The National Spherical Torus Experiment (NSTX)

The NSTX Project will provide a national facility for investigating the fusion physics principles of plasma confinement, heating, and current drive in a low aspect ratio, spherical torus (ST) configuration. The device, designed by a team led by the Princeton Plasma Physics Laboratory (PPPL) and including the Oak Ridge National Laboratory (ORNL), University of Washington, and Columbia University, is now under construction at PPPL. First Plasma is planned for April 1999.

The ST configuration is an alternate confinement concept which is characterized by very high beta, high elongation, high bootstrap fraction, and low toroidal field compared to the conventional high aspect ratio tokamak. NSTX will build on the encouraging results from exploratory experiments such as the PPPL CDX-U (Current Drive Experiment, Upgrade), the START (Small Tight Aspect Ratio Tokamak) at Culham, U.K., and the HIT (Helicity Injected Tokamak) at University of Washington. The NSTX is similar in scale and complementary in testing capabilities to the MAST (Meg-Amp Spherical Tokamak) machine now under construction at Culham.

Main characteristics and ratings of the machine are as follows.

The core of the NSTX machine consists of a narrow center stack (CS) bundle, which contains the inner legs of the Toroidal Field (TF) coil, an Ohmic Heating (OH) solenoid coil, the associated tension cylinder, a pair of inner Poloidal Field (PF) coils, thermal insulation, and a center stack casing that forms the inner wall vacuum vessel boundary. All of the coils utilize water cooled copper conductors. The CS Casing is electrically isolated from the remainder of the machine via ceramic insulator assemblies, which permit the use of Coaxial Helicity Injection (CHI) as an advanced method of current drive. The CS bundle presents one of the main engineering challenges of NSTX and of STs in general; the design maximizes performance while minimizing radial build and the plasma aspect ratio.

The outer vacuum vessel (VV) consists of a cylindrical section with upper and lower domes. It is equipped with ports suitable for mounting a 12-strap High Harmonic Fast Wave (HHFW) RF antenna assembly, and for future attachment of up to two of the Tokamak Fusion Test Reactor (TFTR) Neutral Beam Injection (NBI) systems. There are additional seven 24-inch-diameter ports at the mid-plane for diagnostic access. Four outer PF coil pairs (symmetric about the mid-plane) are mounted directly on the VV.

Plasma Facing Components (PFCs) were designed by ORNL and consist of copper passive plates (to aid plasma stabilization), an inner wall, inboard divertor, and outboard divertor. The PFCs are covered with a combination of carbon fiber composite (CFC) and graphite tiles. The inner wall tiles are mounted via a special rail/pin design, which provides the required mechanical integrity while minimizing the radial build. The PFCs are designed to withstand a temperature of 350oC via the circulation of a high temperature fluid for the outboard and via a DC current for the CS.

NSTX will be installed in the Hot Cell of the D-site facility at PPPL, adjacent to the former TFTR Test Cell. The D-Site infrastructure and equipment including magnet power supplies, and RF sources, cooling water systems, etc., are used extensively to minimize the overall cost of the NSTX facility.

The TF inner leg bundle was manufactured by Everson Electric Company (Bethlehem, PA), and has been delivered to the site. The OH coil, also being built by Everson, is expected to be completed prior to the end of May. The fabrication of the outer legs of the TF coils is well underway. The inner PF coils were manufactured by Magnet Enterprises, International (Oakland, CA), and have been delivered to the site. The Vacuum Vessel fabrication is underway at Process Systems International, Inc. (Westborough, MA).

Background:

In nuclear fusion research the "tokamak" type of plasma confinement has achieved the most successful plasma parameters, and it is expected to achieve ignition in the International Thermonuclear Experimental Reactor (ITER) device. However, tokamaks are relatively large and complex, so alternative plasma confinement schemes are being studied. The compact toroid approach has only been studied in small experiments, but in principle it could afford smaller, less-expensive fusion power plants, so it is becoming increasingly popular.

