High poloidal beta equilibria in the Tokamak Fusion Test Reactor limited by a natural inboard poloidal field null

Abstract

Recent operation of the Tokamak Fusion Test Reactor (TFTR) [Plasma Phys. Controlled Nucl. Fusion Research 1, 51 (1986)] has produced plasma equilibria with values of Λ≡β_p eq+l_i/2 as large as 7, εβ_p dia≡2μ₀ε〈p_⊥〉/〈〈B_p〉〉² as large as 1.6, and Troyon normalized diamagnetic beta [Plasma Phys. Controlled Fusion 2 6, 209 (1984); Phys. Lett. 1 1 0 A, 29 (1985)], β_Ndia≡10⁸〈β_t⊥〉aB₀/I_p as large as 4.7. When εβ_p dia≳1.25, a separatrix entered the vacuum chamber, producing a naturally diverted discharge that was sustained for many energy confinement times, τ_E. The largest values of εβ_p and plasma stored energy were obtained when the plasma current was ramped down prior to neutral beam injection. The measured peak ion and electron temperatures were as large as 24 and 8.5 keV, respectively. Plasma stored energy in excess of 2.5 MJ and τ_E greater than 130 msec were obtained. Confinement times of greater than 3 times that expected from L‐mode predictions have been achieved. The fusion power gain Q_DD reached a value of 1.3×10⁻³ in a discharge with...