H-mode Transition Research Articles

Observations of anomalous momentum transport in Alcator C-Mod plasmas with no momentum input

Anomalous momentum transport has been observed in Alcator C-Mod tokamak plasmas. The time evolution of core impurity toroidal rotation velocity profiles has been measured with a tangentially viewing crystal x-ray spectrometer array. Following the L-mode to EDA (enhanced Dα) H-mode transition in both Ohmic and ion cyclotron range of frequencies heated discharges, the ensuing co-current toroidal rotation velocity, which is generated in the absence of any external momentum source, is observed to propagate in from the edge plasma to the core with a timescale of the order of the observed energy confinement time, but much less than the neo-classical momentum confinement time. The ensuing steady state toroidal rotation velocity profiles in EDA H-mode plasmas are relatively flat, with Vϕ ∼ 50 km s−1, and the momentum transport can be simulated using a simple diffusion model. Assuming that the L–H transition produces an instantaneous edge source of toroidal torque (which disappears at the H- to L-mode transition), the momentum transport may be characterized by a diffusivity, with values of ∼0.07 m2 s−1 during EDA H-mode and ∼0.2 m2 s−1 in L-mode. These values are large compared to the calculated neo-classical momentum diffusivities, which are of the order of 0.003 m2 s−1. Velocity profiles of ELM-free H-mode plasmas are centrally peaked (with Vϕ(0) exceeding 100 km s−1 in some cases), which suggests the presence of an inward momentum pinch; the observed profiles are consistent with simulations including an edge inward convection velocity of ∼10 m s−1. In EDA H-mode discharges which develop internal transport barriers, the velocity profiles become hollow in the centre, indicating the presence of a negative radial electric field well in the vicinity of the barrier foot. Upper single null diverted and inner wall limited L-mode discharges exhibit strong counter-current rotation (with Vϕ(0)∼−60 km s−1 in some cases), which may be related to the observed higher H-mode power threshold in these configurations. For plasmas with locked modes, the toroidal rotation is observed to cease (Vϕ ⩽ 5 km s−1).

Observation of anomalous momentum transport in tokamak plasmas with no momentum input.

Anomalous momentum transport has been observed in Alcator C-Mod tokamak plasmas through analysis of the time evolution of core impurity toroidal rotation velocity profiles. Following the L-mode to EDA (enhanced D(alpha)) H-mode transition, the ensuing cocurrent toroidal rotation velocity, which is generated in the absence of any external momentum source, is observed to propagate in from the edge plasma to the core. The steady state toroidal rotation velocity profiles are relatively flat and the momentum transport can be simulated with a simple diffusion model. Velocity profiles during edge localized mode free (ELM-free) H-modes are centrally peaked, which suggests the addition of inward momentum convection. In all operating regimes the observed momentum diffusivities are much larger than the neoclassical values.

Chapter 6: H-Mode and Pedestal Physics in ASDEX Upgrade

Studies in ASDEX Upgrade of the phenomenology and scaling of the H-mode transition, of edge-localized modes (ELMs), and characterization of the H-mode edge transport barrier carried out in various experimental campaigns between 1996 and 2001 are described. The H-mode transition is recognized by formation of a radial electrical field at the plasma boundary, which in ASDEX Upgrade is detected by an associated increase of the neutral particle charge exchange flux from ripple trapped particles. A scaling for the critical local edge temperature for the H-mode transition threshold is found. Similarity experiments with ASDEX Upgrade and Joint European Torus plasmas for the H-mode transition indicate that the H-mode transition can be obtained at the same values of dimensionless parameters ρ*, ν*, and β at the plasma edge, indicating that the threshold scaling is normally not dominated by atomic physics processes. Energy losses due to ELMs are examined. Different types of ELMs can be obtained, depending on plasma edge temperature and magnetics configuration. An interesting regime is the type II ELMy H-mode for configurations near double null, where the peak heat flux to the target is much reduced compared to large type I ELMs. High-resolution Thomson scattering measurements show that the edge transport barrier width in ASDEX Upgrade shows only weak variations, while the pedestal top electron pressure and pressure gradient strongly depend on the plasma current, or value of Bt/q95.

