Pitch-angle distribution of TAE-induced losses of ICRH accelerated ions on JET

Abstract

Experiments aiming to study the loss of energetic ions carried out in JET using low density plasmas and on-axis minority ion cyclotron resonance heating (ICRH) have been reported in Nabais et al (2010 Nucl. Fusion 50 115006). Though the fast ions accelerated by on-axis ICRH have a distribution in the normalized magnetic moment centred at Λ = 1, experimental measurements have shown that the fast ions whose loss is triggered by tornado modes (toroidal Alfvén eigenmodes (TAE) localized inside the q = 1 surface) reach the scintillator cup with an average Λ significantly higher than one (Λ = 1.1). On the other hand, the losses associated with the presence of TAE localized on the plasma boundary reach the scintillator cup with an average Λ slightly below unity. Since Λ increases while the fast ions drift radially outward due to resonant interaction with the Alfvénic modes, the high values of Λ characterizing the losses triggered by tornado modes indicate these ions have experienced a large radial displacement before being lost, i.e. they were originally localized in the plasma core. These experimental results corroborate the conclusion from Nabais et al (2012 Nucl. Fusion 52 083021) that the combined action of modes localized at different radial locations can lead to large radial transport and loss of ions originally localized in the plasma core. The results presented here also show that measurements of the pitch angle may be used to extract information about the original location of the ions reaching the scintillator cup.