Session TO07: MFE: Negative Triangularity and I-mode

Overview of the DIII-D Negative Triangularity Campaign

In early 2023, a dedicated multi-week experimental campaign was conducted to qualify the negative triangularity (NT) scenario for future reactors on the DIII-D tokamak after the installation of graphite-tile armor on the low-field-side lower outer wall. During this campaign, high confinement (H_98y,2≥1), high current (q₉₅<3), and high normalized pressure plasmas (β_N>2.5) were achieved at high-injected-power in strongly NT-shaped plasmas with δ_avg= - 0.5 and a lower outer divertor X-point that also demonstrated high normalized density (n_e/n_GW≤2), particle confinement comparable to energy confinement, and a detached divertor without impurity seeding, all while maintaining a non-ELMing NT-edge with an electron temperature pedestal, exceeding that of typical low-confinement (L-mode) plasmas. This reactor-relevant regime is accessed over a wide range of DIII-D operational space (plasma current, toroidal field, electron density and pressure) in contrast to other high-performance non-ELMing scenarios that have narrower operating windows. Two scenarios were developed:  an inductive high-gain plasma with usually lower q₉₅ and sawteeth and an advanced quasi-steady-state plasma with higher q₉₅=4 that were sawtooth-free, indicating q_min>1 was obtained.   

On the vertical stability of DIII-D discharges with strong negative triangularity

Negative triangularity (NT) plasmas feature larger Shafranov shifts and more elongated inner flux surfaces than their positive triangularity counterparts, increasing the drive for n=0 modes associated with vertical stability. Interpretive modeling shows that coupling with a non-conformal vessel wall can reduce the growth rates of this instability and help to enable the control of diverted NT discharges on DIII-D [1, 2]. However, due to limitations of the poloidal field coils on DIII-D, increased beta reduces the controllable parameter space for DIII-D NT plasmas, necessitating the development of more advanced control algorithms in the absence of improved hardware. The addition of diagnostic noise and power supply tuning to the TokSys model are needed to capture the time dependent behavior of DIII-D NT discharges, which is able to reproduce experimentally measured growth rates with errors of no more than ∼20%. Implications of these findings for NT reactor design are also discussed.

[1] J. Song et. al. Nucl. Fusion 61 096033 (2021)

[2] A. O. Nelson et. al. Plasma Phys. Control. Fusion 65 044002 (2023)   

Edge Transport Barrier Evolution and H-mode inhibition in DIII-D Negative Triangularity Plasmas*

Diverted negative triangularity plasmas (NT, $delta_{Ave} < -0.2$) in DIII-D remain resiliently in L-mode, without evidence of edge bifurcation in turbulence and flows, but exhibit H-mode-like thermal confinement and normalized pressure $eta_N$. With counter-$I_{p}$ neutral beam injection, the $ extbf{ extit{E}} imes extbf{ extit{B}}$ velocity in the plasma edge can significantly exceed typical positive triangularity (PT) L-mode values; however, the $ extbf{ extit{E}} imes extbf{ extit{B}}$ shearing rate only exceeds the turbulence decorrelation rate within an extremely narrow (few Gyro-radii wide) edge layer. At weakly negative triangularity ($delta_{Ave}> -0.2$), the shearing rate exceeds the decorrelation rate across a wider edge layer, resulting in limit cycle oscillations (LCO [2]) and/or sustained H-mode. Low and intermediate wavenumber edge density fluctuations are reduced intermittently during LCO. We present the L-H transition power threshold parametric dependence on triangularity, neutral beam torque, and plasma current (safety factor $q_{95}$), and edge turbulence and flow correlation analysis across the L-mode/LCO and H-mode regimes. Results from initial gyrofluid stability analysis of the edge/outer core plasma is also shown.   

Full-orbit simulation of charge separation for negative triangularity plasma shapes in DIII-D

In DIII-D negative triangularity (NT) shaped plasmas, H-modes are not observed when the triangularity is stronger than about -0.18 [1]. A characteristic of the H-mode is the presence of a radial electric field at the plasma edge. This field suppresses the edge fluctuations via the ExB shear flow mechanism and creates a a transport barrier. Ion finite orbit width (FOW) effects can cause charge separation at the plasma edge and form the source for the observed electric field. In this presentation, the effects of charge separation due to FOW effects near the plasma edge are presented. Ion orbits are calculated for thermal distributions using the full-orbit following SPIRAL code [2]. Ion densities outside the last closed flux surface (LCFS) were calculated as a function of the the plasma shape. It was found that the ion densities outside the LCFS are comparable for positive triangularity and weakly NTs, where H-modes were observed. For strongly NT shaped without H-mode characteristics, the ion density outside the LCFS is decreased, indicating that the electric field is reduced and therefore, less effective in suppressing edge fluctuations.

