H-mode confinement in JET with enhanced performance by pellet peaked density profiles

Abstract

The combination of two regimes of enhanced performance, the H-mode and the pellet enhanced performance (PEP) mode, has been achieved in JET. The strong enhancement of the central plasma parameters, obtained with pellet injection and subsequent auxiliary heating, is found to persist well into the H-mode phase. A characteristic of the PEP regime is that an improvement of the fusion reactivity over non-pellet discharges is obtained under the condition of nearly equal electron and ion temperatures. A maximum neutron production rate of 0.95 × 10l6 s−1 was obtained in a double-null X-point discharge with 2.5 MW of neutral beam heating and 9 MW of ion cyclotron resonance heating, with central ion and electron temperatures of about 10 keV and a central deuterium density of 8.0 × 1019 m−3. The corresponding fusion product nD(0)τETi(0) is between 7.0 and 8.6 × 1020 m−3·s·keV. The enhanced neutron production is predominantly of thermonuclear (Maxwellian) origin. The compatibility of these regimes is an important issue in the context of tokamak ignition strategies. Several technical developments on JET have played a role in the achievement of this result: (1) the use of low voltage plasma breakdown (0.15 V/m) to permit pellet injection in an X-point configuration before the formation of a q = 1 surface; (2) the elimination of ICRH specific impurities with antenna Faraday screens made of solid beryllium; (3) the use of a novel system of plasma radial position control that stabilizes the coupling resistance of the ion cyclotron heating system.

The history of research into improved confinement regimes

“…Nevertheless, the H-mode edges have been combined with the "high-β p mode" [112], the "PEP mode" [121] and the "NCS mode" [122]. With regard to the two goals of ITER -Q = 10 in an inductive and Q = 5 in a steady-state scenarios, improved confinement will play a major role.…”

Slow Transition of Energy Transport in High-Temperature Plasmas

“…In parallel with these rapidly evolving phenomena, slow evolutions into the state of improved plasma confinement have also been observed in a slowly developing edge transport barrier [12] and in other confinement improved modes. Improved Ohmic confinement mode [13], counter-neutral beam injection improved mode [14], pellet enhanced performance mode [15], radiatively improved mode [16] associated with the change of density profiles without a clear fast transition. Although these improved modes associated with the slow transition are as important as the improved mode with a fast transition, the mechanism of the slow transition has been studied little because of the slow change in transport.…”

Chapter 2: Highlights of the Physics Studies in the FTU

“…modes and is called the PEP mode. 16 In addition, the steep density profiles associated with the PEP modes generate a bootstrap current that might modify the local magnetic shear, therefore further stabilizing some turbulence. 17 This improved confinement phase is not long lasting due to the decay of the density and pressure gradients.…”