Showing papers in "Plasma Physics and Controlled Fusion in 1996"

Edge localized modes (ELMs)

Abstract: The phenomenology of edge localized modes (ELMs), an MHD instability occurring in the edge of H-mode plasmas in toroidal magnetic fusion experiments, is described. ELMs are important to obtain experimental control of the particle inventory of fusion plasmas. From an analysis of the ELM behaviour of different magnetic fusion experiments, three distinct types are identified, namely dithering cycles, type III and type I ELMs. A physical picture of these phenomena is established on the grounds of theoretical models put forward to describe the different ELM phenomena.

Model for the sawtooth period and amplitude

Abstract: A model for sawtooth oscillations in tokamak experiments is outlined. A threshold criterion for the onset of internal kink modes and a prescription for the relaxed profiles immediately after the sawtooth crash have been implemented in a transport code that evolves the relevant plasma parameters. In this paper, applications of this model to the prediction of the sawtooth period and amplitude in projected ITER discharges are discussed. It is found that sawteeth can be stabilized transiently by the fusion alpha particles in ITER for periods that are long on the energy confinement timescale (). The sawtooth period depends on the amount of reconnected flux at the preceding sawtooth crash. When Kadomtsev's full reconnection model is used, the period can exceed 100 s. The sawtooth mixing radius following long duration sawtooth ramps can easily exceed half the plasma minor radius, raising questions about the desirability of transient sawtooth suppression.

Short-pulse laser - plasma interactions

Abstract: Recent theoretical and experimental research with short-pulse, high-intensity lasers is surveyed with particular emphasis on new physical processes that occur in interactions with low- and high-density plasmas. Basic models of femtosecond laser - solid interaction are described including collisional absorption, transport, hydrodynamics, fast electron and hard x-ray generation, together with recently predicted phenomena at extreme intensities, such as gigagauss magnetic fields and induced transparency. New developments in the complementary field of nonlinear propagation in ionized gases are reviewed, including field ionization, relativistic self-focusing, wakefield generation and scattering instabilities. Applications in the areas of x-ray generation for medical and biological imaging, new coherent light sources, nonlinear wave guiding and particle acceleration are also examined.

The role of the electric field in confinement

Abstract: Theories of the electric field effect on toroidal plasma confinement are reviewed with the emphasis placed on recent progress in the areas of anomalous transport, structural formation and bifurcation, research which has been motivated by the discovery of improved confinement. Topics include single-particle physics, such as particle orbits or collisional transport, turbulent transport, transport matrix, structural formation and dynamics, bifurcation, and improved confinement.

Kinetic theory of low-frequency Alfvén modes in tokamaks

Abstract: The kinetic theory of low-frequency Alfven modes in tokamaks is presented. The inclusion of both diamagnetic effects and finite core-plasma ion compressibility generalizes previous theoretical analyses (Tsai S T and Chen L 1993 Phys. Fluids B 5 3284) of kinetic ballooning modes and clarifies their strong connection to beta-induced Alfven eigenmodes. The derivation of an analytic mode dispersion relation allows us to study the linear stability of both types of modes as a function of the parameters characterizing the local plasma equilibrium and to demonstrate that the most unstable regime corresponds to a strong coupling between the two branches due to the finite thermal ion temperature gradient. In addition, we also show that, under certain circumstances, non-collective modes may be present in the plasma, formed as a superposition of local oscillations which are quasi-exponentially growing in time.

