Turbulent ion heating in TCV tokamak plasmas
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Christian Schlatter, Turbulent ion heating in TCV tokamak plasmas, Ph.D. thesis, no. 4479, Ecole polytechnique fédérale de Lausanne (EFPL), CH-1015 Lausanne, Switzerland, July 2009.The thesis in electronic form are identical to the printed edition. Differences might exist with regard to the title page and the use of different fonts.
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The Tokamak à configuration variable (TCV) features the highest electron cyclotron wave power density available to resonantly heat (ECRH) the electrons and to drive noninductive currents in a fusion grade plasma (ECCD). In more than 15 years of exploitation, much effort has been expended on real and velocity space engineering of the plasma electron energy distribution function and thus making electron physics a major research contribution of TCV.
When a plasma was first subjected to ECCD, a surprising energisation of the ions, perpendicular to the confining magnetic field, was observed on the charge exchange spectrum measured with the vertical neutral particle analyser (VNPA). It was soon concluded that the ion acceleration was not due to power equipartition between electrons and ions, which, due to the absence of direct ion heating on TCV, has thus far been considered as the only mechanism heating the ions. However, although observed for more than ten years, little attention was paid to this phenomenon, whose cause has remained unexplained to date.
The key subject of this thesis is the experimental study of this anomalous ion acceleration, the characterisation in terms of relevant parameters and the presentation of a model simulation of the potential process responsible for the appearance of fast ions. The installation of a new compact neutral particle analyser (CNPA) with an extended high energy range (= 50 keV) greatly improved the fast ion properties diagnosis.
The CNPA was commissioned and the information derived from its measurement (ion temperature and density, isotopic plasma composition) was validated against other ion diagnostics, namely the active carbon charge exchange recombination spectroscopy system (CXRS) and a neutron counter. In ohmic plasmas, where the ion heating agrees with classical theory, the radial ion temperature profile was successfully reconstructed by vertically displacing the plasma across the horizontal CNPA line of sight. Active charge exchange measurements, by doping the plasma with ion neutralisation targets injected with the diagnostic neutral beam (DNBI), were used to absolutely calibrate the NPA. Advanced modelling of the measured hydrogenic charge exchange spectra with the neutralisation and neutral transport codes KN1D and DOUBLE-TCV permitted a calculation of the absolute neutral density profiles of the plasma species.
The energisation and the properties of fast ions were studied in dedicated, low density, cold ion, hot electron plasmas, resonantly heated at the second harmonic of the electron cyclotron frequency. The ion acceleration occurs on a characteristic timescale in the sub-millisecond range and comprises up to 20 % of the plasma ions. The number of fast ions nis and their effective temperature Tis are found to depend strongly on the bulk and suprathermal electron parameters, in particular Tis = Teb (electron bulk) and nis ~ Vde (toroidal electron drift speed). The suprathermal electrons, abundantly generated in plasmas subjected to ECCD, are diagnosed with perpendicular and oblique viewing electron cyclotron emission (ECE) antennas and the measured frequency spectra are reconstructed with the relativistic ECE radiation balance code NOTEC-TCV. With steady-state ECRH and ECCD, the fast ion population reaches an equilibrium state. The spatial fast ion temperature profile is broad, of similar shape compared to the bulk ion temperature profile. The hottest suprathermal temperature observed is Tis = 6 keV.
Various potential ion acceleration mechanisms were examined for relevance in the TCV parameter range. The simultaneous wave–electron and wave–ion resonances of ion acoustic turbulence (IAT) show the best correlation with the available experimental knowledge. Ion acoustic waves are emitted by the weakly relativistic circulating electrons and are mainly Landau damped onto the ions. Destabilisation of IAT is markedly facilitated by the important degree of nonisothermicity Te/Ti = 40 of X2 EC heated TCV plasmas.
Efforts were undertaken to consistently model the experimental observations using a numerical experiment. The relevant physics describing IAT was implemented in a finite difference code solving the quasilinear diffusion equation describing the time evolution of the electron and ion distribution functions. The simulations, fed as far as possible with experimentally available information, confirm the growth and saturation of IAT. Electrons and ions are initially preferentially heated in the toroidal direction. As the ions gain energy, the ion waves are damped more efficiently and only modes propagating at oblique angles can still grow, thus accelerating ions into the radial perpendicular direction. The simulation shows that turbulence reaches a steady-state when the ions are sufficiently hot to permanently stabilise IAT. The parameters describing the tail of the modelled equilibrium ion distribution agree quantitatively well with the CNPA measurement.
Preliminary studies investigated on the interaction of fast ions with the sawtooth instability. It is found that the fast ion population in sawtoothing plasmas is transiently enforced with each sawtooth collapse. It is presently thought that the toroidal electric reconnection field lowers the IAT stability threshold thus producing more suprathermal ions.