Speaker
Description
In the transition from L-mode to H-mode, a sheared plasma flow perpendicular to the magnetic field caused by the gradient of a local radial electric field Er is developed. This E × B flow is believed to be crucial for turbulence suppression and the formation of an edge transport barrier, which ultimately leads to better plasma performance. While this is well accepted, there is a long-standing open question about the underlying mechanism for Er generation [1]. Amongst the possible contributors to the Er generation is the ion orbit loss mechanism [2], i.e. the non-ambipolar loss of ions executing orbits close to the plasma boundary [3].
This work studies whether the non-ambipolar loss of fast-ions can act as a source for Er and facilitate the entrance to the H-mode. To address this, dedicated experiments to evaluate the impact of NBI driven fast-ion losses on the Er generation have been carried out at the ASDEX Upgrade tokamak. The Er and fast-ion losses have been characterized using He Spectroscopy and the FILD suite, respectively. The plasma current (Ip = 0.4 – 0.7 MA) has been used as actuator to modify fast-ion losses (as wider banana orbits lead to higher losses) at total input power near the L-H transition. The fast-ion distribution function has also been scanned, using different NBI voltages and injection geometries. The fast-ion losses were successfully modified, and a correlation between the minimum vpol (proxy for Er well) and the total losses to the wall for different NBI sources was found. Interestingly, no LH transition was found during the Ip scans, suggesting that (1) the increased fast-ion losses cannot compensate for the degradation of plasma confinement during the Ip ramps and/or (2) that the heat source loss may be more important than the generation of Er via non-ambipolar loss. The experiments are combined with modeling work to decouple the impact of NBI deposition on fast-ion losses, the edge ion heat flux and toroidal rotation [4], using the TRANSP and the orbit-following ASCOT [5] codes. The ASCOT code is used to benchmark fast-ion losses against FILD measurements and evaluate the beam deposited torque and its contribution to Er via radial force balance.
[1] JW Connor and HR Wilson, Plasma Phys. and Control. Fusion 2000
[2] CS Chang et al., Phys. Rev. Lett. 2017
[3] K Miyamoto, Nuclear Fusion 1996
[4] F Ryter et al., Nuclear Fusion 2014
[5] E Hirvijoki et al. Comput. Phys. Commun. 2014
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