Theoretical and Experimental Investigations of Stochastic Boundaries in Tokamaks

Abstract: This review paper addresses the physics of stochastic boundaries. Although it is focused on the tokamak configuration many features are common to the stochastic boundaries of stellarators. The stochastic properties of magnetic field lines are recalled and related to the spectrum of the radial magnetic perturbation. The stochastic region, referred to as the divertor volume, is shown to be bounded to the edge plasma. Furthermore, the stochastic features discriminate two regions. On short scales, the stochasticity is not effective and parallel transport dominates, this defines the laminar region. On the long scales one recovers the proper stochastic features which characterize the ergodic regime. Theoretical predictions for the transport of energy, current and particles in the divertor volume are analysed for both the laminar and ergodic regimes. A strong increase in electron transport is expected which should lead to a strong increase in the heat diffusivity, a strong increase in the resistivity in the toroidal direction and generally a decrease in the free electron lifetime in the divertor volume. Ambipolarity of particle transport is ensured by a radial electric field. The ion transport, i.e. particle transport, is then more difficult to analyse since one has to consider the strong coupling to the electron temperature field and to the electric potential field. The perturbation level is such that the particle transport induced by the stochasticity remains comparable to the anomalous transport. The experimental data show good agreement with the predictions on electron transport. This translates into a flattening of the edge temperature gradient, a narrowing of the current channel, which probably governs the observed stabilization of MHD activity, and a strong decrease in the lifetime of the runaway electrons. Regarding particle transport, the response is larger than expected and stochastic boundaries are characterized by significant screening properties compared to limiter shots. This property is shown to be a signature of a pumping capability combined with a change of transport properties. Indeed the transport of neutrals is changed since the ratio of the ionization scales to the distance between the recycling surfaces and the separatrix is reduced. Furthermore, a decreased lifetime of the ions at the very edge of the plasma is expected. Screening effects are thus observed for species exhibiting large wall pumping capability, while He and Ne are weakly affected by the stochastic boundary. The plasma properties in the ergodic volume, namely a reduced edge temperature, an increased impurity radiation, an efficient particle screening and the stabilization of MHD activity, have opened the way to radiating layer investigations on Tore Supra. Stable operation has been achieved with 80% of radiated power and radio frequency heating up to 6 MW.

X-ray tomography on the TCV tokamak

Abstract: The TCV tokamak offers outstanding variability of the plasma shape. Using x-ray tomography, the shape of the inner flux surfaces of a poloidal cross section of the plasma can be reconstructed, including fast variations due to MHD activity. Both the hardware and the software of the 200 channel system developed for TCV are described. A new 'dynamical' calibration takes actual plasma parameters into account to determine the spectrum-dependent detector efficiency, resulting in an enhanced quality of reconstructions. Tomographic inversions are obtained using a variety of methods such as maximum entropy, linear regularization and a newly developed method based on the Fisher information. The merits of the different algorithms, which have been implemented as MATLAB functions, are compared. Inversion results are analysed with the help of singular-value decomposition, allowing, for example, identification of MHD modes without using any a priori information on the poloidal mode structure. Recent results on the dependence of sawtooth activity on the plasma triangularity are presented to demonstrate the performance of the soft x-ray tomography system.

Wall conditioning in fusion devices and its influence on plasma performance

Abstract: Proper wall conditioning has turned out to be an essential element for achieving the highest possible plasma performance in present day fusion devices. The main issues are controlling the generation of plasma impurities, liberated by plasma - surface interactions, and controlling the recycling hydrogenic fluxes. The underlying mechanisms are discussed in this paper. The paper presents a review of the different wall conditioning methods. It focuses on low-Z wall coatings (beryllium evaporation, boronization, siliconization, lithium pellet injection) and on helium glow discharge cleaning and assesses their effects on fusion plasmas. New wall conditioning concepts, compatible with steady-state magnetic fields, are discussed in view of future large devices with superconducting coils.

Fluctuation-induced heat transport results from a large global 3D toroidal particle simulation model

Abstract: Turbulent transport simulation results from a large nonlinear three-dimensional (3D) toroidal electrostatic gyrokinetic particle-in-cell simulation model, including global equilibrium effects, are presented. The parallel implementation of the particle simulation model on massively parallel computers has allowed us to perform large-scale simulations of ion temperature gradient-driven turbulence and to include low-n and high-n toroidal mode numbers in a single calculation. Simulation results indicate a strong interaction between these short and long wavelength scales and that this is the possible origin of a Bohm-like ion thermal transport scaling. The inclusion of trapped electron dynamics and self-generated shear flows on the fluctuation dynamics is shown to produce quantitative differences in the ion thermal transport.

