Showing papers in "Plasma Physics and Controlled Fusion in 2004"

Atmospheric-pressure plasma technology

Abstract: Major industrial plasma processes operating close to atmospheric pressure are discussed. Applications of thermal plasmas include electric arc furnaces and plasma torches for generation of powders, for spraying refractory materials, for cutting and welding and for destruction of hazardous waste. Other applications include miniature circuit breakers and electrical discharge machining. Non-equilibrium cold plasmas at atmospheric pressure are obtained in corona discharges used in electrostatic precipitators and in dielectric-barrier discharges used for generation of ozone, for pollution control and for surface treatment. More recent applications include UV excimer lamps, mercury-free fluorescent lamps and flat plasma displays.

Plasma rotation profile measurements using Doppler reflectometry

Abstract: High spatial resolution radial profiles of the perpendicular plasma rotation velocity u⊥ using a dual channel 50–75 GHz Doppler reflectometer system on the ASDEX Upgrade tokamak are presented for a variety of discharge scenarios, including Ohmic, L-mode, H-mode, etc with forward and reversed magnetic field and co- and counter neutral beam injection. The reflectometers have steppable launch frequencies fo = c/λo, with selectable O- or X-mode polarization, giving tokamak edge to mid-radius coverage. Low-field-side antennae (hog-horn antenna pairs) with deliberate tilting (primarily poloidally) produce a Doppler shifted spectrum directly proportional to the perpendicular velocity fD = u⊥k⊥/2π = u⊥ 2sinθt/λo. The incident angle θt between the beam and cut-off layer normal varies with plasma shape, cut-off layer position and refraction. However, typical angles range from 5° to 27° giving a probed turbulence wavenumber, k⊥, range of 1.8–14.3 cm−1, with resulting Doppler shifts fD of up to 5 MHz. The measured perpendicular velocity is u⊥ = vE × B + vphase, which for a typical H-mode is slightly positive in the tokamak scrape-off-layer with a deep negative well across the H-mode steep pressure gradient pedestal region and then following the perpendicularly projected toroidal fluid velocity in the core, should be dominated by the E × B velocity, as the intrinsic phase velocity is predicted to be small, which may allow u⊥ to be interpreted directly as the radial electric field Er profile.

Plasma based charged-particle accelerators

Abstract: Studies of charged-particle acceleration processes remain one of the most important areas of research in laboratory, space and astrophysical plasmas. In this paper, we present the underlying physics and the present status of high gradient and high energy plasma accelerators. We will focus on the acceleration of charged particles to relativistic energies by plasma waves that are created by intense laser and particle beams. The generation of relativistic plasma waves by intense lasers or electron beams in plasmas is important in the quest for producing ultra-high acceleration gradients for accelerators. With the development of compact short pulse high brightness lasers and electron positron beams, new areas of studies for laser/particle beam-matter interactions is opening up. A number of methods are being pursued vigorously to achieve ultra-high acceleration gradients. These include the plasma beat wave accelerator mechanism, which uses conventional long pulse (~100 ps) modest intensity lasers (I ~ 1014–1016 W cm−2), the laser wakefield accelerator (LWFA), which uses the new breed of compact high brightness lasers ( 1018 W cm−2, the self-modulated LWFA concept, which combines elements of stimulated Raman forward scattering, and electron acceleration by nonlinear plasma waves excited by relativistic electron and positron bunches. In the ultra-high intensity regime, laser/particle beam–plasma interactions are highly nonlinear and relativistic, leading to new phenomena such as the plasma wakefield excitation for particle acceleration, relativistic self-focusing and guiding of laser beams, high-harmonic generation, acceleration of electrons, positrons, protons and photons. Fields greater than 1 GV cm−1 have been generated with particles being accelerated to 200 MeV over a distance of millimetre. Plasma wakefields driven by positron beams at the Stanford Linear Accelerator Center facility have accelerated the tail of the positron beam. In the near future, laser plasma accelerators will be producing GeV particles.

