Jet Efda Contributors

Overview of JET results

Scientific and technical activities on JET focus on the issues likely to affect the ITER design and operation. Our understanding of the ITER reference mode of operation, the ELMy H-mode, has progressed significantly. The extrapolation of ELM size to ITER has been re-evaluated. Neoclassical tearing modes have been shown to be meta-stable in JET, and their beta limits can be raised by destabilization (modification) of sawteeth by ion cyclotron radio frequency heating (ICRH). Alpha simulation experiments with ICRH accelerated injected 4 (He) beam ions provide a new tool for fast particle and magnetohydrodynamic studies, with up to 80-90% of plasma heating by fast 4 He ions. With or without impurity seeding, a quasi-steady-state high confinement (H-98 = 1), high density(n(e)/n(GW) = 0.9-1) and high beta (betaN = 2) ELMy H-mode has been achieved by operating near the ITER triangularity ( similar to 0.40-0.5) and safety factor (q(95) similar to 3), at Z(eff) similar to 1.5-2. In advanced tokamak (AT) scenarios, internal transport barriers (ITBs) are now characterized in real time with a new criterion, rhoT(*). Tailoring of the current profile with T lower hybrid current drive provides reliable access to a variety of q profiles, lowering access power for barrier formation. Rational q surfaces appear to be associated with ITB formation. Alfven cascades were observed in reversed shear plasmas, providing identification of q profile evolution. Plasmas with current holes were observed and modelled. Transient high confinement AT regimes with H-89 = 3.3, beta(N) = 2.4 and ITER-relevant q < 5 were achieved with reversed magnetic shear. Quasi-stationary ITBs are developed with full non-inductive current drive, including similar to 50% bootstrap current. A record duration of ITBs was achieved, up to 11 s, approaching the resistive time. For the first time, pressure and current profiles of AT regimes are controlled by a real-time feedback system, in separate experiments. Erosion and co-deposition studies with a quartz micro-balance show reduced co-deposition. Measured divertor thermal loads during disruptions in JET could modify ITER assumptions.

Plasma pressure effect on Alfvén cascade eigenmodes

Tokamak plasmas with reversed magnetic shear are prone to the excitation of Alfven cascade (AC) eigenmodes by energetic particles. These modes exhibit a quasiperiodic pattern of predominantly upward frequency sweeping. Observations also reveal that the AC spectral lines sometimes bend at low frequencies, which is a significant deviation from the shear Alfven wave dispersion relation. This paper shows that the underlying reasons for such bending are the finite pressure of the plasma and the geodesic curvature that precludes shear Alfven perturbations from being strictly incompressible. In addition to the geodesic effect, there are two other pressure effects on shear Alfven waves, which are the convection in the presence of an equilibrium pressure gradient and the toroidicity-induced coupling between shear Alfven waves and acoustic modes. An analytical treatment of the problem enables a parametric comparison of all three mechanisms. The key distinction between the geodesic compressibility and the acoustic c...

Disruptions and disruption mitigation

Detailed disruption related studies have been undertaken at JET in 2001–2003 and are compared and contrasted with results from other tokamaks. An enhanced halo current detection system has allowed more precise asymmetry measurements to be made, confirming the assumptions in the ITER design guidelines. The runaway electron database has been systematically analysed and self-consistent simulations are underway. Disruption heat deposition, which is of fundamental importance for ITER, has been investigated in JET. It has been found that normally only a small fraction of the thermal energy goes to the divertor and that during the thermal quench some energy goes to the outer wall. The neutral point has been experimentally found and a plasma control model has been used to reproduce the dedicated discharges. Helium, neon and argon puffs have been applied as disruption mitigation techniques in JET following the success of recent studies in ASDEX Upgrade, TEXTOR and DIII-D.

Internal transport barrier triggering by rational magnetic flux surfaces in tokamaks

The formation of internal transport barriers (ITBs) has been experimentally associated with the presence of rational q surfaces in both JET and ASDEX Upgrade. The triggering mechanisms are related to the occurrence of magneto-hydrodynamic (MHD) instabilities such as mode coupling and fishbone activity. These events could locally modify the poloidal velocity and increase transiently the shearing rate to values comparable with the linear growth rate of ion temperature gradient modes. For JET reversed magnetic shear scenarios, ITB emergence occurs preferentially when the minimum q reaches an integral value. In this case, transport effects localized in the vicinity of zero magnetic shear and close to rational q values may be at the origin of ITB formation. The role of rational q surfaces in ITB triggering stresses the importance of q profile control for an advanced tokamak scenario and could assist in substantially lowering the access power to these scenarios in next step facilities.

Heat loads on JET plasma facing components from ICRF and LH wave absorption in the SOL

In JET, lower hybrid (LH) and ion cyclotron resonance frequency (ICRF) wave absorption in the scrape-off layer can lead to enhanced heat fluxes on some plasma facing components (PFCs). Experiments have been carried out to characterize these heat loads in order to: (i) prepare JET operation with the Be wall which has a reduced power handling capability as compared with the carbon wall and (ii) better understand the physics driving these wave absorption phenomena and propose solutions for next generation systems to reduce them. When using ICRF, hot spots are observed on the antenna structures and on limiters close to the powered antennas and are explained by acceleration of ions in RF-rectified sheath potentials. High temperatures up to 800??C can be reached on locations where a deposit has built up on tile surfaces. Modelling which takes into account the fast thermal response of surface layers can reproduce well the surface temperature measurements via infrared (IR) imaging, and allow evaluation of the heat fluxes local to active ICRF antennas. The flux scales linearly with the density at the antenna radius and with the antenna voltage. Strap phasing corresponding to wave spectra with lower k? values can lead to a significant increase in hot spot intensity in agreement with antenna modelling that predicts, in that case, an increase in RF sheath rectification. LH absorption in front of the antenna through electron Landau damping of the wave with high N? components generates hot spots precisely located on PFCs magnetically connected to the launcher. Analysis of the LH hot spot surface temperature from IR measurements allows a quantification of the power flux along the field lines: in the worst case scenario it is in the range 15?30?MW?m?2. The main driving parameter is the LH power density along the horizontal rows of the launcher, the heat fluxes scaling roughly with the square of the LH power density. The local electron density in front of the grill increases with the LH launched power; this also enhances the intensity of the LH hot spots.

