JET Tokamak Research Articles

New magnetic real time shape control for MAST

The Mega Ampere Spherical Tokamak (MAST) real time plasma position controller is based on an optical linear camera placed on the mid plane of the vessel. This solution has the advantage of being a direct observation of the Dα emissions coming from the interaction between the boundary of the plasma and neutral gas, but, on the other hand, it restricts the control to the outer radius of the plasma only.A complete chain of tools has been set up to implement, test and simulate a new real time magnetic plasma shape controller based on the rtEFIT code. The complete working path consists of three elements: a linear static relationship between control parameters and current demands, a linear state space model needed to represent the plasma dynamic response in closed loop simulations, and the possibility to run simulations inside the Plasma Control System (PCS). The linear relationship has been calculated using the FIESTA code, which is developed using Matlab at CCFE. The linear state space model was generated using the CREATE-L code developed by the CREATE Consortium. It has already been successfully used to model JET, FTU and TCV tokamaks. Using this working path many simulations have been carried out allowing fine tuning of the control gains before the real experiment. The simulation testing includes the plasma shape control law as implemented in PCS itself, so intensive debugging has been possible prior to operation. Successful control using rtEFIT was established in the second dedicated experiment during the MAST 2011–12 campaign. This work is a stepping stone towards divertor control which is ultimately intended for application to the super-X divertor in the MAST Upgrade experiment.

A fast, magnetics-free flux surface estimation and q-profile reconstruction algorithm for feedback control of plasma profiles

The flux surfaces' layout and the magnetic winding number q are important quantities for the performance and stability of tokamak plasmas. Normally, these quantities are iteratively derived by solving the plasma equilibrium for the poloidal and toroidal flux.In this work, a fast, non-iterative and magnetics-free numerical method is proposed to estimate the shape of the flux surfaces by an inward propagation of the plasma boundary shape, as can be determined for example by optical boundary reconstruction described in Hommen (2010 Rev. Sci. Instrum. 81 113504), toward the magnetic axis, as can be determined independently with the motional Stark effect (MSE) diagnostic. Flux surfaces are estimated for various plasma regimes in the ITER, JET and MAST tokamaks and are compared with results of CRONOS reconstructions and simulations, showing agreement to within 1% of the minor radius for almost all treated plasmas.The availability of the flux surface shapes combined with the pitch angles measured using MSE allow the reconstruction of the plasma q-profile, by evaluating the contour-integral over the flux surfaces of the magnetic field pitch angle. This method provides a direct and exact measure of the q-profile for arbitrary flux surface shapes, which does not rely on magnetic measurements. Results based on estimated flux surface shapes show agreement with CRONOS q-profiles of better than 10%. The impact of the shape of the flux surfaces on the q-profile, particularly the profiles of elongation and Shafranov shift, and offsets in plasma boundary and the magnetic axis are assessed.OFIT+ was conceived for real-time plasma profile control experiments and advanced tokamak operation, and provides quickly and reliably the mapping of actuators and sensors to the minor radius as well as the plasma q-profile, independent of magnetic measurements.

Overview of erosion–deposition diagnostic tools for the ITER-Like Wall in the JET tokamak

This paper presents scientific and technical issues related to the development of erosion–deposition diagnostic tools for JET operated with the ITER-Like Wall: beryllium and tungsten marker tiles and several types of wall probes installed in the main chamber and in the divertor. Markers tiles are the standard limiter and divertor components additionally coated first with a thin sandwich of Ni–Be and Mo–W for, beryllium and tungsten markers, respectively. Both types of markers are embedded in regular arrays of limiter and divertor tiles. Coated W–Be probes are also inserted in the Be-covered Inconel cladding tiles on the central column. Other types of erosion–deposition diagnostic tools are: rotating collectors, deposition traps, louver clips, quartz microbalance and mirrors for the First Mirror Test at JET for ITER. The specific role of these tools is discussed in detail.

