Optimization of the active MHD spectroscopy system on JET for the excitation of individual intermediate and high-n Alfven eigenmodes

The stability of Alfven eigenmodes (AEs) is studied experimentally in the JET tokamak by observing the plasma response to antenna-driven frequency-sweeping perturbations at the plasma edge. During the 2008/9 experimental campaigns, the complete set of the new antennas was operated and AEs with toroidal mode numbers (n) in the intermediate-n range were excited under various plasma conditions. In this paper, we describe the results of the work achieved on the technical aspects of the diagnostic. The antenna currents have been optimized to improve the antenna-plasma coupling. The mode-tracking system has been upgraded for real-time targeting of modes with specific n. As an example of the optimized performance of the diagnostic, the paper concludes with a report on the damping rates of n = 3-5 toroidal AEs that were measured dynamically while the background plasma parameters were evolving in time.

Direct Measurements of the Damping of Alfvén Eigenmodes for an Assessment of their Stability Limits in Tokamak Plasmas

Theodoros Panis

Direct damping rate measurements of AEs are obtained using the active MHD spectroscopy system installed on the JET tokamak. The system was recently equipped with new antennas, designed to study especially the modes of intermediate toroidal mode number n, |n| = 3 – 15, as the AEs of this range are most prone to destabilization by the fast particles in JET and in future burning plasma experiments such as ITER. The broad n-spectrum that is driven by the new antennas and the more localized structure of intermediate-n AEs has important implications for the ability to measure damping rates of intermediate n. To obtain an extended database of high accuracy individual-n measurements, experimental work on technical and engineering aspects was indispensable both on the excitation side and on the detection side. On the excitation side, the electrical model of the AE exciter has been constructed during this thesis. The model is used to determine the operational capabilities of the exciter with the new antennas, to optimize the antenna currents and to design the relevant impedance matching circuits. On the detection side, the excitation of multiple-n, degenerate AEs at close frequencies prompted for a sophisticated method to correctly estimate the n-spectrum of the plasma response. To this end, a sparse spectrum representation method was adapted to deal with the complex and real-time data produced by the active MHD spectroscopy system. The n-decomposition of the plasma response requires an accurate relative calibration of the magnetic pick-up coils. An in situ method was developed and applied for the calibration of the coils using the direct coupling to the new AE antennas. A large collection of damping rate measurements of, mainly, toroidal AEs (TAEs) was obtained during the 2008/2009 JET experimental campaigns following the technical optimization of the antenna system. Selected measurements of |n| = 3, 4 and |n| = 7 TAEs are compared to the plasma models of the numerical codes LEMan and CASTOR. The robustness of the results of the simulations is tested against uncertainties in density and safety factor profiles. The antenna-driven modes are characterized and the dominant damping mechanisms are identified in a number of cases, and confirm in general for a wide n- range the stabilizing role played by the edge magnetic shear. These comparisons underline the importance of the kinetic effects in order to achieve a realistic estimation of the TAE damping. A database of approximately 3000 TAE damping rate measurements in ohmic plasmas is studied. The dependence of the damping rate measurements of |n| = 2 – 7 TAEs on the edge magnetic shear, the edge elongation and the q profile is investigated. The analysis provides experimental evidence that the damping properties of TAEs with |n| = 2 – 7 change as the toroidal mode number n increases, showing that medium-n modes tend to be less damped than low-n modes. In JET plasmas, the turning point for the damping properties to change from low-n to medium-n behavior is found to be at |n| ≈ 3 – 4.

EPFL2010

Optimization of the active MHD spectroscopy system on JET for the excitation of individual intermediate and high-n Alfvén eigenmodes

Patrick Blanchard, Ambrogio Fasoli, Theodoros Panis, Duccio Testa

The stability of Alfv´en eigenmodes (AEs) is studied experimentally in the JET tokamak, by observing the plasma response to antenna-driven frequency-sweeping perturbations at the plasma edge. During the 2008/9 experimental campaigns, the complete set of the new AE antennas was operated and damping measurements for intermediate toroidal mode numbers (n) were collected under various plasma conditions. The quantity and the quality of the measurements reflect a significant effort put in the technical aspects of the diagnostic. We describe the results of the work on the optimization of the antenna currents and on the upgrade of the real-time resonance tracking system that allows one to target in real-time modes with specific n. As an example of the optimized performance of the diagnostic, the paper concludes with a report on the first results regarding the dependence of the measured damping rates of n = 3 and n = ±5 TAEs on background plasma parameters.