Reflectometry techniques for density profile measurements on fusion plasmas

Abstract: Reflectometry applied to the measurement of density profiles on fusion plasmas has been subject to many recent developments. After a brief reminder of the principles of reflectometry, the theoretical accuracy of reflectometry measurements is discussed. The main difficulties limiting the performance, namely the plasma fluctuations and the quality of the transmission lines, are analysed. The different techniques used for reflectometry are then presented grouped into three different categories, depending on the frequency spectrum of the probing wave: single frequency, few discrete frequencies, or broad spectrum. The present status and achievements of actual implementations of these techniques are demonstrated, with an analysis of their respective limitations and merits, as well as foreseen developments. Finally, a discussion of the various reflectometry techniques is made, in particular their ability to cope with plasma fluctuations and complex transmission lines, in view of the application to next step machines and very severe environments.

Stationary magnetic shear reversal experiments in Tore Supra

Abstract: Stable and stationary states with hollow current density profiles have been achieved in Tore Supra with lower hybrid current drive (LHCD) during reduced toroidal magnetic field operation and in weak LH absorption regimes. For these plasma conditions, off-axis LH power deposition profiles are obtained in a reproducible manner when the internal LH caustics prevent central absorption of the waves. In the multipass LH wave propagation regime, the validity of the statistical treatment of stochastic wave diffusion is shown both theoretically and experimentally. When a large fraction of the plasma current (above 50%) is non-inductively sustained by the LH waves, the magnetic shear is reversed in the plasma core, i.e. inside a normalized plasma radius of the order of 0.4. The resulting hollow current density profiles have led to an enhancement of the total electron thermal energy content, up to a factor of 1.6 compared with L-mode discharges. The confinement improvement is attributed to a strong reduction of the electron thermal diffusivity in the central reversed shear region, nearly down to its neoclassical level.

ECRH-assisted start-up in ITER

Abstract: In ITER, the electric field applied for ionization and to ramp up the plasma current may be limited to . In this case, based on established theories of the avalanche process, it is shown that ohmic breakdown in ITER is only possible over a narrow range of pressure and magnetic error field. Therefore, ECRH may be necessary to provide robust and reliable start-up. ECRH can ensure prompt breakdown over a wide range of prefill pressure and error field and can also give control over the initial time and location of breakdown. For ECRH-assisted start-up in ITER, the power and pulse length requirements are essentially determined by the need to ensure burnthrough, i.e. complete ionization of hydrogen and the transition to high ionization states of impurities. A zero-dimensional (0D) code (with inclusion of some 1D effects) has been developed to analyse burnthrough in ITER. The 0D simulations indicate that control of the deuterium density is the key factor for ensuring successful start-up in ITER, where the effects of neutral screening and dynamic fuelling by the ex-plasma volume are also crucial. It is concluded that without ECRH, successful start-up will only be possible over a very restricted range of parameters but 3 MW of absorbed ECRH power will ensure reasonably robust start-up for a broad range of conditions with beryllium impurity. In the case of carbon impurity, even with an absorbed ECRH power of 5 MW one may be restricted to low prefill pressure and/or low carbon concentration for successful start-up.

The physics of edge localized modes (ELMs) and their role in power and particle exhaust

Abstract: In this paper, various types of so-called edge localized mode (ELM), an MHD instability occurring in the edge of H-mode plasmas, are discussed. A phenomenological classification based on experimental observations from several experiments is given. ELMs affect the confinement of both energy and particles, with a stronger degradation of particle confinement. Thus, ELMs provide particle control in the H-mode, allowing steady-state operation at only modest confinement degradation. However, large ELMs may lead to unacceptable power loads on the divertor plates of future large fusion experiments and, thus, active control of ELM type and characteristics is required. At present, three control schemes have been demonstrated, namely dynamic variation of the plasma shape, application of resonant magnetic perturbations and control of the energy flux through the separatrix by additional radiation.