[1] A.O. Nelson et al. NF 62 (2022) 096020

[2] G.J. Kramer et al. PPCF 55 (2013) 025013

  

Investigation of Turbulence Properties in Negative Triangularity Plasmas on DIII-D using Beam Emission Spectroscopy

Negative triangularity (NT) shaped plasmas demonstrate ELM-free plasmas with H₉₈ > 1, β_N ≥ 2.5, and f_GW >1. Turbulence characteristics are measured with localized (k_yρ_s <1), high speed (1 MS/s), 2D (64 channels) density fluctuation measurements using Beam Emission Spectroscopy (BES). Turbulence amplitudes in NT plasmas peak just inside the separatrix, similar to positive triangularity L-mode, but with low core turbulence amplitudes of ñ/n~0.5%, similar to positive triangularity H-modes. Scanning triangularity from -0.01 to -0.12 at fixed power input induces an H-mode to ELM-free NT-edge transition, however turbulence amplitudes decrease from ñ/n of 13% to 7% in the edge and from 0.8% to <0.2% inside ρ≈0.85. Increasing co-beam injected power tends to induce a radially broadened turbulence amplitude profile with lower peak values. Poloidal correlation lengths, decorrelation times and poloidal velocities indicate an electron diamagnetic directed mode with exceptionally broad wavenumber spectrum from k_yρ_s<0.05 to k_yρ_s>0.5 peaked near ρ≈0.95. 2D BES velocimetry is used to assess poloidal flow spectrum and particle flux <Ṽ_rñ>. H-mode like confinement in negative triangularity is achieved by effective turbulence reduction in the core while facilitating an ELM-free edge.   

Observation of a bursty high-frequency edge instability in DIII-D strongly shaped divertered negative triangularity plasmas

During the 2023 DIII-D Negative Triangularity (NT) campaign, nearly all discharges in the diverted “baseline” NT shape ( experienced an edge coherent mode as observed by the magnetics with a mode frequency of 3-10 kHz and  toroidal mode number. The instability is cyclic, with a “burst” frequency of 100-300 Hz. The burst onset drives a reduction in the edge electron temperature (Te) as measured by ECE and ECEI, followed immediately by a recovery period in Te. ECE and BES data show the mode to be radially localized to the outer 10% of toroidal flux, while ECEI shows a possible external mode with the radial mode structure extending beyond the seperatix. Experimental analysis in combination with stability modelling is used to identify the mode. GATO and ELITE are utilized to assess the ideal stability of the discharges, while linear M3D-C1 provides an initial look at non-ideal effects. The primary goal of this work is to identify the type of instability observed in the DIII-D NT experiments, as well as to gain insight into the bursting nature of the instability. Significant work is still needed to fully understand these negative triangularity edge modes, but doing so may better help us understand the unique confinement characteristics of the NT configuration.   

Density peaking and particle transport in negative triangularity discharges on DIII-D

Electron transport in negative triangularity (NT) discharges was found to be an order of magnitude lower than for comparable positive triangularity discharges using gas puff modulation technique to separate the convective and diffusive components. Similar to positive triangularity plasmas, core density peaking increases with decreasing collisionality [1]. A dimensionless collisionality scan demonstrated that  [GM1] the electron density peaking expressed as ne_0.4/ne_0.8 increased from 1.1 to 1.3 when the collisionality decreased by a factor 4. Gas puff modulations were applied to disentangle the contributions from Neutral Beam Injection (NBI) core fueling versus an inward pinch in setting the core density profile [2]. Transport analysis using the Aurora framework, shows particle transport values (for both D and V), at least an order of magnitude lower when compared to positive triangularity for the plasma core. Counter to previous dimensionless collisionality analysis of particle transport, the increase in peaking in NT is due to additional core NBI fueling, not an increase in |V/D| [2].

 References

[1] C. Angioni, et al. PRL 90, 20 (2003), 205003.

