechanisms in the plasma boundary.

Physics and applications of three-ion ICRF scenarios for fusion research

This paper summarizes the physical principles behind the novel three-ion scenarios using radio frequency waves in the ion cyclotron range of frequencies (ICRF). We discuss how to transform mode conversion electron heating into a new flexible ICRF technique for ion cyclotron heating and fast-ion generation in multi-ion species plasmas. The theoretical section provides practical recipes for selecting the plasma composition to realize three-ion ICRF scenarios, including two equivalent possibilities for the choice of resonant absorbers that have been identified. The theoretical findings have been convincingly confirmed by the proof-of-principle experiments in mixed H–D plasmas on the Alcator C-Mod and JET tokamaks, using thermal 3He and fast D ions from neutral beam injection as resonant absorbers. Since 2018, significant progress has been made on the ASDEX Upgrade and JET tokamaks in H–4He and H–D plasmas, guided by the ITER needs. Furthermore, the scenario was also successfully applied in JET D–3He plasmas as a technique to generate fusion-born alpha particles and study effects of fast ions on plasma confinement under ITER-relevant plasma heating conditions. Tuned for the central deposition of ICRF power in a small region in the plasma core of large devices such as JET, three-ion ICRF scenarios are efficient in generating large populations of passing fast ions and modifying the q-profile. Recent experimental and modeling developments have expanded the use of three-ion scenarios from dedicated ICRF studies to a flexible tool with a broad range of different applications in fusion research.

Constructing a new predictive scaling formula for ITER's divertor heat-load width informed by a simulation-anchored machine learning

Understanding and predicting divertor heat-load width λq is a critically important problem for an easier and more robust operation of ITER with high fusion gain. Previous predictive simulation data for λq using the extreme-scale edge gyrokinetic code XGC1 [S. Ku et al., Phys. Plasmas 25, 056107 (2018)] in the electrostatic limit under attached divertor plasma conditions in three major US tokamaks [C. S. Chang et al., Nucl. Fusion 57, 116023 (2017)] reproduced the Eich and Goldston attached-divertor formula results [formula #14 in T. Eich et al., Nucl. Fusion 53, 093031 (2013) and R. J. Goldston, Nucl. Fusion 52, 013009 (2012)] and furthermore predicted over six times wider λq than the maximal Eich and Goldston formula predictions on a full-power (Q = 10) scenario ITER plasma. After adding data from further predictive simulations on a highest current JET and highest-current Alcator C-Mod, a machine learning program is used to identify a new scaling formula for λq as a simple modification to the Eich formula #14, which reproduces the Eich scaling formula for the present tokamaks and which embraces the wide λqXGC for the full-current Q = 10 ITER plasma. The new formula is then successfully tested on three more ITER plasmas: two corresponding to long burning scenarios with Q = 5 and one at low plasma current to be explored in the initial phases of ITER operation. The new physics that gives rise to the wider λqXGC is identified to be the weakly collisional, trapped-electron-mode turbulence across the magnetic separatrix, which is known to be an efficient transporter of the electron heat and mass. Electromagnetic turbulence and high-collisionality effects on the new formula are the next study topics for XGC1.

Experimental observation of low-frequency magnetohydrodynamic instabilities driven by energetic electrons in low hybrid current drive plasmas

Energetic electrons driving low-frequency magnetohydrodynamic instabilities, e.g., electron fishbone (eFB) modes and electron beta-induced Alfvén eigenmodes (eBAEs), are found in co- and counter-current drive low hybrid current drive (LHCD) plasma in HL-2A, respectively. The eBAEs are found in LHCD plasma for the first time. Two branches eFB modes are observed in the core of plasma, and they can transit from the high-frequency one to the low-frequency one continuously. The different mode structures and positions are obtained by tomography of soft x-ray arrays. The frequency jump phenomena of eFB modes are also found. Two eBAEs, with poloidal and toroidal mode numbers m/n = 3/1 and 5/2, are found in the edge of the plasma. The single m/n = 3/1 mode is found when the LHCD power (P LHCD) is 0.44 MW. The strong m/n = 5/2 mode coexists with the weak 3/1 mode when P LHCD = 0.52 MW. Although the current drive direction of LHCD is opposite to the plasma current, the two eBAEs also propagate in the electron diamagnetic drift direction poloidally, which is the same direction as the eFB modes in co-current drive LHCD plasma. With increasing P LHCD (or changed current profile), the value of the edge safety factor (q) decreases. The evolutions of the mode numbers of eBAEs may be related to changes in the edge q factors.

