ARIES Documents -- Meetings ArchiveARIES E*Meeting Conference Call, 17,24 October 2001
Documented by L. Waganer
Agenda: Agenda & Presentation Links
This two-day electronic meeting replaced the September 2001 meeting that was cancelled due to travel restrictions after the World Trade Center disaster. The links to the conference call electronic presentations can be found in the agenda on the ARIES web site shown above.
Bill Dove told the group the fusion budget remains unchanged with a continuing resolution still in place for at least another week. The travel budget continues to be restricted. Bill suggested the team consider proposing a concept or commenting on the technical direction for a burning plasma experiment at the upcoming Snowmass meeting. He said DOE is trying to build a fusion community consensus for the next fusion experiment. Farrokh Najmabadi said that Bill should point out that the ARIES team has provided and will continue to provide technical evidence to help direct and stimulate the developmental pathway for fusion - both for MFE and IFE.
Farrokh discussed the ARIES plans for next year with no budget increase anticipated. The ARIES team will continue to address both IFE and MFE concepts, stressing completion and documentation of the current IFE project. Specific areas, topics, and level of effort have not been finalized yet due to the lack of a firm budget. In FY03 it is anticipated that there might be an assessment, trade studies, and detailed point design of a compact stellarator concept by the ARIES team. Concurrently, there might be a smaller IFE assessment effort.
Systems Analysis and Integration
ARIES IFE Systems - Ron Miller discussed his involvement in the Consistent Design Point Evaluation that will be covered later by RenČ Raffray in his presentation. Ron reviewed the driver and target-scaling algorithms contained in the ARIES System Code. A few parametric examples of these algorithms were shown to illustrate how critical target or driver performance parameters affect the other key plant performance parameters. Ron pointed out other crucial parameters were fixed at nominal values in these parametric scans. Currently, these scans have not been extended to the COE values (as would have been done in the case of a point design.) Ron indicated the repetition rate was generally an independent variable that could be varied to achieve desired plant power levels, however there are limiting criteria that would constrain the overall parameter space. Ron also noted that recirculating power fraction is rather independent of repetition rate.
Heavy Ion Fusion Modeling Update - Wayne Meier reviewed the information he presented in the HIF-IFE meeting held at LLNL in July. Wayne described the starting point for a 1000 MW HIF power plant design as based upon the HYLIFE-II chamber and a hybrid target design. [Note that Ron was using a Close-Coupled Target in the ASC analysis.] The hybrid target allowed larger (5 mm) spot sizes. The target gain scaling from the Lasnex code calculated the yield at 6.7 MJ. Target gain was assumed to scale similarly to the distributed radiator targets. Wayne used his IBEAM driver cost and efficiency scaling code. Driver efficiency was set at 45% and the overall thermal cycle efficiency at 45%. Preliminary COE estimates (for a 1000 MWe plant) minimized around 5 to 6 MJ. Wayne defined some Xe beam requirements to achieve a 5.5 MJ target. He also discussed the key issues to be addressed to obtain the required small spot size, including chromatic aberrations and beam emittance. These data are preliminary as the driver code is being updated to analyze the distributed radiator targets. A new point design may be used as a basis for new VNL and VLT efforts.
Target Fabrication, Injection and Tracking
In-Chamber Target Tracking and Other Target Injection Considerations - Ron Petzoldt presented several viewgraphs to affirm the need for and methods to achieve in-chamber direct drive target tracking. Ron showed that significant target drag occurs even at low chamber gas densities thus inducing displacements along the flight path. These displacements can be predicted if the pressures are accurately determined. Several tracking techniques were discussed. Ron showed that high-energy molecules do not stick to the targets and would not contribute to altering the target optical properties. The convective heat flux on target leading surface is about 3 times higher than target average values, but the temperature rise on the leading edge does not reach the center of the fuel.
Status of IFE Target Fabrication - Dan Goodin presented portions of his recent talk, ěProgress Toward Demonstrating IFE Target Fabrication and Injectionî, D. Goodin, et al. IFSA-2001. Dan summarized the critical issues and fabrication processes for the direct and indirect drive targets. Micro encapsulation is being extensively studied as a method to improve the quality and lower the cost of target production for larger quantity production. A successful method for applying a high-Z (gold) layer has been developed. Coating processes are being quantified. Alternative coatings are being considered along with less stringent target requirements.
Dan discussed possible equipment to layer targets while inside hohlraums. Target systems are also being developed for a commercial-scale Z-pinch plant.
