Fast Response Scintillator Based Detector for MHD Induced Energetic Ion Losses in ASDEX Upgrade

In fusion plasma devices, fast particles i.e. suprathermal ions
generated by heating systems and fusion born a particles must be
well confined, until they have transferred their energy to the
plasma bulk. Signicant loss of these ions may reduce drastically
the heating eficiency and, in addition, may cause damage to plasma
facing components in the vacuum vessel, if it is suficiently
intense and localized. A detailed knowledge of the underlying
physics in particular in the presence of magnetohydrodynamic (MHD)
instabilities is of crucial importance, since these instabilities
can lead to an enhancement of the outwards fast ion radial drift.
The development of a new diagnostic for the study of fast
particle-wave interactions in the ASDEX Upgrade tokamak as well as
the interpretation of the rst measurements have been the aim of
this thesis. The design is based on similar diagnostics that have
been operated in the TFTR tokamak and the W7-AS stellerator. The
fast ion loss detector acts as a magnetic spectrometer, dispersing
fast ions onto a scintillator, with the strike point depending on
their gyroradius (energy) and pitch angle (angle between ion
velocity and magnetic eld line). The emitted light pattern allows
particle identification in the phase space with a high time
resolution. The major new development for the diagnostic used on
ASDEX Upgrade is the use of a very fast scintillator material that
allows sampling rates up to 1 MHz, adequate to study time resolved
interactions between MHD modes and fast particles. Fast Ion Losses
(FIL) were found in the presence of different kinds of MHD
instabilities: time resolved FIL due to Edge Localized Modes (ELMs)
have been directly observed. They show a complex behavior of a
great variety, depending on the ELM substructure. The influence of
ELMs on escaping fast particles is appreciable in the whole lost
particle phase space independent of the fast ion source. FIL could
be measured in the presence of Toroidal Alfv´en Eigenmodes (TAEs)
in ICRH heated discharges. Both species, fast hydrogen and
deuterium ions are affected in a similar way by TAEs. A resonant
process between the TAE frequency and the precession frequency of
the lost ions has been identied by comparisons with HAGIS
simulations as the loss mechanism. A new MHD perturbation has been
observed for the first time during this thesis by means of its
strong influence on the energetic deuterium ion population. The
mode is localized deeply in the plasma core and dominates the uxes
of lost fast deuterium ions in ICRH heated discharges. Finally,
bursts of fast deuterium ions ejected by Neoclassical Tearing Modes
have been detected in discharges with different heating systems. In
pure NBI heated discharges, these ions have energies approximately
equal to the full NBI injection energy and pitch angles
corresponding to ions on passing orbits. A detailed study of the
FIL signal together with Mirnov coil signals revealed that the
losses are due to a diffusive process. According to this,
simulations with the ORBIT code have proven that orbit
stochasticity is a good candidate for the mechanism that causes the
losses of these, in principle well confined, passing ions. These
results revealed the high diagnostic potential of this method,
opening new ways towards a better understanding of the fast ion
physics and therefore will help to predict the behavior of fast
ions in the presence of MHD instabilities for ITER.