A semi-supervised machine learning detector for physics events in tokamak discharges

Databases of physics events have been used in various fusion research applications, including the development of scaling laws and disruption avoidance algorithms, yet they can be time-consuming and tedious to construct. This paper presents a novel application of the label spreading semi-supervised learning algorithm to accelerate this process by detecting distinct events in a large dataset of discharges, given few manually labeled examples. A high detection accuracy (>85%) for H–L back transitions and initially rotating locked modes is demonstrated on a dataset of hundreds of discharges from DIII-D with manually identified events for which only three discharges are initially labeled by the user. Lower yet reasonable performance (∼75%) is also demonstrated for the core radiative collapse, an event with a much lower prevalence in the dataset. Additionally, analysis of the performance sensitivity indicates that the same set of algorithmic parameters is optimal for each event. This suggests that the method can be applied to detect a variety of other events not included in this paper, given that the event is well described by a set of 0D signals robustly available on many discharges. Procedures for analysis of new events are demonstrated, showing automatic event detection with increasing fidelity as the user strategically adds manually labeled examples. Detections on Alcator C-Mod and EAST are also shown, demonstrating the potential for this to be used on a multi-tokamak dataset.

Dimensionless parameter scaling of intrinsic torque in C-Mod enhanced confinement plasmas

A dimensionless parameter dependence study of intrinsic torque has been performed on a database of H- and I-mode plasmas from the Alcator C-Mod tokamak. The torque was determined by comparing intrinsic angular momentum density profiles just before and just after L–H and L–I transitions. The intrinsic torque has been found to scale as , with the parameter ranges 0.3 ⩽ β N ⩽ 1.5, 0.004 ⩽ ρ * ⩽ 0.011 and 0.04 ⩽ ν * ⩽ 0.9. Comparison with results from JET and DIII-D suggests that the intrinsic torque should be normalized by some measure of the device size. Depending upon this normalization, the estimated total intrinsic torques for ITER, SPARC and ARC are ∼20, ∼4 and ∼8 Nm, respectively.

Experimental studies of plasma-antenna coupling with the JET Alfvén Eigenmode Active Diagnostic

This paper presents a dedicated study of plasma-antenna (PA) coupling with the Alfvén Eigenmode Active Diagnostic (AEAD) in JET. Stable AEs and their resonant frequencies f, damping rates γ < 0, and toroidal mode numbers n are measured for various PA separations and limiter versus X-point magnetic configurations. Two stable AEs are observed to be resonantly excited at distinct low and high frequencies in limiter plasmas. The values of f and n do not vary with PA separation. However, |γ| increases with PA separation for the low-f, but not high-f, mode, yet this may be due to slightly different edge conditions. The high-f AE is detected throughout the transition from limiter to X-point configuration, though its damping rate increases; the low-f mode, on the other hand, becomes unidentifiable. The linear, resistive MHD code CASTOR is used to simulate the frequency scan of an AEAD-like external antenna. For the limiter pulses, the high-f mode is determined to be an n = 0 GAE, while the low-f mode is likely an n = 2 TAE. During the transition from limiter to X-point configuration, CASTOR indicates that n = 1 and 2 EAEs are excited in the edge gap. These results extend previous experimental studies in JET and Alcator C-Mod; validate the computational work performed by Dvornova et al (2020 Phys. Plasmas 27 012507); and provide guidance for the optimization of PA coupling in upcoming JET energetic particle experiments, for which the AEAD will aim to identify the contribution of alpha particles to AE drive during the DT campaign.

Progress of Experimental Studies in the HL-2A Tokamak

During the last several years, the HL-2A experiment has made significant progress in the following areas: (1) lower-hybrid wave (LHW) heating and current drive, (2) plasma confinement and turbulent transport, (3) magnetohydrodynamic (MHD) instabilities and energetic particle physics and (4) H-mode and edge localized mode (ELM) control. The results show that the LHW system working in the co-current mode can reach higher driving efficiency and full non-inductive lower-hybrid current drive (LHCD) has been achieved. The intrinsic poloidal torque characterized by the divergence of the residual stress is deduced from synthesis for the first time. The dynamics of spectral symmetry breaking in drift wave turbulence is in good agreement with the development of the poloidal torque to drive the edge poloidal flow. The influence of the cross-phase dynamics on turbulent stress was also investigated. The ion internal transport barrier has been observed in the NBI-heated plasma, and inside the barrier the ion thermal transport is reduced to the neoclassical level. Besides, micro-turbulence is modulated by the rotation frequency of the magnetic island, and this modulation effect is related to a critical island width. Strong E × B shear is found at the island boundary. Three kinds of axisymmetric modes, beta-induced Alfven eigenmode (BAE), toroidal Alfven eigenmode (TAE) and the ellipticity-induced Alfven eigenmode (EAE), are found to be driven unstable by nonlinear mode coupling between Alfven eigenmodes and tearing mode which is well explained by the nonlinear gyrokinetic theory. The fishbone and tearing modes were actively controlled by the electron cyclotron resonance heating (ECRH). The dynamics of the edge plasma flows and turbulence during the L–I–H transition have been dedicatedly investigated. The geodesic acoustic mode (GAM) and limit cycle oscillation (LCO) coexist for a short time and disappear in the H-mode plasma with the increasing of E × B shear flow before the I–H transition, which plays an important role in the turbulence suppression. Different techniques, such as LHW, ECRH, resonant magnetic perturbation (RMP), and impurity seeding by the laser blow-off (LBO) and supersonic molecular beam injection (SMBI), have been successfully applied to control the large ELMs. It has been found that pedestal turbulence enhancement might be responsible for the observed mitigation effect.