Impact of the ITER-like wall on divertor detachment and on the density limit in the JET tokamak

L-mode and H-mode density limits with the ITER-like wall (ILW) have been investigated in the recent experimental campaign and compared with experiments in the JET carbon material configuration. The density limit is up to 40% higher in the JET-ILW than in the JET-CFC machine. This is linked to the formerly higher radiation fraction and, correspondingly, to earlier divertor detachment in the JET-CFC. In the ILW configuration, the discharge demonstrates a stable operation with a completely detached outer divertor in L- and H-mode. In contrary to the well-known “heating power independent” Greenwald limit, the L-mode densities limit increases moderately with rising heating power (∼Pheat0.4) independently of the wall material.The H–L transition constitutes an effective undisruptive density limit for an H-mode plasma. Detachment itself does not trigger the H–L back transition and does not present a limit on plasma density. In the range of neutral beam heating 8–10.5MW, no dependence of the H-mode density limit on the heating power was observed.

Application of deconvolution methods to gamma-radiation spectra of thermonuclear plasma

The possibility of application of deconvolution methods to solving the problem of reconstruction of gamma-radiation spectra of thermonuclear plasma is demonstrated. The DEGAS code is created, which implements the modified algorithms of deconvolution. The results of the code application to the test spectra, the discrete spectra of radiation sources, and the gamma spectra measured during the experiments at the JET tokamak using methods for additional plasma heating are presented. The possibility of reconstruction of spectra with low statistics is shown.

РЕЖИМЫ РАБОТЫ МОДИФИЦИРОВАННОЙ УСТАНОВКИ Т-15

Transport calculations of the operational regimes for the constructed modified T-15 device are presented. The Canonical Profiles Transport Model (CPTM), verified with JET and MAST tokamaks, is used in the simplified version: Te, Ti and current density diffusion equations are solved with the fixed plasma density n profile. L- and H- modes are described by the appropriate modeling of the boundary conditions for the temperatures and plasma density, confirmed by JET experiment. The absorbed power density profile is determined by the simple formulas. The results are presented as graphics and tables, which can be easily used in order to cre ate the scientific device programs, to develop diagnostics or to define the position of the device in modern tokamak experiments.

Particle Flow during Sawtooth Reconnection: Numerical Simulations of Experimental Observations

The core plasma density in ohmic, sawtoothing regime in Tore Supra and JET tokamaks measured by fast-sweeping reflectometry develops a central peak. This peak is surrounded by a density plateau, which extends up to the q = 1 reconnection surface. Such mexican hat like profiles differ from the smooth dome of the temperature profile. Detailed tomographic reconstructions of the density in the poloidal plane show the development of crescent and ring structures during the sawtooth cycle. 3D full MHD nonlinear simulations with the XTOR-2F code recover these structures and show that they can be explained by the MHD activity related to the sawtooth instability. They show also that the electron density evolution is dominated by the perpendicular MHD flows, while the temperature, obtained with ECE measurements is dominated by parallel diffusion.

Calculations to support JET neutron yield calibration: Modelling of the JET remote handling system

After the coated CFC wall to ITER-Like Wall (Beryllium/Tungsten/Carbon) transition in 2010–2011, confirmation of the neutron yield calibration will be ensured by direct measurements using a calibrated 252Cf neutron source deployed by the in-vessel remote handling boom and Mascot manipulator inside the JET vacuum vessel. Neutronic calculations are required to calculate the effects of the JET remote handling (RH) system on the neutron monitors. We developed a simplified geometrical computational model of the JET remote handling system in MCNP. In parallel we developed a script that translates the RH movement data to transformations of individual geometrical parts of the RH model in MCNP. After that a benchmarking of the model was performed to verify and validate the accordance of the target positions of source and RH system with the ones from our model. In the last phase we placed the JET RH system in the simplified MCNP model of the JET tokamak and studied its effect on neutron monitor response for some example source positions and boom configurations.As the correction factors due to presence of the JET RH system can potentially be significant in cases when the boom is blocking a port close to the detector under investigation, we have chosen boom configurations so that this is avoided in the vast majority of the source locations. Examples are given.

