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Effect of the neutral beam injector operational regime on the Alfven eigenmode saturation phase in DIII-D plasma
The aim of this study is to analyze the effect of the neutral beam injector (NBI) operation regime on the saturation phase of the Alfven Eigenmodes (AEs) in DIII-D plasma. Here, the analysis is done using the linear and nonlinear versions of the gyro-fluid code FAR3d. A set of parametric analyses are performed modifying the nonlinear simulation EP $β$ (NBI injection power), EP energy (NBI voltage) and the radial location of the EP density profile gradient (NBI radial deposition). The analysis indicates a transition from the soft (local plasma relaxation) to the hard MHD (global plasma relaxation) limit if the simulation EP $β$ $\unicode{x2A7E}$ $0.02$, leading to bursting MHD activity caused by radial AEs overlapping. MHD bursts cause an enhancement of the EP transport showing ballistic-like features as avalanche-like events. Simulations in the soft MHD limit show an increment of the EP density gradient as the EP $β$ increases. On the other hand, there is a gradient upper limit in the hard MHD limit, consistent with the critical-gradient behavior. AEs induce shear flows and zonal current leading to the deformation of the flux surfaces and the safety factor profile, respectively, particularly strong for the simulation in the hard MHD limit. Simulations in the hard MHD regime show a decrease of the AE frequency in the saturation phase; this is caused by the destabilization of a transitional mode between a $9/3-10/3$ TAE and a $9/3$ RSAE that may explain the AE frequency down-sweeping observed in some DIII-D discharges. Reducing the EP energy in the nonlinear simulations leads to a weakening of the plasma perturbation. On the other hand, increasing the EP energy causes the opposite effect. Nonlinear simulations of off-axis NBI profiles indicate a lower plasma perturbation as the EP density gradient is located further away from the magnetic axis.
Measurement of post-disruption runaway electron kinetic energy and pitch angle during final loss instability in DIII-D
Post-disruption runaway electron (RE) kinetic energy K and pitch angle sin$\vartheta$ are critical parameters for determining resulting first wall material damage during wall strikes, but are very challenging to measure experimentally. During the final loss instability, confined RE K and sin$\vartheta$ are reconstructed during center-post wall strikes for both high impurity (high-Z) and low impurity (low-Z) plasmas by combining soft x-ray, hard x-ray, synchrotron emission, and total radiated power measurements. Deconfined (wall impacting) RE sin$\vartheta$ is then reconstructed for these shots by using time-decay analysis of infra-red imaging. Additionally, deconfined RE K and sin$\vartheta$ are reconstructed for a low-Z downward loss shot by analyzing resulting damage to a sacrificial graphite dome limiter. The damage analysis uses multi-step modeling simulating plasma instability, RE loss orbits, energy deposition, and finally material expansion (MARS-F, KORC, GEANT-4, and finally COMSOL). Overall, mean kinetic energies are found to be in the range $\langle$$K$$\rangle$ $≈ 3-4$ MeV for confined REs. KORC simulations indicate that the final loss instability process does not change individual RE kinetic energy K. In conclusion, confined RE pitch angles are found to be fairly low initially pre-instability, $\langle$sin$\vartheta$$\rangle$ $≈ 0.1 – 0.2$, but appear to increase roughly 2x, to $\langle$sin$\vartheta$$\rangle$ $≈ 0.3 – 0.4$ for both confined and deconfined REs during instability onset in the low-Z case; this increase is not observed in the high-Z case.
