ARIES Documents -- Meetings ArchiveARIES Project Meeting, March 8-9, 2001
Documented by L. Waganer
Host Welcome - Wayne Meier welcomed the ARIES team to Livermore. The off-site location also served as the Tritium Town Meeting immediately preceding the ARIES town meeting.
ARIES Budget and Plans - Farrokh Najmabadi told the group the DOE budgets are continuing to be adjusted with no final values available at the present date. Next years IFE work will probably be funded at roughly half the anticipated level. Thus the planned effort will be not be finished by September 2001 and may extend another year. An evaluation of a compact Stellerator might be a candidate ARIES project as it being considered for a proof-of-principle experiment. The near term project emphasis is to complete the documentation of the dry wall assessment and to begin assessment of the thin, sacrificial liquid wall concepts. Mike Roberts suggested the ARIES team think about a new name that better describes our work scope.
Peer Review - Farrokh Najmabadi noted the recent ARIES Peer Review give the team high marks for the scope and high quality technical work, but they recommended a few. Farrokh has drafted a response being circulated for comment.
Next Meeting/Conference Call - The next meeting was selected to be at San Diego. Several dates were considered with feedback on the candidates. Farrokh selected June 7-8 as the next date. The next conference call date was not discussed.
Summary of ARIES-IFE Tritium Town Meeting
Mark Tillack summarized the ARIES-IFE Tritium Town Meeting held the previous two days. The main purpose of the meeting was to highlight the common tritium aspects between both MFE and IFE power plants, yet understand the basic differences inherent in the two approaches. Both concepts need to achieve tritium self-sufficiency with similar worker and public safety goals. Tritium burn-up fractions and inventories are quite different. Sessions were organized in Fuel Cycle, PFC and Blanket, Safety, and IFE-Specific Issues. Mike Gouge described MFE fueling innovations such as high field side pellet launch, pellet-triggered L-H mode, and disruption mitigation. Continuously regenerating cryopumps are now being used. Vacuum pumping systems can operate with pressures in the 0.1 to 1 Pa range to minimize duct sizes.
Rene Raffray summarized PFC and blanket experience from MFE. Off-normal conditions, such as VDE and disruption, are not relevant to IFE. The issues associated with carbon co-deposition and macroscopic erosion require more examination. Tritium breeding is easier to achieve in IFE. He feels a tritium blanket inventory of less than 200 grams is achievable, but a permeation rate of less than 1-10 Ci/d is very challenging based upon a very low partial pressure.
David Petti said the achievement of a routine public exposure of < 10 mrem/y at the site boundary would translate into a tritium release of < 2-3 Ci/d. To achieve these levels, the tritium permeation through the primary heat exchanger must be more highly controlled. On occupational safety, contamination control is a strong design driver. The accident safety limit is being driven by the tritium release of < 15 g T at HTO and conservative-basis weather conditions. Penetrations are key problem for radiological confinement.
Ken Schultz highlighted the IFE specific findings and recommendations. The blankets are similar except there is more interest in liquids. Tritium processing is similar except for target related issues. The chamber is more sensitive to co-deposition of tritium in carbon surfaces. The extended vacuum boundary presents more difficulty in terms of tritium inventory. The major difference is the potentially large tritium inventory in the target factory. IFE has the potential for tritium-lean targets.
Rich Mattas led a roundtable discussion on H (D/T) and He in liquid surfaces. Much discussion was concentrated on general feasibility, mixed material effects, effects of charge exchange flux, and T-enhanced sputtering. H (T) retention is important for lithium. Other materials have low tritium retention, but helium pumping may be a key issue. The summary report will compare MFE and IFE first wall materials and major issues.
Summary HI Beam Transport Workshop Retreat
Craig Olson summarized the results of a recent HIB transport workshop, which ìfocusedî on non-ballistic forms, i.e., channel transport of beams across the chamber. The most important issues are self-pinch transport, z-discharge channel transport, pinch-like lenses, and pinch effect in neutralized ballistic transport. The beam size should be on the order of 1 mm for a normal target versus 4-6 mm for a hybrid target, with 3.3 MJ versus 6.7 MJ respectively.
Self-pinch transport is dependent upon the initial conditions and forming mechanism into the chamber, e.g., trumpet, beam into the plasma, or preformed pinch. Erosion of the trumpet is a concern.