X-ray generation mechanisms in three-dimensional simulations of wire array Z-pinches

Abstract: Resistive magneto-hydrodynamic (MHD) simulations are used to evaluate the influence of three-dimensional inhomogeneities on x-ray power production in wire array Z-pinches. In particular, we concentrate on simulations of wire array Z-pinch experiments on the MAGPIE generator at Imperial College. An initial temperature perturbation is used to stimulate variations in wire core ablation rates that result in a highly non-uniform final implosion. Results indicate that x-ray power production is governed by the symmetry of the implosion surface and by the rate at which current can transfer to the axis through a three-dimensional debris field that trails behind the main implosion. The peak power is ultimately limited by the growth of MHD instabilities in the stagnated pinch. The individual contributions of the implosion kinetic energy, compression of the stagnated pinch, ohmic heating and MHD instabilities to the radiation yield are quantified. The onset of m = 1 instabilities is found to provide an efficient mechanism for dissipation of the magnetic energy surrounding the stagnated pinch. The formation of a helical plasma column not only allows the magnetic field to do work in driving an expansion of the helix but also enhances the ohmic heating by elongating the path of the current through the pinch. The effect of these energy sources combined is to increase the radiation yield to typically 3½ times the kinetic energy of the implosion. Simulations of arrays with different wire numbers, wire material and with nested arrays are used to examine the mechanisms that influence the peak soft x-ray power. In the simulations, peak power can be increased by: increasing the number of wires (which improves the implosion symmetry), by increasing the atomic number of the material (which increases the compressibility of the plasma) and by using a nested inner array (which brings the mass and the current to the axis more efficiently than a single array).

Turbulence spreading into the linearly stable zone and transport scaling

Abstract: We study the simplest problem of turbulence spreading corresponding to the spatio-temporal propagation of a patch of turbulence from a region where it is locally excited to a region of weaker excitation or even local damping. A single model equation for the local turbulence intensity, I(x, t), includes the effects of local linear growth and damping, spatially local nonlinear coupling to dissipation and spatial scattering of turbulence energy induced by nonlinear coupling. In the absence of dissipation, front propagation into the linearly stable zone occurs with the property of rapid progression at small t, followed by slower sub-diffusive progression at late times. The turbulence radial spreading into the linearly stable zone reduces the turbulent intensity in the linearly unstable zone and introduces an additional dependence on the ρ* ≡ ρi/a to the turbulent intensity and the transport scaling. These are in broad, semi-quantitative agreement with a number of global gyrokinetic simulation results with zonal flows and without zonal flows. Front propagation stops when the radial flux of fluctuation energy from the linearly unstable region is balanced by local dissipation in the linearly stable region.

Dust in fusion devices—a multi-faceted problem connecting high- and low-temperature plasma physics

Abstract: Small particles with sizes between a few nanometers and a few 10 µm (dust) are formed in fusion devices by plasma–surface interaction processes. Though it is not a major problem today, dust is considered a problem that could arise in future long pulse fusion devices. This is primarily due to its radioactivity and due to its very high chemical reactivity. Dust formation is particularly pronounced when carbonaceous wall materials are used. Dust particles can be transported in the tokamak over significant distances. Radioactivity leads to electrical charging of dust and to its interaction with plasmas and electric fields. This may cause interference with the discharge but may also result in options for particle removal. This paper discusses some of the multi-faceted problems using information both from fusion research and from low-temperature dusty plasma work.

Physics of transport in tokamaks

Abstract: This paper is an overview of recent results relating to turbulent particle and heat transport, and to the triggering of internal transport barriers (ITBs). The dependence of the turbulent particle pinch velocity on plasma parameters has been clarified and compared with experiment. Magnetic shear and collisionality are found to play a central role. Analysis of heat transport has made progress along two directions: dimensionless scaling laws, which are found to agree with the prediction for electrostatic turbulence, and analysis of modulation experiments, which provide a stringent test of transport models. Finally the formation of ITBs has been addressed by analysing electron transport barriers. It is confirmed that negative magnetic shear, combined with the Shafranov shift, is a robust stabilizing mechanism. However, some well established features of internal barriers are not explained by theory.