Impact of divertor material on neutral recycling and discharge fueling in DIII-D
We report that experiments with the lower divertor of DIII-D during the Metal Rings Campaign (MRC) show that the fraction F of atomic D in the total recycling flux is material-dependent and varies through the ELM cycle, which may affect divertor fueling. Between ELMs, F C ~ 10% and FW ~ 40%, consistent with expectations if all atomic recycling is due to reflections. During ELMs, F C increases to 50% and F W to 60%. In contrast, the total D recycling coefficient including atoms and molecules R stays close to unity near the strike point where the surface is saturated with D. During ELMs, R can deviate from unity, increasing during high energy ELM-ion deposition (net D release) and decreasing at the end of the ELM which leads to ability of the target to trap the ELM-deposited D. The increase of R > 1 in response to an increase in ion impact energy Ei has been studied with small divertor target samples using Divertor Materials Evaluation System (DiMES). An electrostatic bias was applied to DiMES to change E i by 90 eV. On all studied materials including C, Mo, uncoated and W-coated TZM (>99% Mo, Ti, and Zr alloy), W, and W fuzz, an increase of E i transiently increased the D yield (and R) by ~10%. On C there was also an increase in the molecular D 2 yield, probably due to ion-induced D 2 desorption. Despite the measured increase in F on W compared to C, attached H-mode shots with OSP on W during MRC did not demonstrate a higher pedestal density. About 8% increase in the edge density could be seen only in attached L-mode scenarios. The difference can be explained by higher D trapping in the divertor and lower divertor fueling efficiency in H- versus L-mode.
Evolution of ELMs, pedestal profiles and fluctuations in the inter-ELM period in NBI and ECH dominated discharges in DIII-D
In DIII-D, it has been observed that ELM frequency decreases by 40% and ELM spacing becomes more regular in time when heating is changed from pure neutral beam injection (NBI) to predominantly electron cyclotron heating (ECH) in ITER Similar Shape plasmas. In comparison with pure the NBI discharges, pedestal fluctuations in magnetics and density increase in the ECH dominated discharges. Recovery of the pedestal profiles like electron density (ne), temperature (Te) and pressure (pe) shows marked differences for these two heating schemes. Average profiles in the last 30% of the ELM cycle shows higher Te, lower ne, and similar pe at the pedestal top for the ECH discharge when compared to the NBI discharge. Here, the gradient of Te (∇Te) is also steeper at the pedestal in the ECH discharge. Magnetic fluctuations show 3 distinct modes in 13~116 kHz in the ECH discharges only. ne fluctuations show two modes evolving in the inter-ELM period of the ECH discharge, a low frequency (400 kHz) quasi-coherent mode (LFQC) and high frequency (~2 MHz) broadband (HFB) fluctuations. Evolution of these modes has marked correspondence with the inter-ELM ∇Te recovery. A sharp decrease in the Dα baseline is observed whenever the LFQC weakens and the HFB grows, prior to each large ELM. Transport coefficients obtained from TRANSP shows that MTM and/or TEM are plausible candidates for the observed fluctuations. Linear gyrofluid simulation (TGLF) corroborates this characterization. TGLF shows that the linear growth rate of the most dominant mode peaks at ion-scale (kθρs ~ 0.4) at the pedestal steep gradient and the frequency is in the electron diamagnetic direction. It is proposed that increased fluctuations in the ECH dominated case, due to increased ∇Te, caused an increase in fluctuation-driven transport in the pedestal and slowed the pedestal recovery between ELMs, leading to a reduction in the ELM frequency.
Energy and particle balance during plasma detachment in a long-leg divertor configuration
Abstract Comprehensive studies of energy and particle balances in the transition to plasma detachment in an alternative divertor configuration with long outer legs are shown. Numerical simulations are performed with the 2D code suite SOLPS 4.3, using a disconnected double null grid with narrow, tightly baffled long poloidal leg divertors at the outer lower target and outer upper target. A particle count scan is performed using the ‘closed gas box’ model, where the tunable parameter in the simulations is the total number of deuterium particles in the simulation space and all other parameters are held fixed, including a constant input power and trace neon impurity radiation, to assess the physics of the transition to detachment in the system as the particle count increases. Three main aspects of the physics of divertor detachment are addressed: the criteria for the local onset of divertor detachment in each of the divertors, the distribution of heat flux and other plasma parameters between the four divertors as each divertor transitions to detachment, and the role of perpendicular transport in the transition to the detached regime. A synergistic mechanism by which the cross-field transport is reduced by factors associated with the onset of plasma recombination effects is identified. These results are compared to the existing understanding of the physics of the transition to plasma detachment in standard divertors.
