Diagnostic Long-Term Plan for the DII-D

Present Diagnostic Capabilities

The DIII-D plasma diagnostic set is made up of more than 50 instruments built and operated by the DIII-D national program. This ensemble of instruments is the most complete of any tokamak in the world and routinely produces the high quality data required to fuel the DIII-D scientific research program. The DIII-D diagnostics set includes extensive divertor and edge measurement capability, plasma core profile measurements of density, temperature and plasma current and a large suite of fluctuation diagnostics. Many of the diagnostic systems on DIII-D have been installed and are routinely operated by collaborators. A complete list of the diagnostic systems and descriptions can be found on the DIII-D Diagnostic Web page.

Diagnostic System Initiatives

There are additions and upgrades to the diagnostics that will have a large pay-back in terms of the scientific output of the DIII-D research program. These additions are motivated by new areas of research requiring either new measurement capability or changes in the DIII-D machine hardware or, in some limited number of cases such as the core Thomson scattering diagnostic, to fill in gaps in our existing measurement capability. In addition to the diagnostic improvements described in this section, we expect to implement other unforeseen diagnostics or improvements during the five-year span of this plan.

Table of contents

Radiative Divertor Diagnostics

To fully take advantage of the science that can be learned as a result of the installation of the lower Radiative Divertor Project (RDP) (Phase lB), the systems to diagnose conditions in this divertor will be upgraded by re-aiming or refurbishing existing diagnostics and the addition of some entirely new ones. The anticipated improvements to our present capabilities are summarized in Table 1.

This list represents a conscious decision to concentrate enhancements in one divertor, the lower divertor, so that a complete set of measured plasma parameters will exist against which sophisticated models can be compared. The new configuration does not lend itself to the use of Thomson scattering electron temperature measurements. Thus, the present divertor Thomson system will be used to benchmark a variety of spectroscopic techniques for diagnosing local plasma parameters; these techniques include: (1) determination of ne from Stark broadening transitions arising from high n levels, (2) determination of Te from line ratios, and (3) discrimination of ionizing plasmas from recombining by means of Balmer series line ratios in deuterium. Maturation of these techniques is essential for reliable diagnosis of plasma parameters in the slot divertor. The spectroscopic techniques listed above the same ones on which ITER will most likely rely as a consequence of the difficulties its high on flux imposes on line-of-sight diagnostics. RF reflectometry or interferometry and a new insertion path for the fast reciprocating probe are additional measures under consideration for help in replacing the loss of the Thomson scattering data in the divertor. An essential part of the plan will be a software effort to automate the analysis of data from the spectroscopic diagnostics above so that processed information will be available to the whole scientific staff for a large number of discharges discharge times as is the case with DIII-D's sophisticated Thomson scattering systems. The development of a reliable spectroscopic determination of critical plasma parameters in the divertor will be a major program element.

Table 1: Phase 1B Divertor Diagnostics
Diagnostic Institution Divertor Comment
Lower Upper
Power loss measurements
Vertical IR TVs LLNL X X Re-aim
Foil bolometers GA X New
Under-baffle, 4-ch bolo arrays LLNL, GA X New
Filtered line monitors (Da, etc.) ORNL X X Re-aim
Fast pressure gauges (inner and outer plenums, private flux) ORNL X X 3 new (out of 5 total)
Magnetic probes on baffles GA X X New in lower
Fixed Langmuir probes SNL X X Rebuild probe tips, upper and lower
Tangential TV imaging, visible LLNL X Relocate
Tangential TV imaging, ultraviolet Hampton U. X New
Vertical TV camera, visible GA X Re-aim
EUV SPRED (10-160 nm) LLNL, PPPL X Re-aim
Visible survey spectrometer ORNL, GA X X New fibers, upper
High resolution, visible spectrometer ORNL, GA
Vertical viewchords X X New fibers, upper
Toroidal viewchords (for flow velocity) X Relocate
RF reflectometers (3 ch) UCLA X New
Multilayer mirror spec. JHU X New

Further improvements to divertor diagnostics will be undertaken during the vent for modification of the RDP hardware (Phase 2B) late in the DIII-D Five-Year Plan. A list of these diagnostic additions is given in Table 2.

