David Johnson and Robert Kaita

NSTX Research Forum

Princeton Plasma Physics Laboratory

February 5-7, 1997

One of the underlying reasons for the NSTX Physics Forum was to derive community input in the consideration of NSTX diagnostics. As the physics Working Groups discussed the elements of the NSTX research program, it was necessary to consider the tools needed for these studies. All of the working groups had a number of presentations on diagnostic issues, which are appropriately recorded in the separate Working Group Summaries.

There were two diagnostic presentations at the plenary sessions, one given by R. Kaita on the Baseline Diagnostic Set at the Introductory Session, and another by D. Johnson on a Summary of Needed Measurements at the Summary Session. This is a written summary of the Forum discussion on diagnostics, including the need for further effort in the area of profile diagnostics. This report further tabulates the list of measurement needs, taking advantage of the Expert Group’s evaluation of the ITER diagnostics, and identifies diagnostic topics where presentations were made at the forum.

NSTX provides an attractive environment for diagnostic installations. Diagnostic access is excellent with wide angle views readily available. In the initial years of NSTX research, eight midplane ports can be allocated to diagnostic use. The toroidal field is low and major disruptions is likely rare so that eddy current forces will be readily accommodated by the diagnostics systems.

However, relative to the state of the art in tokamak diagnostics, large differences in the spherical torus plasma and field parameters will require renewed diagnostics development and innovation. For example, with the low TF and high densities anticipated, ECE diagnostics will not be possible for measuring Te(r,t) in the plasma core. Motional Stark Effect measurements of the poloidal field and hence J(r,t) may become inadequate, because the reduced v x BT Stark splitting and polarization fraction are expected to require increased accuracy for the polarimeter measurement. Strong paramagnetic to diamagnetic variations in the toroidal field expected in spherical torus plasmas during auxiliary heating will complicate the interpretation of poloidal flux measurements. The large variations in field pitch strongly alters the viewing geometry for BES measurements, and the antenna launch geometry for reflectometer measurements. Identification of the MHD poloidal mode numbers for this geometry will require an increased number of magnetic pickup coils near the inboard, center stack tiles. The reduced temperatures for NSTX at high densities will increase the edge background light, challenge the accuracy of CHERS measurements, and call for sensitive x-ray imaging detectors to achieve adequate response to high frequency MHD. The new plasma regimes anticipated for NSTX therefore call for a fresh and innovative look at a large number of diagnostic techniques.

The first two challenges cited above, measuring Te(r,t) and the poloidal field [(and J(r,t)] profiles, were discussed extensively at the forum. The results of discussion should be useful in improving the plan for the baseline diagnostics on NSTX in these two areas. The baseline plan includes the utilization of an existing single-pulse TVTS system and an existing MSE system, because of budget limitations.

In this regard, several Working Groups strongly recommended a multipulse, multichannel, Nd:YAG-based Thomson scattering system, particularly in view of the lack of effective ECE measurements of Te(r,t). Reuse of an existing ruby laser system would still require considerable effort to install new collection optics, and would be inherently limited to single pulse capability. This limitation was viewed as a severe handicap. The needed Nd:YAG technology is straightforward, relatively costly, but modular in construction, permitting phased installation to fit resource constraints.

The suggestion to use the electron Bernstein waves to measure Te(r,t) was also generally favored by the Working Groups. If further study confirms this potential, this technique could be tried on an existing device such as the CDX-U, prior to installation on NSTX.

The signal to noise ratio for the existing MSE system will be seriously degraded due to the reduced polarization fraction. Changes in the physics mechanisms and the techniques of measurements are needed to achieve the required precision in pitch angle. The potentially large radial electric fields and the large variability in toroidal field will further complicate the interpretation of the signals. Possible enhancements of the MSE technique such as by laser fluorescence was suggested. Alternative techniques were also discussed, including Faraday rotation, and a heavy ion beam probe. A task group to review, analyze and compare the merits of these options for measuring the poloidal field and current profile was suggested by several of the Working Groups.

A related critical core profile parameter is ne(r,t). Both Thomson scattering measurements of Te(r,t) and Faraday rotation measurements of the poloidal field involve techniques capable of measuring ne(r,t) as well. Clearly, the core profile measurement alternatives need further evaluation.

The discussion of these core profile measurements at the Forum was extensive due to their importance in confinement, heating and stability studies, as well as the large improvements they would bring to the baseline diagnostics systems in NSTX. While this area received significant discussion at the forum, many valuable comments were presented also on other measurements.

A summary of the diagnostics discussed at the forum is presented in the tables below, adapted from the list of measurements being considered for the ITER engineering design. Measurements are grouped into three tables covering the following areas:

i) essential for machine protection and plasma control

ii) necessary for plasma control in specific studies (i.e.. profile control)

iii) necessary for specific physics studies.

Candidate techniques for each measurement of the NSTX plasma are listed, including the NSTX baseline and "day one" diagnostics. Also listed are key considerations of the measurement, and the Forum participants who presented ideas on the techniques of measurement.

Advances in innovative diagnostics will enable effective investigation of the very exciting fusion and plasma sciences of the spherical torus plasma. With first plasma scheduled for April 1999, there is time for developing important new tools of measurement. NSTX has excellent access and a user-friendly environment for diagnostics. This summary of diagnostics discussion and the tabulation of ideas and contributions provide initial information for use by researchers interested participating in advancing the diagnostics capabilities of the NSTX scientific research. We look forward to discussing the exciting opportunities of collaboration in this important area of NSTX scientific research.