[2] S. Mordijck et al. Nuclear Fusion 60, 6 (2020), 066019.   

Characterization of density limit in negative triangularity plasmas on DIII-D tokamak

The operational density limit in negative triangularity (NT) plasmas has been explored on the DIII-D tokamak, particularly its dependence on plasma current and auxiliary power. The results show that at low power input, the density limit increases as the power input increases, but the power dependence becomes weaker once the input power exceeds 5 MW. The highest achieved density in NT plasmas does not exhibit a clear dependence on the plasma current. In this experiment, it was possible to achieve a high Greenwald fraction of the line-average density (f_G=1.5-2) by using a high neutral beam injection (NBI) power input and an unfavorable ▽B drift configuration at a lower plasma current of 0.6 MA. When the line-averaged density reached the density limit, there was a noticeable collapse of the edge electron temperature and the mean shear flow profiles, indicating edge cooling. However, in high-power discharges near the density limit, disruptive avoidance could be achieved. Furthermore, as the line-averaged density approached the Greenwald limit, the intermediate-k edge density fluctuations at midplane were observed to increase, but these fluctuations became saturated once the Greenwald limit was exceeded.   

First Integration of negative triangularity plasmas with high core radiation fraction

High performance, high power, highly radiative and robustly ELM free discharges have been achieved in negative triangularity (NT) shape enabled by the DIII-D armored campaign. A wide parameter range has been achieved with a core radiated power fraction up to 0.85 and a Greenwald density greater than 1 in some cases. Core radiation limits have been explored and achieved for Ne, Ar and Kr as seeded impurities while scanning key parameters such as plasma density, current, toroidal magnetic field direction, impurity rate and gas seeding location. It was also found that adding N₂ mantle radiation leads to a reduction in heat flux at the target while the energy confinement remained constant or improved. Matched discharges at high core radiation fraction where only the toroidal field direction was changed yield to similar global parameters but very different divertor conditions which are analyzed with SOLPS-ITER modeling. BES measurements reveal reduction in turbulence during the impurity injection which correlates with changes in transport. High core radiation fraction with BetaN values greater than 2 have been sustained as long as impurity accumulation was controlled to prevent radiative collapse and excess dilution, highlighting a path for an optimized core-edge and power exhaust solution with NT shaping.   

Divertor detachment characterization in negative triangularity discharges in DIII-D via UEDGE modeling

Edge fluid modeling of the first detachment experiments in diverted negative triangularity discharges in DIII-D is presented using UEDGE including cross-field drifts. Detachment experiments were performed at strong (baseline shape, δ~-0.5) and intermediate negative triangularity (hybrid shape, ) and intermediate negative triangularity (hybrid shape, δ~-

0.2) in DIII-D via an increase in upstream density or impurity seeding. UEDGE simulations with both the ion Grad-B drift directed into (Forward B T ) and out of (Reverse B T ) the active divertor are included for the baseline shape and the ion Grad-B drift into the active divertor in the hybrid shape. Upstream density scans are performed with UEDGE to reach a detached plasma and to quantitatively recover the experimental roll-over of the ion saturation current on the outer target by the Langmuir Probes. Consistent with experiments, simulations reproduce: a higher fraction of Greenwald density required to reach detachment onset in negative triangularity compared to positive triangularity; a ~40% higher

density to reach detachment onset with Forward B T than with Reverse B T ; deep detachment only in the Forward B T case. The effect of longer connection length and changes in radial transport in the hybrid shape compared with the baseline shape are investigated in the simulations.   

Integrated modeling of highly radiative plasmas with negative triangularity shaping

A reactor-class fusion device with high radiation fraction from noble gas impurities and with negative triangularity (NT) shaping could have confinement on the scale of an H-mode plasma while intrinsically avoiding edge localized modes (ELMs). Highly radiative positive triangularity (PT) plasmas have demonstrated enhanced confinement while remaining in L-mode by dispersing heat from the core before it reaches the edge [1]. Separately, NT plasmas have demonstrated enhanced confinement in low radiation fraction plasmas [2]. In this work, we use the integrated modeling code STEP (Stability Transport Equilibrium Pedestal) to self-consistently study the core of highly radiative plasmas with noble gas radiators in both PT and NT configurations. Our workflow is validated with experimental data from DIII-D. We extend our analysis to reactor-relevant conditions and study fusion power balance in the PT and NT model reactors ARCH [3] and MANTA, respectively. We find that there is an impurity fraction that optimizes confinement before the plasma undergoes radiative collapse in both PT and NT.

