EFDA-JET-CP(01)02/27

Extreme Shear Reversal in JET Discharges

Discharges in JET have been studied where a combination of fast current ramp with Lower Hybrid heating and Current Drive (LHCD) is used to produce a reversed shear q-profile, Fig.1. The choice of a fast Ip ramp means that the current is still diffusing into the core when the main experiment starts. The diffusion process is slowed by the heating action of the LHCD which raises Te and increases conductivity, h||, [1]. The LHCD also drives a current parallel to the plasma current with an absorption radius peaked off-axis, which tends to remove current from nearby regions, further reducing the on-axis current density[2]. In this scenario a transport barrier can be established in the electron channel (eITB) which persists into the main heating phase. This is manifest as steep gradient in Te at r/a = 0.2-0.3 with a flattening of the profile inside this barrier region. Also during this phase there are pe-riodic, 'sawtooth-like', relaxations[3] in central Te with accompanying positive transients in outer channels from the escaping heat pulses, Fig.1. The inversion radius of these collapses liesin the steep gradient part of the Te profile.
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