DIII-D RESEARCH OPPORTUNITIES FORUM FOR THE 2008 EXPERIMENTAL CAMPAIGN
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| Title | 391: Optical Sensor for Feedback Control of Resistive Wall Modes | ||
| Name: | Matthew Lanctot ( |
Affiliation: | Columbia University |
| Research Area: | RWM Physics | Presentation time: | Not requested | Co-Author(s): | Gerald Navratil, Andrea Garofalo, Holger Reimerdes, Ioan Bogatu, and Jin-soo Kim |
| Description: | We propose a proof-of-principle experiment whose primary goal is to demonstrate improved closed-loop performance of the n=1 RWM feedback control system by including soft x-ray photodiode detectors as optical sensors in the feedback algorithm. | ||
| Experimental Approach/Plan: | An evaluation of the method will be made by comparing the performance of the existing magnetic sensor algorithm to the multiple sensor algorithm. This experiment should follow other experiments that propose to measure the internal structure of the RWM at high and low rotation. Results from such experiments would aid in developing the new control algorithm. Tuning of the control system could occur during one or two two-hour experiments followed by a half-day session to demonstrate the effectiveness of the approach. | ||
| Background: | Initially, external radial magnetic field sensors were used to detect and track the n=1 resistive wall mode on DIII-D. Subsequent simulations suggested and experimental results indicated improved performance of the feedback control system using internal poloidal field probes. Johnson has shown that while magnetic probes can measure the helical mode structure at the plasma surface, the internal kink structure of the RWM can be detected using photodiodes that measure the soft x-ray emissivity. This is possible because the emissivity is a strongly varying function of density and temperature, two flux surface quantities, and the kink mode disturbs the equilibrium magnetic field. Since the present control system relies only on magnetic sensors, the RWM amplitude must exceed the magnetic noise level (~ 5-10 Gauss) at the vessel wall before triggering the feedback control system. This may be too late for the control system to provide full stabilization of the mode. Ongoing upgrades to the SXR diagnostic will make available at least three toroidally-separated measurements at 12 radial locations, which could in principle be used to track the slowly growing RWM before it is detected on the magnetic sensors. Early detection of a small amplitude RWM may improve the control system performance at low rotation. | ||
| Resource Requirements: | At least 4 real-time digitizer channels are needed to feed SXR signals to the PCS. Modifications to the existing feedback algorithm is necessary to include SXR measurements in the feedback logic. | ||
| Diagnostic Requirements: | 4 SXR cameras in R+1 ports at 45, 90, 165, and 195 degrees, MSE, CER, magnetics, Thomson | ||
| Analysis Requirements: | CERFIT | ||
| Other Requirements: | Success in this experiment depends upon the completion of work focused on characterizing the internal mode structure of the RWM using diagnostics located at multiple toroidal locations. | ||
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