Starlite Project Meeting Minutes
9-10 December, 1996
Princeton Plasma Physics Laboratory


Attendees: L. Bromberg, S. Dean, B. Dove, L. El-Guebaly, P. 
Heitzenroeder, 
S. Jardin, C. Kessel, T.K. Mau, R. Miller, F. Najmabadi, W. Reiersen, D. 
Steiner, 
R. Stambaugh, , D-K. Sze, M. Tillack, L. Waganer

Guests: Numerous guests from PPPL

Attachments: Meeting agenda

Summary

The kickoff meeting for the Advanced Concept Studies Program was held at PPPL on 9-10 December 1996, mainly to define and initiate the next year's study effort. The draft of the Starlite ARIES-RS final report is currently being reviewed within the team. The results of the recent low aspect ratio (LAR) assessment, both physics and engineering, were reviewed along with more recent LAR physics results by PPPL and GA. Technical experts from PPPL briefed the group on the goals, requirements, and preliminary design information of the NSTX experiment to be constructed at PPPL.

General

Steve Jardin welcomed the Starlite ARIES group to PPPL and reviewed the two-day agenda. Farrokh Najmabadi summarized the main areas to be investigated by the team over the next year or two. The principal focus of the study will be to evaluate key elements of the LAR (AKA the spherical tokamak (ST)) approach in more detail. This independent evaluation will provide information to help guide decision makers on the direction and benefits of future LAR machines. This LAR investigation will be the majority effort this year, with lessor efforts in two other areas. The second area will be the investigation of selected technical issues common to a general class of advanced fusion concepts, such as the ability to handle higher surface and volume heat loads (higher power density). The third effort will be to investigate alternate non-electric applications for fusion. L. Waganer will lead this effort.

As a sidebar, F. Najmabadi noted that the Starlite Utility Advisory Committee recently reviewed the ARIES-RS results. While satisfied with the design and projected technical performance, they noted the overall design did not meet their expected COE goal to be 20% cheaper than the likely competition.

Bill Dove stated that he was pleased with the ARIES-RS results and is looking forward to seeing the published final report. He noted the main result of achieving a high performance design with good safety features and an attractive environmental approach. He expects DOE to conduct a formal review of the final study results. He also mentioned the need for the conceptual electrical power plants to achieve attractive COE projections in, or below, the range of the competition. He would like to extend the outreach of the ARIES-RS results and not solely rely on the current approach of presenting papers at technical conferences. He is expecting the group to investigate some alternate approaches and applications for fusion.

Bill Dove related that the system studies budget has been cut almost in half, down to $1.5M. This budget will likely remain at this level for the near future. This lower funding level has reduced the breadth and scope of the team. The main players are UCSD, University of Wisconsin, and PPPL plus several other important participants from other institutions. Don Steiner asked if DOE has abandoned the idea of a demo or commercial power plant. No, rather DOE and Congress view the current embodiments as too large and expensive. They are asking the community to find new, innovative ways to provide a power source with smaller developmental steps. For the moment, the community should concentrate on the physics and technology aspects. Farrokh commented that our group should concentrate on the power plant design, identify the critical R&D needs, look at the end products, and highlight constraints that may be relaxed (or eliminated). We do not need to define the R&D pathways or estimate their costs (at this point).

Status of LAR Tokamaks

Martin Peng, program manager for the PPPL NSTX, reviewed the worldwide activities on the low aspect ratio or spherical tokamaks. There is a great deal of new experimental results being released and reviewed. The results indicate high stable average total beta (25-45%) with dynamic pressure-gradient driven current fractions in the range of 50-90%. These plasmas need conductive shells to get high beta values (without shells the total beta will be around 25%). The results also indicate high confinement values. They have strong magnetic wells and pressure gradients. Martin noted that theoretical data would suggest the ST plasmas will have thick scrape-off layers and diverter channels inferring easier power handling capabilities for these machines.

Several new ST machines are being planned and developed to build upon new experimental results. Non-inductive formed plasmas are being demonstrated with coaxial helicity injection (CHI) in HIT, SPEX, and TXT. Pressure-gradient driven currents are being created in CDX-U, and DIII-D. Efficient heating and current sustainment will permit steady-state operations in CDX-U, HIT-II, SPHEX, and START. Some of the ST machines operating are START (Culham, 91), HIT (U. of Wash), CDX-U (PPPL, 94), TS-3 (U. of Tokyo, 93), Medusa (U. of WI, 94), and TST (U. of ??, 95). Under construction are ETE (INPE, Brazil), Globus-M (IOFFE), MAST (Culham, 98?), and Pegasus (U. of WI).