Update on IFE Safety Work for ARIES - Lee Cadwallader, Theron Marshall, and Phil Sharpe presented David Pettiís viewgraphs on the IFE Safety Work. Target fabrication safety and reliability, risk assessment (including accident-initiating events), coolant hazards, and aerosol consideration for chamber clearing are being studied and evaluated. Lee summarized the highlights from the Micron Technology visit to determine manufacturing similarities and quality assurance techniques of chips and targets.
Accident initiating events are being compiled using a Master Logic Diagram (MLD) for the IFE SOMBRERO design. The same MLD process has begun for the HYLIFE II design.
Theron Marshall told the team that work is commencing on candidate coolant hazards that are based upon chemical toxicity and chemical reactivity in air and with other materials, such as concrete and water. A table of Temporary Emergency Exposure Levels (TEEL) for aerosol inhalation of chemical substances was compiled for candidate IFE coolants. Radiological concerns were compared in five areas:
Phil Sharpe discussed the issues of aerosol generation and transport within the IFE chamber. Using the aerosol general dynamic equation, he analyzed both lead and tungsten vapor in the chamber for a range of saturation pressures. He concluded that dust particle concentrations generated from homogeneous nucleation are sufficiently large so that particle growth and coagulation for both Pb and W occur faster than chamber clearing times.
Safety Assessment for Potential Target Materials: Radioactivity vs. Chemical Toxicity - Susana Reyes presented radioactivity and chemical toxicity safety assessments for potential target materials, namely the high-Z materials of lead and mercury as opposed to the more traditional gold-gadolinium. Previous safety work indicated that both lead and mercury meet the criteria for contact dose rate, waste disposal rating and accident dose. LLNL has re-evaluated the accident dose considering conservative weather conditions. To meet the 1- rem goal, the release of Hg must be limited to 4.2 kg and Pb to 20 kg. Chemical toxicity issues have yet to be addressed. The TEEL-2 criteria level was adopted as the basis for comparison. Using this criteria, release rates of 56 mg/s was the limiting case for Hg and 28 mg/s for Pb. Assuming a circular pool of liquid, the radius would be 13 m or 440 km for Hg and Pb, respectively.
Susana concluded that activation results show that both Hg and Pb are acceptable when analyzing the contact dose rates and waste disposal rating. For radioactivity releases from accidents at the target fabrication facility, Hg is the most hazardous material. But, segregation of the Hg-inventory in the plant would make the 1-rem limit goal achievable. In case of accidents involving the power plant primary coolant loop, Pb seems to pose a greater radiological hazard due to its higher inventory suspended in the Flibe coolant flow. The chemical assessment did not indicate a strong preference. The high volatility of Hg suggests it a more hazardous material option because of the high saturation concentration at normal temperatures. Overall, the chemical toxicity is the key issue from the Safety and Environmental point of view.
Chamber Physics and Chamber Clearing
Dry Wall Response to HIB (Close Coupled) Target: Refinement of T-Xe and R-Xe Operating Windows - Don Haynes presented a summary of his analysis of the heavy ion beam (HIB) target being used with a SOMBRERO gas-protected, graphite-lined chamber. Further BUCKY calculations have refined the operating windows of (1) xenon vs. equilibrium temperature for a fixed 6.5 m radius and (2) radius vs. xenon density for a fixed 1000C equilibrium temperature. Don noted the minimum Xe density varies greatly between the chamber radii of 3 and 6.5 m. Although the yield for the SOMBRERO DD target and the HIB target are similar, over 25% of the HIB target yield is in x-rays as compared to 5% for the SOMBRERO DD and 1% for the NRL Au-coated targets. The HIB target x-ray spectra lies between the SOMBRERO and the NRL targets, which indicates a buffer gas has to be more dense than SOMBRERO to protect the graphite wall at 6.5 m. Combinations of buffer gas densities and wall surface temperatures were shown to protect the wall from vaporization (< one monolayer loss/shot) for all three target types. Wall temperatures in excess of 1000C could be achieved in the 6.5 m radius chamber if appropriate buffer gas density is used.
When the wall temperature was held at 1000C and the radius reduced, the required xenon density remained less than a torr down to a radius of 4.5 m. However, at a 3-m chamber radius, the required pressure increased to 23 torr. At the smaller radii, the chamber gas remains very hot and continues to heat the wall throughout most of the cycle. Don postulated that the equilibrium temperatures would stabilize at or below 1000C. He said at these elevated temperatures, there is a need to analyze the time between short duration radiation effects and the introduction of the next target. It was suggested that Don do additional analysis to better define the chamber conditions at radii between 3.0 m and 4.5 m.
