J.B. Lister

A modern plasma controller tested on the TCV tokamak

A high-order, multivariable, modern plasma controller has been designed using H-infinity optimal control techniques and tested oil the Tokamak Configuration Variable (TCV) tokamak. An initial design for the control of the plasma current, position, and shape parameters is described. The design process was based on the CREATE-L linearized model of TCV, and the controller was implemented oil a digital processor. The results demonstrated that the required performance was delivered and the controller response was in good agreement with predictions using the model.

The control of tokamak configuration variable plasmas

AbstractThe general control of tokamak plasmas has evolved considerably over the past few years with an increase in the plasma pulse length, an increase in the control of additional heating and fueling, and an increase in the degree to which the shape of the plasma can be varied. The Tokamak Configuration Variable (TCV) is specifically designed to explore the operational benefits of plasma shaping over a wide variety of plasma shapes. Consequently, considerable attention has been given to the control of the poloidal field coil currents that impose the desired shape. All aspects of the control of TCV plasmas, from the diagnostic measurements to the power supplies, via particular control algorithms and overall supervision are discussed.

Design and experimental testing of a robust multivariable controller on a tokamak

Describes the design and the experimental validation of a multivariable digital controller for a Tokamak, the Tokamak a configuration variable (TCV). The design of the controller is based on a linearized model of the plasma confined in the Tokamak. The plant is multiple-input-multiple-output (MIMO) and the various outputs are strongly coupled. Moreover the plant is open-loop unstable. The scope of the controller is to stabilize the plasma and to guarantee some closed-loop performance in terms of decoupling among the plant outputs. The proposed controller is composed of two nested loops: one is devoted to the vertical stabilization, the other, designed using the /spl Hscr//sub /spl infin// technique, guarantees the control of the plasma current and of the plasma shape. After massive simulations, this controller has been successfully tested on the plant. The experimental results show a significant improvement of the performance with respect to those obtained with a proportional integral derivative (PID) MIMO controller, that was used before on the plant.

Self-consistent simulation of plasma scenarios for ITER using a combination of 1.5D transport codes and free-boundary equilibrium codes

Self-consistent transport simulation of ITER scenarios is a very important tool for the exploration of the operational space and for scenario optimization. It also provides an assessment of the compatibility of developed scenarios (which include fast transient events) with machine constraints, in particular with the poloidal field coil system, heating and current drive, fuelling and particle and energy exhaust systems. This paper discusses results of predictive modelling of all reference ITER scenarios and variants using two suites of linked transport and equilibrium codes. The first suite consisting of the 1.5D core/2D SOL code JINTRAC (Wiesen S. et al 2008 JINTRAC-JET modelling suite JET ITC-Report) and the free-boundary equilibrium evolution code CREATE-NL (Albanese R. et al 2003 ISEM 2003 (Versailles, France); Albanese R. et al 2004 Nucl. Fusion 44 999), was mainly used to simulate the inductive D-T reference Scenario-2 with fusion gain Q = 10 and its variants in H, D and He (including ITER scenarios with reduced current and toroidal field). The second suite of codes was used mainly for the modelling of hybrid and steady-state ITER scenarios. It combines the 1.5D core transport code CRONOS (Artaud J.F. et al 2010 Nucl. Fusion 50 043001) and the free-boundary equilibrium evolution code DINA-CH (Kim S.H. et al 2009 Plasma Phys. Control. Fusion 51 105007).

Investigation of the consistency of magnetic and soft x-ray plasma position measurements on TCV by means of a rapid tomographic inversion algorithm

A rapid algorithm for tomographic inversion is presented that allows post-discharge reconstruction of the soft x-ray (SXR) emissivity evolution in the Tokamak a configuration variable (TCV). Simultaneously, it determines the centre of gravity of the emissivity that may serve as a reliable measurement of the plasma position, that is independent of magnetic measurements. Tests of the algorithm were performed using shaped phantom datasets and all past SXR measurements were processed to obtain plasma position statistics. Multiple linear regression is applied in matching the results with the magnetic axis data. Systematic functionalities including magnetic field dependence and drift of the magnetic axis are observed. The joint resolution limit of magnetic and SXR position measurements at TCV is derived from the residual discrepancies and equals 2.2 and 3.1 mm in the radial and vertical directions, respectively.

Alfven eigenmode experiments in tokamaks and stellarators

In tokamaks and stellarators, measurements of electromagnetic fluctuations in the presence of resonant particle drive, including fusion-produced alphas, reveal the excitation of Alfven eigenmodes (AE), related under certain conditions to a degradation in the fast-particle confinement. The balance between the drive and the background damping is investigated using active diagnostic systems to excite and measure the AE spectrum in terms of frequencies and damping rates. At JET, saddle-coil antennae drive low toroidal mode number (n 1%) during the creation of the magnetic X-point. In the presence of resonant fast particles, information on the instability drive is obtained: low-n modes are found to be stable in the presence of NBI with upsilon(parallel to)/upsilon(A) P-thresh, with 2.5 MW < P-thresh < 5 MW; under these conditions, intrinsically driven TAE and EAE are clearly observed in the magnetic fluctuation spectra, with no measurable effect on the plasma performance.