The Small Tight Aspect Ratio Tokamak (START) experiment in the UK achieved remarkable results in 1994-96, which have stimulated worldwide interest in this field. Therefore, the UK is building the Meg-Ampere Spherical Tokamak (MAST) experiment to pursue this concept further. Some other laboratories around the world are now planning or operating compact toroid experiments. For example, the National Spherical Torus Experiment (NSTX) is under construction at Princeton, USA; a spheromak experiment is under construction in Livermore, USA; the GLOBUS-M spherical torus experiment is under construction in Russia; Japan has the TST and FIX experiments; and several developing countries have expressed interest in compact toroid research. This fall we will hold a TCM on "Spherical Torus" (compact toroid) research.

Here, the word "compact" refers to toroidal devices with low aspect ratios (R/a < 2.5, where R = major radius, a = minor radius of the plasma ring). This is a broad definition that includes devices with center posts (such as spherical tokamaks). There are several types of compact toroids:

• spheromaks
• field reversed configurations (FRCs)
• spherical tori
• other compact toroidal devices.

There are also several means of forming and sustaining the plasmas, including:

• magnetic induction
• radiofrequency and microwave current drive
• neutral beam injection
• external electrodes
• injection of many small compact toroids from plasma guns
• bootstrap current.

The main technical problems facing compact toroid research are:

• plasma formation without excessive impurity ingress
• plasma stability against magnetohydrodynamic instabilities, such as the tilt mode, especially at large ratios of (plasma size)/(ion gyroradius)
• plasma current sustainment by non-inductive means
• plasma fueling for density sustainment
• plasma-wall interactions.

Budgetary constraints have limited compact toroid experiments to small sizes and modest plasma parameters. There is a lack of a large experimental database, as exists for tokamaks, so it is difficult to develop confinement scaling relations with confidence. Therefore, it is not clear which configurations and sustainment methods will ultimately prove to be superior for the economical production of fusion energy. Recently several new compact toroid experiments have been constructed or approved, but there is not yet effective international coordination of these efforts. This IAEA Coordinated Research Project (CRP) will aim to coordinate world research efforts and to promote the comparison of various compact toroid configurations.

The proposed CRP pertains to comparatively long-term plasma confinement of medium-density plasmas, and not to brief confinement of fast-pulsed high-density plasmas (n > 10²² m^-3), even if they have compact shapes. This CRP does not include stellarators or reversed field pinches.

Objective:

The specific objective of this CRP is to compare various compact toroid configurations with regard to:

• plasma stability
• plasma sustainment techniques
• both experiments and theoretical models
• relative advantages and disadvantages
• prospects for fusion energy production.

The individual research projects can be theoretical models, experiments, or both. They may involve other related phenomena, such as plasma heating, fueling, turbulence, edge effects, impurities, and diagnostics, but the focus must be on understanding plasma stability and sustainability. The research results will be individual annual reports, reports of Research Coordination Meetings, a final report summarizing the results and conclusions, papers in the IAEA Fusion Energy Conference proceedings, and publications in scientific journals, such as Nuclear Fusion.. Research collaborations will be formed that may be continued after the CRP. The recommendations for future research may include international collaboration on a large joint experimental project

Activities:

1. We will form a communication network of interested laboratories in the fall of 1998 and discuss what each is working on.

2. At a Research Coordination Meeting (RCM) in late 1998, we will write a report summarizing the current state of compact toroid research and the plans for the CRP. (One possible date for the RCM could be 29-31 October).

3. The participants will continue to discuss their research problems and progress by electronic means in the months after the meeting. This will include some joint studies arranged at the RCM.

4. At the second RCM in 2000, we will write a report with the preliminary comparison of the various configurations studied.

5. At the third RCM in 2002, we will write a final report summarizing:
   • the research results of each participant
   • the advantages and disadvantages of the various configurations and helicity injection methods<
   • conclusions, with regard to the development of fusion power plants.

6. Then we will edit and publish the final report in a technical journal or as a IAEA Technical Document (TECDOC).

Possible Participating Countries :

Possible research contracts are Argentina, Brazil, China, Egypt, India, Russia, Turkey, Ukraine, and Venezuela. Possible research agreements are Australia, France, Germany, Italy, Japan, UK, and USA. Other countries may be added, if appropriate.

