Diagnostic Measurement Needs of the DIII-D Tokamak Program

Diagnostic Overview

Plasma diagnostics play a key role in developing the scientific basis for fusion energy, with physical measurements providing the link between theory/modeling and experiment. New measurements and diagnostics will be needed continually to further our understanding. Much of the diagnostic development and diagnostic implementation on DIII-D in the past several years has been done in collaborations between General Atomics (the prime contractor for DIII-D) and other institutions. These collaborations have been extremely successful and the DIII-D program intends to expand the number of collaborations in the area of diagnostics in the coming years.

The basic fusion science pursued in the DIII-D program is organized into four topical areas; Confinement, Stability, Boundary and Heating and Current Drive. Key measurements needed to advance our understanding in each of these areas has been identified by the DIII-D staff.

The multi-institutional DIII-D Executive Committee has considered the future DIII-D diagnostic needs and made the following lists under the various fusion topical areas. The items in the lists are in priority order with the highest priority at the top.

Contact Rejean Boivin for more information about diagnostics on DIII-D.

Confinement and Transport

The DIII-D program plans to take up the challenge of finally achieving an understanding of the basic processes underlying plasma transport from turbulence. In order to take up this challenge, new plasma measurements are needed as listed below. The theory and computational situation in transport has progressed to the point where being able to calculate transport rates from turbulence is a realistic possiblity over the next several years. Two and three dimensional pictures exhibiting what theorists currently believe to be the major physical processes are now becoming available. The experimental situation requires an increase in measurement capability to keep up with this strongly progressing theory capability.

High k fluctuations. While long wavelength turbulence associated with ion transport is better understood at this point, electron transport remains anomalous and of large magnitude for most regimes of tokamak operation. Short wavelength electron temperature gradient modes in the wavenumber range 2 < k_perp < 200 cm-1 are believed to cause some of this anomalous electron transport. Thus, measurements of any turbulence field (density, temperature, potential, magnetic field) at these wavenumbers is desired.

Zonal Flow Identification Nonlinear simulations of turbulent transport indicate that zonal flows are nonlinearly driven by the turbulence, and act in turn to regulate the level of turbulence and resulting turbulent transport. While necessary to bring simulations into agreement with transport measurements, such zonal flows have not been clearly identified experimentally. Zonal flows are predicted to be radially localized (several ion gyroradii) but poloidally and toroidally uniform (n=0, m=0) electrostatic potential structures. Experimental observation and characterization is required.

Magnetic Fluctuations Simulations and limited measurements have suggested that magnetic fluctuations may contribute to anomalous electron transport in the core. Thus magnetic fluctuation measurements at low and high wavenumber are desired.

Turbulence Imaging While most measurements of turbulence are 0D, 1D, or single point, turbulence in a tokamak is inherently 2D. Thus large field-of-view and high sensitivity 2D measurements of core and edge turbulence are required. Such measurements will assist with direct measurements of shear flows, eddy structures, as well as nonlinear interactions and energy cascades.

Ion Temperature Fluctuations Different turbulent modes (e.g., ion temperature gradient, and trapped electron modes) cause different relative fluctuation levels among turbulent fields. Measurement of Ion temperature fluctuations, to complement density fluctuation measurements, will provide a more direct confirmation of the types of modes driving ion transport. Furthermore, such measurements may potentially allow for direct turbulent energy flux measurements.

Turbulent Flux Measurements While most fluctuation measurements obtain a single fluctuating field, measurements of the full turbulent flux in the core are needed. The electrostatic particle flux can be obtained by correlation of density and potential fluctuations, while energy flux and electromagnetically driven fluxes can in principle be obtained with measurements of correlated density, potential, temperature and magnetic fluctuations.

Contact Jim DeBoo and/or Keith Burrell for more information about transport diagnostics on DIII-D.

Stability

The DIII-D program is a world leader in plasma stability research and has excellent diagnostics in this area. But, certain key measurement improvements are needed to resolve particular physics issues.

Beam-Ion Distribution Diagnostics that give additional information about the radial distribution of beam ions are needed for both confinement and stability studies. One possibility is to use scintillating fibers that measure the radial profile of the neutron flux.

Current Profile Measurement MHD stability in a tokamak is critically dependent on the current density profile. The DIII-D MSE current profile diagnostic has difficulty with high density plasmas because of limited beam penetration needed for the measurement. The MSE diagnostic also cannot unambiguously discriminate between the current profile and a radial electric field without additional views. A lithium beam diagnostic is now being developed for DIII-D to make this measurement in the edge pedestal.

ELM Instability Characterization ELM instabilities are driven by pressure and current gradients near the separatrix. Their mode structure is difficult to measure with standard magnetic probes because they are typically transient in time, and spatially localized to a relatively small region of the plasma surface. Detailed measurements of their toroidal, poloidal, and radial mode structure are needed for comparison with theoretical models.

