Detection of neutral beam deuterons acceleration by 2nd harmonic radio frequency heating in ASDEX Upgrade via neutron spectrometry

Second harmonic absorption of radio frequency in the ion cyclotron range of frequency (ICRF) by fast D is expected in the case of H minority heating at the fundamental cyclotron frequency, provided the ion Larmor radius is of the order of the RF-wavelength, as it is for neutral beam injection (NBI)-D ions. In tokamaks a strong increase of neutron production is observed when such an ICRF heating is applied on top of NBI, often referred to as 'synergy effect'.

On ASDEX Upgrade a scintillator-based compact neutron spectrometer (CNS) is available outside the torus hall, with perpendicular view through the plasma equatorial plane, measuring pulse height spectra (PHS), which allow to infer the ion distribution functions, in particular the suprathermal tail.

The measured PHS exhibit clear energetic tails when ICRF is applied in the presence of NBI, proving there's an overproportional tail in the fast-ion distribution at the high energy end. The PHS in the CNS line-of-sight are modelled coupling kinetic solvers with wave codes. Unfolding the PHS into neutron emission spectra allows to quantify the maximum fast ion energy with significant occurrence, showing a non-negligible population around 500 keV, significantly higher than the NBI energy. A sensitivity study assesses the maximum fast ion energy compatible with the measured PHS, confirming the presence of 500 keV D ions. (paper)