Showing papers in "Plasma Physics and Controlled Fusion in 2007"

A quarter-century of H-mode studies

Abstract: The H-mode is a confinement mode of toroidal plasmas, which may make the goals of fusion possible—the development of a clean energy source at competitive electricity costs. The most challenging aspect of the H-mode physics is the sudden disappearance of the edge turbulence whereas its driving forces—the gradients—increase. As the physics behind the H-mode is subtle many features are not yet clarified. There is, however, substantial experimental and theoretical evidence that turbulent flows, which normally limit the confinement, are diminished by sheared poloidal flow residing at the plasma edge. There are many conceivable mechanisms giving rise to sheared flow. The most intriguing of these is that fluctuations themselves induce the flow, which acts back to its generating origin and annihilates the turbulence. This review concentrates mostly on the transition physics, describes one line of understanding the H-mode in more detail, recalls some of the older observations and summarizes the achievements in the H-mode for both tokamaks and stellarators. (Some figures in this article are in colour only in the electronic version)

Edge turbulence measurements in toroidal fusion devices

Abstract: This paper reviews measurements of edge plasma turbulence in toroidal magnetic fusion devices with an emphasis on recent results in tokamaks. The dominant feature of edge turbulence is a high level of broadband density fluctuations with a relative amplitude δn/n ~ 5–100%, accompanied by large potential and electron temperature fluctuations. The frequency range of this turbulence is ~10 kHz–1 MHz, and the size scale is typically ~0.1–10 cm perpendicular to the magnetic field but many metres along the magnetic field, i.e. the structure is nearly that of 2D 'filaments'. Large intermittent bursts or 'blobs' are usually observed in the scrape-off layer. Diagnostic and data analysis techniques are reviewed and the main experimental results are summarized. Recent comparisons of experimental results with edge turbulence theory are discussed, and some directions for future experiments are suggested.

Electron Bernstein wave heating and diagnostic

Abstract: This paper gives a review on the experiments with electron Bernstein waves (EBWs) in fusion devices. The different methods of EBW generation are described and compared with experimental results. The influence of density fluctuation and parametric instability on the conversion efficiency is discussed. The related experiments are reported. The EBW propagation is calculated by ray-tracing codes. The results are used to analyse EBW emission, heating and current drive experiments in stellarators and tokamaks. With high power microwave sources EBWs have been excited over a wide range of frequencies for plasma heating and current drive. The experimental results demonstrated that EBW can efficiently heat over-dense plasmas. The local power deposition allows the generation of heat waves for transport studies. Due to their electrostatic character, EBWs can achieve parallel refractive indices (N||) larger than 1, which is favourable for efficient current drive. This could be confirmed by a first current drive experiment. The EBWs also express a strong cyclotron damping, which enabled efficient heating at higher harmonics in several experiments.

Industrial applications of atmospheric non-thermal plasma in environmental remediation

Abstract: Atmospheric non-thermal plasma (NTP) has been recognized as an important tool for the abatement of pollutants and for promoting various chemical reactions in gas or in liquid. Indoor air cleaners have been mass-produced and proved to be effective for the removal of odour and allergen. NTP has various potential applications in environmental remediation, such as the removal of volatile organic pollutants, simultaneous removal of NOx and soot in diesel exhaust and sterilization of air and water. To improve the efficiency of plasma chemical processes, a combination of NTP and catalysts/absorbents is effective. Synergetic effects have been recognized; however, its mechanism remains subject to further investigations. The generation of non-thermal discharge plasma and several industrial applications are presented in this paper.

Modelling of dynamics and transport of carbon dust particles in tokamaks

Abstract: The transport of dust particles in tokamak fusion devices is studied using computer simulations with the dust transport code, DUSTT. Recent developments in modelling with the DUSTT code are reported. The improved model of dust dynamics in edge plasmas takes into account several additional effects, including thermionic and secondary electron emission which affects dust charging and heating, dust grain size effect on thermal radiation, and the presence of impurities in the plasma. It is shown that thermionic emission leads to enhanced dust heating by the plasma that boosts destruction of dust particles. The zone structure of tokamak plasmas is introduced for a qualitative analysis of dust survivability conditions. It is shown that a dust particle can experience net deposition in relatively cold carbon-contaminated plasma regions. Trajectories of sample dust particles in the DIII-D tokamak are simulated and analysed using the zone plasma description. Statistical averaging over an ensemble of particle trajectories is used to obtain spatial distributions of dust characteristics in the edge plasma of tokamaks. It is shown that transport of dust in tokamaks can significantly enhance penetration of carbon impurities towards the core plasma.