Overview of NSTX Experiment

Masa Ono presented an overview of the planned NSTX experiment to be developed by PPPL, ORNL, and Columbia University. The device will be a spherical tokamak at an aspect ratio of 1.25. It will investigate non-inductive start-up, current stabilization, profile control, plasma confinement and transfer, pressure-limited and self-driven currents, scrape-off layer and divertor physics, stability margins, and resilience to disruption. Its mission is related to the Volume Neutron Source (VNS). It is designed to have a high total beta (25-45%); high pressure-gradient driven current fraction (50-90%); fully relaxed, non-inductively sustained current profiles; and operate in a collisionless regime. The plasma current will be 1 MA for a pulse duration of 5 seconds. Heating will be 5 MW of NBI, 3 MW of fast wave RF, and 400 kW of 25 GHz ECRH. The center column will be fully replaceable. The construction phase began in FY1996 and operation of the experiment is expected to begin in 1999. The experiment will be located in the TFTR D-site hot cell.

Outlook for New Energy Technologies in the 21st Century

Prof. Robert Socolow, Director of the Princeton University Center for Energy and Environmental Studies, reviewed his thoughts on the US' and world's energy future and possible sources to satisfy our expected energy needs. The general perception is that energy is not an urgent problem to be solved. Rather, the question of adequate energy resources, and companion question of environmental impact, may be postponed to some future time when the level of urgency increases. However, he feels these important issues must be dealt with in the near future. He noted that most researchers feel that by 2050, the world's energy demand will double or triple. There is not sufficient energy capability and resources to meet this demand without significant environmental impact. PCAST used the groundrule that, in the near-term, the greenhouse effect will be acknowledged but the principal energy source will continue to be fossil fuels. Fusion could help as an energy source in the future, but Rob feels fusion is handicapped by three current perceptions: (a) the unit size must be very large, (b) it is mainly an electric generating device, and (c) it is a nuclear process. He believes fusion can co-exist with fission as an energy supplier. He thinks our ultimate competition will be "green" fossil fuel energy sources that release no carbon products to the atmosphere, resulting in a hydrogen economy. He views this to be very beneficial to the commercialization of an efficient fuel cell.

ARIES Engineering Assessment of the LAR Tokamak

Mark Tillack reviewed the results of the recently completed ARIES assessment of several critical LAR engineering elements. To help assess these elements in the context of an actual machine, a commercial LAR device was defined to deliver the requisite 1000 MWe to the grid. The derived machine characteristics determined the machine size and performance. Notable characteristics were the recirculating power requirements that influenced the machine size. A shield was employed with the copper center post to help lessen the electrical resistance increase, minimize the embrittling effects, and enable Class C waste privileges. When asked the reason for choosing the aspect ratio of 1.25, Farrokh said that both aspect ratios of 1.4 and 1.25 were evaluated and were judged to be roughly similar, except for a higher total beta for the 1.25 case which was the determining factor for selection. C. Kessel mentioned that this next year's activity will be to continue to explore and optimize the LAR aspect ratio parameter for a commercial device.

Examination of NSTX Engineering Issues

John Spitzer, PPPL, presented the main parameters of the NSTX machine: major radius of 85.4 cm, minor radius of 68.0 cm, aspect ratio of 1.26, and plasma current of 1.0 MA. He felt that the center stack (post) was the most critical NSTX system. Within that system, the high-strength porcelain insulator, which electrically isolates the center stack from the vacuum vessel and coils and allows differential movement between the TF and OH coils, is the most difficult system to design. Electrical isolation is required because of the coaxial helicity injection (CHI) current drive. The bellows and insulator have undergone several design changes to accommodate necessary configuration changes. The vacuum chamber from the S-1 Spheromak experiment is being modified for this experiment. Several TFTR neutral beam coils are being used for the NSTX poloidal field coils. Active and passive vertical stability plates are shown inside the vacuum chamber. Both single and double null operation are being planned. Divertors are not shown at present but are being developed.

