Thermal Ion Orbit Loss in Diverted Tokamaks and its Role Approaching the L-H Transition
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Thermal Ion Orbit Loss in Diverted Tokamaks and its Role Approaching the L-H Transition


The exact nature of the physics governing the L-H transition seen in tokamak magnetic confinement experiments has eluded fusion researchers for several decades. To date, a first principles model for the transition does not exist. The improved particle and energy confinement realized by the suppression of turbulence in the post-transition H-mode motivates an understanding of the transition and the empirically known conditions necessary for its initiation, generically an input power threshold with key sensitivities to the edge electron density, main ion mass and charge, plasma configuration, divertor conditions, ∇B drift direction, etc. Modern consensus that an increase in the E � B shear at the plasma edge is responsible for the turbulence suppression and formation of a transport barrier invigorates research into possible driving mechanisms. The loss of thermal ions from the imperfectly confining magnetic field of a tokamak manifests as a steady-state radial current in the edge and has long been suspected to play a role in the generation of the E � B shear and hence the L-H transition.The body of this thesis presents the development of a model for the steady-state thermal orbit loss based on the identification of the phase-space loss cone. The presented model boasts several improvements over other loss cone models found in the literature, largely rooted in the careful consideration of local pitch angle scattering on ions within and near the velocity-space boundaries of projections of the phase-space loss cone to observation points in configuration-space. The probability that ions within the loss cone will be lost on a first orbit is estimated by comparing the rates of collisionally scattering out of the loss cone to the periods of orbit loss. The steady-state is determined by the rates of collisional loss cone refueling modified by the statistical chance of first orbit loss. A competition arises between the sufficiently large temperatures necessary for appreciable parts of the distribution to interact with the loss cone and the reduced rate of collisional refueling of high energy ions. The steady-state orbit loss current calculated by the model exhibits several features of the experimentally measured L-H transition power threshold not present in other models. The orbit loss current displays branching behaviors in the edge density, peaking at densities similar to those minimizing the required transition power on ASDEX Upgrade. Additionally, the loss current features the suspected strong ∇B drift direction asymmetry of the orbit loss. The unfavorable drift configuration requires about a factor of two greater input power to produce a similar orbit loss current seen in the favorable drift, again echoing a known behavior of the power threshold. Other explored features that suggest a promising connection between the thermal orbit losses and the transition are the main ion mass and the horizontal position of the X-point. The orbit loss current has been implemented into the edge fluid transport code SOLPS. The first order plasma response to the current is studied over the high-density branch of the loss current. The leading order effect is an increase in the magnitude of the edge Er well and the associated E � B shear. Over the explored parameter space, the input power necessary to reach some threshold Er magnitude lessens on the order of ∼ 10–20% in the presence of the loss current. Thermal ion orbit loss appears capable of influencing the onset of the L-H transition.

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