A control-oriented model of the current profile in tokamak plasma

Abstract: This paper proposes a control-oriented approach to the tokamak plasma current profile dynamics. It is established based on a consistent set of simplified relationships, in particular for the microwave current drive sources, rather than exact physical modelling. Assuming that a proper model for advanced control schemes can be established using the so-called cylindrical approximation and neglecting the diamagnetic effects, we propose a model that focuses on the flux diffusion (from which the current profile is inferred). Its inputs are some real-time measurements available on modern tokamaks and the effects of some major actuators, such as the magnetic coils, lower hybrid (LHCD), electron and ion cyclotron frequency (ECCD and ICRH) systems, are particularly taken into account. More precisely, the non-inductive current profile sources are modelled as 3-parameters functions of the control inputs derived either from approximate theoretical formulae for the ECCD and bootstrap terms or from experimental scaling laws specifically developed from hard x-ray Tore Supra data for the LHCD influence. The use of scaling laws in this model reflects the fact that the operation of future reactors will certainly depend upon a great number of scaling laws and specific engineering parameters. The discretization issues are also specifically addressed, to ensure robustness with respect to discretisation errors and the efficiency (in terms of computation time) of the associated algorithm. This model is compared with experimental results and the CRONOS solver for tore supra tokamak.

Review: Pellet injection experiments and modelling

Abstract: During the last decade, significant progress has been made in the field of pellet injection with (1) the identification of the drift of the deposited material in the inhomogeneous magnetic field that opened the possibility of fuelling efficiently the plasmas from the high-field side of the torus, (2) the technique to mitigate ELMS in H-mode discharges with shallow pellet injection at high frequency and (3) with the development of high density, high performance scenarios close to the ITER requirements. Both the experimental and theoretical aspects of this domain are reviewed in this paper.

Global full-f gyrokinetic simulations of plasma turbulence

Abstract: Critical physical issues can be specifically tackled with the global full-f gyrokinetic code GYSELA. Three main results are presented. First, the self-consistent treatment of equilibrium and fluctuations highlights the competition between two compensation mechanisms for the curvature driven vertical charge separation, namely, parallel flow and polarization. The impact of the latter on the turbulent transport is discussed. In the non-linear regime, the benchmark with the Particle-In-Cell code ORB5 looks satisfactory. Second, the transport scaling with p. is found to depend both on p. itself and on the distance to the linear threshold. Finally, a statistical steady-state turbulent regime is achieved in a reduced version of GYSELA by prescribing a constant heat source.

Plasma wall interaction and its implication in an all tungsten divertor tokamak

Abstract: ASDEX Upgrade has recently finished its transition towards an all-W divertor tokamak, by the exchange of the last remaining graphite tiles to W-coated ones. The plasma start-up was performed without prior boronization. It was found that the large He content in the plasma, resulting from DC glow discharges for conditioning, leads to a confinement reduction. After the change to D glow for inter-shot conditioning, the He content quickly dropped and, in parallel, the usual H-Mode confinement with H factors close to one was achieved. After the initial conditioning phase, oxygen concentrations similar to that in previous campaigns with boronizations could be achieved. Despite the removal of all macroscopic carbon sources, no strong change in C influxes and C content could be observed so far. The W concentrations are similar to the ones measured previously in discharges with old boronization and only partial coverage of the surfaces with W. Concomitantly it is found that although the W erosion flux in the divertor is larger than the W sources in the main chamber in most of the scenarios, it plays only a minor role for the W content in the main plasma. For large antenna distances and strong gas puffing, ICRH power coupling could be optimized to reduce the W influxes. This allowed a similar increase of stored energy as yielded with comparable beam power. However, a strong increase of radiated power and a loss of H-Mode was observed for conditions with high temperature edge plasma close to the antennas. The use of ECRH allowed keeping the central peaking of the W concentration low and even phases of improved H-modes have already been achieved.

