Implementation and validation of swept density reflectometry for integrated data analysis at ASDEX Upgrade.

In the tokamak ASDEX Upgrade, Integrated Data Analysis (IDA) is used to infer plasma quantities, such as electron density, using heterogeneous data sources. Essential is forward modeling from the parameter space into the data space with physically reasonable models for probabilistic evaluation. This paper presents a new forward model for O-mode profile reflectometry, a necessary prerequisite for Bayesian inference and inclusion in IDA. An efficient forward model based on the analytic solution for a piece-wise linear density description allows IDA to overcome problems associated with the established determination of cut-off locations via Abel inversion and Bottollier-Curtet's method. Instead of using a hard-coded initialization for densities below the first measured cut-off density, other diagnostics, such as the lithium beam, are used to analyze the shape of the initial part of the profile. Error propagation from the measured data, and other uncertain sources, to the uncertainties in the density profile and also its gradient is an intrinsic property of the probabilistic approach, which benefits from the joint analysis. Missing or ambiguous data do not prevent the profile evaluation, but only increase the uncertainty for densities in the affected range. Density profiles together with their uncertainties are determined by the joint analysis of complementary diagnostics, with the newly added reflectometry closing a gap in the outer core region. A stand-alone inversion based on the new forward model, including uncertainty quantification, is introduced, optionally providing an n(R) profile with uncertainties and a gradient. This method is a candidate for real-time analysis, providing error bars.

Compact Radiative Divertor Experiments at ASDEX Upgrade and Their Consequences for a Reactor.

We present a novel concept to tackle the power exhaust challenge of a magnetically confined fusion plasma. It relies on the prior establishment of an X-point radiator that dissipates a large fraction of the exhaust power before it reaches the divertor targets. Despite the spatial proximity of the magnetic X point to the confinement region, this singularity is far away from the hot fusion plasma in magnetic coordinates and therefore allows the coexistence of a cold and dense plasma with a high potential to radiate. In the compact radiative divertor (CRD) the target plates are placed close to this magnetic X point. We here report on high performance experiments in the ASDEX Upgrade tokamak that indicate the feasibility of this concept. Despite the shallow (projected) field line incidence angles of the order of θ_{⊥}=0.2°, no hot spots were observed on the target surface monitored by an IR camera, even at a maximum heating power of P_{heat}=15 MW. And even with the X point located exactly on the target surface and without density or impurity feedback control, the discharge remains stable, the confinement good (H_{98,y2}=1), hot spots absent, and the divertor in a detached state. In addition to its technical simplicity, the CRD scales beneficially to reactor-scale plasmas that would benefit from an increased volume of the confined plasma, more space for breeding blankets, smaller poloidal field coil currents, and-potentially-an increased vertical stability.