Profile stiffness and global confinement

Abstract: This paper analyses the properties of a critical gradient transport model based on a few assumptions: electrostatic gyroBohm scaling law, existence of an instability threshold and finite background transport below the threshold. A quantitative criterion of stiffness is proposed, which provides the means for a quantitative assessment and inter-machine comparison. It is also shown that this transport model is compatible with a two-term scaling law of global confinement, as proposed recently by the International Tokamak Physics Activity–Confinement Data Base and Modelling Topical Group. This model has also been applied to analyse a variety of experiments mostly using electron heat modulation on JET, ASDEX-Upgrade, TORE SUPRA and FTU. The thresholds are found to be in the expected domain for micro-instabilities in tokamaks. However, the stiffness factor is found to cover a broad range of variation.

The Physics of Inertial Fusion

Abstract: The growing effort in inertial confinement fusion (ICF) research, with the upcoming new MJ class laser facilities, NIF in USA and LMJ in France, and the upgraded MJ z-pinch ZR facility in the USA, makes the appearance of this book by Atzeni and Meyer-ter-Vehn very timely. This book is an excellent introduction for graduate or masters level students and for researchers just entering the field. It is written in a very pedagogical way with great attention to the basic understanding of the physical processes involved. The book should also be very useful to researchers already working in the field as a reference containing many key formulas from different relevant branches of physics; experimentalists will especially appreciate the presence of `ready-to-use' numerical formulas written in convenient practical units. The book starts with a discussion of thermonuclear reactions and conditions required to achieve high gain in ICF targets, emphasizing the importance of high compression of the D-T fuel, and compares the magnetic confinement fusion and inertial confinement fusion approaches. The next few chapters discuss in detail the basic concepts of ICF: the hydrodynamics of a spherically imploding capsule, ignition and energy gain. This is followed by a thorough discussion of the physics of thermal waves, ablative drive and hydrodynamic instabilities, with primary focus on the Rayleigh--Taylor instability. The book also contains very useful chapters discussing the properties of hot dense matter (ionization balance, equation of state and opacity) and the interaction of laser and energetic ion beams with plasma. The book is based on and reflects the research interests of the authors and, more generally, the European activity in this area. This could explain why, in my opinion, some topics are covered in less detail than they deserve, e.g. the chapter on hohlraum physics is too brief. On the other hand, the appearance in the book of an interesting chapter on the concept of fast ignition is also a reflection of the research interests of the authors. Altogether, the book is very well written, contains a wealth of useful information and the reviewer highly recommends it to the interested reader.

Characterization of peeling-ballooning stability limits on the pedestal

Abstract: The peeling–ballooning model for edge localized modes (ELMs) and pedestal constraints, based on ideal MHD instabilities driven by pressure gradients and current in the edge barrier region, has been broadly applied toward understanding ELM and pedestal behaviour in a number of tokamak experiments. Due in part to multiple driving terms, multiple wavelengths and second stability access physics, the peeling–ballooning stability limits which are proposed to constrain the pedestal and trigger ELMs depend sensitively on many details of the tokamak equilibrium. Here we present a technique for characterizing these stability constraints as a function of important parameters, using carefully constructed model equilibria. We discuss comparisons of calculated stability constraints to observed pedestal behaviour, in which an encouraging level of agreement is found. We then present results of an extensive series of calculations which characterize the peeling–ballooning stability constraints as a function of pedestal width, magnetic field, plasma current, density, and triangularity.