Transport-driven toroidal rotation with general viscosity profile
Abstract Using the assumption of a weak normalized turbulent viscosity, usually valid in practice, the modulated-transport model (Stoltzfus-Dueck 2012 Phys. Plasmas 19 055908) is generalized to allow the turbulent transport coefficient to vary in an arbitrary way on radial and poloidal position. The new approach clarifies the physical interpretation of the earlier results and significantly simplifies the calculation, via a boundary-layer asymptotic method. Rigorous detailed appendices verify the result of the simple boundary-layer calculation, also demonstrating that it achieves the claimed order of accuracy and providing a concrete prediction for the strong plasma flows in the immediate vicinity of the last closed flux surface. The new formulas are used to predict plasma rotation at the core-edge boundary, in cases with and without externally applied torque. Dimensional formulas and extensive discussion are provided, to support experimental application of the new model.
Wall heating by subcritical energetic electrons generated by the runaway electron avalanche source *
Abstract Subcritical energetic electrons (SEEs) produced by the runaway electron (RE) avalanche source at energies below the runaway threshold are found to be the primary contributor to surface heating of plasma-facing components (PFCs) during final loss events. This finding is supported by theoretical analysis, computational modeling with the Kinetic Orbit Runaway electrons Code (KORC), and qualitative agreement with DIII-D experimental observations. The avalanche source generates significantly more secondary electrons below the runaway threshold, which thermalize rapidly when well-confined. However, during a final loss event, the RE beam impacts the first wall, and SEEs are deconfined before they can thermalize. Additionally, because the energy deposition length decreases faster than energy, the deposited energy density, and thus the maximum PFC surface temperature change, is larger for SEEs than REs. KORC simulations employ an analytic first wall to model particle deconfinement onto a non-axisymmetric wall composed of individual tiles. PFC surface heating is calculated using a 1D model extended to include an energy-dependent deposition length scale. Simulations of DIII-D qualitatively agree with infrared (IR) imaging only when SEEs from the avalanche source are included. These results demonstrate that SEEs are the dominant contributor to PFC surface heating and indicate that the avalanche source plays a critical role in the PFC damage caused during final loss events. The prominence of SEEs also has important implications for interpreting IR imaging, one of the primary diagnostics for RE-wall interaction diagnosis, despite REs dominating the energy and current density. This result improves predictions of wall damage due to post-disruption REs to estimate material lifetime and design RE mitigation systems for ITER and future reactors.
The root cause of disruptive NTMs and paths to stable operation in DIII-D ITER baseline scenario plasmas
Analyses of the DIII-D ITER Baseline Scenario database support that the disruptive m,n=2,1 magnetic islands are pressure gradient driven, non-linear instabilities seeded in a sequence of stochastic transient magnetic perturbations, and that the current profile relaxation does not affect the m,n=2,1 island onset rate. At low torque, these Neoclassical Tearing Modes are most commonly seeded by non-linear 3-wave coupling when the differential rotation between the q=1 & q=2 rational surfaces approaches zero. Lack of statistically significant difference between the current profiles of stable and unstable states, as well as lack of correlation between the tearing mode onset rate and the current profile relaxation both reject causality between the current profile evolution and the 2,1 magnetic island onsets in these plasmas. These support that preserving the differential rotation between the q=1 and q=2 rational surfaces is key to long pulse stable operation in the plasma scenario planned for ITER, while optimization of the current profile within the explored parameter space may lead to much weaker improvements than sustaining the differential rotation.