Well-defined, preformed channels can be formed with a low power laser, but a laser beam is necessary for each channel plus an additional channel for the return current. Also a prepulse can be used to form the channel. A z-pinch can also be used to preform a channel in the chamber.
ARIES-IFE Study Presentations
Systems Analysis and Integration
Systems Results - Ron Miller presented polynomial fits to the NRL target/gain data obtained from Bodner, et al at the June 2000 Madrid conference and the close-coupled Callahan-Miller and Tabak HIF target/gain data. These target gain fits will be input into the IFE systems code to conduct parametric trade studies. Ron also represented the IFE power flow diagram using the Definitive Scenario in terms of an influence diagram. The free variables mentioned included beam spot size, beam energy, ion, ion charge state, choice of target chamber, repetition rate, and ion range. The intent is to understand the influence of each of these free variables on the output dependent variables that determine the attractiveness of the overall plant. It was suggested the more traditional power flow diagrams be included to help visualize the process.
Heavy Ion Driver Model Update - Wayne Meier explained his modifications to the heavy ion driver systems code. He has been adding more design parameters that can be varied in a single run to allow more rapid parameter scans, such as total driver energy, ion mass, final ion energy, minimum pulse duration, and final pulse duration on target after drift compression. The capability to calculate both near-term (e.g., IRE) and long-term (power plant driver) cases has been added. HI driver costs were graphically shown for A = 85 and 133 amu and full-size/close-coupled targets. Wayne also showed likely heavy ion target operating parameter space (gain vs. energy) considering close-coupled and full-size targets, a range of yields, and range of beam spot sizes. He examined a few cases to determine a reasonable driver parameter set to yield a small, 2-mm spot size. Coupling these data into his driver model yielded a solution that suggested 100 beams or so with an initial pulse duration on the order of 15 ms. Data on emittance, neutralization, final focus length, and focusing half angle were determined for a given spot size. The next steps are to improve spot size model, simplified driver cost and efficiency scaling, and combine all submodels into an integrated performance and cost model.
Direct Drive Gain Update - Wayne Meier showed a new, better curve fit for the direct drive, laser (1/4 mm) target with a low a, zooming beam. He provided the data to Ron Miller for the systems code. Wayne observed the new gain curve is relatively flat beyond 1.5 MJ. This would suggest the COE would optimize at lower beam energy cases with lower yield and higher repetition rates to achieve the necessary power level. Typical assumptions with this curve might yield E = 1.5 MJ, Y = 204 MJ, RR = 13.6 Hz for 1 GWe. Repetition rates this high may not be feasible with target and chamber constraints.
Chamber Wall Engineering
Assessment of Carbon and Tungsten Dry Wall Chamber Walls Under IFE Energy Depositions - Rene Raffray is wrapping up the dry wall chamber wall material assessment. Lifetime of the wall material is a key issue. Tungsten and carbon are the most viable materials to accommodate the energy deposition and provide required lifetime without any protective gas in chamber. Rene has quantified the X-ray, debris ion and fast burn ion spectra for the NRL direct drive target. The wall geometry and material composition are modeled to determine temperature and sublimination responses. Thermal and sublimination loss results for both C and W were given for a range of coolant temperatures for the baseline chamber and wall geometry. For carbon, sublimination loss is minimal over the range of coolant temperatures unless energy deposition is increased by a factor of 2-3. A tungsten wall has a much shallower photon energy deposition than a carbon wall, but ion energy deposition is somewhat deeper. Tungsten will not sublime, but the wall must remain below melting point of 3410C. In the example case, the tungsten peak wall temperature was around 1438C, but chamber/wall geometry and target are determining factors. Both C and W offered encouraging results with comparable thermal and sublimination results.
Rene showed results of a more detailed analysis of the carbon fibrous wall (free-standing fibers embedded in a cooled carbon fiber infiltrated substrate.) The fibrous wall (at a shallow incidence angle) did exhibit a lower maximum temperature than a flat carbon plate. The disadvantage is the significantly higher surface area is a greater safety concern for oxidation and trapping of tritium.
Rene discussed the importance of the ion energy deposition parameter on the dry wall material lifetime results relating to sputtering. In addition, other mass transfer mechanisms (physical sputtering, chemical sputtering, and radiation enhanced sublimination) will influence the wall erosion lifetime performance. These mechanisms are dependent upon the characteristics of the target and wall parameters, thus more investigation is required in these areas.
