DIII-D RESEARCH OPPORTUNITIES FORUM FOR THE 2008 EXPERIMENTAL CAMPAIGN
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| Title | 202: Development of Model-based Current Profile Control at DIII-D | ||
| Name: | Eugenio Schuster
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Affiliation: | Lehigh University |
| Research Area: | Model based Control | Presentation time: | Requested | Co-Author(s): | John Ferron, Tim Luce, Mike Walker, Dave Humphreys, Tom Casper, Bill Meyer, Yongsheng Ou |
| Description: | The objective of this experiment is to implement open-loop controllers developed for the regulation of the q profile evolution during the early phase of the discharge, including ramp-up and beginning of the flattop, with the ultimate goal of achieving a desired target profile at some time during the first part of the flattop phase. The experiment will allow not only to evaluate the performance of the proposed controllers but also to validate the simplified models for current profile evolution used for the control synthesis. The validation of these models is crucial to decide about the viability of using them for model-based closed-loop control design. The experiment will also contribute to evaluate the correctness of actuation constraints considered during the control synthesis. The evaluation of control feasibility through use of open loop trajectories and the validation of the control model are both key prerequisites for the next step, which is to implement a feedback controller to drive the q profile to the desired target. | ||
| Experimental Approach/Plan: | The open-loop control laws will be expressed as time trajectories for the actuators: total plasma current, average plasma density, and non-inductive current drive (NBI, ECH) power. Special care will be put in reproducing in the experiment those initial conditions for the poloidal magnetic flux considered for the synthesis of the open-loop control laws. Comparison between desired and achieved time trajectories for the actuators will enable the redefinition of actuation constraints and the redevelopment of new open-loop controllers that could be tested later in the date. The evolution of the poloidal magnetic flux, plasma density and plasma temperature will be used for model validation. Different initial and target profiles will be considered mainly in L-mode but we also intend to carry out part of the experiment in H-mode. We also plan to consider different initial conditions and evolutions for plasma density and temperature to determine how they affect our control-oriented model. The ultimate goal of these experiments is the validation of a simplified dynamic model that could be used for closed-loop control.
Note: A separate proposal is being submitted to test closed-loop model-based control strategies for current profile regulation. Our goal is to carry out these open-loop experiments early in the experimental campaign, before the closed-loop experiments. | ||
| Background: | Setting up a suitable current profile has been demonstrated to be a key condition for the achievement of advanced scenarios with improved confinement and possible steady-state operation. The current approach at DIII-D focuses on creating the desired current profile during the plasma current ramp-up and early flattop phases with the aim of maintaining this target profile during the subsequent phases of the discharge.
The development of model-based current profile controllers aims at saving long trial-and-error periods of time currently spent by fusion experimentalists trying to manually adjust the time evolutions of the actuators to achieve the desired current profile at some pre-specified time during the early flattop phase. A simplified dynamic model describing the evolution of the poloidal flux, and therefore the q profile, during the inductive phase of the discharge has been proposed. Since this simple model serves not only as a fast simulation test-bed but also as the mathematical model used for the design of both open-loop and closed-loop model-based controllers, its validation is simply crucial. An initial experiment in July 2007 showed promising results. However, further and more in-depth validation experiments are still necessary. Open-loop controllers satisfying many constraints of the actuators have been designed using Extremum Seeking and Nonlinear Programming techniques, and successfully tested in simulations, to match a desired q profile within a predefined time window during the flattop phase of the tokamak discharge. Based on the promising results obtained in simulation studies, it is anticipated that the scheme can play an important role in fusion plasma physics experiments at the DIII-D tokamak. | ||
| Resource Requirements: | Machine time: 1 day
Note: Some coordination with the Steady-State Scenario group might allow use of piggybacks or individual shots on their experimental days. Beams, ECH. | ||
| Diagnostic Requirements: | Core and tangential Thomson, CER, CO2, magnetics, MSE, ECH diagnostics, a reasonable set of fast ion instability diagnostics (UF interferometers, FIR scattering, ECE at 500 kHz, fast magnetics with fast delay set in the current ramp), FIDA. | ||
| Analysis Requirements: | -- | ||
| Other Requirements: | -- | ||
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