Proposal for New Innovative High Heat Flux Blanket

Dai-Kai Sze presented a new blanket concept that may accommodate the high heat flux environment of a LAR, have a high coolant exit temperature, and still use low-activation materials. He proposed using high pressure helium as the coolant. The coolant first enters the first wall region that is an enclosing shell of ferritic steel (FS) around the entire blanket module. The coolant flows radially inward toward the plasma; and then at the center of the module, it returns through the central breeder, reflector, and shield regions. The structural material for the inner regions is SiC which contains the breeder material and insulates the FS from the higher blanket material. The blanket, reflector, and shield will bring the coolant temperature to superheater temperatures of approximately 1000 C. This will enable the use of the higher efficiency Brayton cycle thermal conversion system. The main helium coolant stream will also be used as the tritium purge stream. This is a new concept, so it will have to be thoroughly evaluated.

Applying the Integrated-Blanket-Coil to the ST Central Column

Don Steiner explained how the Integrated-Blanket-Coil (IBC) concept of uses a liquid metal to both cool a component and conduct a current through it. This liquid metal concept would avoid the waste disposal problems and eliminate the radiation damage problems inherent with a copper center post. Maintenance would also be significantly simplified if the life of the center post is extended. If the temperature of the liquid metal can be elevated, some of the captured heat can be more efficiently recovered. The downside arguments are that lithium and sodium are more resistive, which translates into more power loss and MHD effects that may induce significant pumping power requirements. Don will lead the assessment of the IBC concept for the center column application.

Summary of Engineering Tasks and Planning

M. Tillack reviewed the Engineering Group's tasks and organization for FY97. The primary focus will be on a LAR design study with some contributions toward more general MFE design innovation and assessment of critical issues. ANL will work on blanket concept development and power density studies. PPPL will concentrate on magnet systems and design/analysis of the center post. The University of Wisconsin will provide design, neutronic analysis, mechanical analysis, materials assessment, and safety support for the centerpost, blanket, and the overall system. UCSD will provide the plant configuration and maintenance development, design integration, CAD design development, thermal analysis, and divertor design development.

Mark also defined general (ARIES-LAR) milestones to be completed in certain time periods:

Dec 96 - Mar 97:
Concept Development
Mar 97 - Jun 97:
Concept Selection (Bill Dove suggested a mid-point design report here.)
Jun 97 - Sep 97:
Design Evaluation
Sep 97 - Dec 97:
Final System Analysis (F. Najmabadi suggested a zeroth-order integrated design point then.)
The group recognized that there are at least four independent LAR or ST design concepts representative of power production systems that need to be objectively compared. This comparison will highlight the inherent assumptions and the underlying precepts that lead to differing configurations and performance projections. The four concepts identified are: GA (Stambaugh), Culham, Peng, and ARIES-LAR (Mar97 version). These concept comparisons will be led by Ron Miller (systems) and supported by M. Tillack (engineering), S. Jardin (physics), and D. Steiner (general). Results are to be reported at the March meeting.

Review of LAR Physics Assessment

Steve Jardin reviewed the equilibrium and vertical stability analysis results that encompassed a range of LAR plasma configurations involving aspect ratios, elongations, triangularity, with and without conducting shells, and with and without active vertical stability feedback coils. As the aspect ratio decreases, the self-consistent elongation and triangularity increase at rates which depend on the current profile and pressure. As the elongation and triangularity become more pronounced, conducting walls and/or feedback coils must be added to maintain stability. At higher elongation conditions, the conducting wall must be located closer to the plasma surface. It is estimated that, with conducting walls and active feedback, LARs may be operated at elongations of 1.5 to 2 times the natural elongation for that condition. MHD and bootstrap current analyses found stable conditions with high bootstrap fractions, but current drive was required for both on-axis and off-axis locations. Since the off-axis current drive (CD) is very inefficient, cases with high bootstrap current but requiring only on-axis CD were sought and a few cases were found. The downside was that, whereas the less efficient cases were stable to all kink modes, the more efficient bootstrap cases required difficult stability analyses for higher order kink modes. Steve suggested a more complete vertical stability study at higher elongation conditions, extension of the MHD stability study to higher elongations and more shapes, and development of an understanding of the bridging between high-bootstrap, high beta LAR plasmas and the reverse shear plasmas at higher aspect ratios. Chuck Kessel will lead the latter study.