2009

Real-Time Control of Tokamak Plasmas: from Control of Physics to Physics-Based Control

Federico Alberto Alfredo Felici

Stable, high-performance operation of a tokamak requires several plasma control problems to be handled simultaneously. Moreover, the complex physics which governs the tokamak plasma evolution must be studied and understood to make correct choices in controller design. In this thesis, the two subjects have been merged, using control solutions as experimental tool for physics studies, and using physics knowledge for developing new advanced control solutions. The TCV tokamak at CRPP-EPFL is ideally placed to explore issues at the interface between plasma physics and plasma control, by combining a state-of-the-art digital real-time control system with a flexible and powerful set of actuators, in particular the electron cyclotron heating and current drive system (ECRH/ECCD). This unique experimental platform has been used to develop and test new control strategies for three important and reactor-relevant tokamak plasma physics instabilities, including the sawtooth, the edge localized mode (ELM) and the neoclassical tearing mode (NTM). These control strategies offer new possibilities for fusion plasma control and at the same time facilitate studies of the physics of the instabilities with greater precision and detail in a controlled environment. The period of the sawtooth crash, a periodic MHD instability in the core of a tokamak plasma, can be varied by localized deposition of ECRH/ECCD near the q = 1 surface, where q is the safety factor. Exploiting this known physical phenomenon, a sawtooth pacing controller was developed which is able to precisely control the time of appearance of the next sawtooth crash. It was also shown that each individual sawtooth period can be controlled in real-time. A similar scheme is applied to H-mode plasmas with type-I ELMs, where it is shown that pacing regularizes the ELM period. The regular, reproducible and therefore predictable sawtooth crashes obtained by the sawtooth pacing controller have been used to study the relationship between sawteeth and NTMs. It is known that post-crash MHD activity can provide the "seed" island for an NTM, which then grows under its neoclassical bootstrap drive. Experiments are shown which demonstrate that the seeding of 3/2 NTMs by long sawtooth crashes can be avoided by preemptive, crash-synchronized EC power injection pulses at the q = 3/2 rational surface location. NTM stabilization experiments in which the ECRH deposition location is moved in real-time with steerable mirrors have shown effective stabilization of both 3/2 and 2/1 NTMs, and have precisely localized the deposition location that is most effective. Studies of current-profile driven destabilization of tearing modes in TCV plasmas with significant amounts of ECCD show a great sensitivity to details of the current profile, but failed to identify a stationary region in the parameter space in which NTMs are always destabilized. This suggests that transient effects intrinsically play a role. Next to instability control, the simultaneous control of magnetic and kinetic plasma profiles is another key requirement for advanced tokamak operation. While control of kinetic plasma profiles around an operating point can be handled using standard linear control techniques, the strongly nonlinear physics of the coupled profiles complicates the problem significantly. Even more, since internal magnetic quantities are difficult to measure with sufficient spatial and temporal resolution —even after years of diagnostic development— routine control of tokamak plasma profiles remains a daunting and extremely challenging task. In this thesis, a model-based approach is used in which physics understanding of plasma current and energy transport is embedded in the control solution. To this aim, a new lightweight transport code has been derived focusing on simplicity and speed of simulation, which is compatible with the demands for real-time control. This code has been named RAPTOR (RApid Plasma Transport simulatOR). In a first-of-its-kind application, the partial differential equation for current diffusion is solved in real-time during a plasma shot in the TCV control system using RAPTOR. This concept is known in control terms as a state observer, and it is applied experimentally to the tokamak current density profile problem for the first time. The real-time simulation gives a physics-model-based estimate of key plasma quantities, to be controlled or monitored in real-time by different control systems. Any available diagnostics can be naturally included into the real-time simulation providing additional constraints and removing measurement uncertainties. The real-time simulation approach holds the advantage that knowledge of the plasma profiles is no longer restricted to those points in space and time where they are measured by a diagnostic, but that an estimate for any quantity can be computed at any time. This includes estimates of otherwise unmeasurable quantities such as the loop voltage profile or the bootstrap current distribution. In a first closed-loop experiment, an estimate of the internal inductance resulting from the real-time simulation is feedback controlled, independently from the plasma central temperature, by an appropriate mix of co- and counter- ECCD. As a tokamak plasma evolves from one state to another during plasma ramp-up or ramp-down, the profile trajectories must stay within a prescribed operational envelope delimited by physics instabilities and engineering constraints. Determining the appropriate actuator command sequence to navigate this operational space has traditionally been a trial-and-error procedure based on experience of tokamak physics operators. A computational technique is developed based on the RAPTOR code which can calculate these trajectories based on the profile transport physics model, by solving an open-loop optimal control problem. The solution of this problem is greatly aided by the fact that the code returns the plasma state trajectory sensitivities to input trajectory parameters, a functionality which is unique to RAPTOR. This information can also be used to construct linearized models around the optimal trajectory, and to determine the active constraint, which can be used for time-varying closed-loop controller design. This physics-model-based approach has shown excellent results and holds great potential for application in other tokamaks worldwide as well as in future devices.