Pedestal Physics Research Articles

Fast automated analysis of high-resolution reflectometer density profiles on DIII-D

High temporal (⩾10 µs) and spatial (⩾4 mm) resolution electron density profiles can now be automatically obtained on a between-shot basis from profile reflectometry data on the DIII-D tokamak. Fast, automated analysis is made possible by a new between-shot analysis package, details of which are presented. Analysis of X-mode data covering ∼0–3 × 1019 m−3 with 1 ms duration data bursts every 5 ms for 5–6 s, for a total of ∼1000 profiles, takes 4–5 min, allowing the profiles to be viewed and used between shots. Additional details are provided of how the system density coverage at the above temporal and spatial resolution was recently expanded to (0–6.4) × 1019 m−3 using a novel simultaneous, dual-polarization (O- and X- mode) measurement technique. Automatic (but not between-shot) analysis for the complete O- and X-mode data set (1.5 GB for each discharge, about 100 000 profiles at 25 µs temporal resolution) is available after the experiment. The improved capabilities of the system are illustrated using new results from a variety of areas such as edge localized mode dynamics, L–H transition and pedestal physics, impurity deposition and ITB studies, and direct comparisons with Thomson data are presented.

H-mode pedestal, ELM and power threshold studies in NSTX

H-mode operation plays a crucial role in National Spherical Torus Experiment (NSTX) research, allowing higher beta limits due to reduced plasma pressure peaking, and long pulse operation due to high bootstrap current fraction. Here, new results are presented in the areas of edge localized modes (ELMs), H-mode pedestal physics and power threshold studies. ELMs of several types as reported by higher aspect ratio tokamaks have been observed: (1) large, Type I ELMs, (2) intermediate-sized Type III ELMs and (3) tiny ELMs. Many high performance discharges in NSTX have the tiny ELMs (recently termed Type V), which have some differences as compared with small-magnitude ELM types in the published literature. A divertor multifaceted axisymmetric radiation from the edge (MARFE) on the inboard leg provides an effective light source to examine the effect of the ELMs on the divertor plasma; it is clear that only the large ELMs burn through the MARFE. The time difference between observation of the ELM flux at the outer and inner targets is substantially longer for the smallest ELMs as compared with the large ELMs. In addition, the visible light patterns show ‘finger-like’ striations during the tiny ELMs. H-mode pedestal studies have commenced, with the observation that the pedestal contains between 25% and 33% of the total stored energy, and the NSTX pedestal energy agrees reasonably well with a recent international multi-machine scaling. A power threshold identity experiment between NSTX and the Mega-Amp Spherical Tokamak shows comparable loss power at the L–H transition in balanced double-null discharges. Both machines require more power for the L–H transition as the balance is shifted toward lower-single null. High-field side gas fuelling enables more reliable H-mode access in NSTX, but does not always lead to a lower power threshold, e.g. with a reduction of the duration of early heating.

High-resolution dual-polarization frequency modulated reflectometer density profile measurements on DIII-D

A novel dual-polarization technique has been applied to the frequency-modulated electron density profile reflectometer on DIII-D. In this application, both O- and X-mode waves are simultaneously launched from a common high-performance solid-state source, and simultaneous measurements of both O- and X-mode reflections from corresponding cutoffs serve to expand the density coverage. The system consists of both Q-band (33–50 GHz) and V-band (50–72 GHz) subsystems, their combination covers a density range from 0 to 6.4×1019 m−3, with a time resolution of ⩾10 μs and spatial resolution of ⩾4 mm. The dual-polarization technique was implemented successfully in the Q-band subsystem during 2003 and detailed information for H-mode pedestal physics studies was provided. It has also recently been applied to the V-band subsystem, and extended contributions to H-mode pedestal physics and particle transport studies are expected during the 2004 DIII-D campaign.

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.

A 1-D predictive model for energy and particle transport in H-mode

In the last few years, significant progress has been made in the understanding of H-mode plasmas (e.g. ion temperature profile stiffness, pedestal physics, etc). Based on this improved understanding, a set of rules (models) comprising a physics picture of the H-mode has been implemented in the ASTRA code in order to improve the understanding of experimental observations and ultimately to provide a predictive capability for ITER complementary to the scaling relations. The model has been verified for consistency with experimental observations in ASDEX-UG and JET plasmas. Numerical coefficients for the transport, required because of simplifications or missing quantitative information, are determined for one plasma (e.g. from JET) and then held constant for all others (JET, ASDEX-UP, ITER). After benchmarking the model to experimental results, it was also applied to ITER. It predicts that Q = 10 can be achieved in ITER but only with at least a 50% deep fuelling contribution (inside the H-mode pedestal). However, in existing machines as well as in our model runs for existing machines, gas puffing is sufficient to achieve the observed density pedestal and line average densities. A second important result from the predictive runs for ITER is that electron energy transport in the plasma core, the neoclassical transport in the pedestal and the CX losses at the plasma edge are important constraints for a better performance. Thus future theoretical and experimental work should concentrate on these areas in order to improve our predictions.