The collisionality dependence of tokamak -limits

Abstract: An interpretation of the collisionality dependence of , which has been observed in many tokamaks, is presented in terms of neoclassical tearing mode theory. Magnetic islands are predicted to be driven to large saturated widths by the perturbed bootstrap current, provided their width exceeds a certain threshold value. This width is of the order of the ion banana width for low collisionality plasmas, but is substantially larger in collisional plasmas (though still within the banana regime); below this threshold value the stabilizing ion polarization current causes an island to decay. Assuming that `background' MHD events such as ELMs or sawteeth can provide an initial `seed' magnetic island of width (through toroidal coupling, for example) one can develop a model equation which retains the essential properties of neoclassical tearing mode theory and is able to reproduce many of the experimental trends associated with the reduced -limit.

The tungsten divertor experiment at ASDEX upgrade

Abstract: Tungsten-coated tiles, manufactured by plasma spray on graphite, were mounted in the divertor of the ASDEX Upgrade tokamak and cover almost 90% of the surface facing the plasma in the strike zone. Over 600 plasma discharges have been performed to date, around 300 of which were auxiliary heated with heating powers up to 10 MW. The production of tungsten in the divertor was monitored by a W I line at 400.8 nm. In the plasma centre an array of spectral lines at 5 nm emitted by ionization states around W XXX was measured. From the intensity of these lines the W content was derived. Under normal discharge conditions W-concentrations around or even lower were found. The influence on the main plasma parameters was found to be negligible. The maximum concentrations observed decrease with increasing heating power. In several low power discharges accumulation of tungsten occurred and the temperature profile was flattened. The concentrations of the intrinsic impurities carbon and oxygen were comparable to the discharges with the graphite divertor. Furthermore, the density and the limits remained unchanged and no negative influence on the energy confinement or on the H-mode threshold was found. Discharges with neon radiative cooling showed the same behaviour as in the graphite divertor case.

Fast transport changes and power degradation in the W7-AS stellarator

Abstract: The turbulent heat transport in high-temperature fusion plasmas is not understood. It increases with heating power. The physical mechanism of this process is investigated in the W7-AS stellarator. The fundamental question addressed here is whether transport is governed by local plasma parameters or by a global quantity. Three different types of experiments with critical sensitivity are carried out. They can consistently be described on the basis of a non-local dependence of the transport coefficient on the global heating power.

Internal transport barrier with improved confinement in the JT-60U tokamak

Abstract: Characteristics of the internal transport barrier (ITB) were studied. The region of steep and , i.e. the ITB front, propagated from the core outwards. The thickness of the ITB front was about 3 cm. The ITB worked as a particle transport barrier as well as a thermal transport barrier for ions. The threshold heating power for ITB formation strongly increased with electron density and was independent of the toroidal magnetic field. ITB with was sustained for twice the global energy confinement time . A repetitive relaxation phenomenon at ITB was observed, which induced spikes like ELMs but had a different poloidal distribution.

Ion-beam and neutron production in a low-energy plasma focus

Abstract: Experimental results related to deuteron-beam production and its correlation with the neutron yield and hard x-ray production in a low-energy plasma focus device (30 kV, 5 kJ) are presented. After performing corrections in the ion energy spectrum (within an ion energy range from 50 keV to 1.5 MeV) for elastic and inelastic interactions of the ions with the background gas, it is found that the ion density at low ion energy is very high and plays an important role in the neutron production of this plasma focus device.