Plasma Control Diagnostics

The advanced tokamak program relies to a large extent on the ability to control the pressure, current and Er profiles. The diagnostics required to generate control signals for this task largely exist on DIII-D, however, there are improvements to those diagnostics that are needed to fully implement the plasma control program. The temperature and density profiles generated from the Thomson scattering diagnostic can be used for real-time plasma control with some changes to the Thomson scattering data acquisition system. The biggest improvement in the diagnostic systems in the area of plasma control is an upgrade to the motional Stark effect (MSE) diagnostic to generate real-time Er profiles. When viewing a co- and counter-injected neutral beam simultaneously with two separate MSE systems the radial electric field Er can be determined with good spatial and temporal resolution. The Er field can easily be extracted from the raw MSE pitch angle data and could be used as a real-time feedback signal for control of Er .

Table 2: Phase 2B Divertor Diagnostics

Diagnostic Institution Divertor Comment
Lower Upper
Power loss measurements
Real-time image processing for IR TVs LLNL X X New data acquisition hardware
Under-baffle, 4-ch bolo arrays LLNL X Copy lower
Under-baffle Da monitors ORNL X New, in-vessel fibers
Under-baffle spectroscopy ORNL X New, in-vessel fibers
X-point reciprocating probe UCSD, SNL X Port move and rebuild
Reflectometer or interferometer UCLA X New or copy of lower
Thomson scattering LLNL X New diagnostic
Partial pressure in all plenums with fast pressure gauges ORNL/GA X X New diagnostic
Laser fluorescence of Da X New diagnostic
Normal incidence spectrometers ORNL/GA X X New, divertor and core

Using simultaneous view of co- and counter-injected beamlines provides the maximum sensitivity of the MSE measurement to Er. This viewing geometry also provides the best spatial resolution across the entire plasma radius.

The direct measurement of Er is demonstrated in Fig. 1 for a recent discharge using the new MSE system which features a tangential and radial view of a single beam line. In Fig. 1(c), the effective vertical field (assuming Er = 0) is plotted for a tangential chord (solid line) and a radial chord (dashed line) at a radius of R~2 m. If Er were zero, then these two curves would track one another. The separation of the two curves during the ELM-free period from 2 to 2.25 s is an indication of the buildup of radial electric field. Er is calculated directly from the MSE measurements as shown in Fig. 1(d). The temporal evolution of Er follows closely the time evolution of the plasma toroidal rotation in Fig. 1(e) obtained from charge-exchange measurements of carbon impurities. The maximum time response of the MSE Er measurement is 1 ms with an RMS noise resolution of ~7 kV/m. The curve in Fig. 1(c) was generated using a 5 ms sliding boxcar average giving somewhat better resolution. Possible systematic errors in Er due to spatial averaging in the radial chords and calibration are a factor of 2-3 larger than uncertainties due to noise.

Figure 1. Example of direct measurement of the readial electric field on DIII-D using the MSE diagnostic

The use of MSE for measuring the radial electric field has been demonstrated; using this technique for a real-time feedback measurement of Er could be accomplished by adding the new MSE channels to the plasma control system (PCS) data acquisition system. The addition of the counter neutral beam and the MSE channels to view the counter neutral beam will substantially improve the spatial resolution of the Er measurement.

Electron Transport Diagnostics

The remaining challenge in understanding core turbulence and transport in fusion plasmas is determining what mechanisms are responsible for anomalous electron transport and how can electron transport be controlled? In order to address this electron transport issue, new diagnostic measurement capabilities are required. At present, only relatively low wavenumber (long wavelength) density turbulence is measured in the core of DIII-D, using FIR scattering, beam emission spectroscopy (BES) and reflectometer systems. However, several other mechanisms such as short wavelength turbulence, or magnetic turbulence may be responsible for the anomalous electron transport. To further understand these possible transport mechanisms, we will expand the turbulence diagnostic coverage as follows:

Search for and measure electron mode turbulence using a scattering system specifically modified for the purpose. Theoretically predicted electron mode turbulence (such as e modes) have wavelength 20-60 times shorter than that responsible for ion transport.

