Engineering the Alcator C-Mod MSE diagnostic: Solutions to reactor-relevant diagnostic challenges
Deploying precise optical diagnostics in harsh reactor environments poses significant engineering challenges including limited access, large forces, time-varying systematic errors, reflections, low signal-to-background levels and a need for in-situ calibration. Fielding a polarization-based Motional Stark Effect (MSE) diagnostic on Alcator C-Mod has proven to be difficult due to similar challenges, which has led to the development of solutions that enable polarization angle measurements with an accuracy of 0.2 degrees.
Engineering upgrades to the Motional Stark Effect diagnostic on Alcator C-Mod
The Motional Stark Effect (mse) diagnostic deployed on the alcator c-mod tokamak previously experienced unacceptable calibration drift and sensitivity to partially-polarized background light that limited its ability to measure magnetic field pitch-angles. The need to identify the cause of the polarization drift and to characterize sources of error motivated the development of a robotic calibration system consisting of a motorized three axis positioning system and a light source capable of generating arbitrary polarization states. The system produces linear polarization angles with accuracy of < 0.05◦, operates during maintenance periods, has a Lambertian and laser illumination source, and captures the complex diagnostic polarization response using a Fourier expansion of the detector signals in terms of even harmonics of the input polarization angle. This study guided the fielding of a first-of-a-kind inter-shot calibration (isc) system for the mse diagnostic—a long sought capability. The isc inputs high quality polarized light at four carefully chosen polarization angles into the diagnostic objective lens within ten seconds of the plasma discharge using a single high-precision moving part internal to the vacuum vessel. Fiber optics are used to sequentially illuminate the calibration polarizers via backlight scattering similar to an LCD screen, fully filling the view of each of the ten mse sightlines with polarization angles repeatable to better than 0.05◦. The system has operated inside the vacuum for over 8500 cycles and is used to reconstruct the diagnostic calibration on a variety of timescales. This system was used to identify thermal-stress induced birefringence in several of the diagnostic’s lenses as the cause of polarization calibration drift. Gold-plated radiative heat-shields and low-conduction lens and periscope mounts thermally isolate the in-vacuum periscope, and a forced-flow active thermal control system regulates the ex-vessel periscope and vacuum window. This successfully controls the optic stress in thermal environment ranging ±150◦C. A high-throughput narrow-bandpass filter-based polychromator was developed as a new configuration for mse detectors. The system allows high-spectral selectivity at an étendue of > 15mm2 sr for four simultaneous polarization measurements on the same viewing sightline. It sequentially passes light among four different < 0.9nm FWHM temperature-tuned filters to custom avalanche photodiode detectors operating at NA = 0.6. The polychromator is imaging, non-vignetting, high transmission, utilizes mostly off-the shelf optics, is easily aligned, remotecontrolled and can be replicated at relatively low cost. This enables wavelength-interpolation of the mse background light and simultaneous detection of different parts of the Stark manifold. Thesis Supervisor: Steven D. Scott Principal Research Physicist, Princeton Plasma Physics Laboratory Thesis Reader: Ian H. Hutchinson Professor, Department of Nuclear Science and Engineering
Numerical analysis and experimental observation of guidewire motion in a blood vessel model.
We have developed a computer-based system to simulate a guidewire in blood vessels for surgical planning, intra-operative assistance, and to facilitate the design of new guidewires. In this study, we compared simulation results with experimental results for validation of the simulation system. First, we inserted a commercial guidewire into a poly (vinyl alcohol) hydrogel blood vessel model using a two-axis automatic stage and measured the position of the guidewire tip and the contact force between the guidewire and the vessel. The experimental apparatus can be used not only for the validation of numerical analyses, but also as a simulation system. Second, similarly to the experiment, the motion of the guidewire in the blood vessel model was calculated when the proximal part of the guidewire model was pushed and twisted. The model of the guidewire is constructed with viscoelastic springs and segments, and the proximal part of the guidewire model is constrained by the fixed catheter model. Collisions between the guidewire and the vessel are calculated, and the contact forces are determined according to the stiffness of the vessel wall. The same tendency was seen in the trajectories and the contact force of both the experimental and simulated guidewire tips.