Nonlinear analysis of turbulence across the L to H transition

Abstract: Nonlinear time series analysis and wavelet bicoherence are used to characterize the changes to microturbulence in the radial electric field shear layer across the L to H transition in DIII-D. Time series of fluctuating floating potential and ion saturation current from the reciprocating probe array have been analysed for skewness and kurtosis as indications of intermittency in the turbulence. Provided the time series are carefully selected for stationarity, the skewness S and kurtosis K are consistent with Gaussian probability distributions in both L- and H-modes. Wavelet bicoherence analysis reveals the presence of intermittency in the coupling due to sum frequencies above 400 kHz in both the L- and H-modes. The intermittency is most pronounced in the H-mode, with the intervals of strong coupling correlated with changes in local edge plasma conditions. These results appear consistent with predictions from self-organized criticality models.

Diagnostics for experimental thermonuclear fusion reactors

Abstract: The book Diagnostics for experimental thermonuclear reactors gathers together the papers presented at the Workshop on Diagnostics for ITER organized by the International School of Plasma Physics in Varenna, Italy, in August 1995. The authors had clearly done an enormous amount of work in preparation for the Workshop, and this book will serve as an excellent tutorial for anyone involved in diagnostic design for any future fusion device, let alone one with the complex design issues of ITER. This book has three very important strengths. The first is in the focus in meeting requirements for the measurements of plasma parameters set down by the ITER organization. These measurements appear formidable, but the standards of physics studies and code development on the current devices require such localization and accuracy. In the descriptions of the diagnostic equipment, there is little detail about the physics of the measurement of the specific techniques presented, but a great deal about the practical implementation of such techniques. For example, there is considerable discussion about microwave reflectometry for measurement of electron density profiles, a rapidly evolving technique but one which requires special consideration in the flattish profiles and high electron temperatures anticipated for ITER. Another case is spectroscopy where measurements in the core of such parameters as the density of the helium ash, left in the core of the plasma after the alpha-particles have given up their energy to the surrounding plasma, for which a seed beam is essential. There is extensive treatment of neutron spectroscopic techniques to benefit from the production of copious neutrons for new and better plasma measurement. The second strength is in its summation of the state of the art of diagnostics on the currently operating devices. It is seldom possible to find summarized so conveniently the experience of some of the most able diagnostic physicists in the world. While some of the measurements are explicitly described, elsewhere the quality of measurement is implicit and the publications making use of those measurements are referenced. There is significant information about the capability of magnetic diagnostics for providing plasma control capability. The section on diagnostics of the divertor plasmas brings together well the measurements needed for this very important region. It is a very rapidly growing area of measurement, and very important in the near term for qualifying computer code predictions of the behaviour of divertor plasmas. Spectroscopy and total radiation in the divertor are very well treated and it is clear that vacuum ultraviolet measurement will be an important tool. The third strength is in its comprehensiveness, covering the whole range of diagnostics from the most sophisticated newly developed techniques down to Langmuir probes and pressure gauges (though the reader will find that these last two become fairly sophisticated in an ITER-like environment). There is good coverage of the measurement of the confined alpha particles using collective scattering, not yet fully demonstrated on an operating tokamak, and also discussion of the use of laser-induced fluorescence in divertor measurements, particularly of the neutral species, which could be very important with a highly radiating divertor plasma. As in any such collection of papers the referencing and figures are not always of the highest quality, but nevertheless, the overall presentation is surprisingly good. The book will be extremely valuable for graduate students and for anyone looking for a perspective on the best possible ways to measure the plasmas in high temperature magnetic fusion devices, including stellarators as shown in two of the closing papers on the diagnostics for the W7-X and LHD stellarators, both now under construction.

Runaway electron measurements in the JET tokamak

Abstract: The perpendicular x-ray emission up a to few MeV of runaway electrons has been measured in JET low-density ohmic discharges by means of the fast electron bremsstrahlung profile monitor. A diffusion model simulating the temporal evolution of the line-integrated x-ray signals is used to determine the runaway radial transport coefficient in the central region of the plasma ( for r/a<0.5); a comparison is made with the predictions of magnetic and electrostatic turbulent transport theories and limits on the level of radial magnetic field fluctuations are found.

