Survey of pellet enhanced performance in JET discharges

Abstract

Pellet enhanced performance (PEP) has been observed in a number of JET discharges at various plasma conditions, in both L and H modes, with the H multiplier (the confinement enhancement factor over the Goldston confinement time) covering the range from 1 to 4, and with plasma currents from 1 MA to 4.1 MA. Most of the PEP plasmas have been created by refuelling with pellets of 4 mm diameter injected at 1.2 km/s. PEPs show an improved central confinement with an effective heat conductivity reduced by factors of approximately 2-5 relative to otherwise comparable discharges. This is possibly related to the inverted shear in the plasma core due to the large local bootstrap current density. The limitations in the PEP performance seem to be set by at least two mechanisms: impurity behaviour, MHD activity or a combination of both. In certain discharges, MHD modes seem to be able to check the often observed impurity accumulation. Too much MHD mode activity, however, easily destroys the enhanced confinement of the PEP discharge. The stability of the ballooning modes has been studied and the PEP plasma core is found to be in the second stability region against ballooning modes or close to marginal stability. In a number of discharges complex high (m,n) modes have been observed with the soft X-ray cameras. The behaviour of the low (m,n) MHD modes can only be understood by considering the detailed evolution of the inverted q profile, which exists in a given discharge

Turbulence Mechanisms of Enhanced Performance Stellarator Plasmas

“…The density peaking is a markedly different mechanism from those reported in various PEP tokamak experiments, involving either a reversed magnetic shear, due to a large bootstrap current, or high collisionality in medium-scale devices [31][32][33][34][35][36]. To see why, we employ an axisymmetric configuration with aspect ratio similar to that of W7-X and a standard value for the magnetic shear,ŝ ¼ 0.76 [37].…”

Improved confinement induced by tangential injection of compact torus into the Saskatchewan Torus-Modified (STOR-M) tokamak

“…Central fuelling of a tokamak reactor can significantly improve tokamak confinement as demonstrated in pelletenhanced performance. 1 Localized central fuelling provides a means to control the plasma density and pressure profiles which may also enhance bootstrap current density leading to modification of magnetic shear in the plasma core. In a tokamak reactor, central fuelling is desired to achieve a high fuel burn-up rate and low tritium recycling.…”

Degraded Confinement and Turbulence in Tokamak Experiments

“…Necessary condition: a strong particle source on axis. This can be done in a continuous way by massive neutral beam injection by which TFTR has reached its 'supershot' regime or in a more dynamic situation by pellet injection as Pellet-Enhanced Performance ('PEP') mode in JET [13], see Fig.4. Improvement: the higher the value of n e (0)/ n e , the better.…”