Using Plasma Physics to Weigh the Photon

Abstract: The currently accepted value for the upper bound for the photon mass, mph, is 22 orders of magnitude less than the electron mass. As the mass mph is so incredibly small, it has essentially no effect on atomic and nuclear physics; and it is very difficult to improve this estimate by laboratory experiments. However, even a very small mass may have a significant effect on astrophysical phenomena occurring on a scale exceeding the photon Compton length (where for the currently accepted mass). A set of magnetohydrodynamic equations (assuming a finite photon mass) are used to analyze properties of the solar wind at Pluto's orbit. This yields an improved (reduced) by a factor of 70 estimate of the photon mass. Possible opportunities and challenges for the further reduction of the upper limit for mph based on the properties of larger-scale astrophysical objects are discussed.

Negative ion RF sources for ITER NBI: status of the development and recent achievements

Abstract: For heating and current drive the neutral beam injection system for ITER requires a deuterium beam with an energy of 1 MeV for up to 1 h. In order to inject the required 17 MW the ion source has to deliver 40 A of negative ion current. For an accelerated current density of 200 A m−2 at the specified source pressure of 0.3 Pa the extraction area is 0.2 m2 resulting in a large area source of 1.5 × 0.6 m2. Two types of sources have been under discussion, the filamented arc source and the inductively driven RF source, the latter now having been chosen for the ITER reference design. The development of negative ion RF sources, which fulfil these specifications is being carried out at the Max–Planck-Institut fur Plasmaphysik at three test facilities in parallel. The required current densities at the ITER relevant pressure have been achieved and even exceeded in a test facility equipped with a small ion source (extraction area of 0.007 m2) at limited pulse length (<4 s). The extraction area can be extended up to 0.03 m2 and the pulse length up to 3600 s at a second test facility which is dedicated to long pulse operation experiments where pulses up to 800 s have already been achieved. The ion source at the third test facility has roughly the full width and half the height of the ITER source but is not equipped with an extraction system. The aim is to demonstrate the size scaling and plasma homogeneity of RF ion sources. First results from different diagnostic techniques (optical emission spectroscopy and Langmuir probe) are very promising.

Micro-tearing modes in the mega ampere spherical tokamak

Abstract: Recent gyrokinetic stability calculations have revealed that the spherical tokamak is susceptible to tearing parity instabilities with length scales of a few ion Larmor radii perpendicular to the magnetic field lines. Here we investigate this 'micro-tearing' mode in greater detail to uncover its key characteristics and compare it with existing theoretical models of the phenomenon. This has been accomplished using a full numerical solution of the linear gyrokinetic?Maxwell equations. Importantly, the instability is found to be driven by the free energy in the electron temperature gradient as described in the literature. However, our calculations suggest it is not substantially affected by either of the destabilizing mechanisms proposed in previous theoretical models. Instead the instability is destabilized by interactions with magnetic drifts and the electrostatic potential. Further calculations reveal that the mode is not significantly destabilized by the flux surface shaping or the large trapped particle fraction present in the spherical tokamak. Its prevalence in spherical tokamak plasmas is primarily due to the higher value of plasma ? and the enhanced magnetic drifts due to the smaller radius of curvature.

Plasma wall interaction and its implication in an all tungsten divertor tokamak. Invited Paper

Abstract: ASDEX Upgrade has recently finished its transition towards an all-W divertor tokamak, by the exchange of the last remaining graphite tiles to W-coated ones. The plasma start-up was performed without prior boronization. It was found that the large He content in the plasma, resulting from DC glow discharges for conditioning, leads to a confinement reduction. After the change to D glow for inter-shot conditioning, the He content quickly dropped and, in parallel, the usual H-Mode confinement with H factors close to one was achieved. After the initial conditioning phase, oxygen concentrations similar to that in previous campaigns with boronizations could be achieved. Despite the removal of all macroscopic carbon sources, no strong change in C influxes and C content could be observed so far. The W concentrations are similar to the ones measured previously in discharges with old boronization and only partial coverage of the surfaces with W. Concomitantly it is found that although the W erosion flux in the divertor is larger than the W sources in the main chamber in most of the scenarios, it plays only a minor role for the W content in the main plasma. For large antenna distances and strong gas puffing, ICRH power coupling could be optimized to reduce the W influxes. This allowed a similar increase of stored energy as yielded with comparable beam power. However, a strong increase of radiated power and a loss of H-Mode was observed for conditions with high temperature edge plasma close to the antennas. The use of ECRH allowed keeping the central peaking of the W concentration low and even phases of improved H-modes have already been achieved.

