Improved Confinement in JET Hybrid Discharges

A new technique has been developed to produce plasmas with improved confinement relative to the H98,y2 scaling law on the JET Tokamak. In the mid size Tokamaks ASDEX Upgrade and DIII-D heating during the current formation is used to produce a flat q-profile with a minimum close to 1. On JET this technique leads to q-profiles with similar minimum q but opposite to the other Tokamaks not to an improved confinement state. By changing the method utilising a faster current ramp with temporary higher current than in the flattop (current over shoot) plasmas with improved confinement (H98,y2 = 1.35) and good stability (bN 3) have been produced and extended to many confinement times only limited by technical constraints. This paper will introduce the q-profile forming method, the consequent current diffusion and will discuss the effect on the kinetic profiles in phases without the presence of Neo-classical Tearing Modes (NTMs).
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