Observation of High Recycling Steady H-mode edge and compatibility with improved core confinement mode on JFT-2M

A new operational regime has been discovered on JFT-2M under the boronized first wall condition to produce the High Recycling Steady (HRS) H-mode, which is characterized by good energy confinement (H89P ∼ 1.6) at a high density of around 70% of the Greenwald density (ne/nGW), low radiated power fraction, and complete disappearance of large ELMs. Accompanying the HRS H-mode transition, coherent magnetic fluctuation is seen on magnetic probes at the vessel wall. These coherent fluctuations have a frequency of the order of 10–100 kHz with significant variation. They are recognized as important in enhancing particle transport, whose characteristics are similar to the Enhanced Dα (EDA) H-mode regime reported from Alcator C-Mod. The H′-mode, previously observed on JFT-2M, had features in common with the EDA mode in terms of the coherent density fluctuation, but it appeared transiently. The HRS operating regime is also similar to the EDA mode, except that HRS is seen even at low q95 (2 < q95 ⩽ 3). The most important feature of the HRS H-mode edge condition is compatibility with an improved core confinement mode at a high density without large ELMs. We have demonstrated that an internal transport barrier can be produced under a HRS H-mode edge condition, achieving βNH89P ∼ 6.2 at ne/nGW ∼ 70%, transiently.

Design of a New Type of Stub Tuner in ICRF Experiment

In the Ion Cyclotron Range of Frequency (ICRF) heating experiment, impedance matching is of great practical significance, because wide variations in antenna loading are observed within the discharge, in tokamaks operating in H-mode. A sudden decrease in antenna loading accompanying the L-mode to H-mode transition typically occurs on a timescale of a few milliseconds, as does the increase in loading at the H- to L-mode transition. Therefore, it is necessary to match dynamically in the transmission line between the generator output and the antenna input connections [1]. A new type of stub tuner being developed utilizes the difference in radio-frequency wavelengths between gas and liquid due to different relative dielectric constants. The impedance matching can be adjusted in realtime in an attempt to track the variations in the antenna loading. Since there are no mechanically moving parts in the short ends of stub, the change can be more convenient and safe, moreover, it can withstand higher voltage without breakdown. This system device will be applied in the HT-7 superconductor Tokamak ICRF experiment.

H-mode threshold and dynamics in the National Spherical Torus Experiment

Edge parameters play a critical role in high confinement mode (H-mode) access, which is a key component of discharge optimization in present day toroidal confinement experiments and the design of next generation devices. Because the edge magnetic topology of a spherical torus (ST) differs from a conventional aspect ratio tokamak, H-modes in STs exhibit important differences compared with tokamaks. The dependence of the National Spherical Torus Experiment (NSTX) [C. Neumeyer et al., Fusion Eng. Des. 54, 275 (2001)] edge plasma on heating power, including the low confinement mode (L-mode) to H-mode (L-H) transition requirements and the occurrence of edge-localized modes (ELMs), and on divertor configuration is quantified. Comparisons between good L-modes and H-modes show greater differences in the ion channel than the electron channel. The threshold power for the H-mode transition in NSTX is generally above the predictions of a recent International Tokamak Experimental Reactor (ITER) [ITER Physics Basis Editors, Nucl. Fusion 39, 2175 (1999)] scaling. Correlations of transition and ELM phenomena with turbulent fluctuations revealed by gas puff imaging and reflectometry are observed. In both single-null and double-null divertor discharges, the density peaks off-axis, sometimes developing prominent “ears” which can be sustained for many energy confinement times, τE, in the absence of ELMs. A wide variety of ELM behavior is observed, and ELM characteristics depend on configuration and fueling.

Survey of H-mode transition and confinement from ASDEX Upgrade `H-mode standard shot'

The H-mode standard shot of ASDEX Upgrade provides an overviewof the long term evolution of machine conditions and scatter ofH-mode power threshold and confinement. This type-I ELMy H-mode isrun every day of plasma operation since 1999.After a long opening for in-vessel work,the power threshold drops rapidly by about 50% over100 discharges and then much slower by about 20% over the remaining 1000plasma dischargesmade in a campaign.At the natural density without gas puffing, confinement increases by about30%over about 500 discharges. Afterwards thepeak-to-peak scatter is about 20%, smaller athigh density.Conditioning causes changes of edge density and temperature,similar to the confinement degradation observed with gas puffing.

H-mode threshold power in TCV Ohmic plasmas

L-mode to H-mode (LH) transitions are observed in a large variety of plasma conditions in TCV Ohmic plasmas. The power flowing through the separatrix at the LH transition differs by up to a factor of two from the predicted H-mode threshold power. Variations in this deviation are due to the large operational domain covered by the transitions as well as from the different scenarios identified to access the H-mode regime. LH transitions obtained during the plasma current ramp at low plasma current, elongation and triangularity have a threshold power below the standard expression while transitions at high current and triangularity or during the stationary phase of the discharge have a threshold power above the standard expression.