Liquid Wall Chamber Dynamics - Bob Peterson described the time sequence of how the prompt x-rays create vapor from the liquid wall. After vapor formation, it protects the underlying liquid and wall structure from debris ions. At much longer time scales, the vapor recondenses on the wall and equilibrium conditions return, thus allowing another cycle to commence. Bob showed the simulation codes, inputs and outputs relating to the target output, liquid dynamics, and chamber recovery. Radiation transport and hydrodynamics are crucial to evaluating the chamber gas performance. Spectra and energy partitioning of the different targets are important to the vaporization of the liquid walls. Chamber clearing dominates the repetition rate for low chamber gas density, wetted wall chamber concepts. Another effect to consider is the inertial recoil from the rapid vaporization resulting in a large impulse to the bulk liquid and substructure (~100 Pa-s).
Propagation of HI Beams in Chamber Metal Vapor Atmosphere - Craig Olson summarized two ballistic and two pinch transport approaches for the dry-wall, wetted-wall, and thick liquid-wall chamber concepts and discussed their pros, cons, and features in each of the chambers. Craig explained that several of these combinations of beam transport and chamber concepts have been documented in prior conceptual power plant studies.
Vacuum ballistic is not being considered presently, as this approach requires many beams or very low pressures around 0.1 mtorr.
Neutralized ballistic uses a converging charged beam that is neutralized just before the chamber wall or inside the chamber with a preformed plasma, neutralizing gas cell, or photoionization of the chamber vapor. Photoionization of the plasma permits a good beam spot size at a chamber radius of 2.65 m. The neutralized ballistic transport distance across the chamber is limited to around 3 meters.
Assisted pinch transport allows smaller chamber entrance holes, eases focusing requirements, and reduces accelerator costs. Additional hardware of a laser and z-discharge equipment is required to form an ionized channel to the target. The issues of a suitable insulator at the chamber entrance and the stability of the beams are key to the success of this approach. Simulations are indicating a negligible growth of the hose instability. Transport across chamber with assisted pinch can exceed the distances associated with neutralized ballistic transport.
The self-pinch transport approach uses no hardware to form the channel to direct the beams to the target. The beams are focused outside the chamber and are highly stripped to induce the self-pinch effect. Issues include beam front erosion, aiming/tracking, multiple beam effects, and beam/plasma stability. The self-pinch is compatible with an intermediate chamber pressure.
Craig noted that there are no showstoppers for any of the transport schemes. The neutralized ballistic transport has the most mature theory and photo-ionization greatly improves the transport up to 3 m. Assisted pinch is sensitive to gas conductivity, but modeling predicts sufficient decay length to suppress deleterious self-pinch effects. Self-pinch transport has an attractive chamber pressure regime between 10-150 mtorr xenon, but efficiency of transport and 3-D stability issues need to be resolved.
Chamber Wall Engineering
Design Considerations for Beam Port Insulator Rings - Simon Yu discussed the engineering considerations for the channel assisted pinch transport. The beam dynamics must be controlled on a 10s of nanosecond timescale and the channel formation on a 10s of microseconds timescale. The channel must have breakdown without any breakdown to the chamber walls. Beam port insulator rings are designed to inhibit breakdown to chamber walls. The beam channel first must be made slightly conducting with a short (10 ns) laser pulse, followed by a longer electrical prepulse that deposits energy through Joule heating and subsequent hydrodynamic expansion to form the deep density channel, and finally a high conductivity axial current is imposed on the channel to guide the beam to the target. Simon discussed several options for the design of the insulating ring at the port, limits on the depth of the channel, and the anticipated energy loss in the reduced density channel. These issues are key to determining the propagation window for the assisted pinch mode. Near term investigations are concentrating on the channel hydrodynamics and the properties and design of the port insulator. Simon thought the assisted pinch approach is compatible with the dry wall as well as the wetted wall chamber concept.
Remaining Action Items on Dry Chamber Wall - RenČ Raffray presented the results of his more accurate analysis to determine the spatial temperature profiles within the dry wall based upon temporal energy deposition due to arrival of photons and ions at the first wall. The more accurate results were similar to prior more simplified analyses, thus the prior results were affirmed.
Assessment of the Overlap Points - RenČ Raffray presented his results to self-consistently analyze the driver, chamber, and power plant parameters to determine attractive design and operating regimes for both direct and indirect drive targets. RenČ assumed a tungsten-coated SiC first wall. He presented common parametric studies that traded repetition rate (and net power out) to maintain a constant outlet coolant temperature (and cycle efficiency) as well as parametric studies that traded chamber radius and coolant temperature at a given repetition rate. Although there was consistency in the analysis, there was no metric (e.g., COE) to determine the optimum regime. Since there is no baseline design, establishing the data to define the metric cannot be done.