Measurements of fast-ion acceleration at cyclotron harmonics using Balmer-alpha spectroscopy

Abstract: Combined neutral beam injection and fast wave heating at the fourth and fifth cyclotron harmonics accelerate fast ions in the DIII-D tokamak. Measurements with a nine-channel fast-ion D-alpha (FIDA) diagnostic indicate the formation of a fast-ion tail above the injection energy. Tail formation correlates with enhancement of the d–d neutron rate above the value that is expected in the absence of fast-wave acceleration. FIDA spatial profiles and fast-ion pressure profiles inferred from the equilibrium both indicate that the acceleration is near the magnetic axis for a centrally located resonance layer. The enhancement is largest 8–10 cm beyond the radius where the wave frequency equals the cyclotron harmonic, probably due to a combination of Doppler-shift and orbital effects. The fast-ion distribution function calculated by the CQL3D Fokker– Planck code is fairly consistent with the data. (Some figures in this article are in colour only in the electronic version)

Tokamak edge turbulence: background theory and computation

Abstract: The basic scales of motion and computational requirements for low frequency fluid drift turbulence are summarized in tutorial fashion, with emphasis on the tokamak edge region. Parameters are given by experimental observations, but the computations are otherwise done from first principles. Edge turbulence is fundamentally electromagnetic and nonlinear, not treatable by standard linear or secondary instability analysis. Energetic character is determined by diagnosis of the terms in the energy theorem within the fully developed saturated phase. The spectra of the fluctuations and transport always extend to below the ion gyroradius scale. Direct coupling of pressure fluctuations and E-cross-B eddies through the parallel current is always active. Edge turbulence derives its character from steep gradients, with a parallel/perp scale ratio larger than 100, rather than from collisional effects. Collisionality is neither absent nor strongly dominant for electrons, but very weak for ions. Fluctuations in the axisymmetric component, including the Pfirsch–Schluter currents, are dynamically integrated into the turbulence. Time scales are one to two orders of magnitude shorter than the ion collision time, hence significant delays occur in the response of heat fluxes and viscosity to temperature gradients and flows. Hence the need for a trans-collisional gyrofluid model to treat cases with comparable ion and electron temperature. Two orders of magnitude in spatial scales and three in time scales are typically involved.

Thin film deposition by means of atmospheric pressure microplasma jet

Abstract: An RF microplasma jet working at atmospheric pressure has been developed for thin film deposition application. It consists of a capillary coaxially inserted in the ceramic tube. The capillary is excited by an RF frequency of 13.56 MHz at rms voltages of around 200–250 V. The plasma is generated in a plasma forming gas (helium or argon) in the annular space between the capillary and the ceramic tube. By adjusting the flows, the flow pattern prevents the deposition inside the source and mixing of the reactive species with the ambient air in the discharge and deposition region, so that no traces of air are found even when the microplasma is operated in an air atmosphere. All these properties make our microplasma design of great interest for applications such as thin film growth or surface treatment. The discharge operates probably in a γ -mode as indicated by high electron densities of around 8×10 20 m −3 measured using optical emission spectroscopy. The gas temperature stays below 400 K and is close to room temperature in the deposition region in the case of argon plasma. Deposition of hydrogenated amorphous carbon films and silicon oxide films has been tested using Ar/C2H2 and Ar/hexamethyldisiloxane/O2 mixtures, respectively. In the latter case, good control of the film properties by adjusting the source parameters has been achieved with the possibility of depositing carbon free SiOx films even without the addition of oxygen. Preliminary results regarding permeation barrier properties of deposited films are also given.

Collisionality dependent transport in TCV SOL plasmas

Abstract: Results are presented from probe measurements in the low field side scrapeoff layer (SOL) region of TCV during plasma current scan experiments. It is shown that with decreasing plasma current the radial particle density profile becomes broader and the fluctuation levels and turbulence driven radial particle flux increase. In the far SOL the fluctuations exhibit a high degree of statistical similarity and the particle density and flux at the wall radius scale inversely with the plasma current. Together with previous TCV density scan experiments, this indicates that plasma fluctuations and radial transport increase with plasma collisionality. Such a collisionality dependence is consistent with a recent theory for radial blob motion, which suggests that filamentary structures become electrically disconnected from the target sheaths at large collisionality and thus experience less sheath dissipation. This increases the radial convective transport and is possibly linked to the discharge density limit.