Hydrogenic fast-ion diagnostic using Balmer-alpha light

Abstract: Hydrogenic fast-ion populations are common in toroidal magnetic fusion AQ1 devices, especially in devices with neutral beam injection. As the fast ions orbit around the device and pass through a neutral beam, some fast ions neutralize and emit Balmer-alpha light. The intensity of this emission is weak compared with the signals from the injected neutrals, the warm (halo) neutrals and the cold edge neutrals, but, for a favourable viewing geometry, the emission is Doppler shifted away from these bright interfering signals. Signals from fast ions are detected in the DIII-D tokamak. When the electron density exceeds ∼7×10 19 m −3 , visible bremsstrahlung obscures the fast-ion signal. The intrinsic spatial resolution of the diagnostic is ∼5 cm for 40 keV amu −1 fast ions. The technique is well suited for diagnosis of fast-ion populations in devices with fast-ion energies (∼30 keV amu −1 ), minor radii (∼0.6 m) and plasma densities (10 20 m −3 ) that are similar to those of DIII-D. AQ2 (Some figures in this article are in colour only in the electronic version)

Scaling laws of density fluctuations at high-k on Tore Supra

Abstract: Anomalous transport in tokamaks is generally attributed to turbulent fluctuations. Since a large variety of modes are potentially unstable, a wide range of short-scale fluctuations should be measured, with wavenumbers from kρi ~ 0.1 to kρi 1. In the Tore Supra tokamak, a light scattering experiment has made possible fluctuation measurements in the medium- and high-k domains where a transition in the k-spectrum is observed: the fluctuation level decreases much faster than usual observations, typically with a power law S(k) ≡ k−6. A scan of the ion Larmor radius shows that the transition wavenumber scales with ρi around kρi ~ 1.5. This transition indicates that a characteristic length scale should be involved to describe the fluctuation nonlinear dynamics in this range. The resulting very low level of fluctuations at high-k does not support a strong effect of turbulence driven by the electron temperature gradient. For this gyroradius scan, the characteristics of turbulence also exhibit a good matching with predictions from gyro-Bohm scaling: the typical scale length of turbulence scales with the ion Larmor radius, the typical timescales with a/cs; the turbulence level also scales with ρi, according to the mixing length rule.

Plasma synthesis of single-crystal silicon nanoparticles for novel electronic device applications

Abstract: Single-crystal nanoparticles of silicon, several tens of nanometres in diameter, may be suitable as building blocks for single-nanoparticle electronic devices. Previous studies of nanoparticles produced in low-pressure plasmas have demonstrated the synthesis of nanocrystals 2–10 nm diameter but larger particles were amorphous or polycrystalline. This work reports the use of a constricted, filamentary capacitively coupled low-pressure plasma to produce single-crystal silicon nanoparticles with diameters between 20 and 80 nm. Particles are highly oriented with predominantly cubic shape. The particle size distribution is rather monodisperse. Electron microscopy studies confirm that the nanoparticles are highly oriented diamond-cubic silicon.

Reconstruction of the magnetic perturbation in a toroidal reversed field pinch

Abstract: A new method to obtain the radial profile of the magnetic perturbation in a toroidal force-free plasma having a circular cross section is developed. The toroidal geometry produces poloidal harmonics in the equilibrium quantities (at the leading order m = ±1, n=0), which act as mediators between perturbations with the same toroidal number and different poloidal numbers. The approach proposed here, based on the contravariant representation of the magnetic field in flux co-ordinates, is formally simple and rigorous and maintains a nice similarity with the cylindrical treatment. The method is quite general and can be applied to any circular low-beta plasma. In this work we describe its application to the Reversed Field eXperiment (RFX) plasma. It is customary in reversed field pinches to approach the analysis of MHD instabilities by using a cylindrical geometry. Nonetheless, the effect of a more realistic toroidal geometry can play an important role, and indeed we found that the toroidal effects on the magnetic perturbations are not negligible.