Initial studies of core and edge transport of NSTX plasmas

Rapidly developing diagnostic, operational, and analysis capability is enabling the first detailed local physics studies to begin in high β plasmas of the National Spherical Torus Experiment (NSTX). These studies are motivated in part by the observation of energy confinement times in neutral-beam-heated discharges that are favourable with respect to predictions from the ITER-89P scaling expression. For plasmas heated with neutral beaminjection (NBI), analysis based on profile measurements suggests that electron heat conductionis the dominant thermal loss channel. Cases where early analysis indicates that ionthermal conduction may be exceptionally low is motivating studies of possible sources of ion heating not presently accounted for by classical collisional processes. Gyrokinetic microstability studies indicate that long wavelength turbulence with kθρi~0.1-1 may be stable or suppressed by E×B shear in these plasmas, while modes with kθρi~50 may be robust. High harmonic fast wave (HHFW) heating efficiently heats electrons on NSTX, and studies have begun using it to assess transport in the electron channel. Regarding edge transport, H-mode transitions occur with either NBI or HHFW heating. The power required for L- to H-mode transitions far exceeds that expected from empirical ELM-free H-mode scaling laws derived from moderate aspect ratio devices. Finally, initial fluctuation measurements made with two techniques are permitting the first characterizations of edge turbulence.

H-mode plasmas in the MAST spherical tokamak

Operating scenarios in the Mega-Ampère Spherical Tokamak (MAST) have been recently developed to access routinely H-mode plasmas over a wide range of plasma conditions. Gas refuelling at the inboard mid-plane has proved invaluable in facilitating L- to H-mode transitions. Already, a comprehensive range of H-mode behaviour has been observed providing similar and contrasting comparisons with conventional tokamaks. For example, similarities are found in H-mode edge and global evolutions, increases in confinement and ELM behaviour. In contrast, H-mode threshold powers are about an order of magnitude higher in MAST than those predicted by conventional aspect ratio tokamak scalings. At the highest βθ values obtained so far, transient confinement times of ~100 ms are achieved and calculations indicate that bootstrap currents may approach 50% of the plasma current.

Characteristics of the first H-mode discharges in the national spherical torus experiment.

We report observations of the first low-to-high ( L-H) confinement mode transitions in the National Spherical Torus Experiment. The H-mode energy confinement time increased over reference discharges transiently by 100-200%, as high as approximately 100 ms. This confinement time is approximately 2 times higher than predicted by a multimachine scaling. Thus the confinement time of spherical tori has been extended to a record high value, leading to an eventual revision of confinement scalings. Finally, the power threshold for H-mode access is >10x higher than predicted by an international scaling from conventional aspect-ratio tokamaks, which could lead to new understanding of H-mode transition dynamics.

Enhancement of mode-converted electron Bernstein wave emission during National Spherical Torus Experiment H-mode plasmas

A sudden, threefold increase in emission from fundamental electrostatic electron Bernstein waves (EBW) which mode convert and tunnel to the electromagnetic X-mode has been observed during high energy and particle confinement (H-mode) transitions in the National Spherical Torus Experiment (NSTX) plasma [M. Ono, S. Kaye, M. Peng et al., in Proceedings of the 17th IAEA Fusion Energy Conference (IAEA, Vienna, Austria, 1999), Vol. 3, p. 1135]. The mode-converted EBW emission viewed normal to the magnetic field on the plasma midplane increases when the density profile steepens in the vicinity of the mode conversion layer, which is located in the plasma scrape off. The measured conversion efficiency during the H-mode is consistent with the calculated EBW to X-mode conversion efficiency derived using edge density data. Calculations indicate that there may also be a small residual contribution to the measured X-mode electromagnetic radiation from polarization-scrambled, O-mode emission, converted from EBWs.

Evidence for edge gradients as control parameters of the spontaneous high-mode transition

Measurements in the DIII-D tokamak [J. L. Luxon et al., Plasma Physics and Controlled Fusion Research, 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. I, p. 159] show that edge gradients of ion and electron temperature and pressure, ∇Ti, ∇Te, ∇Pi, and ∇Pe, are good candidates for parameters which respond to the heating power to control the fundamental physics of the spontaneous Low-mode (L-mode) to High-mode (H-mode) transition. These gradients are measured in the region where the H-mode transport barrier forms and they are found to consistently increase in time during the L-phase of discharges which make a transition to H-mode. Moreover, for a fixed magnetic configuration, there is a well-defined boundary between the L-mode and H-mode states in the ∇Te−∇Pe operational space. The values of Ti, Te, Pi, and Pe, measured at this location, show smaller relative changes during the L-phase of these discharges, indicating that the scale lengths of these quantities are decreasing as the L–H transition is approached in time. These results are consistent with several theoretical models in which gradients of pressure or temperature cause the transition.