Optimized phasing conditions to avoid edge mode excitation by ICRH antennas

An ion cyclotron resonance heating (ICRH) antenna system must launch radio frequency (RF) power with a wavenumber spectrum which maximizes the coupling to the plasma. It should also ensure good absorption while minimizing the wave interaction with the plasma edge. Such interactions lead to impurity release, whose effect has been measured far from the antenna location (Klepper et al. 2013; Wukitch et al. 2017; Perkins et al. 2019) and can involve the entire scrape-off layer. In the normal heating scenario, for which the frequency of the waves launched by the antenna is larger than the ion cyclotron frequency of the majority ions $\omega &gt; \omega _{\textrm {ci},\textrm {maj}}$ , release of impurities due to ICRH can be affected by minimizing the low $|k_{\parallel }| &lt; k_0$ power spectrum components of the antenna. Impurity release can be the result of low central absorption of the waves or power transfer from the fast to the slow wave due to the presence of a confluence in the plasma edge. In ASDEX Upgrade (AUG), a reduction of heavy impurity release by ICRH in the plasma was qualitatively well correlated to the parallel electric field and RF currents flowing around the antenna (Bobkov et al. 2017). In this article, we first show a correlation between the reduction in impurity release by ICRH in AUG and the rejection of the low $|k_{\parallel }| &lt; k_0$ region of the antenna power spectrum. We show that the same correlation holds for results obtained in the Alcator C-Mod tokamak. Finally, using this idea, we reproduce ICRH induced impurity release behaviour in a not yet published experiments of JET, and make predictions for ITER and DEMO.

Robust control of q-profile and βp using data-driven models on EAST

• A novel H ∞ robust control scheme is developed for the current profile control using data-driven models. • A decoupling technique enables the control scheme to exhibit multi-functional characteristics. • An anti-windup technique can compensate the effects from moderate time-delays and actuator dynamics. • An efficient procedure is provided to support the H-mode steady-state operation on EAST. • Three general performance indexes are proposed to characterize the current profile control performance. A new robust feedback controller for the safety factor profile and poloidal plasma pressure parameter has been developed using a two-time-scale data-driven model. The model describes the linear plasma responses of the ι profile and β p with respect to auxiliary heating & current drive (H&CD) powers, around a typical plasma equilibrium in an H-mode steady-state plasma discharge on EAST. The feedback controller comprises a carefully designed low-pass filter for the timescale separation, a decoupling module and three local controllers synthesized from the H ∞ norm optimization and the singular value decomposition. The actuators are the lower hybrid current drive (LHCD) system at 4.6 GHz and the ion cyclotron resonance heating (ICRH) system at 33 MHz. Taking into account the actuation dynamics, an anti-windup technique is employed to condition the controller online aiming to attenuate the negative effects from moderate time delays and power saturations. Extensive nonlinear closed-loop simulations with the METIS code suggest that high β p and negative central magnetic shear that characterize advanced tokamak plasma scenarios can be achieved and sustained on EAST with good tracking performance and reasonable robustness via the proposed control scheme. The feedback control of the core ι profile and β p with a range of time delays, power saturations and varying weighting functions are evaluated numerically, compared and discussed. The control robustness to plasma parameter uncertainties including the line-averaged plasma density 〈 n ¯ e 〉 , the H-mode enhancement factor H factor and the effective charge number Z eff are assessed and analyzed.

Comparison between mirror Langmuir probe and gas-puff imaging measurements of intermittent fluctuations in the Alcator C-Mod scrape-off layer

Statistical properties of the scrape-off layer plasma fluctuations are studied in ohmically heated plasmas in the Alcator C-Mod tokamak. For the first time, plasma fluctuations as well as parameters that describe the fluctuations are compared across measurements from a mirror Langmuir probe (MLP) and from gas-puff imaging (GPI) that sample the same plasma discharge. This comparison is complemented by an analysis of line emission time-series data, synthesized from the MLP electron density and temperature measurements. The fluctuations observed by the MLP and GPI typically display relative fluctuation amplitudes of order unity together with positively skewed and flattened probability density functions. Such data time series are well described by an established stochastic framework that models the data as a superposition of uncorrelated, two-sided exponential pulses. The most important parameter of the process is the intermittency parameter, $\gamma = {\tau _{d}} / {\tau _{w}}$ , where ${\tau _{d}}$ denotes the duration time of a single pulse and ${\tau _{w}}$ gives the average waiting time between consecutive pulses. Here we show, using a new deconvolution method, that these parameters can be consistently estimated from different statistics of the data. We also show that the statistical properties of the data sampled by the MLP and GPI diagnostic are very similar. Finally, a synthetic GPI signal using only plasma parameters sampled by the MLP shows qualitatively different fluctuation statistics from the measured GPI signal.

