E. de la Luna

Active control of type-I edge localized modes with n = 1 and n = 2 fields on JET

Recent experiments on JET have shown that type-I edge localized modes (ELMs) can be controlled by the application of static low n = 1 external magnetic perturbation fields produced by four external error field correction coils (EFCC) mounted far away from the plasma between the transformer limbs. When an n = 1 field with an amplitude of a few mT at the plasma edge (the normalized poloidal flux, ?, is larger than 0.95) is applied during the stationary phase of a type-I ELMy H-mode plasma, the ELM frequency rises from ~30?Hz up to ~120?Hz. The energy loss per ELM normalized to the total stored energy, ?WELM/W, decreased from 7% to values below the resolution limit of the diamagnetic measurement (<2%). Transport analysis using the TRANSP code shows up to 20% reduction in the thermal energy confinement time because of density pump-out, but when normalized to the IPB98(y, 2) scaling the confinement time shows almost no reduction. Stability analysis of controlled ELMs suggests that the operational point with n = 1 perturbation field moves from the intermediate-n peeling?ballooning boundary to the low-n peeling boundary, and the radial width of the most unstable mode reduced from ~3% down to ~1% of the normalized minor radius. The first results of ELM control with n = 2 fields on JET demonstrate that the frequency of ELMs can be increased by a factor of 3.5 with the present capability of the EFCC power supply. During the application of the n = 1, 2 fields, a reduction in the absolute ELM size (?WELM) and ELM peak heat fluxes on the divertor target by roughly the same factor as the increase in the ELM frequency has been observed. The reduction in heat flux is mainly due to the drop in particle flux rather than a change in the electron temperature. Similar plasma braking effects have been observed with n = 1 and n = 2 external fields when the same EFCC coil current was applied. Compensation of the density pump-out effect has been achieved by means of gas fuelling in low triangularity plasmas. An optimized fuelling rate to compensate the density pump-out effect has been identified. When the n = 1 field is applied in plasmas with reduced toroidal rotation and density due to increased TF ripple of 8%, both the magnitude of the toroidal braking and density pump-out are found to become smaller; however, the increase in the ELM frequency with the n = 1 field is still observed. Active ELM control by externally applied fields may offer an attractive method for next-generation tokamaks, e.g. ITER.

A two-time-scale dynamic-model approach for magnetic and kinetic profile control in advanced tokamak scenarios on JET

Real-time simultaneous control of several radially distributed magnetic and kinetic plasma parameters is being investigated on JET, in view of developing integrated control of advanced tokamak scenarios. This paper describes the new model-based profile controller which has been implemented during the 2006–2007 experimental campaigns. The controller aims to use the combination of heating and current drive (H&CD) systems—and optionally the poloidal field (PF) system—in an optimal way to regulate the evolution of plasma parameter profiles such as the safety factor, q(x), and gyro-normalized temperature gradient, ρ ∗ (x). In the first part of the paper, a technique for the experimental identification of a minimal dynamic plasma model is described, taking into account the physical structure and couplings of the transport equations, but making no quantitative assumptions on the transport coefficients or on their dependences. To cope with the high dimensionality of the state space and the large ratio between the time scales involved, the model identification procedure and the controller design both make use of the theory of singularly perturbed systems by means of a two-time-scale approximation. The second part of the paper provides the theoretical basis for the controller design. The profile controller is articulated around two composite feedback loops operating on the magnetic and kinetic time scales, respectively, and supplemented by a feedforward compensation of density variations. For any chosen set of target profiles, the closest self-consistent state achievable with the available actuators is uniquely defined. It is reached, with no steady state offset, through a near-optimal

Characterization of pedestal parameters and edge localized mode energy losses in the Joint European Torus and predictions for the International Thermonuclear Experimental Reactor

This paper presents the experimental characterization of pedestal parameters, edge localized mode (ELM) energy, and particle losses from the main plasma and the corresponding ELM energy fluxes on plasma facing components for a series of dedicated experiments in the Joint European Torus (JET). From these experiments, it is demonstrated that the simple hypothesis relating the peeling-ballooning linear instability to ELM energy losses is not valid. Contrary to previous observations at lower triangularities, small energy losses at low collisionality have been obtained in regimes at high plasma triangularity and q95∼4.5, indicating that the edge plasma magnetohydrodynamic stability is linked with the transport mechanisms that lead to the loss of energy by conduction during type I ELMs. Measurements of the ELM energy fluxes on the divertor target show that their time scale is linked to the ion transport along the field and the formation of a high energy sheath, in agreement with kinetic modeling of ELMs. Higher...

Integrated scenario in JET using real-time profile control

The recent development of real-time measurements and control tools in JET has enhanced the reliability and reproducibility of the relevant ITER scenarios. Diagnostics such as charge exchange, interfero-polarimetry, electron cyclotron emission have been upgraded for real-time measurements. In addition, real-time processes like magnetic equilibrium and q profile reconstruction have been developed and applied successfully in real-time q profile control experiments using model based control techniques. Plasma operation and control against magnetohydrodynamic instabilities are also benefiting from these new systems. The experience gained at JET in the field of real-time measurement and control experiments operation constitutes a very useful basis for the future operation of ITER scenarios.