Measure core magnetic turbulence using a cross-polarization scattering system, as on Tore Supra, or using enhanced scattering at the upper hybrid layer as proposed by Russian and Dutch groups.

Improve the central density fluctuations measurements by upgrading the phase contrast imaging (PCI) diagnostic by adding a central viewing set of sightlines.

It should be noted that in addition to their role in the search for electron transport mechanisms, these diagnostics will also generally enhance our physics measurement capability. For example, a magnetic turbulence diagnostic will allow us to address the long unresolved issue of the relative importance of electrostatic and magnetic turbulence in core transport. More detailed descriptions of the three diagnostics follow.

Study of Small-Scale Turbulence Via Scattering

Turbulent structures with wavelengths approaching the ion gyroradius can be measured using coherent scattering, but are far too small for core turbulence measurement techniques based on multiple point correlation analysis. Therefore, we propose to reconfigure the current FIR scattering system (which currently probes larger structure turbulence at small scattering angles) to allow short wavelength density fluctuations to be detected. A number of technical issues such as the specific scattering geometry optimum for use in DIII-D, effects of E infty B convection and Doppler shifts, and wavenumber resolution must be solved in a developmental effort. The availability of an existing collective scattering system on DIII-D offers a cost effective opportunity to resolve this issue.

Magnetic Field Fluctuation Measurements

Recent work on Tore Supra using mode conversion scattering has suggested that such fluctuations may prove important in explaining the anomalous electron channel. However, many concerns continue to exist related to the diagnostic technique of cross polarization scattering in a fusion plasma and the specifics of the Tore Supra work. It is anticipated that these will be resolved during the proposed plan period and that installation of a magnetic field fluctuation diagnostic on DIII-D will take place in around three years. Such a system will require an inside launch and an outside receive capability, a combination which does not currently exist on DIII-D. The major developmental issue will, therefore, initially focus on establishing such a launch/receive arrangement and determining the details of antenna design and associated plasma facing components. The feasibility of applying a second magnetic scattering technique utilizing enhanced scattering from the upper hybrid layer will also be evaluated.

Measurement of Turbulent Temperature Fluctuations

Electron temperature fluctuations can be measured using correlation ECE radiometer systems. However, the field of view of the current ECE system utilized for electron temperature measurements is not suitable for fluctuation studies as the wavenumber sensitivity is too limited. Consequently, for fluctuation measurements a new optimized antenna system is required so as to obtain the desired wavenumber sensitivity. Installation of a new optimized system on DIII-D should be complete within the year, as well as development of the required data acquisition and software analysis routines.

It is anticipated that diagnostic development aspects of this work will be supported through the UCLA Advanced Diagnostic Development Program, in addition to DIII-D program support.

Central Thomson Scattering

The multipoint, multilaser Thomson scattering system that currently operates on DIII-D has been a critical diagnostic for the research program on DIII-D. The detailed temperature and density profiles routinely produced by the Thomson scattering diagnostic are essential to the scientific progress of the research program on DIII-D. The Thomson scattering diagnostic uses a vertical laser beam path. Due to vertical port locations, the laser beam does not pass through the center of most discharges and typically provides data from about 0.25 of the minor radius outward. The Central Thomson Upgrade will allow Thomson scattering measurements to be extended from R = 194 cm to the magnetic axis at R = 165 cm covering the center of essentially all of the discharges made in DIII-D. This extension is critical for such diverse areas as: (1) core transport analysis and stability in pressure peaked discharges, (2) modeling of current profile evolution and q0 control algorithms, (3) modeling of high performance discharges, and (4) high density experiments.