Resistive Wall Mode Identification and Evolution. Resistive Wall Modes are difficult to detect at low amplitude with magnetic probes because they rotate very slowly, or not at all; growth of a non-rotating mode becomes difficult to distinguish from changes in the axisymmetric equilbrium fields. Measurements of the mode amplitude and phase that remain reliable at low amplitudes are needed in order to feedback stabilize these modes with nonaxisymmetric control coils.

Alven Mode Eigenfunctions Beam driven Alfven eigenmodes and energetic particle modes are regularly observed by magnetic diagnostics. The toroidal mode number is known accurately but accurate measurements of the radial and poloidal structure are desired for comparison with theoretical models.

Contact Ted Strait for more information about stability diagnostics on DIII-D.

Boundary and Divertor

DIII-D has a very comprehensive set of divertor and SOL diagnostics. But, some critical holes in the measurement capability exist. These holes must be filled to resolve critical physics issues.

Main Ion Temperature Profile. The main ion temperature, Ti, is a basic plasma parameter important for understanding particle, energy and impurity transport. This measurement is difficult because Ti cannot be inferred from techniques that measure the impurity ion temperature as they may be very different in the SOL and divertor. Extension of any techniques to measure Ti to 2D profiles is highly desirable.

Plasma Ion Flow Profile. Plasma flow is an important parameter of the boundary plasma that affects impurity transport as well as particle and energy balance. It is important to measure both the flow both parallel to the magnetic field and perpendicular flow that arises due to cross field drifts. The flow profile is expected to have strong spatial variations such that high resolution 2D profiles are needed. Besides the main ion flow, measurements of impurity flows, intrinsic or injected, are also desired.

Fast Transients in the SOL and Divertor. Transients, such as ELMs, deposit energy and particles into the SOL which can quickly flow to the divertor. These transients can impose severe constraints on the design of future divertors. On DIII-D measurements of density and temperature profiles with ~10 microsecond response time are needed to advance our understanding of these events.

Radial Transport in the outer SOL and Divertor. The poloidal profiles and underlying mechanisms of radial in the SOL and divertor are not well measured. An understanding of radial transport will be needed to predict divertor heat flux profiles and main chamber particle fluxes in future devices. Measurements of density, temperature and potential fluctuations, as well as turbulent flux, are important in advancing our understanding of SOL and divertor radial transport. Extending these transport measurements to a poloidal profile are particularly needed.

Neutral Density and Impurity Source Profile. The neutral density and impurity source profile are uncertain because of uncertain plasma and charge-exchange neutral flux to the main chamber wall. Present techniques make measurements in limited isolated locations, or are subject to considerable uncertainty due interpretation of visible spectroscopy More direct techniques for imaging neutral and low charge state impurity profiles are needed.

Innovative Imaging Techniques. The SOL and divertor is an inherently 2D problem with strong gradients both perpendicular and parallel to the magnetic field. Imaging systems, which lend themselves to high resolution 2D profiles, have mostly been limited to visible radiation measurements where spectroscopic interpretation can be uncertain. Imaging techniques for more direct measurements of most all plasma parameters, from plasma and neutral density to charge-exchange flux, could greatly advance boundary research. Coupled with this a difficulty of diagnostic access for the newer more closed divertors. Imaging techniques that extend measurements into restrictive geometries is needed.

In-situ Wall Surface Conditions. Divertor target erosion, impurity generation and tritium retention are important issues for future large magnetic fusion devices. These processes are not currently well understood, or characterized. Measurements of erosion, redeposition, and co-deposition rates are needed in both the divertor and main chamber wall surfaces. In-situ, or real time, measurements are needed to correlate changes in these rates with changes in plasma conditions.

Contact Rejean Boivin for more information about boundary diagnostics on DIII-D.

Heating and Current Drive

DIII-D has a major emphasis on current drive for steady-state and local profile control. Improved diagnostics are needed for the overall current profile and to fully understand the physics ofelectron cyclotron current drive.

Central Current Profile The advanced tokamak concept attempts to control the current density profile in order to optimize performance. Plasma current profile control is carried out on DIII-D with Electron Cyclotron Current Drive and other current profile control tools. Measurements of the central current profile with good spatial resolution are needed to assess the effectiveness of these current drive control tools. The DIII-D MSE diagnostic has difficulty in that it cannot unambiguously discriminate between the current profile and a radial electric field without additional views. A diagnostic that can overcome this difficulty and make measurements with high spatial resolution is needed.

Electron Energy Distribution Spatial Profile. Plasma current profile control on DIII-D with Electron Cyclotron Current Drive relies on driving a population of fast electrons in the toroidal current direction. Measurement of the electron energy distribution, and its spatial profile, is needed to model the current drive process.

Contact Ron Prater for more information about Heating and Current Drive diagnostics on DIII-D.