Sustainment and modification of reversed magnetic shear by LHCD on JT-60U

Abstract: In the JT-60U tokamak a pre-formed reversed magnetic shear configuration was sustained non-inductively by means of lower hybrid current drive (LHCD). It was formed by neutral beam heating at current build-up and had a reversed shear region extending to 65% of the minor radius. The configuration was sustained for 7.5 s, and the extent of the reversed shear region was kept as large as 55% of the minor radius until the very end. The plasma was stable against MHD activities. It is also demonstrated that LHCD could modify reversed magnetic shear keeping plasma parameters, such as the plasma current and the toroidal magnetic field, the same.

Ordinary-mode reflectometry: modification of the scattering and cut-off responses due to the shape of localized density fluctuations

Abstract: Ordinary wave reflectometry in a plasma containing a localized density perturbation is studied with a one-dimensional (1D) model. The phase response is studied as a function of the wavenumber and position of the perturbation. It is shown that it strongly depends upon the perturbation shape and size. For a small perturbation wavenumber, the response is due to the oscillation of the cut-off layer. For larger wavenumbers, two regimes of resonant Bragg scattering are found: for a broad perturbation, the phase response is an image of the perturbation itself; for a narrow perturbation, it is an image of the Fourier transform. These features are enhanced for a broadband perturbation (modulated square wave) and scattering can occur over the whole region up to the cut-off. Furthermore, in that case there is a specific behaviour at the cut-off due to the sharp boundary effects of this perturbation. Because of this peculiarity, the phase response obtained for a damped square perturbation reproduces the results of an earlier experiment (Rhodes et al, Rev. Sci. Instrum. 1992).

Heating and current drive regimes in the ion cyclotron range of frequency

Abstract: The fast magnetosonic wave, launched in the ion cyclotron range of frequency (ICRF) is one of the main heating schemes in tokamak plasmas Great flexibility of the heating and current drive scenarios is possible, playing both on the various wave - particle interaction mechanisms and on the mode conversions that the fast wave can experience When properly designed, an ICRF system can be employed for ion or electron heating and current drive, with the asset of localized deposition profiles Almost all the existing tokamaks are now using ICRF heating and current drive, and an ICRF system is being studied for ITER

A new scheme for heterodyne polarimetry with high temporal resolution

Abstract: A new scheme for making combined interferometric and polarimetric measurements on tokamak plasmas with high temporal resolution is presented The method, which can be regarded as a generalization of techniques suggested by other workers, can achieve bandwidths in excess of 100 kHz using an optically pumped far-infrared triple laser source A bandwidth of 10 kHz with an accuracy of better than has already been demonstrated Calibration measurements as well as first plasma measurements obtained with the new approach are presented and evaluated

Onset condition for ELMs in JT-60U

Abstract: For the standard shape of JT-60U at low triangularity , the onset condition for giant ELMs is clearly correlated with the high-n ballooning limit in the first stability regime over wide ranges of plasma parameters (, ...). The limit of the normalized edge pressure gradient (-parameter) increases with elongation (1.5 - 1.8) and internal inductance . Recently, a new connection of the poloidal field coils enabled us to scan triangularity from the original value of up to (currently limited at MA). The -scan showed that the limit of edge density, edge pressure and the -parameter increase with . When both and are high (, ), minute-grassy ELMs appear. The edge -parameter during the minute-grassy ELMs can be higher than that for the onset of giant ELMs.