3-D Equilibrium Reconstructions

MHD instabilities play a major role in limiting plasma performance and in catastrophic disruptions. Both theory and simulations suggest that the degradation of the plasma performance and plasma disruptions are due to the growth of magnetic islands driven by these MHD instabilities. The development of a systematic technique and tools to experimentally reconstruct the magnetic island structures is crucial to improve the understanding and subsequent control of these instabilities. 2-D equilibrium reconstructions have been extremely useful in the studies of instabilities, however, many of the catastrophic instabilities driven by static error fields, mode locking, or a resistive wall are nonrotating or rotate slowly and do not lend themselves to 2-D analysis. This task is to extend the 2-D reconstruction to 3-D reconstructions by modeling the distortions of the magnetic surfaces and growth of magnetic islands in three dimensions using an extended set of toroidally displaced diagnostic measurements. The task consists of two main elements; construction of the extended set of diagnostics and the code development required to produce the 3-D reconstructions. The code development is an extension of the EFIT 2-D reconstruction currently used on DIII-D.

The diagnostic requirements center around making measurements of poloidal and toroidal distortion of the magnetic surfaces both at the surface of the plasma and internal to the plasma. Many instabilities of interest have low toroidal mode numbers (2/1,3/2) and this allows us to reduce the requirements on the toroidal resolution of the diagnostics. There are at least four diagnostics that can play an important role in providing this information - they are magnetics, ECE, x-ray arrays, and MSE. A limited number of additional poloidal arrays at new toroidal locations of each of these diagnostics can be used to reconstruct the 2/1,3/2 and possible higher distortions in the plasma. Other diagnostics that will be considered are a tangential viewing x-ray system for improved resolution of high m,n modes and improved spatial resolution BES.

The magnetic diagnostics can play by far the most important role due to the relative ease of installing large numbers of sensors. The addition of four poloidal arrays of 24 measurements (12 B and 12 Br) each separated by 90 deg poloidally would allow the reconstruction of modes up to n=3 and m=7. The ECE and x-ray arrays would provide internal measurements of Te and x-ray emission (x-ray emission is a function of impurity concentrations, electron density and temperature). If we assume that these parameters are constant on flux surfaces, then the measurements can be used to determine the internal magnetic structure. An additional toroidally displaced ECE system will be installed and the existing toroidal array of x-ray detectors could be upgraded with additional detectors. Finally, assuming the addition of the counter neutral beam injector, a new MSE diagnostic could be installed giving information on distortions in the current profile.

Current Profile Measurements at High Densities

The MSE system is a very effective tool for current profile measurements at moderate densities. However, in DIII-D at densities approaching 1.5 infty 1014cm - 3, strong attenuation of the diagnostic neutral beam makes measurement of current profile in the inner half of the plasma impractical. In ITER, because of the larger size of the device, this problem can be even more severe unless high intensity neutral beams with energies >500 keV are developed. We propose to develop a new current profile diagnostic system that uses a submillimeter laser instead of a neutral beam for probing the plasma. The new concept is based on the well known Fizeau effect and uses a new interferometer concept which is the key for making this measurement possible.

The system measures the line integral of current density, in contrast to the MSE which measures the poloidal component of the magnetic field. This system is, therefore, most sensitive near the magnetic axis of the plasmas. For the DIII-D applications, the proposed system would be complementary to the existing MSE system, whereas for future devices conceivably a complete profile system, based on this concept, could be installed. We propose a two-phase development plan for the system. In the first phase, we would design and install a single channel system. Once the behavior of the system is well documented, we proceed to the second phase where we would install an array of four to five channels to unfold the current profile in the inner half of the plasma.

The Fizeau effect is a well-known phenomenon causing a small phase shift of an electromagnetic wave traveling through a moving a dielectric medium. This phase shift is of the order of VD / c of the normal phase for a stationary medium, where VD is the drift speed of the medium and c is the velocity of light in vacuum.

A practical problem with the measurement of the current density-dependent term is its small value compared to the phase shift for a stationary medium, since small changes in geometry or plasma density result in far more phase shift than that due to plasma motion. As a result in tokamaks with standard interferometers, measurement of the first order term is impractical. This difficulty is overcome with the interferometer arrangement shown in Fig. 2. In the arrangement shown, the reciprocity theorem guarantees perfect cancellation of the zero order term while the contribution due to the motion of the medium is doubled.