Overview of LAR Current Drive Scoping Study Results

T.K. Mau reviewed the CD requirements, RF techniques investigated, analysis codes used, and physical system optimization parameters. His conclusions were: CD power requirements depend on bootstrap current fraction and alignment with overall current requirement and off-axis CD very inefficient; hence, both off-axis and edge bootstrap alignment are very crucial. He concluded that it is possible to sustain equilibrium with efficient on-axis CD, but off-axis CD is still required for startup and detailed profile control. The trapped particle degradation to CD efficiency for off-axis drive needs to be modeled more accurately. Chiu at GA suggested that the previous Starlite off-axis CD efficiency estimates may be pessimistic. He would also like to look at NBI and electron Bernstein wave CD techniques.

Examination of Physics Issues at Low Aspect Ratio

Stan Kaye stated the mission of NSTX is to investigate plasma confinement and particle transport in a larger LAR device, plasma stability and resilience to disruption, non-inductive plasma initiation and current sustainment, and scrape off layer and divertor physics. The NSTX is expected to exhibit a high average total beta (25-45%), have a high pressure gradient driven current factor (50-90%), and have a fully-relaxed plasma with a non- inductively driven current. This will be one of the only LAR or ST experiments that will have high power auxiliary heating of the plasma. The START experiment is the other one. Global confinement results from NSTX are hoped to bridge the gap to conventional tokamaks. High beta operation is a consequence of low aspect ratio, but ballooning modes are a limiting factor. The conducting shell is essential for a vertically-stabilized plasma. Plasma rotation is important for effective wall stabilization. For to use high harmonic fast wave, bootstrap current, CHI and NBI. They are expecting the divertor heat flux to be in the range of 8 to 16 MW/m2. If the steady-state heat flux exceeds 12 MW/m2, they will sweep the flux lines on the divertor to spread the maximum heat flux.

GA Physics Perspective on Potential of a LAR Device

Ron Stambaugh presented a series of LAR machines encompassing a physics performance machine, a pilot plant, a power plant, and a DHe3 machine. He stated that GA stability study results indicate that a LAR can be constructed and operated with high beta(n), high beta(t), 100% bootstrap current fraction, and stable to low n kinks and n=0 with a conducting shell. He would propose curve fitting his data with an expression relating beta(n) to the inverse of the square of the aspect ratio, (bn ń 1/A2). He also thinks the pressure gradient at the edge can be greater than zero and a radiative mantle can be used on the edge to minimize the heat load on the divertor. He also feels the center post should be bare, without a shield. This is because he feels the embrittlement and the radiation-induced resistance increase in the centerpost copper is not a problem and that the requirement for Class C disposal could be waived. He also believes that copper is mandatory for use in all divertors including commercial machines. The comparison of the LAR concepts will help resolve some of these differences.

Summary of Physics Tasks and Planning

T.K. Mau led off with a proposal to more thoroughly investigate CD system options, namely:
On axis:
Electron Bernstein Wave
RF Helicity Injection
NBI
DC or Coaxial Helicity Injection
Off axis:
Mode Conversion
ECH
He also proposed implementing a CURRAY model to estimate the efficiency of the HFFW system:
- Investigate sensitivity of power to current profiles
- Determine CD system efficiency with respect to aspect ratio and electron temperature
- Investigate sensitivity to startup scenarios and parameters

Chuck Kessel proposed in the area of plasma vertical stability and control to:

- Determine the natural plasma shape (elongation and triangularity) in a uniform vertical field as a function of plasma inductance, beta (poloidal), aspect ratio, and plasma current.
- Address requirements for vertical position control to produce elongation and triangularity values larger than natural values.

In the area of ideal MHD stability analysis, he proposed to:

- Examine the ideal MHD stability of plasmas in the range between 1.25 and 2.5
- Examine how RS plasmas connect to LAR plasmas
- Examine the bootstrap(BS) current fraction versus betan dependence as a function of aspect ratio to determine if there is a more optimal value
- Examine more extreme plasma shaping (e.g. elongation ~ 4) for stability and BS benefits

Ron Stambaugh proposed to:

- Provide (physics) consultation to the project
- Redo beta limit optimization study with finite edge pressure gradient
- Determine beta limit from A-1.2 to 1.8 with reduced parameter variation
- Investigate self-consistent 1-D transport simulations of unusual profile current drive requirements and confinement times
- Briefly look at divertor issues from particle control and pumping
- Assess upper limit on elongation reactive power versus elongation/natural elongation
- Assess the need for divertor coils
- Assess non-transformer startup capabilities using the PF coils only.