Error field predictability and consequences for ITER
Abstract ITER coil tolerances are re-evaluated using the modern understanding of coupling to least-stable plasma modes and an updated center-line-traced model of ITER’s coil windings. This reassessment finds the tolerances to be conservative through a statistical, linear study of n = 1 error fields (EFs) due to tilted, shifted misplacements and nominal windings of central solenoid and poloidal field coils within tolerance. We also show that a model-based correction scheme remains effective even when metrology quality is sub-optimal, and compare this to projected empirical correction schemes. We begin with an analysis of the necessity of error field correction (EFC) for daily operation in ITER using scalign laws for the EF penetration threshold. We then consider the predictability of EF dominant mode overlap across early planned ITER scenarios and, as measuring EFs in high power scenarios can pose risks to the device, the potential for extrapolation to the ITER Baseline Scenario (IBS). We find that carefully designing a scenario matching currents proportionally to those of the IBS is far more important than plasma shape or profiles in accurately measuring an optimal correction current set.
Runaway electron current reconstitution after a nonaxisymmetric magnetohydrodynamic flush
Benign termination of mega-ampere (MA) level runaway current has been convincingly demonstrated in recent JET and DIII-D experiments, establishing it as a leading candidate for runaway mitigation on ITER. This comes in the form of a runaway flush by parallel streaming loss along stochastic magnetic field lines formed by global magnetohydrodynamic instabilities, which are found to correlate with a low-Z injection that purges the high-Z impurities from a post-thermal-quench plasma. Here, in this study, we show the competing physics that govern the postflush reconstitution of the runaway current in an ITER-like reactor where significantly higher current is expected. The trapped “runaways” are found to dominate the seeding for runaway reconstitution, and the incomplete purge of high-Z impurities helps drain the seed but produces a more efficient avalanche, two of which compete to produce a 2–3 MA step in current drop before runaway reconstitution of the plasma current.
A Coaxial Helicity Injection System for Nonsolenoidal Startup Studies on the PEGASUS-III Experiment
Not Available
A Method to Identify the Heat Flux From Photons and Neutrals at the Divertor Target
The Surface Eroding Thermocouple (SETC) is a diagnostic designed for fast surface heat flux measurement and has been successfully applied in DIII-D to investigate divertor detachment. A novel method has been developed of using a combination of surface-facing and recessed SETCs to distinguish between the heat flux from charged particles, and that from neutrals and radiative heating. Furthermore, this technique suggests that at least 20% of the total incident heat flux is attributable to the latter sources not trapped in the magnetic field in a fully detached open divertor condition. It not only indicates the non-charged particle heating significantly contributes to the total surface heat flux in detached divertor conditions, but also demonstrates the feasibility of using two operational modes of SETCs to distinguish between the two surface heating mechanisms.
SPARC 3D Field Physics and Support of the Non-Axisymmetric Coil Assessment
Commonwealth Fusion Systems (CFS) is deploying SPARC, a net-energy tokamak, by 2025. A notable challenge facing all tokamak approaches to fusion energy production is maintaining a stable plasma and thereby steady energy production. “Disruptions” to plasma operation are often encountered in devices when they operate at high normalized pressure, high normalized density, or high normalized current. SPARC is unique in that it is expected to demonstrate Q>=2 in a plasma with low normalized pressure and density, but with a more modest current limit buffer sufficient to avoid disruptions. Due to both the high normalized current and high magnetic field of the SPARC design, it is expected to decrease its relative resilience to instabilities driven by non-axisymmetry in the externally applied magnetic field, termed ‘error fields’. To prevent error field driven instabilities, a primary line of defense is strict engineering tolerances in magnet fabrication and installation. A secondary line of defense is a purpose-built set of magnetic coils to correct these errors, termed error field correction coils (EFCCs). The overarching goals of this INFUSE project were to evaluate the effectiveness of the EFCCs in the SPARC design and provide guidance as to how to improve this effectiveness.