The bubble regime of laser plasma acceleration: monoenergetic electrons and the scalability

Abstract: The bubble regime of electron acceleration in ultra-relativistic laser plasma is considered. It has been shown that the bubble can produce ultra-short dense bunches of electrons with quasi-monoenergetic energy spectra. The first experiment in this regime done at LOA has confirmed the peaked electron spectrum (Faure J et al 2004 Nature at press). The generated electron bunch may have density an order of magnitude higher than that of the background plasma. The bubble is able to guide the laser pulse over many Rayleigh lengths, thus no preformed plasma channel is needed for high-energy particle acceleration in the bubble regime. In this work we discuss a simple analytical model for the bubble fields as well as the scaling laws.

A comparison of experimental measurements and code results to determine flows in the JET SOL

Abstract: Two reciprocating probe systems, at the same poloidal position at the top of the JET torus but toroidally separated by 180degrees, have been used to measure parallel flow in the scrape-off layer (SOL) of lower single-null, diverted plasmas. One system uses the entrance slit plates of a retarding field analyser to record upstream and downstream flux densities, whilst the second employs two pins of a nine-pin turbulent transport probe. Measurements have been made for both forward and reversed toroidal field directions. The results from both probe systems are similar. In the forward field direction, that is with the ion B x del(B) over right arrow drift direction downwards towards the divertor, a strong parallel flow is measured at the top of the machine in the direction from the outer to the inner divertor. The flow generally has a low value, Mach number M similar to 0.2, close to the separatrix, but rises in the region of high magnetic shear close to the separatrix to a maximum of M similar to 0.5 some 20 mm outside the separatrix. In contrast, for a reversed field, the measured flow is small (close to zero) throughout much of the SOL but rises near the separatrix to a value equal in both magnitude and direction to that observed in the forward field. There is thus some symmetry in the flow with respect to field reversal but with a symmetry axis given by a positive offset of around M similar to 0.2. This paper presents simulations using the EDGE2D/Nimbus code, which predicts very low values of parallel flow Mach number near the probe position. The possibility of impurities released from the probe surfaces increasing the flow velocity is explored using the code.

Bifurcation in the MHD behaviour of a self-organizing system: the reversed field pinch (RFP)

Abstract: Within the framework of MHD modelling the RFP is shown to develop turbulent or laminar regimes switching from the former to the latter in a continuous way depending on the strength of dissipative forces (the higher they are the more laminar is the corresponding regime). In either of these cases interesting features can be observed such as the occurrence of quasi-periodic relaxation events involving reconnection processes, or the formation of stationary helical symmetric configurations. The first case corresponds to the conventional turbulent dynamo in the RFP where perturbations with multiple helical harmonic content are present. The second case corresponds to a global single helical deformation of the current channel. This simpler configuration is associated with a laminar electrostatic dynamo field and may also be found as a solution of a helical Ohmic equilibrium problem where a finite beta is necessary. The continuity of the transition between the two regimes suggests that the simple helical symmetric solution can provide a fruitful intuitive description of the RFP dynamo in general. Many of the MHD predictions are in good agreement with experimental findings and suggest possible improvements for the confinement properties of the RFP configuration.

Spectroscopic determination of the internal amplitude of frequency sweeping TAE

Abstract: From an understanding of the processes that cause a marginally unstable eigenmode of the system to sweep in frequency, it is shown how the absolute peak amplitude of the mode can be determined from the spectroscopic measurements of the frequency sweeping rate, e.g. with Mirnov coils outside the plasma. In a first attempt to implement such a diagnostic calculation, the MISHKA code (Mikhailovskii A B et al 1997 Plasma Phys. Rep. 23 844) is used to determine the global mode structure of toroidal Alfven eigenmodes (TAEs) (Cheng C Z et al 1985 Ann. Phys. (NY) 161 21) observed in the MAST spherical tokamak (Sykes A et al 2001 Nucl. Fusion 41 1423). Simulations using the HAGIS code (Pinches S D 1996 PhD Thesis The University of Nottingham, Pinches S D et al 1998 Comput. Phys. Commun. 111 131) are then made, replicating the experimentally observed sweeping phenomena. The fundamental theory is then used together with these simulation results to predict the internal field amplitude from the observed frequency sweeping. The calculated mode amplitude is shown to agree with that obtained from Mirnov coil measurements.