Steve Jardin summarized all the future physics effort as:

- UCSD (TK Mau) will investigate the divertor power balance on a 0-D basis and determine the RF startup scenario.
- GA will assess the ideal stability limit for an aspect ratio and elongation search and p'>0 at edge. GA will also do transport calculations using self consistent profiles with neoclassical ci and a radiative mantle. On the divertor, GA will determine the X-point geometry as well as the natural divertor geometry.
- PPPL will complete the natural stability study with maximum and minimum elongation values (aspect ratio and elongation search) and the physics connection to the RS regime. They will also determine the startup scenario, including non-RF solutions.

Innovative Approaches Applied to ARIES Systems Analysis

With the reduced LANL funding level, Ron Miller will be responsible for all the systems analysis and code modeling efforts. He noted the continuing trend within fission plants to increase the capacity factor so that a majority of US plants have a 3-year average above 70%, with many plants above 85%. Ron elucidated some new concepts and techniques in the field of Systems Analysis that may bring new analysis tools to the fusion systems studies. Ron discussed the Response Surface Methodology and how that approach of a search over a multi-dimensional parameter space might lead to better results. This is opposed to our historical approach of a local parameter search about a discrete design point (strawman). The difficulty lies in the need to fully define and parameterize the engineering and physics database for all systems so that they work and interact properly and seamlessly over the entire multi-dimensional parameter space being surveyed. It was judged we do not have sufficient resources, manpower, and time to accomplish the fully parametric approach.

Wrap-up

Mark Tillack summarized the ARIES-RS Final Report Chapter status, which is mainly in the draft stage and being reviewed by chapter co-authors and senior editors. Delinquent chapters are the blanket, divertor, and the magnet systems. Completion of final drafts are expected in January, 1997. It was decided to have four meetings per year, with the next one to coincide with the US/Japanese Reactor Studies Workshop @ UCSD near the end of March 1997. The second may be in June, perhaps at UWisc. Also a meeting may be coordinated with the 17th IEEE/NPSS meeting to be held in San Diego 6-10 October 1996.

Agenda

ARIES Low-A Kickoff Meeting
Princeton Plasma Physics Laboratory

Monday Dec. 9, B318 PPPL

AM ------------------

8:30   coffee and donuts

9:00 - S. Jardin  -    Discussion of Agenda, Logistics, Status of Reports

9:15 - F. Najmabadi   Opening remarks, project directions, meeting objectives

9:30 - Bill Dove      OFES perspectives 

9:45 - Martin Peng    Status of worldwide activities on LAR tokamaks

10:30 -               break

10:45 - Masa Ono      Overview of the NSTX experiment

11:30 - Rob Socolow    The outlook for new energy technologies in the 21st century
	(Princeton University, Center for Energy & Environmental Studies)
         [Note:  This talk has a Required Reading prerequisite 
         Chapter 1 of the PCAST report, by Socolow and Holdren]

PM ------------------

2:00 - Mark Tillack
Review of ARIES engineering assessment of the LAR tokamak

2:30 - J. Spitzer
NSTX engineering issues

3:00 - Dai Kai Sze
Innovative blanket concept He/(FS+SiC)/Li2O/Be
 
3:15 - D. Steiner
Applications of IBC to spherical tokamaks

3:30-5:30
Discussion of engineering issues and tasks

7:00 Dinner at Good Time Charley’s (Route 27 in Kingston)

     Tuesday Dec. 10  B318 PPPL

AM ------------------

8:30  

9:00 - S.Jardin, T.K. Mau
       Review of ARIES physics assessment of the LAR tokamak

9:30 - Stan Kaye
       Princeton plasma physics activities supporting the LAR tokamak

10:00 - Ron Stambaugh
       Edge physics and other issues for the LAR tokamak

10:30 -  Discussion of physics issues and tasks

      T.K. Mau ... comments on current drive options
      C. Kessel ... comments on stability studies

12:00 - Other Topics:

R. Miller.. "Innovations in Systems Analysis"

PM ------------------

2:00 - 3:00
Discussion of scope and direction of ARIES project
   LAR tokamak design and analysis
   Non-electric applications
   Critical issues and design innovations

3:00 - 4:00
Discussion of FY97 work plan

4:00 Adjourn