Inference of experimental radial impurity transport on Alcator C-Mod: Bayesian parameter estimation and model selection

We present a fully Bayesian approach for the inference of radial profiles of impurity transport coefficients and compare its results to neoclassical, gyrofluid and gyrokinetic modeling. Using nested sampling, the Bayesian impurity transport inference (BITE) framework can handle complex parameter spaces with multiple possible solutions, offering great advantages in interpretative power and reliability with respect to previously demonstrated methods. BITE employs a forward model based on the pySTRAHL package, built on the success of the well-known STRAHL code (Dux 2003 Fusion Sci Technol. 44 708–15), to simulate impurity transport in magnetically-confined plasmas. In this paper, we focus on calcium (Ca, Z = 20) laser blow-off injections into Alcator C-Mod plasmas. Multiple Ca atomic lines are diagnosed via high-resolution x-ray imaging crystal spectroscopy and vacuum ultra-violet measurements. We analyze a sawtoothing I-mode discharge for which neoclassical and turbulent (quasilinear and non-linear) predictions are also obtained. We find good agreement in diffusion across the entire radial extent, while turbulent convection and density profile peaking are estimated to be larger in experiment than suggested by theory. Efforts and challenges associated with the inference of experimental pedestal impurity transport are discussed.

Physics basis for the ICRF system of the SPARC tokamak

Ion cyclotron range of frequencies (ICRF) heating will be the sole auxiliary heating method on SPARC for both full-field (Bt0 ~ 12 T) D–T operation and reduced field (Bt0 ~ 8 T) D–D operation. Using the fast wave at ~120 MHz, good wave penetration and strong single-pass absorption is expected for D–T(3He), D(3He), D(H) and4He(H) heating scenarios. The dependences of wave absorption on${k_\parallel }$,3He concentration, resonance location, antenna poloidal location and losses on alpha particles and ash have been studied. The antenna loading has been assessed by comparison with the Alcator C-Mod antennae. An antenna spectrum of${k_\parallel }\sim 15\text{--}18\,{\textrm{m}^{ - 1}}$is shown to be good for both core absorption and edge coupling. For the control of impurity sources, the antenna straps are rotated ~10° to be perpendicular to the B field and the straps can run with different power levels in order to optimize the antenna spectrum and to minimize the image current on the antenna frame. Combining the physics constraints with the SPARC port design, maintenance requirement and contingency against antenna failure during D–T operation, we plan to mount on the inner wall of the vacuum vessel a total of 12 4-strap antennae in 6 ports while keeping 3-strap antennae that are insertable and removable on port plugs as the backup option.

Study of turbulence-induced refraction of lower hybrid waves using synthetic scrape-off layer filaments

Turbulence-induced refraction effects to lower hybrid (LH) wave propagation and current drive are studied using synthetic scrape-off layer (SOL) blob/filament fields. A synthetic 3D, field-following, blob turbulence model is implemented in the ray-tracing/Fokker-Planck (RTFP) codes GENRAY/CQL3D. In Alcator C-Mod, the blob field is shown to significantly affect LH ray-trajectories, leading to increased on-axis damping and smoother current profiles. This effect depends on the average blob size and amplitude. In addition, the diffusion of ray-trajectories in phase-space caused by turbulence increases the robustness of the RTFP model. A modified launch spectrum, acting as a proxy for parametric decay instability (PDI) effects, is included in simulations with the blob model. A synergy between the modified launch spectrum and turbulence-induced refraction results in synthetic hard x-ray profiles that agree with experiment. Lastly, the blob model is used to predict the effect of SOL turbulence on DIII-D high-field side (HFS) LH launch. Assuming low turbulence amplitude in the HFS SOL (∼5%), turbulence-induced refraction is predicted to have little effect on current drive efficiency.

Generation of localized lower-hybrid current drive by temperature perturbations

Despite high demonstrated efficiency, lower-hybrid current drive (LHCD) has not been considered localized enough for neoclassical tearing mode (NTM) stabilization in tokamaks. This assessment must be reconsidered in view of the radiofrequency current condensation effect. We show that an island with a central hot spot induces significant localization of LHCD. Furthermore, in steady state tokamaks where a significant amount of current is provided by LHCD, passive stabilization of NTMs may occur automatically, particularly as islands become large, without requiring precise aiming of the wave power.