Impact of toroidal rotation on ELM behaviour in the H-mode on JT-60U

Abstract: The mitigation of the large pulsed heat loads induced by edge-localized modes (ELMs) on the divertor plates is one of the most important issues for a tokamak fusion reactor. However, ELMs have been completely suppressed in the quiescent H-mode (QH-mode) plasmas produced in the DIII-D tokamak (see Burrell K H et al 2002 Plasma Phys. Control. Fusion 44 A253). One of the key conditions for producing QH-mode plasmas is that the direction of neutral beam injection (NBI) should be opposite to that of the plasma current (i.e. ctr-NBI), which then leads to the toroidal rotation velocity being in the counter direction to the plasma current. By using various combinations of NBI lines in JT-60U, it has been possible to investigate the impact of the toroidal rotation velocity on ELM behaviour by changing the toroidal momentum input in a detailed manner for similar absorbed NB heating power. It has been determined that the ELM frequency decreases with increased counter toroidal rotation velocity at the plasma edge even to the point of the ELMs disappearing. In addition, the magnitude of the pulsed Dα signal at the divertor decreases with decreasing ELM frequency. These results indicate that it is possible to control the ELM behaviour through the toroidal momentum input.

The role of energetic particles in fusion plasmas

Abstract: In the burning fusion plasmas of next step devices such as ITER (2001 ITERFEAT Outline Design Report IAEA/ITER EDA/DS/18 (Vienna: IAEA) p 21), the majority of the heating of the fusing fuel will come from the plasma self-heating by fusion born α-particles. Recent advances in theoretical understanding, together with the development of new diagnostic techniques, make this a timely opportunity to survey the role of energetic particles in fusion plasmas and how it projects to future burning plasma devices. (Some figures in this article are in colour only in the electronic version)

Power deposition outside the divertor in ASDEX Upgrade

Abstract: The heat deposition outside the divertor region in ASDEX Upgrade was measured using thermography and Langmuir probes, as well as thermometry and cooling water calorimetry for a type-I ELMy H-mode discharge with low density ( ). The total deposited energy to non-divertor components was found to be below 8% of the total plasma energy throughput. The main part of this energy is deposited onto the low field side limiters. The radial heat flux deposition profile measured with an infrared camera at the protection limiter is the radial continuation of the heat flux profile measured in the lower outer divertor mapped to the mid-plane. Twenty-five per cent of the plasma energy loss by ELMs is detected at non-divertor components. The large area of the central column is loaded with about 10% and the ion cyclotron resonance heating and protection limiters with 15%. The ELM contribution to the non-divertor energy deposition is about 60% of the total non-divertor load. A systematic investigation of ELM energy deposition outside the divertor as a function of plasma and edge parameters is in progress.

Radial propagation of Type-I ELMs on JET

Abstract: The propagation of edge localized modes (ELMs) towards the main chamber could lead to potentially serious consequences in ITER (Federici G et al 2001 Nucl. Fusion 41 1967). This effect was studied in JET by examining the interaction of a large sample (>200) of Type-I ELMs with an array of Langmuir probes on the outboard limiter. High clearance discharges were used with a separatrix to wall distance of 8?18?cm. The average ELM duration was measured as ~ 200 ? 50??s, the average radial propagation velocity of the ELM plasmoid in the scrape-off layer as v? ~ 0.45 ? 0.1?km?s?1 or as a fraction of the sound speed, v?/cs ~ 0.2%, and its diffusion coefficient, D?, as 500 ? 100?m2?s?1?a significant increase over inter-ELM levels. The above velocities are in good agreement with the lower (? loss) limit of the sheath-limited model of plasmoid propagation (Krasheninnikov S I 2001 Phys. Lett. A 283 368, Krasheninnikov S I et al 2002 29th EPS Conf. on Controlled Fusion and Plasma Physics (Montreux, Switzerland)).