Upgrade of the present JET shape and vertical stability controller

The development of linear model of the present JET (Joint European Torus) electromagnetic system has allowed the design of an optimized Plasma Shape Controller. Quite extreme plasma boundaries (elongation k = 1.9 and triangularity = 0.6) are shown to be controllable also in the presence of large variations of the values of poloidal beta (0.1 < beta(p) < 0.6) and/or internal magnetic inductance (0.8 < l(i) < 1.4). The developed model gives also an excellent simulation of the growth rate of the experimental vertical disruptions

Modeling and control of TCV

A new approach to the modeling and control of tokamak fusion reactors is presented. A nonlinear model is derived using the classical arguments of Hamiltonian mechanics and a low-order linear model is derived from it. The modeling process used here addresses flux and energy conservation issues explicitly and self-consistently. The model is of particular value, because it shows the relationship between the initial modeling assumptions and the resulting predictions. The mechanisms behind the creation of uncontrollable modes in tokamak models are discussed. A normalized coprime factorization H/sub /spl infin// controller is developed for the Tokamak a/spl grave/ Configuration Variable (TCV), CRPP-EPFL, Lausanne, Switzerland, tokamak using the linearized model, which has been extensively verified on the TCV and JT-60U, JAERI, Naka, Japan, tokamaks. Recent theory is applied to reduce the controller order significantly whilst guaranteeing a priori bounds on the robust stability and performance. The controller is shown to track successfully reference signals that dictate the plasmas shape, position and current. The tests used to verify this were carried out on linear and nonlinear models.

The separatrix response of diverted TCV plasmas compared with the predictions of the CREATE-L model

The response of ohmic, single null diverted (SND), non-centred plasmas in TCV to poloidal field coil stimulation has been compared with the predictions of the linear CREATE-L MHD equilibrium response model. The closed loop responses of directly measured quantities, reconstructed parameters and the reconstructed plasma contour have all been examined. Provided that the plasma position and shape perturbation were small enough for the linearity assumption to hold, agreement between the model and experiment was good. For some stimulations the open loop vertical position instability growth rate changed significantly. This led to oscillations in the vertical position that were unpredicted and illustrated the limitations of a linear model. A different model was developed with the assumption that the flux at the plasma boundary is frozen and the predictions of this model were also compared with experimental results. It proved not to be as reliable as the CREATE-L model for some simulation parameters, showing that the experiments were able to discriminate between different plasma response models. The closed loop response was also found to be sensitive to changes in the modelled plasma shape. It was not possible to invalidate the CREATE-L model, despite the extensive range of responses excited by the experiments.

Improving tokamak vertical position control in the presence of power supply voltage saturation

The control of the current, position and shape of an elongated cross-section tokamak plasma is complicated by the so-called instability of the current vertical position. Linearized models all share the feature of a single unstable eigenmode, attributable to this vertical instability of the plasma equilibrium movement, and a large number of stable or marginally stable eigenmodes, attributable to zero or positive resistance in all other model circuit equations. Due to the size and therefore cost of the ITER tokamak, there will naturally be smaller margins in the poloidal field coil power supplies, implying that the feedback control will experience actuator saturation during large transients due to a variety of plasma disturbances. Current saturation is relatively benign, due to the integrating nature of the tokamak, resulting in a reasonable time horizon for strategically handling the approach to saturation which leads to the loss of one degree of freedom in the feedback control for each saturated coil. On the other hand, voltage saturation is produced by the feedback controller itself, with no intrinsic delay. This paper presents a feedback controller design approach which explicitly takes saturation of the power supply voltage into account when producing the power supply demand signals. We consider the vertically stabilizing part of the ITER controller (fast controller) with one power supply and therefore a single saturated input. We consider an existing ITER controller and enlarge its region of attraction to the full null controllable region by adding a continuous nonlinearity into the control. In a system with a single unstable eigenmode and a single stable eigenmode we have already provided a proof of the asymptotical stability of the closed loop system, and we have examined the performance of this new continuous nonlinear controller. We have subsequently extended this analysis to a system with a single eigenmode and multiple stable eigenmodes. The method requires state feedback control, and therefore a reconstruction of the states is indispensable. We discuss the feasibility of extracting these states from the available diagnostic information as well as other implementation details. As a complement to our ITER simulations we confirm the enlargement of the region of attraction by the new controller by a JET simulation.