The beta scaling of energy confinement in ELMy H-modes in JET

The disagreement between the weak dependence of the energy confinement time on normalized pressure, β, observed in dedicated scans and the strongly negative dependence in the confinement scaling laws used for the design of next step tokamaks and future reactors, remains an outstanding problem. As such, scans of β have been undertaken in single null, low triangularity (δ ≈ 0.2) ELMy H-mode plasmas in JET with the MarkIIGB-SRP divertor. The scans varied β by a factor of 2.8 (normalized β from 0.72 to 2.04) and covered a range of magnetic fields (1.5-2.3 T), plasma currents (1.5-2.75 MA) and safety factors (q₉₅ = 2.8 and 3.3). A weak β dependence was observed both globally (B₀τ_E varied less than 9% across any one scan) and locally. A scan within Type I ELMy H-modes suggests that this weaker dependence is not due to ELM regimes. A statistical analysis indicates that these results are consistent with log-linear regressions performed on a wide JET database of ELMy H-modes, if correlations in this database are considered.