Study of Type III ELMs in JET

Abstract: This paper presents the results of JET experiments aimed at studying the operational space of plasmas with a Type III ELMy edge, in terms of both local and global plasma parameters. In JET, the Type III ELMy regime has a wide operational space in the pedestal ne – Te diagram, and Type III ELMs are observed in standard ELMy H-modes as well as in plasmas with an internal transport barrier (ITB). The transition from an H-mode with Type III ELMs to a steady state Type I ELMy H-mode requires a minimum loss power, PTypeI. PTypeI decreases with increasing plasma triangularity. In the pedestal ne – Te diagram, the critical pedestal temperature for the transition to Type I ELMs is found to be inversely proportional to the pedestal density (Tcrit ∝ 1/n) at a low density. In contrast, at a high density, Tcrit, does not depend strongly on density. In the density range where Tcrit ∝ 1/n, the critical power required for the transition to Type I ELMs decreases with increasing density. Experimental results are presented suggesting a common mechanism for Type III ELMs at low and high collisionality. A single model for the critical temperature for the transition from Type III to Type I ELMs, based on the resistive interchange instability with magnetic flutter, fits well the density and toroidal field dependence of the JET experimental data. On the other hand, this model fails to describe the variation of the Type III ne – Te operational space with isotopic mass and q95. Other results are instead suggestive of a different physics for Type III ELMs. At low collisionality, plasma current ramp experiments indicate a role of the edge current in determining the transition from Type III to Type I ELMs, while at high collisionality, a model based on resistive ballooning instability well reproduces, in term of a critical density, the experimentally observed q95 dependence of the transition from Type I to Type III ELMs. Experimental evidence common to Type III ELMs in standard ELMy H-modes and in plasmas with ITBs indicates that they are driven by the same instability.

The beta scaling of energy confinement in ELMy H-modes in JET

Abstract: The disagreement between the weak dependence of the energy confinement time on normalized pressure, β, observed in dedicated scans and the strongly negative dependence in the confinement scaling laws used for the design of next step tokamaks and future reactors, remains an outstanding problem. As such, scans of β have been undertaken in single null, low triangularity (δ ≈ 0.2) ELMy H-mode plasmas in JET with the MarkIIGB-SRP divertor. The scans varied β by a factor of 2.8 (normalized β from 0.72 to 2.04) and covered a range of magnetic fields (1.5–2.3 T), plasma currents (1.5–2.75 MA) and safety factors (q95 = 2.8 and 3.3). A weak β dependence was observed both globally (B0τE varied less than 9% across any one scan) and locally. A scan within Type I ELMy H-modes suggests that this weaker dependence is not due to ELM regimes. A statistical analysis indicates that these results are consistent with log–linear regressions performed on a wide JET database of ELMy H-modes, if correlations in this database are considered.

Polymorphous silicon thin films produced in dusty plasmas: application to solar cells

Abstract: We summarize our current understanding of the optimization of PIN solar cells produced by plasma enhanced chemical vapour deposition from silane–hydrogen mixtures. To increase the deposition rate, the discharge is operated under plasma conditions close to powder formation, where silicon nanocrystals contribute to the deposition of so-called polymorphous silicon thin films. We show that the increase in deposition rate can be achieved via an accurate control of the plasma parameters. However, this also results in a highly defective interface in the solar cells due to the bombardment of the P-layer by positively charged nanocrystals during the deposition of the I-layer. We show that decreasing the ion energy by increasing the total pressure or by using silane–helium mixtures allows us to increase both the deposition rate and the solar cells efficiency, as required for cost effective thin film photovoltaics.

