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.

RADCAM-A radiation camera system combining foil bolometers, AXUV diodes, and filtered soft x-ray diodes

Measurements of radiated power are critical for characterizing and optimizing tokamak performance. The RADCAM system, comprising arrays of foil bolometers, Absolute eXtreme UltraViolet (AXUV), and filtered soft x-ray diodes, has been constructed to provide improved measurements of plasma radiation on "Tokamak a Configuration Variable" (TCV). An overview of the physical geometry, electronics, and design of the system is provided. The construction of the bolometer foils together with the improved sensitivity characteristics resulting from the inclusion of an anti-reflection carbon coating are presented. The large number of lines of sight in RADCAM are shown to significantly increase the spatial resolution over the legacy system. The system calibration procedure is detailed, and the mean system sensitivity is shown to vary by less than 5% over 1000 discharges. Additionally, the methodology for cross-calibration of the AXUV diodes with the bolometer foils is presented and applied to generate high temporal resolution measurements. The RADCAM radiation camera system is a compact, versatile system that is demonstrated to provide high resolution profiles of the radiated power in TCV.

Physically principled reflection models applied to filtered camera imaging inversions in metal walled fusion machines

Ray-tracing techniques are applied to filtered divertor imaging, a diagnostic that has long suffered from artifacts due to the polluting effect of reflected light in metal walled fusion machines. Physically realistic surface reflections were modeled using a Cook-Torrance micro-facet bi-directional reflection distribution function applied to a high resolution mesh of the vessel geometry. In the absence of gonioreflectometer measurements, a technique was developed to fit the free parameters of the Cook-Torrance model against images of the JET in-vessel light sources. By coupling this model with high fidelity plasma fluid simulations, photo-realistic renderings of a number of tokamak plasma emission scenarios were generated. Finally, a sensitivity matrix describing the optical coupling of a JET divertor camera and the emission profile of the plasma was obtained, including full reflection effects. These matrices are used to perform inversions on measured data and shown to reduce the level of artifacts in inverted emission profiles.

Description of complex viewing geometries of fusion tomography diagnostics by ray-tracing

Ray-tracing techniques are applied to bolometry, a diagnostic where the finite collection volume is particularly sensitive to the machine and detector configuration. A technique is presented that can handle arbitrarily complex aperture and collimator geometries, neglecting reflection effects. Sight lines from the ASDEX Upgrade bolometer foils were ray-traced with a path tracing algorithm, where the optical path is represented by a statistical bundle of ray paths connecting the foil surface with the slit geometry. By using the full 3D machine model for the detector box and first wall, effects such as occlusion and vignetting were included in the calculation of the bolometer's étendue. Inversion matrices calculated with the ray-tracing technique were compared with the more conventional single-ray approach and shown to be naturally more constrained, requiring less regularisation. The two models were tested on a sample radiation scenario, and the common single-ray approximation is shown to be insufficient. These results are particularly relevant for the divertor where strong emission gradients may be present. The technique developed generalises well to arbitrarily complex viewing geometries and collimators, opening up a new design space for bolometer configurations that might not normally have been considered.

KDI: A wireless ECoG recording platform with impedance spectroscopy, electrical stimulation and real-time, lossless data compression

A power-efficient modular wireless platform has been designed for prototyping and pre-clinical evaluations of neural recording implants. This Kit for Designing Implants (KDI) is separated in function specific modules of 34×34mm which can be assembled as needed. This paper presents the design of new modules for this existing wireless KDI platform. These modules cover the functionalities of electrical stimulation for BCI neurofeedback, impedance spectroscopy for monitoring tissue reaction around implanted electrodes and a real-time lossless data compression algorithm for ECoG signals. This algorithm has been implemented using two different hardware solutions and its performances compared. The design and evaluation of these modules are a first step towards the inclusion of these functionalities into the next generation of WIMAGINE(®) implants.

Application of AXUV diode detectors at ASDEX Upgrade

In the ASDEX Upgrade tokamak, a radiation measurement for a wide spectral range, based on semiconductor detectors, with 256 lines of sight and a time resolution of 5 μs was recently installed. In combination with the foil based bolometry, it is now possible to estimate the absolutely calibrated radiated power of the plasma on fast timescales. This work introduces this diagnostic based on AXUV (Absolute eXtended UltraViolet) n-on-p diodes made by International Radiation Detectors, Inc. The measurement and the degradation of the diodes in a tokamak environment is shown. Even though the AXUV diodes are developed to have a constant sensitivity for all photon energies (1 eV-8 keV), degradation leads to a photon energy dependence of the sensitivity. The foil bolometry, which is restricted to a time resolution of less than 1 kHz, offers a basis for a time dependent calibration of the diodes. The measurements of the quasi-calibrated diodes are compared with the foil bolometry and found to be accurate on the kHz time scale. Therefore, it is assumed, that the corrected values are also valid for the highest time resolution (200 kHz). With this improved diagnostic setup, the radiation induced by edge localized modes is analyzed on fast timescales.

[Long-term follow-up with various Dacron bifurcation prostheses. 5-years results of a prospective randomized study]

In the frame of a prospective randomised study various Dacron bifurcation prostheses with or without velours or with or without gelatin impregnation were implanted in one hundred patients. Fifty-four patients could be reexamined clinically and sonographically after an average of 66 months. A total of 23 patients had died during the period of observation. A comparison of the Fontaine stages during reexamination indicated in 72.3% of cases an operation-related improvement as compared to the condition in the preoperative stage. Sonographically, an average dilatation of 17.3% could be proved. A significant difference due to material or impregnation was not observed. After 72 months the limb salvage amounted to 83.0%. A comparison of the various types of prostheses regarding their limb salvage could indicate no significant difference in the course of 72 months. Based on the only slightly differing results within the period of observation, only the more comfortable intraoperative handling leads to giving preference to the use of double velours prostheses in comparison to prostheses without velours. A positive influence of impregnation with gelatin could not be established.