Impact of toroidal magnetic field direction on integrated ELM-stable operation and divertor power exhaust via boron powder injection in EAST
We report the first in-depth comparison of the impact of toroidal magnetic field direction on solid boron injection used for Edge-Localized Mode (ELM) control, power exhaust, and core high-Z impurity control in the Experimental Advanced Superconducting Tokamak. With favorable ion ∇B drift towards the upper X-point in an upper-single-null configuration, boron injection effectively suppresses ELMs, produces a detachment of the inner divertor target, and leads to improved energy confinement. ELM suppression in this configuration is accompanied by the excitation of an Edge Harmonic Mode. In contrast, with unfavorable ion ∇B drift away from the upper X-point, boron injection also suppresses ELMs but leads to a more symmetric detachment state of both the inner and outer divertor targets, while plasma energy confinement is slightly degraded despite similar boron injection levels; a different low-frequency coherent mode without multiple harmonics is observed. Measurements from toroidally separated views show that the divertor response to boron injection is essentially toroidally symmetric, supporting the use of two-dimensional SOLPS-ITER modeling with a toroidally uniform impurity source. These experimental observations are qualitatively consistent with SOLPS-ITER simulations, which highlight the critical role of E × B drift effects in setting the Bt-dependent in–out asymmetry of detachment and in asymmetrically transporting particles and injected impurities within the scrape-off layer and private-flux region. These findings underscore the importance of drift physics and real-time wall conditioning in controlling low-Z impurity transport and optimizing edge solutions for integrated, ELM-stable, high-performance tokamak operation.
Quasilinear theory and modelling of gyrokinetic turbulent transport in tokamaks
The theory, development, and validation of reduced quasilinear models of gyrokinetic turbulent transport in the closed flux surface core of tokamaks is reviewed. In combination with neoclassical collisional transport, these models are successful in accurately predicting core tokamak plasma temperature, density, rotation, and impurity profiles in a variety of confinement regimes. Refined experimental tests have been performed to validate the predictions of the quasilinear models, probing changes in the dominant gyrokinetic instabilities, as reflected in fluctuation measurements, cross-phases, and transport properties. These tests continue to produce a deeper understanding of the complex mix of instabilities at both electron and ion gyroradius scales.
Variable magnetic field electron spectrometer to measure hot electrons in the range of 50–460 keV
Resonance absorption (RA) occurs when a p-polarized electromagnetic wave, obliquely incident on an inhomogeneous plasma, tunnels past its turning point and resonantly excites an electron plasma wave (EPW) at the critical density. This phenomenon is important, for instance, in the direct drive approach to inertial fusion energy and is a particular example of a wider phenomenon in plasma physics known as mode conversion, which is crucial for heating magnetic fusion devices, such as tokamaks, via RF heating. Direct measurement of these RA-generated EPW accelerated hot electrons, with energy in the range of a few tens to a few hundreds of keV, is a challenging task due to the relatively low deflecting magnetic fields needed. The solution described here is a magnetic electron spectrometer (MES) with a continually changing magnetic field, lower at the entrance of the MES and gradually increasing toward the end, that enables the measurement of a wide spectral range of electrons with energies between 50 and 460 keV. Electron spectra taken in a LaserNetUS RA experiment were acquired from plasmas generated by irradiating polymer targets with the combination of an ~300 ps pulse followed by a series of ten high intensity 50–200 fs duration laser pulses from the ALEPH laser at Colorado State University. In conclusion, the high intensity beam is designed as spike trains of uneven duration and delay pulses in order to modify the RA phenomenon.
Observation of long-radial-range-correlation in turbulence in high-collisionality high-confinement plasmas on DIII-D
Abstract We report on the observation of spatially asymmetric turbulent structures with a long radial correlation length in the core of high-collisionality H -mode plasmas on DIII-D tokamak. These turbulent structures develop from shorter wavelength turbulence and have a radially elongated structure. The envelope of turbulence spans a broad radial range in the mid-radius region, leading to streamer-like transport events. The underlying turbulence is featured by intermittency, long-term memory effect, and the characteristic spectrum of self-organized criticality. The amplitude and the radial scale increase substantially when the shearing rate of the mean flow is reduced below the turbulent scattering rate. The enhanced long-radial-range-correlated (LRRC) transport events are accompanied by apparent degradation of normalized energy confinement time. The emergence of such LRRC transport events may serve as a candidate explanation for the degrading nature of H-mode core plasma confinement at high collisionality on DIII-D tokamak.