[Paper Review] Simultaneous access to high normalized current, pressure, density, and confinement in strongly-shaped diverted negative triangularity plasmas

C. Paz-Soldan, C. Chrystal+13 more|arXiv (Cornell University)|Sep 7, 2023

Magnetic confinement fusion researchPhysics and Astronomy3 citations

TL;DR

This study demonstrates in DIII-D tokamak experiments that strongly-shaped, diverted negative triangularity (NT) plasmas simultaneously achieve high normalized current (q₉₅ > 3), pressure (βₙ > 3), density (f_GW > 1), and H-mode confinement (H₉₈ > 1), with robust stability and no edge-localized mode (ELM) activity. The results show that NT plasmas can access a high-performance, stable operating space comparable to conventional positive triangularity plasmas, significantly advancing their viability for fusion energy applications.

ABSTRACT

Strongly-shaped diverted negative triangularity (NT) plasmas in the DIII-D tokamak demonstrate simultaneous access to high normalized current, pressure, density, and confinement. NT plasmas are shown to exist across an expansive parameter space compatible with high fusion power production, revealing surprisingly good core stability properties that compare favorably to conventional positive triangularity plasmas in DIII-D. Non-dimensionalizing the operating space, edge safety factors below 3, normalized betas above 3, Greenwald density fractions above 1, and high-confinement mode (H-mode) confinement qualities above 1 are simultaneously observed, all with a robustly stable edge free from deleterious edge-localized mode instabilities. Scaling of the confinement time with engineering parameters reveals at least a linear dependence on plasma current although with significant power degradation, both in excess of expected H-mode scalings. These results increase confidence that NT plasmas are a viable approach to realize fusion power and open directions for future detailed study.

Motivation & Objective

• To demonstrate that strongly-shaped, diverted negative triangularity (NT) plasmas can simultaneously access high normalized current, pressure, density, and confinement.
• To assess the stability and performance of NT plasmas under conditions relevant to fusion power production, particularly in the absence of edge-localized mode (ELM) instabilities.
• To evaluate whether NT plasmas can match or exceed the performance of conventional positive triangularity (PT) plasmas in key fusion-relevant parameters.
• To investigate the scaling of energy confinement time (τ_E) with engineering parameters in NT configurations, comparing to standard H-mode scalings.
• To identify operational and stability constraints limiting plasma performance in NT configurations and inform future improvements in shape control and current drive.

Proposed method

• Conducted experimental campaigns on the DIII-D tokamak using temporary graphite 'armor' components to enable a poloidal divertor configuration in strongly-shaped (δ ≈ -0.5) negative triangularity plasmas.
• Measured and analyzed plasma parameters including q₉₅, βₙ, f_GW, H-mode confinement (H₉₈), and energy confinement time (τ_E) over a range of plasma currents (0.5–1.1 MA), toroidal fields (1.0–2.1 T), and input powers (0.5–12 MW).
• Used non-dimensional scaling to compare performance against standard H-mode scaling laws (e.g., ITER H97), evaluating confinement quality relative to expectations.
• Applied engineering parameter regression to quantify the dependence of τ_E on plasma current and other key parameters, identifying at least linear scaling with current.
• Employed the OMFIT integrated modeling framework for data analysis and validation of plasma state and transport characteristics.
• Focused on discharges with stationary phases ≥3τ_E to ensure reliable confinement measurements and statistical robustness.

Experimental results

Research questions

• RQ1 Can strongly-shaped, diverted negative triangularity plasmas simultaneously achieve high normalized current, pressure, density, and confinement without ELMs?
• RQ2 How does the confinement performance of NT plasmas compare to standard H-mode scalings and conventional positive triangularity plasmas?
• RQ3 What is the dependence of energy confinement time (τ_E) on plasma current and other engineering parameters in NT configurations?
• RQ4 To what extent do stability constraints limit the achievable plasma current and elongation in NT plasmas compared to conventional configurations?
• RQ5 What are the operational and physics limitations affecting the absolute performance of NT plasmas, particularly in terms of volume and current?

Key findings

• Strongly-shaped, diverted NT plasmas in DIII-D simultaneously achieved q₉₅ > 3, βₙ > 3, f_GW > 1, and H₉₈ > 1, indicating high performance across all major fusion-relevant normalized parameters.
• The plasmas exhibited robust edge stability with no observed edge-localized mode (ELM) activity, even at high normalized pressure and density, confirming the ELM-free advantage of the NT configuration.
• Energy confinement time (τ_E) showed at least a linear dependence on plasma current, with significant power degradation exceeding standard H-mode scalings.
• Despite lower plasma volume (14–15 m³) and reduced elongation (1.4–1.5) compared to typical DIII-D positive triangularity plasmas (18–19 m³, 1.8–1.9), the NT plasmas maintained high confinement quality.
• The operating space for NT plasmas was found to be expansive and stable, challenging prior assumptions that NT configurations suffer from severe stability penalties limiting performance.
• The results indicate that NT plasmas are a viable path to high-performance fusion conditions, motivating improvements in shape control and current drive to enhance absolute performance.