Applications:

If you would like to apply for a research contract or research agreement to participate in this CRP, please request the application forms from:

and submit your applications to that address, specifying that the research is to be part of CRP number F1.30.07 entitled "Comparison of Compact Toroid Configurations". Proposals should state how your research will contribute to the theme of the CRP. Proposals are due 15 August 1998, but proposals received after that date may still be considered.

General Information on IAEA Coordinated Research Projects:

The purposes of an IAEA Coordinated Research Project are:

• to advance the state of the art

• to provide coordination for work in many countries

• to reach conclusions regarding the subject matter

• to publish a document of technical value

• to encourage cooperation between developing countries and advanced countries.

A CRP typically lasts 3-5 years and involves about 5-15 participants, which are usually institutes, government labs, or university labs. A joint research topic is selected, and each participant works on an appropriate aspect of the problem. A Research Coordination Meeting (RCM) is held about every 18-24 months.

Participants from developing countries receive small research contracts (3-5 k$/year) from the IAEA. Participants from advanced countries have "agreements" with the IAEA, but do not receive financial support for the research. The IAEA pays the travel expenses of all participants to the RCMs, which are either in Vienna or at one of the participating Institutes.

The steps in setting up a CRP are approximately as follows:

1. The idea is discussed by Agency staff and sometimes at a consultants meeting. The list of potential topics and participating institutes is refined.
2. Potential participants are told informally the Agency is considering a CRP. An external advisory group (the IFRC) may be consulted.
3. A formal proposal is submitted to the IAEA "Research Coordination Committee" (RCC). After approval by the RCC, a formal announcement is sent out to potential participants.
4. Proposals are submitted and evaluated.
5. Contracts are awarded or agreements are signed with the selected participants. The CRP begins.
6. RCMs are held about every 18-24 months. Participants submit annual progress reports.
7. In the final year, each participant reports in detail the research and accomplishments. These results are combined, edited, and published by the IAEA (or by a technical journal).

In the field of plasma physics and nuclear fusion, the IAEA has the following CRPs recently completed, in progress, or in preparation (shortened titles):

• Software development for numerical simulation and data processing
• Plasma heating and diagnostics systems
• Lifetime prediction for a fusion reactor first wall
• Plasma-interaction induced erosion of fusion reactor materials
• Radiative cooling rates of fusion plasma impurities
• Reference data for thermomechanical properties of fusion reactor plasma facing materials
• Tritium retention and release from fusion reactor plasma facing components
• Atomic and plasma-wall interaction data for fusion reactor divertor modeling
• Engineering, Environmental, and Industrial Applications of Plasma Physics and Fusion Technology.

20th Symposium on Fusion Technology - SOFT 98

IAEA Technical Committee Meeting on Steady-State Tokamak Operations

17th IAEA Fusion Energy Conference

Last December, we explored the electronic mail as a possible alternative to the printed newsletter. Thanks to those who responded to our request and sent their E-mail addresses. In the future, we do plan to distribute the newsletter by E-mail. We also plan to use the E-mail service to distribute ANS-FED information when an item cannot wait for the regular biannual editions of the newsletter. Few ANS members have expressed interest in receiving hard copies of the newsletter. We encourage you all to advise us if you get access to the Web site or have a new E-mail address.

The ANS-FED Web site address is included at the beginning of the E-mail for those who prefer to go to the Web to read the newsletter. The articles are listed at the beginning of the E-mail to alert the readers to the subject to look for in the text. The "Chairman's Message" and the division-related topics will be published on a regular basis. As we begin to target a wider audience in the fusion community, the newsletter will cover a broad spectrum of subjects that emphasize the engineering and technology aspects of fusion. While some articles are devoted to the national fusion activities, we do plan to continue covering the international activities as they drew a wide interest in the U.S.

Please contact us or any of the Executive Committee members if you have any suggestions or comments.

The content of this newsletter represents the views of the authors and the FED Executive Committee and does not constitute an official position of any U.S. governmental department or international agency