[1] S Coda et al 2022 Plasma Phys. Control. Fusion 64 014004

[2] A O Nelson et al 2022 Nucl. Fusion 62 096020

[3] S J Frank et al 2022 Nucl. Fusion 62 126036   

Geometric dependencies of the mean ExB shearing rate in negative triangularity tokamaks

Negative triangularity(NT) tokamaks exhibit better confinement than positive triangularity(PT) tokamaks. ExB shearing is well known to reduce turbulence and improve confinement. We present a comparative study of the poloidal distribution of mean ExB shearing rate for PT and NT tokamaks. The effects of flux surface up-down asymmetry due to asymmetric upper and lower triangularities is also considered. Both direct eddy straining and effects on Shafranov shift feedback loops are examined. Shafranov shift increases the shearing rate at all poloidal angles for all triangularities, due to flux surface compression. The maximum shearing rate bifurcates at a critical triangularity δ_crit ≤0. Thus, the shearing rate is maximal off the outboard mid-plane for NT, while it is maximal on the outboard mid-plane for PT. For up-down asymmetric triangularity, the usual up-down symmetry of the shearing rate is broken. The shearing rate at the out board mid-plane is lower for NT than for PT, suggesting that the shearing efficiency in NT is reduced. Implications for turbulence stabilization and confinement improvement in high-β_p NT and ITB discharges and experimental suggestions will be discussed.   

Manipulating Turbulence via Flux-Surface Triangularity and Electron Cyclotron Heating

Based on TCV discharges with large triangularity |δ|, the impact of extreme |δ| > 0.6 on microturbulence is assessed. Focusing on profiles for an experimental δ = 0.3 in a TEM turbulence regime, nonlinear gyrokinetic simulations are found to be consistent with the experimental flux. Varying δ, growth rates are suppressed at large |δ|, with negative δ experiencing stronger stabilization and little dependence on radial wavenumber; this is the case irrespective of collisionality and despite more energy being available for instability and turbulence at δ < 0, suggesting a mechanism by which flux-surface shaping can avoid available energy being injected by the instability. Heat fluxes dip as |δ| > 0.6, although the ion fluxes rise at strongly negative δ, where a transition to ITG modes occurs. Quasilinear transport modeling captures trends well in an ITG regime, whereas TEM-based predictions tend to overestimate fluxes at large positive δ. This is due to substantially stronger and larger-scale zonal flows at positive δ.

Preliminary gyrokinetic results are shown of electron cyclotron heating, with explicit heat deposition during simulations. The generation of corrugations in the electron temperature and electric field can, depending on the deposition location, regulate - and indeed lower - TEM fluxes. A concurrent effect is the local destabilization of ETG modes that can drive additional flux.   

Withdrawn

I-mode is an important alternative operational scenario for burning plasmas. A possible theoretical understanding is presented for a unique turbulent transport phenomenon in the I-mode regime, i.e., the so-called transport decoupling between heat and particle in tokamak edge plasmas. Based on our particle simulations by running gyrokinetic toroidal code (GTC), we found that a particular instability can account for such experimental phenomenon, which makes it the major candidate for experimentally observed weakly coherent modes (WCMs) turbulence. This instability is driven by steep electron temperature gradient up to a certain value, with characteristic time scale around transit time of passing electrons while the spatial scale falling in the range of ion's poloidal gyro-radius. A crucial feature about this instability is that neither ions nor passing electrons can be treated adiabatically, which distinguishes it from the conventional drift-like instabilities such as ion temperature gradient modes (ITGs), trapped electron modes (TEMs) and normal electron temperature gradient modes (ETGs). Those non-adiabatic responses for both ions and electrons indeed excited this unique instability and could be saturated by typical flow shearing suppression mechanism including low frequency zonal flow and geodesic acoustic modes (GAMs) instead of poloidal mean flow to give rise to the considerable particle and heat transport compared with experimentally observed values. A detailed discussion about this instability and its nonlinear saturation mechanism which may account for experimentally observed WCMs and associated turbulent transport will also be presented in this talk.
Keywords: I-mode, WCMs, Transport decoupling.