Space and High Energy Experiments Advanced Electronic Systems 2012

Abstract This paper is a research survey of the WILGA Symposium work. It presents a digest of technical effort results shown by young researchers from different universities during the Jubilee XXXth SPIE-IEEE-Photonics Society of Poland Wilga 2012 symposium on Photonics and Internet Engineering. Topical tracks of the symposium embraced: nanomaterials and nanotechnologies for photonics, sensory and nonlinear optical fibers, object oriented design of hardware, photonic metrology, optoelectronics and photonics applications, photonics-electronics co-design, optoelectronic and electronic systems for astronomy and high energy physics experiments, JET tokamak and pi-ofthe sky experiments development. The symposium is an annual summary in the development of numerable Ph.D. theses carried out in this country in the area of advanced electronic and photonic systems. It is also a great occasion for SPIE, IEEE, OSA and PSP students to meet together in a large group spanning the whole country with guests from this part of Europe. A digest of Wilga references is presented [1]-[60]. This paper is the first part of the digest focused on astronomy, space, astroparticle physics, accelerators, and high energy physics experiments.

Successive identification of surface heat flux and thermophysical properties of plasma facing components inside the JET tokamak: numerical feasibility

We present here the numerical feasibility of using two thermal diagnostics (IR surface and embedded thermocouple temperature measurements), that outfit the same carbon tile inside the JET Fusion reactor and whose combination enables to identify, on one hand, a surface heat flux history and, on the other hand, the spatial and time variation of the thermal resistance of an unknown deposited surface carbon layer (SCL). The Conjugate Gradient Method (CGM) and the adjoint state were applied to perform these two identifications.

Characterization of time varying thermophysical property of a surface layer: Numerical feasibility for JET tokamak tiles

The present paper deals with the thermal characterization of the Surface Carbon Layer (SCL) usually poorly attached to the Plasma Facing Component (PFC) inside the nuclear fusion machines (Tokamak). A nonlinear unsteady calculation combined with the Conjugate Gradient Method (CGM) and the adjoint state is achieved in order to characterize in situ the time variation of the thermal resistance Rth of a surface layer. A numerical feasibility is presented with a realistic virtual experiment design. The accuracy of the method is examined by using simulated exact and inexact infrared measurements obtained on the SCL surface. The advantages of applying the CGM with the adjoint state in the present study, are that no prior information is needed on the time variation and the initial guess of the unknown thermal resistance.

Ubiquity of non-diffusive momentum transport in JET H-modes

A broad survey of the experimental database of neutral beam heated baseline H-modes and hybrid scenarios in the JET tokamak has established the ubiquity of non-diffusive momentum transport mechanisms in rotating plasmas. As a result of their presence, the normalized angular frequency gradient R∇ω/ω is higher than expected from momentum diffusion alone, by about unity in the core (r/a ∼ 0.3), rising to near 5 close to the edge, where its contribution to the total gradient is comparable to the gradient associated with the diffusive flux. The magnitude and parameter dependences of the non-diffusive contribution to the gradient are consistent with a theoretically expected pinch, which has its origin in the vertical particle drift resulting from the Coriolis force. Linear gyrokinetic calculations of the pinch number RV/χφ and the Prandtl number χφ/χi are in good agreement with the experimental observations, with similar dependences on R/Ln, q and ε = r/R. A contribution due to residual stresses may also be present, but could not be identified with certainty.