Edge localized modes: recent experimental findings and related issues

Abstract: Edge localized mode (ELM) measurements in many tokamaks, including ASDEX-Upgrade, DIII-D, JET, JT-60U and MAST, are reviewed, which includes progress in experimental observations at the plasma edge region by means of fast-time resolved diagnostics with high precision, such as scanning probe, radial interferometer chord, BES and tangentially viewing fast-gated camera at the midplane. ELM dynamics data show that the majority of the ELM particle and energy transport should be dominated by ion convection physics and associated timescales. Furthermore, recent diagnostic upgrades on many tokamaks reveal the ELM filament structure and their complex motion towards radial, poloidal and toroidal directions. Approaches to control the Type-I ELMs, in addition to the alternative scenarios to Type-I ELMy H-mode operation (so-called, small/no ELM regimes) are also a key area of research for current tokamaks, which demonstrated a high confinement (being comparable to that of Type-I ELMy H-mode plasmas at similar parameters) in the absence of large, ELM induced, transient heat/particle fluxes to the divertor targets. Although tolerable ELM regimes are obtained in existing devices, their application to ITER is uncertain. Issues of these regimes towards further experiments and power deposition on divertor targets and main chamber wall are discussed.

Application of powerful quasi-steady-state plasma accelerators for simulation of ITER transient heat loads on divertor surfaces

Abstract: The paper presents the investigations of high power plasma interaction with material surfaces under conditions simulating the ITER disruptions and type I ELMs. Different materials were exposed to plasma with repetitive pulses of 250 µs duration, the ion energy of up to 0.6 keV, and the heat loads varying in the 0.5–25 MJ m−2 range. The plasma energy transfer to the material surface versus impact load has been analysed. The fraction of plasma energy that is absorbed by the target surface is rapidly decreased with the achievement of the evaporation onset for exposed targets. The distributions of evaporated material in front of the target surface and the thickness of the shielding layer are found to be strongly dependent on the target atomic mass. The surface analysis of tungsten targets exposed to quasi-steady-state plasma accelerators plasma streams is presented together with measurements of the melting onset load and evaporation threshold, and also of erosion patterns with increasing heat load and the number of plasma pulses.

Momentum confinement at low torque

Abstract: Momentum confinement was investigated on DIII-D as a function of applied neutral beam torque at constant normalized beta βN, by varying the mix of co (parallel to the plasma current) and counter neutral beams. Under balanced neutral beam injection (i.e. zero total torque to the plasma), the plasma maintains a significant rotation in the co-direction. This 'intrinsic' rotation can be modeled as being due to an offset in the applied torque (i.e. an 'anomalous torque'). This anomalous torque appears to have a magnitude comparable to one co neutral beam source. The presence of such an anomalous torque source must be taken into account to obtain meaningful quantities describing momentum transport, such as the global momentum confinement time and local diffusivities.Studies of the mechanical angular momentum in ELMing H-mode plasmas with elevated qmin show that the momentum confinement time improves as the torque is reduced. In hybrid plasmas, the opposite effect is observed, namely that momentum confinement improves at high torque/rotation. GLF23 modeling suggests that the role of E × B shearing is quite different between the two plasmas, which may help to explain the different dependence of the momentum confinement on torque.

ELM resolved energy distribution studies in the JET MKII Gas-Box divertor using infra-red thermography