Stationary regimes of improved confinement in Tore Supra

Abstract: Recent stationary improved confinement experiments with current density profile modifications in non-inductive Tore Supra operation are reported. Significant progress has been obtained by achieving long duration discharges using lower hybrid (LH) waves: (i) 2 min in the 4 T improved confinement LHEP (LH enhanced performance) regime, at a current MA, loop voltage V, LH power MW (, are the electron energy content and Rebut - Lallia - Watkins L-mode prediction), (ii) 75 s long in a fully non-inductive LHEP regime (, V, MA, MW) using a new plasma control scheme, (iii) 30 s at 1.7 MA ( V, MW) in the L-mode regime. MHD stability in full current drive operation and the role of weak - or reversed - central magnetic shear in the central LHEP electron temperature transition (reproduced in full current drive in a systematic manner and measured by independent electron temperature diagnostics) are discussed. Stationary high- experiments were performed with fast-wave direct electron heating (FWEH) in a large range of operation: 42 - 76 MHz and - 3.9 T. Maximum coupled power of 9.5 MW is obtained in the 48 MHz/2T configuration with good electron heating performance and improved confinement at high density (central density ). By increasing the magnetic shear in the gradient region, stationary improved confinement states (6 MW during 5 s, ) are reached in a reproducible manner with poloidal beta, , approaching 1 and 40% of bootstrap current. The high-bootstrap FWEH experiments have been recently combined with LHCD at reduced loop-voltage ( V and up to 70% of non-inductive current). A total injected power up to 7.2 MW (LH: 3.6 MW, FW: 3.6 MW) has produced stationary improved confinement with a peaked electron temperature profile (central value up to 8.0 keV) at a central density of .

MHD-mode stabilization by plasma rotation in TEXTOR

Abstract: An experimental investigation into rotating MHD modes has been performed in the TEXTOR tokamak. The effects on the stability of the MHD tearing modes of coupling between m/n = 2/1 and 1/1 modes and of the slowing down of the mode rotation by wall friction have been studied. Tangential neutral beam injection (NBI) has been used to change the toroidal rotation to observe the influence of plasma rotation on the development of MHD modes. Two phases in the rotation frequency behaviour of the modes can be distinguished: a slow reversible decrease and a fast exponential decay, which starts with a short transient increase in frequency. The last phase finally results in mode locking, which mostly leads to a disruption. By injecting low power NBI in the last phase of the slowing-down process, mode locking could be prevented. The mode amplitude and rotation frequency have been stabilized, without major changes to the pressure profile. By decreasing the central plasma velocity, using NBI, a coupling of the two modes has been forced. The coupling of the modes had a destabilizing effect on the MHD activity.

A review of the dimensionless parameter scaling studies

Abstract: The theoretical basis of the dimensionless parameter scaling technique is derived and the limitations in its application are discussed. The use of the technique is illustrated by the production on JET of a steady-state ITER similarity pulse having the same and collisionality as the ignited ITER. The key issue of the scaling of the transport with the main dimensionless parameter is discussed in detail. Finally, possible shortcomings of the technique are examined.

Full steady-state operation in Tore Supra

Abstract: In order to produce fully non-inductive, lower hybrid (LH) driven discharges in a systematic and reproducible manner, new operation modes have been studied on the superconducting Tore Supra tokamak. To cope with some uncertainties in the LH current drive efficiency (e.g. profile dependences), the plasma current is not imposed a priori, but evolves freely until the equilibrium (which depends on the LH power level) is reached. The voltage applied on the primary circuit no longer controls the plasma current. In an `open loop' scenario in which this voltage is preset and constant, the timescale required to attain the equilibrium is the longest characteristic time of the coupled plasma - poloidal field coils system ( s). In order to obtain a stationary state faster, a new feedback scheme has been implemented in which the primary circuit voltage is controlled in such a way that the flux consumption vanishes. It is shown that this operation mode allows full steady-state to be reached within a characteristic time of a few seconds. The underlying physics is described and a detailed analysis of the experiments is made. It is shown, in particular, that this operation scenario generates stable stationary plasmas with improved confinement, so that the so-called `LHEP' regime can be extrapolated to continuous operation.