Improved performance of ELMy H-modes at high density by plasma shaping in JET

TLDR

The study investigates how plasma shaping influences high‑density ELMy H‑mode performance in JET, using a magnetic boundary geometry relevant to ITER’s Q = 10 design. The experiments employed single lower‑null plasmas with standard q profiles, neutral‑beam heating, gas fuelling, and triangularity δ≈0.45–0.5 and elongation κ≈1.75. The results confirm that higher triangularity improves confinement and permits higher steady‑state densities, achieving up to 1.1 nGR (≈95 % nGR) at 2.5 MA/2.7 T with H98≈1 and βN≈2, while the stored thermal energy remains roughly constant with increasing density under Type I ELMs.

Abstract

We present the results of experiments in JET to study the effect of plasma shape on high density ELMy H-modes, with geometry of the magnetic boundary similar to that envisaged for the standard Q = 10 operation in ITER. The experiments described are single lower null plasmas, with standard q profile, neutral beam heating and gas fuelling, with average plasma triangularity δ calculated at the separatrix ~0.45-0.5 and elongation κ~1.75. In agreement with the previous results obtained in JET and other divertor Tokamaks, the thermal energy confinement time and the maximum density achievable in steady state for a given confinement enhancement factor increase with δ. The new experiments have confirmed and extended the earlier results, achieving a maximum line average density ne~1.1nGR for H98~0.96. In this plasma configuration, at 2.5 MA/2.7 T (q95~2.8), a line average density ~95% nGR with H98 = 1 and βN~2 are obtained, with plasma thermal stored energy content Wth being approximately constant with increasing density, as long as the discharge maintains Type I ELMs, up to nped~nGR (and ne~1.1nGR).