Figure 2

Laser Pumping to Improve Beam Emission Spectroscopy Diagnostics

Two key diagnostics on DIII-D would be significantly improved by using laser pumping to affect the population of the n=3 level in the neutrals injected by the neutral beam injectors. The BES diagnostic could achieve an improvement in signal to noise by about a factor of 6 through this technique, shortening the minimum time resolution by the same factor. Data analysis for the charge exchange recombination (CER) system could be greatly eased by using the same laser pumping to modulate the charge exchange signal. These improvements in the diagnostics would be beneficial in the study of plasma transport processes.

The basic idea of using laser pumping to improve the BES signal was invented by Fonck at the University of Wisconsin. The fundamental concept uses a laser tuned to the 656.3 nm transition between the n=2 and n=3 levels of deuterium to illuminate the region being examined and thus increase the population of the n=3 state. The signal that the BES system detects is due to spontaneous emission from n=3 to n=2. At readily achievable laser intensities, it is possible to saturate the transition, making the population of the n=3 and n=2 states equal. Since the n=2 state population in the absence of the laser is significantly larger than the n=3 state population, this results in an appreciable enhancement of the BES signal. A proof-of-principle test of this technique is being pursued as part of the work the Wisconsin group is doing at DIII-D. If it is successful, further development would be required to apply this technique to all the BES channels.

BES is a key diagnostic for measuring density fluctuations in the plasma. The time resolution of this measurement, at present, is typically hundreds of milliseconds owing to insufficient signal. Even so, significant changes in plasma turbulence have been measured. However, other diagnostics, for example FIR scattering, suggest that the turbulence can change even faster. (FIR scattering has excellent time resolution but poor spatial resolution.) If this laser pumping scheme is successful, the minimum time resolution for the BES system could be decreased by about a factor of six, which would be quite significant.

The CER system is used for a number of measurements on DIII-D ion temperature, poloidal and toroidal rotation, impurity density, and radial electric field. Analysis of the data is difficult owing to the complexity of the spectra. One way to greatly simplify the analysis is to modulate the charge exchange signal and subtract the spectra taken with the modulation off from the spectra taken with the modulation on. Such modulation of the signal has been done, for example, by modulating the neutral beams. However, owing to equipment limitations, such modulation tends to be slow (50 to, at best, 100 Hz) and it is frequently the case that the plasma changes during this time. This makes time slice subtraction ineffective, since the interfering lines in the spectra have also changed. In cases where the subtraction is successful, analysis of the resulting CER spectrum is trivial, because it consists of a single Gaussian peak. Laser pumping would allow much more rapid modulation, decreasing the plasma changes occurring between the CER measurement and the background measurement thus improving the effectiveness of the background subtraction.

By using laser pumping of the population of the higher n levels of the deuterium in the neutral beam, the charge exchange signal can also be modulated, since the cross sections increase rapidly with n level. In the case of the CER measurement, either resonant pumping of the n=2 to n=3 transition or photo ionization of the levels with n=2 and greater could be used to effect the modulation. In one case, the charge exchange signal would increase, in the second it would decrease. Since the photo ionization technique affects more levels, it would probably have the larger amplitude; however, it may require significantly greater laser power. We intend to assess both techniques, and develop the best.

Diagnostic Neutral Beam

There are three diagnostics which could be significantly improved by a diagnostic neutral beam. The MSE diagnostic measures the field line pitch angle from which the plasma current profile is calculated and the radial electric field Er in the plasma. A diagnostic neutral beam directed counter to the plasma current would significantly improve the real time Er measurement and the plasma current profile measurement. The CER system is used to measure ion temperature, poloidal and toroidal rotation, impurity density, and Er. A diagnostic beam, capable of being operated at a higher beam modulation rate and higher beam power density with a smaller diameter beam, would increase the CER signal and improve spatial resolution. The improvement in signal-to-noise ratio is of particular importance at the core of the plasma where beam attenuation now limits the accuracy of the measurement. The BES diagnostic would benefit from a diagnostic beam with higher power density to improve time resolution, and its measurement might, based on theoretical estimates, be improved by use of a helium diagnostic neutral beam.