Relevance of Uncorrelated Lorentzian Pulses for the Interpretation of Turbulence in the Edge of Magnetically Confined Toroidal Plasmas

Recently, it has been proposed that the turbulent fluctuations measured in a linear plasma device could be described as a superposition of uncorrelated Lorentzian pulses with a narrow distribution of durations, which would provide an explanation for the reported quasiexponential power spectra. Here, we study the applicability of this proposal to edge fluctuations in toroidal magnetic confinement fusion plasmas. For the purpose of this analysis, we introduce a novel wavelet-based pulse-detection technique that offers important advantages over existing techniques. This technique allows for extracting the properties of individual pulses from the experimental time series, and for quantifying the distribution of pulse duration and energy as well as temporal correlations. We apply the wavelet technique to edge turbulent fluctuation data from the W7-AS stellarator and the JET tokamak, and find that the pulses detected in the data do not have a narrow distribution of durations and are not uncorrelated. Instead, the distributions are of the power-law type, exhibiting temporal correlations over scales much longer than the typical pulse duration. These results suggest that turbulence in open and closed field line systems may be distinct, and cast doubts on the proposed ubiquity of exponential power spectra in this context.

Plasma isotopic effect on the damping rate of toroidal Alfvén eigenmodes with intermediate toroidal mode numbers**This paper follows up the contribution presented at the 12th IAEA Technical Committee Meeting on Energetic Particles in Magnetic Confinement Systems (Austin, TX, 8–10 September 2011).

This paper reports on the results of recent experiments performed on the JET tokamak on toroidal Alfvén eigenmodes (TAEs) with toroidal mode number (n) in the range up to |n| = 7. The stability properties of these medium-n TAEs are investigated experimentally using a set of compact in-vessel antennas, providing a direct and real-time measurement of the frequency, damping rate and amplitude for each individual toroidal mode number. The measurements, reported here, of the damping rate (γ/ω) for these medium-n modes were obtained during a deuterium to helium to hydrogen changeover experimental campaign, and are used to infer the effect of the plasma effective isotopic composition (AEFF) on the stability properties of these medium-n TAEs. We find that the damping rate of n = 1 TAEs decreases approximately as γ/ω ∼ 1/AEFF as reported previously, but only for modes whose frequency is close to the centre of the n = 1 toroidal gap and for density and current profiles giving an open gap structure. Conversely, for n ⩾ 3 TAEs we find that their damping rate approximately increases as γ/ω ∼ AEFF.

Study of ICRH scenarios for thermal ion heating in JET D–T plasmas

Various ion cyclotron resonance heating (ICRH) scenarios relevant for the D–T phase of the JET tokamak are studied. Recent ICRH experiments in JET (3He)–D and (3He)–H plasmas confirmed the possibility of electron heating enhancement in the mode conversion (MC) regime due to the constructive interference of the reflected fast waves. Such a heating enhancement in D–T plasma is investigated first for JET-like conditions for both dipole and +π/2 ICRH antenna phasing, and for T concentration varied from 0% to 100%. It is shown that most of the MC scenarios at comparable concentrations of D and T species suffer from a parasitic absorption by fusion-born alpha-particles and NBI-produced fast ions whereas the impact of such fast ions in the minority heating (MH) ICRH schemes is substantially smaller. A possibility of ion heating enhancement due to the interference effect is shown for the MH scenarios. It is found that thermal ion heating becomes dominant in tritium-rich plasmas with T concentration ∼80%. The efficiency of ion heating in such a scenario is compared with the alternative 3He minority ICRH scenario in D : T = 50 : 50 plasmas.

First fusion proton measurements in TEXTOR plasmas using activation technique

MeV particle loss measurements from fusion plasmas, in particular alpha particles, remain difficult in large fusion devices and further R&D is needed for ITER. This paper describes the first attempt to measure 3 MeV escaping fusion protons emitted from TEXTOR tokamak plasmas using activation technique. This technique was successfully demonstrated, initially, in 2006 on the JET tokamak. An ion camera equipped with a collimator and several types of activation detectors was installed inside the TEXTOR vacuum vessel to perform these measurements. After irradiation, the detectors were analyzed using ultra low level gamma-ray spectrometry at the HADES underground laboratory. 3 MeV escaping fusion protons were detected in larger number -~6 times more--compared to earlier measurements using this technique on JET. Another major progress was the reduction of the cooling time by a factor of 50, which made possible to detect radionuclides with half-life of less than 90 min.