Abstract: Using infra-red (IR) thermography, power loads onto the MKII Gas-Box divertor targets have been investigated in Type-I ELMy H-Mode plasmas at JET in medium current discharges (I-p = 2.6MA and B-T = 2.7 T). Heat fluxes are calculated from the measured divertor target tile surface temperatures taking into account the influence of co-deposited surface layers on tile surfaces. This is particularly important when estimating the energy deposition during transient events such as ELMs. Detailed energy balance analysis is used, both from IR and tile embedded thermocouples, to demonstrate an approximately constant ELM-averaged in/out divertor target asymmetry of approximate to 0.55 and to show that the ELM in/out energy deposition ratio ranges from 1 : 1 to 2 : 1. The inter-ELM in/out ratio is close to the ELM-averaged value at low pedestal collisionalities and decreases down to values close to zero when the inner target plasma detaches at the highest pedestal collisionalities. The fraction of ELM transported energy is observed to behave differently for the inner and the outer divertor. At higher pedestal collisionalities nearly the full inner target load is due to the ELMs whereas for the outer target the ELM transported energy never exceeds values of approximate to 0.3 of the total energy deposited there. The fraction of ELM energy arriving at the divertor compared with the pedestal loss energy in JET is found to be in the range of 0.75 for small ELMs down to 0.4 for large ELMs systematically decreasing with normalized ELM size. Since ITER is bound to use small ELMs the corresponding ELM wall load is expected to be small. The latter experimental result is in fair agreement with the observation that larger ELMs tend to travel faster across the SOL than smaller ELMs. However, a comparison of the presented data with models of ELM perpendicular transport is not conclusive due to the large experimental errorbars and uncertainties in the model assumptions.

The effect of ion-scale dynamics on electron-temperature-gradient turbulence

Abstract: This work reports on numerical studies of small-scale electron-temperature-gradient (ETG) turbulence embedded in large-scale turbulence driven by both ion-temperature-gradient (ITG) modes and trapped-electron modes. To begin with, we find that the simplified adiabatic-ion model of ETG turbulence does not always saturate nonlinearly, suggesting that corrections to the purely adiabatic ion response are required for robust saturation. Our results also qualitatively confirm a prediction of Holland and Diamond that the back-reaction of ETG on ITG turbulence is insignificant. For the parameters studied, we find that ETG turbulence levels are reduced as ion driving gradients are increased. This result is at least partially explained by linear physics. An important practical result of this work is the finding that most of the electron energy transport arises from ion scales (kθρi 1.0.

Evolution of the pedestal on MAST and the implications for ELM power loadings

Abstract: Studies of the pedestal characteristics and quantities determining edge-localized mode (ELM) energy losses in MAST are presented. High temperature pedestal plasmas have been achieved which have collisionalities one order of magnitude lower than previous results . A stability analysis performed on these plasmas shows them to be near the ballooning limit. The fraction of pedestal energy released by an ELM as a function of collisionality on MAST is consistent with data from other devices. The evolution of the filamentary structures observed during ELMs has been studied and has shown that they exist near to the last closed flux surface for the time over which the majority of particles and energy are being released from the pedestal region into the scrape off layer. A simple model has been developed, which is in reasonable agreement with the observed ELM energy losses and target profiles.

Magnetic self organization, MHD active control and confinement in RFX-mod

Abstract: RFX-mod is a reversed field pinch (RFP) experiment equipped with a system that actively controls the magnetic boundary. In this paper we describe the results of a new control algorithm, the clean mode control (CMC), in which the aliasing of the sideband harmonics generated by the discrete saddle coils is corrected in real time. CMC operation leads to a smoother (i.e. more axisymmetric) boundary. Tearing modes rotate (up to 100 Hz) and partially unlock. Plasma–wall interaction diminishes due to a decrease of the non-axisymmetric shift of the plasma column. With the ameliorated boundary control, plasma current has been successfully increased to 1.5 MA, the highest for an RFP. In such regimes, the magnetic dynamics is dominated by the innermost resonant mode, the internal magnetic field gets close to a pure helix and confinement improves.

Global gyrokinetic particle simulations with kinetic electrons

Abstract: A toroidal, nonlinear, electrostatic fluid-kinetic hybrid electron model is formulated for global gyrokinetic particle simulations of driftwave turbulence in fusion plasmas. Numerical properties are improved by an expansion of the electron response using a smallness parameter of the ratio of driftwave frequency to electron transit frequency. Linear simulations accurately recover the real frequency and growth rate of toroidal ion temperature gradient (ITG) instability. Trapped electrons increase the ITG growth rate by mostly not responding to the ITG modes. Nonlinear simulations of ITG turbulence find that the electron thermal and particle transport are much smaller than the ion thermal transport and that small scale zonal flows are generated through nonlinear interactions of the trapped electrons with the turbulence.