Tritium transport experiments on the JET tokamak

Abstract: An overview is given of the experimental method, the analysis technique and the results for trace tritium experiments conducted on the JET tokamak in 2003. Observations associated with events such as sawtooth collapses, neo-classical tearing modes and edge localized modes are described. Tritium transport is seen to approach neo-classical levels in the plasma core at high density and low q(95), and in the transport barrier region of internal transport barrier (ITB) discharges. Tritium transport remains well above neo-classical levels in all other cases. The correlation of the measured tritium diffusion coefficient and convection velocity for normalized minor radii r/a = [0.65, 0.80] with the controllable parameters q95 and plasma density are found to be consistent for all operational regimes (ELMy H-mode discharges with or without ion cyclotron frequency resonance heating, hybrid scenario and ITB discharges). Scaling with local physics parameters is best described by gyro-Bohm scaling with an additional inverse beta dependence.

ELM characteristics in MAST

Abstract: Edge localized mode (ELM) characteristics in a large spherical tokamak (ST) with significant auxiliary heating are explored. High confinement is achieved in mega ampere spherical tokamak (MAST) at low ELM frequencies even though the ELMs exhibit many type III characteristics. These ELMs are associated with a reduction in the pedestal density but no significant change in the pedestal temperature or temperature profile, indicating that energy is convected from the pedestal region into the scrape-off layer. Power to the targets during an ELM arrives predominantly at the low field outboard side. ELM effluxes are observed up to 20 cm from the plasma edge at the outboard mid-plane and are associated with the radial motion of a feature at an average velocity of 0.75 km s−1. The target balance observed in MAST is potentially rather favourable for the ST since H-mode access is facilitated in a regime where ELM losses flow mostly to the large wetted area, outboard targets and, in addition, the target heat loads are reduced by an even distribution of power between the upper and lower targets.

Lithium beam charge exchange diagnostic for edge ion temperature measurements at the ASDEX Upgrade tokamak

Abstract: Charge exchange recombination spectroscopy, utilizing a fast lithium beam as the recombination source, has been successfully applied to measuring edge ion temperature profiles of fully stripped carbon and helium ions in Ohmic, L-mode and H-mode plasmas with a radial resolution of about 6 mm. The temperatures of the carbon and helium ions agree within their respective experimental error bars in discharges where both could be measured. Depending on the plasma scenario, impurity content, beam penetration, etc, either helium or carbon can be the better choice for accurate edge measurements. The impact of edge localized modes (ELMs) on this diagnostic method has been noted. Temperature profiles in between ELMs can be determined as long as the ELM frequency is less than half the frame rate of the detector, i.e. less than about 60 Hz. In a first application, accurate measurements in L-mode plasmas with dominant electron heating and low density show much higher ion temperatures than electron temperatures across the separatrix.

Doppler reflectometry with optimized temporal resolution for the measurement of turbulence and its propagation velocity

Abstract: Doppler reflectometry selects electron density perturbations with a finite wave vector K⊥ by a line of sight which is non-perpendicular with respect to the reflecting layer. This provides, simultaneously, a local measurement of their propagation velocity v⊥(K⊥) and of changes in their fluctuation amplitude ˜ n(K⊥). In the multichannel Doppler reflectometer at the Stellarator W7-AS the antenna system and signal detection are optimized for maximum temporal resolution in order to study transport bifurcations such as the transitions between L- and H-mode. For the conditions in W7-AS the quantities v⊥ and ˜ n can be measured with a temporal resolution of less than 10 µs as shown for the example of an H to L back-transition. The dual antenna system installed allows us to supplement Doppler reflectometry with a simultaneous time-offlight measurement, which is the complementary method to determine v⊥.

EDGE2D modelling of edge profiles obtained in JET diagnostic optimized configuration

Abstract: 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