4/15/99

For some time people have thought of liquid walls as an attractive solution to the technology problems of high power density plasma configurations for MFE, and as (nearly) essential for the pulsed wall loading conditions in IFE. A flowing, renewable surface could be eroded, evaporated and even be broken apart with no permanent adverse affects on a structure requiring frequent maintenance and replacement. Alpha particle energy could be removed without conduction through a solid wall and the associated thermal stress and creep failure modes, and the energy could be extracted at higher temperatures for efficient energy conversion. If a liquid wall of sufficient depth could be formed, radiation damage and waste disposal issues for solid structures could be significantly ameliorated or completely eliminated.

All these benefits are possible, if only liquid walls could be made to work!

The most obvious issue with liquid walls in MFE, assuming that they can be formed without splash, is that the vacuum required for current successful plasma experiments will be compromised by the relatively high evaporation rate of the hot liquid in the plasma chamber. This concern has led to the formulation of the idea that there is a temperature limit, corresponding to an acceptable evaporation flux, above which liquid walls will kill the operation of the plasma. This temperature limit will be inextricably linked to the ability of the plasma edge to screen neutral atoms and molecules before they enter the core plasma, accounting for the fact that the plasma edge behavior itself will likely be influenced by this large neutral flux.

For IFE, a similar issue is the ability to clear the chamber of vaporized and/or spalled liquid wall material so that targets can be injected and the driver beams can propagate to the targets through the residual vapor and debris at a pulse rate of several shots per second.

Other issues have sprung up as the analysis of liquid walls has advanced slowly over the years, and now more rapidly as part of the APEX project. For MFE these issues include:

• Conflicting need for low surface temperature for plasma compatibility and high bulk outlet temperature for efficient energy conversion.

• High mass-flow rates needed to form thick liquid walls leads to low bulk temperature rise, and requires large pumping power, significant pumping equipment, and large piping (especially for gravity drainage from the vacuum chamber).

• Electromagnetic fluctuations and currents from the plasma can couple to liquid metal walls and exert a significant influence on its flow behavior.

• The liquid walls may act as particle pumps, especially in the limiter/divertor region, which change the effective recycling behavior and plasma-operating regime.

• Effect on other essential reactor systems like vacuum pumping, tritium recovery, plasma fueling, RF antennae, diagnostic and neutral beam penetrations, may require radically redesigned systems.

For IFE the above issues can be rephrased as:

• Conflicting need for low surface temperature for driver beam propagation to the target, and cryo-target compatibility and high bulk outlet temperature for efficient energy conversion.

• High mass-flow rates needed to form thick liquid walls leads to low bulk temperature rise, and require large pumping power, significant pumping equipment, and large piping (especially for gravity drainage from the vacuum chamber).

• Effect of liquid and vapor on other essential reactor systems like vacuum pumping, tritium recovery, final optics, diagnostic and driver beam penetrations, may require radically redesigned systems.

The subtopic questions posed to the Fusion Community are designed to try to extract from people of various physics and technology backgrounds their views of the precise issues facing liquid walls, and the associated modeling and experiments needed to establish the feasibility and attractiveness of liquid wall concepts as a new paradigm for fusion reactor design.

Mohamed Sawan (UWM), Ed Lee (LBNL), Steve Payne (LLNL), Rich Mattas and Dai-Kai Sze (ANL), Dale Meade or Dick Majeski (PPPL), Mike Ulrickson (SNL), Per Peterson (UCB), Tom Rognlien (LLNL)

1. Do liquid walls really have the potential to yield a more attractive fusion energy product? What is research and development path required to address feasibility and, subsequently, engineering design issues in a timely, economically realistic manner? What is the real impact on other reactor technology systems? (Moir, Sawan)

 

2. What modeling and experiments are required to establish the hydrodynamic feasibility of various thick liquid wall configurations for MFE and IFE? (Morley, Peterson)

 

3. What plasma modeling and experiments are required to determine the criteria of compatibility of liquid walls with acceptable tokamak or emerging concepts plasma operation (e.g. allowable surface temperature?) Will plasma operation with liquid walls be fundamentally different than with dry walls? Does it make sense to have a liquid divertor only, with solid first walls or solid divertor with liquid walls? (Meade or Majeski, Ulickson, Rognlien)

 

4. What modeling and experiments are needed to determine the real limits on residual amounts of vaporized wall material in IFE reactors? Are there driver propagation, focusing modes, and final optics more compatible with liquid walls? Will residual liquid vapor and droplets affect target and driver propagation? (Lee, Payne)

 

5. Is there a clearly superior choice of working liquid? Is Flibe a feasible liquid based on plasma contamination (MFE), molecular recombination and condensation (IFE), tritium breeding, and structural material compatibility? Is lithium vapor pressure simply too high to make an attractive liquid wall? Will MHD effects and interaction with the plasma exclude either Flibe or liquid metals as viable working liquids? How important are activation and chemical reactivity properties in affecting materials compatibility, waste disposal, and accident response? (Mattas, Sze)

A draft of the report will be done in advance of the meeting so that it can be distributed to all participants signed up technology Q1. The report will be modified following the Snowmass discussions to reflect the community view of each subtopic. The report outline follows very closely the subtopic outline. The chairmen will contribute to all sections, collect contributions from the authors, and serve as editors.

The primary authors are encouraged to discuss the subject among and gain the assistance of others from their respective organizations. Primary authors should also assemble a list of key advances in Engineering Science for the R&D associated with their subtopic. This list will be forwarded to the organizers of technology cross-cutting questions.

1. Introduction - Attractiveness and potential of Liquid Walls: R. Moir, M. Sawan

 

2. Hydrodynamic feasibility of various thick liquid wall configurations for MFE and IFE: N. Morley, P. Peterson

 

3. Compatibility of liquid walls with plasma operation: D. Meade, M. Ulrickson, T. Rognlien

 

4. Limits on residual amounts of vaporized wall material in IFE reactors: E. Lee, S. Payne

 

5. Choice of working liquid: R. Mattas, D-K Sze

 

6. Summary of key issues and required modeling and experiments: R. Moir, N. Morley

In order to distribute a draft in advance of the Snowmass meeting, the following schedule is proposed.

Two two-hour discussion periods during the first week of Snowmass are anticipated for the subtopics. Moir and Morley will act as chairmen for these two discussion sessions.

Before the one-hour session in the second week, the core working-group will modify will preliminary report to reflect the conclusion of the discussion sessions. This will be presented during the one-hour session in the second week. Following the summer study, the final report will be prepared based on the discussion at Snowmass.