Active control of type-I edge localized modes on JET

The operational domain for active control of type-I edge localized modes (ELMs) with an n = 1 external magnetic perturbation field induced by the ex-vessel error field correction coils on JET has been developed towards more ITER-relevant regimes with high plasma triangularity, up to 0.45, high normalized beta, up to 3.0, plasma current up to 2.0 MA and q95 varied between 3.0 and 4.8. The results of ELM mitigation in high triangularity plasmas show that the frequency of type-I ELMs increased by a factor of 4 during the application of the n = 1 fields, while the energy loss per ELM, ΔW/W, decreased from 6% to below the noise level of the diamagnetic measurement (<2%). No reduction of confinement quality (H98Y) during the ELM mitigation phase has been observed. The minimum n = 1 perturbation field amplitude above which the ELMs were mitigated increased with a lower q95 but always remained below the n = 1 locked mode threshold. The first results of ELM mitigation with n = 2 magnetic perturbations on JET demonstrate that the frequency of ELMs increased from 10 to 35 Hz and a wide operational window of q95 from 4.5 to 3.1 has been found.

Multichannel electron cyclotron emission radiometry in TJ-II stellarator

Electron cyclotron emission measurements are routinely performed in TJ-II stellarator by means of a multichannel heterodyne radiometer. The radiometer is absolutely calibrated and measures the temperature profile with high temporal resolution. The description of the radiometer, the calibration procedure, and some data taken during the occurrence of fast phenomena in the plasma (edge localized mode-like events and central temperature crashes) are presented in the article.

Development of steady-state scenarios compatible with ITER-like wall conditions

A key issue for steady-state tokamak operation is to determine the edge conditions that are compatible both with good core confinement and with the power handling and plasma exhaust capabilities of the plasma facing components (PFCs) and divertor systems. A quantitative response to this open question will provide a robust scientific basis for reliable extrapolation

MHD stability analysis of diagnostic optimized configuration shots in JET

The plasma edge MHD stability is analysed for several JET discharges in the diagnostic optimized configuration. The stability analysis of Type I ELMy plasmas shows how after an edge localized mode (ELM) crash the plasma edge is deep in the stable region against low- to intermediate-n peeling?ballooning modes. As the pressure gradient steepens and the edge current builds up, the plasma reaches the low- to intermediate-n peeling?ballooning mode stability boundary just before the ELM crash. Increasing the plasma fuelling by gas puffing makes the second stability access against high-n ballooning modes narrower until it closes completely and the ELMs change from Type I to Type III. Reducing the plasma heating has a similar effect. Increasing the safety factor at the plasma edge improves the stability against low- to intermediate-n modes allowing steeper pressure gradients to develop before an ELM crash.

Hydrogen plasmas with ICRF inverted minority and mode conversion heating regimes in the JET tokamak

During the initial operation of the International Thermonuclear Experimental Reactor (ITER), it is envisaged that activation will be minimized by using hydrogen (H) plasmas where the reference ion ...

Development of real-time diagnostics and feedback algorithms for JET in view of the next step

Real-time control of many plasma parameters will be an essential aspect in the development of reliable high performance operation of next step tokamaks. The main prerequisites for any feedback scheme are the precise real-time determination of the quantities to be controlled, requiring top quality and highly reliable diagnostics, and the availability of robust control algorithms.A new set of real-time diagnostics was recently implemented on JET to prove the feasibility of determining, with high accuracy and time resolution, the most important plasma quantities. Some of the signals now routinely provided in real time at JET are: (i) the internal inductance and the main confinement quantities obtained by calculating the Shafranov integrals from the pick-up coils with 2 ms time resolution; (ii) the electron temperature profile, from electron cylotron emission every 10 ms; (iii) the ion temperature and plasma toroidal velocity profiles, from charge exchange recombination spectroscopy, provided every 50 ms; and (iv) the safety factor profile, derived from the inversion of the polarimetric line integrals every 2 ms. With regard to feedback algorithms, new model-based controllers were developed to allow a more robust control of several plasma parameters.With these new tools, several real-time schemes were implemented, among which the most significant is the simultaneous control of the safety factor and the plasma pressure profiles using the additional heating systems (LH, NBI, ICRH) as actuators. The control strategy adopted in this case consists of a multi-variable model-based technique, which was implemented as a truncated singular value decomposition of an integral operator. This approach is considered essential for systems like tokamak machines, characterized by a strong mutual dependence of the various parameters and the distributed nature of the quantities, the plasma profiles, to be controlled. First encouraging results were also obtained using non-algorithmic methods like neural networks, which have been successfully applied to non-linear and ill-posed problems, for example the determination of the divertor radiated power.The real-time hardware and software architectures adopted are also described with particular attention to their relevance to ITER.