EDGE2D modelling of edge profiles obtained in JET diagnostic optimized configuration

Nine type-I ELMy H-mode discharges in diagnostic optimized configuration in JET are analysed with the EDGE2D/NIMBUS package. EDGE2D solves the fluid equations for the conservation of particles, momentum and energy for hydrogenic and impurity ions, while neutrals are followed with the two-dimensional Monte Carlo module NIMBUS. Using external boundary conditions from the experiment, the perpendicular heat conductivities χi,e and the particle transport coefficients D, v are varied until good agreement between code result and measured data is obtained. A step-like ansatz is used for the edge transport parameters for the outer core region, the edge transport barrier and the outer scrape-off layer. The time-dependent effect of edge localized modes on the edge profiles is simulated with anadhoc ELM model based on the repetitive increase of the transport coefficients χi,e and D. The values of the transport coefficients are matched to experimental data mapped to the outer midplane, in the course of which radial shifts of experimental profiles of the order of 1 cm caused by the accuracy limit of the equilibrium reconstruction are taken into account. Simulated divertor profiles obtained from the upstream transport ansatz and the experimental boundary conditions agree with measurements, except a small region localized at the separatrix strike points which is supposed to be affected by direct ion losses. The integrated analysis using EDGE2D modelling, although still limited by the marginal spatial resolution of individual diagnostics, allows the characterization of profiles in the edge/pedestal region and supplies additional information on the separatrix position. The steep density gradient zone inside the separatrix shrinks compared to the electron temperature

Approximate method to extract the pure Faraday and Cotton–Mouton effects from polarimetry measurements in a tokamak

Polarimetry, a powerful diagnostic tool, can provide information on the density and magnetic field, utilizing the Faraday and the Cotton–Mouton effects in a magnetized plasma. Both effects contribute to the change of the polarization of an electromagnetic wave traversing a magnetized plasma such that, unless both effects are small, the measurable output polarization does not provide the pure effects in terms of Faraday rotation angle and Cotton–Mouton phase shift angle, which are directly related to simple line-of-sight integrals of the electron density times certain components of the magnetic field. In view of the importance of the latter, a new formalism has been developed, which delivers narrow limits to these quantities in terms of analytic formulae that contain nothing but the input and output polarization parameters. The approach is valid for vertical lines of sight in toroidal devices whose toroidal field dominates over other field components perpendicular to the line of sight. Examples relating to measurements at JET demonstrate the capabilities of the method.

Electron cyclotron emission radiometer upgrade on the Joint European Torus (JET) tokamak

The capabilities of the Joint European Torus (JET) electron cyclotron emission (ECE) diagnostics have recently been extended with an upgrading of the heterodyne radiometer. The number of channels has been doubled to 96 channels, with a frequency separation corresponding to 2.6u2002m for the X-mode due to harmonic overlap) at almost all magnetic field values used at JET (1.7u2002T<BT<4u2002T), while maintaining the high sensitivity and spectral resolution of the previous system. In this paper an overview of the upgraded radiometer is presented along with some results showing its performance.

Boundary plasma energy transport in JET ELMy H-modes

Power deposition profiles on JET MkIIGB divertor plates were measured for a variety of plasma conditions. Comparison of experimental measurements with theoretical predictions of two dozen candidate models of perpendicular heat conductivity suggests that a combination of classical ion conduction and ion orbit loss from the pedestal region dominates radial energy transport in the scrape-off layer (SOL), at least in the vicinity of the separatrix, during the inter-edge localized mode (ELM) phase of H-mode discharges. Although not a conclusive proof of suppression of (ion) turbulence, it is consistent with the hypothesis that the edge transport barrier extends into the SOL during the inter-ELM phase. Extrapolations to ITER predict an outer target power width of 3 ± 1 mm-omp (mapped to the outer mid-plane) at the entrance into the divertor volume.

Sawtooth control in fusion plasmas

Clear observations of early triggering of neo-classical tearing modes by sawteeth with long quiescent periods have motivated recent efforts to control, and in particular destabilize, sawteeth. One successful approach explored in TCV utilizes electron cyclotron heating in order to locally increase the current penetration time in the core. The latter is also achieved in various machines by depositing electron cyclotron current drive or ion cyclotron current drive close to the q = 1 rational surface. Crucially, localized current drive also succeeds in destabilizing sawteeth which are otherwise stabilized by a co-existing population of energetic trapped ions in the core. In addition, a recent reversed toroidal field campaign at JET demonstrates that counter-neutral beam injection (NBI) results in shorter sawtooth periods than in the Ohmic regime. The clear dependence of the sawtooth period on the NBI heating power and the direction of injection also manifests itself in terms of the toroidal plasma rotation, which consequently requires consideration in the theoretical interpretation of the experiments. Another feature of NBI, expected to be especially evident in the negative ion based neutral beam injection (NNBI) heating planned for ITER, is the parallel velocity asymmetry of the fast ion population. It is predicted that a finite orbit effect of asymmetrically distributed circulating ions could strongly modify sawtooth stability. Furthermore, NNBI driven current with non-monotonic profile could significantly slow down the evolution of the safety factor in the core, thereby delaying sawteeth.