Bulk plasma rotation in the TCV tokamak in the absence of external momentum input

Abstract: Carbon ion velocity profiles are measured in TCV with a charge exchange diagnostic using a negligibly perturbing diagnostic neutral beam. These ‘intrinsic’ rotation profiles are measured up to the plasma edge in the toroidal and poloidal directions for both limited and diverted plasma configurations in Ohmic plasmas and in the presence of strong second harmonic electron cyclotron heating (ECH). Absolute toroidal velocities are shown to scale with peak ion temperature and inversely with plasma current. The plasma edge rotation is always small in limited configurations but evolves smoothly with the core density for diverted configurations. A strong intrinsic rotation builds up in the plasma core in the counter-current direction for limited configurations but is observed in the co-current direction for diverted plasmas. Unexpectedly, above a given density threshold, the rotation profile reverses to the co-current direction for limited configurations (and surprisingly, in the counter-current direction for diverted configurations). This threshold density is found to depend on plasma current, the presence of ECH and the magnetic topology. Poloidal velocity measurements are used to deduce the radial electric field change across the transition. A strong dependence of the rotation profile on plasma triangularity is reported and possible physics models for these observations are discussed. The origin of the momentum drive, its reversal and its magnitude are not yet clearly understood even for these relatively ‘simple’ experimental configurations. (Some figures in this article are in colour only in the electronic version)

Active control of type-I edge localized modes on JET

Abstract: 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.

Radial correlation length measurements on ASDEX Upgrade using correlation Doppler reflectometry

Abstract: The technique of correlation Doppler reflectometry for providing radial correlation length Lr measurements is explored in this paper. Experimental Lr measurements are obtained using the recently installed dual channel Doppler reflectometer system on ASDEX Upgrade. The experimental measurements agree well with theory and with Lr measured on other fusion devices using different diagnostic techniques. A strong link between Lr and plasma confinement could be observed. From the L- to the H-mode, an increase in the absolute value of Er shear was detected at the same plasma edge region where a decrease in Lr was measured. This observation is in agreement with theoretical models which predict that an increase in the absolute shear suppresses turbulent fluctuations in the plasma, leading to a reduction in Lr. Furthermore, Lr decreases from the plasma core to the edge and decreases with increasing plasma triangularity δ. The experimental results have been extensively modelled using a 2-dimensional finite difference time domain code. The simulations confirm that Doppler reflectometry provides robust radial correlation lengths of the turbulence with high resolution and suggests that Lr is independent of the turbulence wavenumber k⊥ and its fluctuation level.

The physics of sawtooth stabilization

Abstract: Long period sawteeth have been observed to result in low-beta triggering of neo-classical tearing modes, which can significantly degrade plasma confinement. Consequently, a detailed physical understanding of sawtooth behaviour is critical, especially for ITER where fusion-born a particles are likely to lead to very long sawtooth periods. Many techniques have been developed to control, and in particular to destabilize the sawteeth. The application of counter-current neutral beam injection (NBI) in JET has resulted in shorter sawtooth periods than in Ohmic plasmas. This result has been explained because, firstly, the counter-passing fast ions give a destabilizing contribution to the n=1 internal kink mode-which is accepted to be related to sawtooth oscillations-and secondly, the flow shear strongly influences the stabilizing trapped particles. A similar experimental result has been observed in counter-NBI heated plasmas in MAST. However, the strong toroidal flows in spherical tokamaks mean that the sawtooth behaviour is determined by the gyroscopic flow stabilization of the kink mode rather than kinetic effects. In NBI heated plasmas in smaller conventional aspect-ratio tokamaks, such as TEXTOR, the flow and kinetic effects compete to give different sawtooth behaviour. Other techniques applied to destabilize sawteeth are the application of electron cyclotron current drive (ECCD) or ion cyclotron resonance heating (ICRH). In JET, it has been observed that localized ICRH is able to destabilize sawteeth which were otherwise stabilized by a co-existing population of energetic trapped ions in the core. This is explained through the dual role of the ICRH in reducing the critical magnetic shear required to trigger a sawtooth crash, and the increase in the local magnetic shear which results from driving current near the q=1 rational surface. Sawtooth control in ITER could be provided by a combination of ECCD and co-passing off-axis negative-NBI fast ions.