Neutron field parameter measurements on the JET tokamak by means of super-heated fluid detectors

The neutron field parameters (fluence and energy distribution) at a specific location outside the JET Torus Hall have been measured by means of super-heated fluid detectors (or "bubble detectors") in combination with an independent, time-of-flight, technique. The bubble detector assemblies were placed at the end of a vertical line of sight at about 16 m from the tokamak mid plane. Spatial distributions of the neutron fluence along the radial and toroidal directions have been obtained using two-dimensional arrays of bubble detectors. Using a set of three bubble detector spectrometers the neutron energy distribution was determined over a broad energy range, from about 10 keV to above 10 MeV, with an energy resolution of about 30% at 2.5 MeV. The very broad energy response allowed for the identification of energy features far from the main fusion component (around 2.45 MeV for deuterium discharges).

Wave–particle resonances and redistribution/losses of fast ions in tokamaks

Enhanced fast ion losses, mostly in the range of energies from around 1.2 to 2.4 MeV, were measured during the activity of tornado modes in the JET tokamak. Tornado modes are TAE localized inside the q = 1 surface, which do not extend to the outer regions of the plasma. Thus, it is necessary to find an explanation on how such modes can lead to the loss of fast ions. In this paper, a mechanism that allows explaining the loss of fast ions triggered by tornado modes is proposed. This mechanism is based on the combined effect of tornado modes and global TAEs over the fast ions (global TAEs were always observed along with the tornado modes in the experiments in which enhanced losses were measured). Tornado modes would trigger the process of loss by resonantly interacting with the fast ions near the centre of the plasma and transporting the ions to a more peripheral region where tornado modes and global TAEs coexist. The TAE would then transport convectively the fast ions, most efficiently through the first bounce resonances (p = 1), all the way to the plasma edge eventually leading to its loss. This mechanism of loss is supported by calculations carried out with the CASTOR-K code.

A Software Tool for the Design of the Current Limit Avoidance System at the JET Tokamak

A current limit avoidance (CLA) system has been proposed to avoid current saturation in the poloidal field (PF) coils of the Joint European Torus tokamak when the eXtreme Shape Controller is used to control the plasma shape. CLA uses the redundancy of the PF coil system to automatically obtain almost the same plasma shape with a different combination of control currents. This paper describes the set of graphic tools that has been recently developed to aid the design of the CLA parameters by nonexpert users.

Improved confinement in JET hybrid discharges

A new technique has been developed to produce plasmas with improved confinement relative to the H98,y2 scaling law (ITER Physics Expert Groups on Confinement and Transport and Confinement Modelling and Database ITER Physics Basics Editors and ITER EDA 1999 Nucl. Fusion 39 2175) on the JET tokamak. In the mid-size tokamaks ASDEX upgrade and DIII-D heating during the current formation is used to produce a flat q-profile with a minimum close to 1. On JET this technique leads to q-profiles with similar minimum q but opposite to the other tokamaks not to an improved confinement state. By changing the method utilizing a faster current ramp with temporary higher current than in the flattop (current overshoot) plasmas with improved confinement (H98,y2 = 1.35) and good stability (βN ≈ 3) have been produced and extended to many confinement times only limited by technical constraints. The increase in H98,y2-factor is stronger with more heating power as can be seen in a power scan. The q-profile development during the high power phase in JET is reproduced by current diffusion calculated by TRANSP and CRONOS. Therefore the modifications produced by the current overshoot disappear quickly from the edge but the confinement improvement lasts longer, in some cases up to the end of the heating phase.