Initial performance results of the DIII-D Divertor 2000

A major upgrade of the DIII-D divertor, with the goal of enhancing impurity and density control and increasing the thermal pulse length limit of advanced tokamak (AT) plasmas has been successfully completed and commissioned. The integrated system that includes independent cryopumps at both the inner and the outer legs of the divertor, private flux region and outboard baffles, and improved graphite divertor armor, has been successfully applied to a variety of plasma conditions. Comparison of similar discharges before and after the upgrades show that with the new divertor the core plasma neutral source and carbon content are lower by as much as 50%. Calculations supported by preliminary infra-red (IR) camera measurements show that the new graphite armor design increases the limit on the discharge duration, due to temperature of the tile edges reaching sublimation point, by an order of magnitude. With the new system we have been able to control the density of high confinement H-mode plasmas to less than 1/3 of the Greenwald limit. It is observed that with divertor pumping during the current ramp phase the wall particle inventory and consequently the density rise after the H-mode transition can be significantly reduced.

Investigations of H-mode plasmas triggered directly by pellet injection in the DIII-D tokamak.

The transition from low-confinement (L-mode) to high-confinement (H-mode) plasmas has been directly produced by injecting frozen deuterium pellets in the DIII-D tokamak. H-mode transitions were produced at edge electron and ion temperatures below the L-mode values. This implies that a critical edge temperature is not necessary for H-mode transitions. The experimentally determined edge plasma parameters were well below those predicted by several theories of the H-mode transition to trigger the H-mode, indicating a need for revision of these theories.

Toroidal momentum confinement in neoclassical quasilinear theory in tokamaks

The neoclassical quasilinear theory is employed to study the toroidal momentum confinement in tokamaks. The electrostatic fluctuation-induced toroidal stress is calculated. It is found that besides the conventional diffusion term, there is also a convective term in the toroidal momentum equation. In the case of a weak toroidal momentum source, the logarithmic gradient of the toroidal flow is proportional to the linear combination of the logarithmic gradients of the plasma pressure and the temperature. This conclusion is insensitive to both the renormalization procedure and the magnitude of the fluctuations. The theory is applied to explain the changing of the toroidal flow direction after low mode (L-mode) to high mode (H-mode) transition observed in some experiments.

Synergy of multiviewing spectroscopic diagnostics on COMPASS-D (invited)

In the COMPASS-D tokamak three multiviewing diagnostic systems are combined by using the same collection optics. HeI line ratios from a helium thermal jet provide detailed edge Te(r) and ne(r) profiles. The spectrometer also provides the Dα profile. A Doppler spectrometer system provides Ti and Vθ profiles using HeII emission. The Te and ne profiles are used in extracting neutral densities from the Dα emissivities, and the radial electric field from the Doppler measurements. The behavior of Te(r), ne(r), 2nD(r), Er, and Vθ and their gradients can now be compared in detail to investigate the H-mode transition physics.

Millimeter-wave reflectometry for electron density profile and fluctuation measurements on NSTX

A millimeter-wave reflectometry system for electron density profile and fluctuation measurements is being developed and installed on the National Spherical Torus Experiment. The initial frequency coverage will be in the bands 12–18, 20–32, and 33–50 GHz, provided by frequency-tunable solid-state sources. These frequencies correspond to O-mode cutoff densities ranging from 1.8×1012 to 3.1×1013 cm−3, which will span both the plasma core (ρ=r/a&amp;lt;0.8) and edge (ρ&amp;gt;0.8) regions. Operated as a broadband swept-frequency (frequency-modulated continuous-wave) reflectometer, the diagnostic is expected to provide routine (shot-to-shot) time- (⩽50 μs) and spatially resolved (∼1 cm) density profiles. The previous hardware can be easily reconfigured as a fixed-frequency reflectometer for density fluctuation measurements. The combination of measurements would be valuable for studying phenomena such as possible L- to H-mode transitions and edge-localized modes.

First results of biasing experiments on the T-10 tokamak.

The first results of the electrode biasing experiments performed on the T-10 tokamak are presented. Positive voltage applied to an electrode inserted inside the tokamak limiter results in the H-mode transition characterised by a decrease of Dα emission intensity, a rise of line-average plasma density and an increase in energy lifetime. Growth of core electron and ion temperatures during the electrode biasing implies the formation of the thermal barrier in addition to the particle transport barrier.

H-mode operational boundaries

Recent results on the understanding and scaling of several of the operationalboundaries found in ELMy H-mode plasmas are presented. Divertor geometry isfound to be important in determining the H-mode transition power andtemperature. Recent experiments from different tokamaks find apparentlyconflicting evidence for the role of the edge neutral density in controllingthe H-mode transition and further intermachine comparisons are clearlywarranted. Significant progress has been made both on the understanding andon the avoiding of the H-mode density limit. Models based on divertordetachment andon MARFE physics reproduce the empirical density limit scalings. The densitylimit in ELMy H-modes has been increased by changing the profile of thedensity in the divertor and in the main plasma. Highly radiating ELMy H-modesare currently being studied and display some of the favourable characteristicsof the TEXTOR radiative improved confinement mode.