EFDA-JET-CP(11)04/10

Comparison of Different Improved H-mode Scenarios on ASDEX Upgrade and JET

Within the last few years significant progress has been made on the hybrid scenario on JET. By introducing the so called current over-shoot technique significant improved confinement with H89y2 = 1.3 has been reached. This technique implements a fast current ramp to a high current followed by a current ramp down to remove some current from the edge. The effect is two fold. Firstly, the core q-profile is flatter than in the previous attempts utilising early NBI heating or LHCD and secondly, the magnetic shear is increased at the edge for a shorter period of time due to quick current diffusion there. Having achieved this of course some questions arise. Why does JET need different q-profile shaping techniques than ASDEX Upgrade to produce hybrid like q-profiles? Does the same method work on other machines or is it specific to JET? As a follow up experiment the current over shoot technique has been used on ASDEX Upgrade and some preliminary experiments in a low d configuration have been done. Within a few pulses a similar quasi steady performance as on JET in a low d shape has been reached without the excitation of low n-number NTM's (H98y2 = 1.3, bN = 2.5), transiently much higher performance has been reached and further experiments are planned. This indicates that similar q-profile shaping methods can lead to similar performance on different devices. In a pulse repeated without current over-shoot good performance was reached which is similar to the JET results. The absolute and also the dimensionless parameters are rather different between the two machines. The main difference is a line averaged electron density of about 6·1019 m-3 on ASDEX Upgrade and 3.6·1019 m-3 on JET leading to different n* and r* at similar bN values. Within this paper a detailed analysis of the different pulses on ASDEX Upgrade will be shown. The paper will discuss the different heating scenarios and current ramps and their influence on the q-profile development. This will be put into context with the improved Hmode scenarios developed at JET and the scenarios developed at ASDEX Upgrade e.g. low density, low beta plasmas with high confinement or very high density, high beta plasmas using nitrogen seeding with good confinement.
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