Scoping Analysis of Chamber Recovery Time for Sacrificial Wall Concept - RenČ Raffray explained his initial effort on a scoping analysis to determine the time scale of the recondensation of the chamber vapor to properly establish equilibrium conditions for the next shot. The UCLA/UCSD RECON code, initially developed by M. Tillack for the Prometheus study, was used to model the thermodynamics of the vapor, liquid layer, and solid wall structure when heated by the x-rays and ions. The present version of the RECON code was recalibrated against a documented Prometheus data set. The results with the current code indicated the Pb vapor temperature data were identical and the vapor pressures were similar but higher by a factor of 5 or so. Using RECON on the current sacrificial chamber approach showed the temperature of the background gas at different pressures (1 mtorr and 10 mtorr) did not affect the Pb vapor temperature history, but the background gas pressure did have a 10-fold effect on the pressure for the time history. Two coolant temperatures, 100 K apart, induced a vapor pressure difference of 3-5 fold. The effect of increased wall conduction and convection altered the shape of the pressure response curves, but pressures were similar until greater than 0.10 sec, after which the results significantly differed.
Design Considerations for Thin Liquid Film Wall Protection Systems - Said Abdel-Khalik described his approach to assess the feasibility of attached thin liquid film protection schemes along with identifying optimum design parameters and design windows for these systems. The HEIGHTS-IFE/SPLASH-IFE code developed by A. Hassanein of ANL is being used to evaluate the multi-layer structure of liquid film and solid wall, the liquid stability, and macroscopic erosion and splashing of the liquid layer. The preliminary analysis determined the minimum film thickness and wetting rate (average film velocity) for a stable dry patch (not a desirable attribute). For the fluids of interest (lithium, lithium-lead, and Flibe), a film thickness of ~1 mm and an average film velocity of ~1 m/s exceed the values required for a stable dry patch in isothermal films on vertical adiabatic surfaces. Higher values of either parameter would ensure the surface would remain covered by a continuous thin liquid film. Saidís next investigation will involve hydrodynamics of thin liquid films and response of liquid film/solid wall systems to microexplosions.
Chamber Aerosol Cleaning - Leslie Bromberg was unable to attend the conference call. This material will be presented at some future date.
Chamber and Final Optics Nuclear Analysis
Target Recycling: Problem Definition and Preliminary Analysis for ARIES-IFE/HIB - Laila El-Guebaly posed the questions to be answered regarding target recycling; identify how much waste is generated by the targets and should any candidate target material be recommended for exclusion? To answer these questions, she estimated target inventory as compared to nuclear island waste inventory, determined key elements for recycling, and developed solutions for potential recycling problems. To understand the magnitude of the issue, each year there will be from 10 to 20 tonnes of activated hohlraum materials stored in repositories or recycled. She presented a list of heavy metals being considered as target materials. She concluded hohlraum wall materials constitute only 0.6% of the cumulative volume and less than 4 % of the cumulative (waste) mass. Laila described a typical recycling process for these materials. Recycling should only be considered if recycling is imposed as a top-level program requirement. She also presented several reasons why recycling target materials could be detrimental. Laila then discussed the activation consequences of the temporal and energy distribution on the waste products.
Shock Interface and Shock Structure Interactions - Mark Anderson, Fusion Technology Institute, UW Madison, prefaced his talk by describing that many IFE target chambers may experience significant hydrodynamic motion in the chamber. Both the dry wall, gas-protected chambers and liquid-protected chambers are expected to exhibit this effect. The University of Wisconsin has a Shock Tube Laboratory with experimental shock tubes that can model this phenomenon. Mark discussed several types of instabilities that might occur in the chamber. The first model he analyzed was a set of sequential results for a set of cooling tubes with a shock wave propagating perpendicular to the tubes. A Richtmyer-Meshkov instability was observed in the cooling tube interactive shocks. Future work will involve a shock driven into a film supported water sheet above a single cylinder test section.
Assessment of Off-Normal Events
Update of Off-Normal Event Assessment - Don Steiner noted that the Off-Normal Assessment is awaiting inputs from other team members. Earlier in the call, Lee Cadwallader reported on the visit of an ARIES group to Micron Technology to gather information on abnormal events - cause and consequence. He is also awaiting data from the chamber engineering group on the consequence of abnormal target shots on the chamber walls.
HI Magnet Engineering
Design Analysis and Layout of HI Focusing Magnets - Leslie Bromberg was unable to attend the conference call. This material will be presented at some future date.