Rene offered cautious optimism that a dry wall chamber with a carbon or tungsten surface would be viable option without a protective gas within a reasonable design window of target and chamber parameters. However, he mentioned that additional work is needed to address many remaining unknowns.
Parametric Results for Gas-Filled Chamber Dynamics Analysis - Bob Peterson showed a flow chart of variables and processes that influence and determine first wall erosion and target heating for a dry wall concept. He has completed most of the action items from the last ARIES meeting: knock-on ions with finer time resolution, a useful form for ion density/flux, wall load as a function of time, wall temperature vs. depth, assessment of W and SiC armor, scaleup of NRL target yield, and revisit of zero pressure case for NRL target. He explained the advantage of the BUCKY code is its ability to model both radiation and hydrodynamics of the target output and chamber dynamics when filled with an attenuating gas.
Bob compared the physical and performance parameters for the Sombrero, NRL radiation tailored - wetted foam, and the NRL wetted foam direct drive, laser targets. Concentrating on the NRL target, Bob used BUCKY to calculate the implosion/burn/explosion geometry and compare the results to the NRL calculations. There are only slight differences in the yield output from the two sources. Bob is trying to import the NRL laser deposition model into the BUCKY code to yield better agreement of the results. He believes the spectra are dependent upon the yield, but for the moment, the team is assuming the spectra are independent of yield. Bob has also used John Perkinsí target output results for the NRL target.
Migrating ITER Tritium and PFC Materials Experience to IFE - Gianfranco Federici summarized his ITER experience that might apply to IFE materials and tritium interaction. He stressed the need to have a robust interface between the plasma and the in-vessel hardware components with little or no erosion. Carbon was not a favorite choice for ITER due to tritium retention. Disruptions were significant design drivers in ITER plasma facing materials. He was cautionary about co-deposition of tritium in areas shielded from the plasma, such as beam tubes in IFE.
Chamber Engineering Wrap-up - Rene Raffray summarized that erosion for dry wall materials is a significant engineering issue. Spectra from target calculations are temporially distributed to analyze the erosion of the wall material and the thermal response throughout the wall component. Other erosion mechanisms are being investigated for this application. A majority of the work used the NRL target, but the HI target is planned for inclusion. He is planning to complete a draft write-up of analysis approach, input data, and results.
Findings and Recommendations from Wetted-Wall Designs - Jeff Latkowski reviewed the design features and advantages of wetted-wall designs. A liquid coolant fed through nozzles or through porous walls absorbs the surface heat load and presents a renewable wall that protects the underlying blanket structure. The size of the chamber is expected to be smaller than a dry wall chamber, hence is likely to be less costly with a longer wall and blanket lifetime. Osiris had a flexible carbon cloth with oozing flibe coolant and a liquid metal floor covered with coolant spray. Flibe was the primary coolant. Prometheus had a more rigid, but porous SiC first wall that was renewed with liquid lead. The primary coolant was helium at a higher temperature than the first wall lead coolant, thus higher thermal conversion efficiencies were possible. The HIBALL-II chamber was cooled with LiPb oozing through woven SiC tubes hung vertically. KOYO uses a porous SiC structure with oozing LiPb coolant. Most of the chamber designs were cylindrical with flat or hemispherical ends.
Wetted-wall concepts typically have porous first wall materials that the coolant will ooze through to renew the first wall surface. Jeff discussed with the group the approach of a spray or nozzle arrangement to provide a renewable surface, but on first principles, it did not seem possible to renew a portion of the wall surface within the time between shots, e.g. < 0.1 second, so this approach was shelved. Flow around chamber penetrations was thought to be a serious engineering challenge, but a solution should be possible. More of a serious problem is the ability to sufficiently wet the porous surface. The solution offered was to coat the material with a material that will wet with the chosen coolant.
Response to the isochoric heating is common to all liquid and dry wall concepts. Flow up the beam lines and condensation of materials on colder surfaces remain problems for all the concepts. Rapid replacement of the blanket is a must for all concepts.
Chamber Nuclear Analysis
Activation Issues for Candidate Coating/Hohlraum Materials - Laila El-Guebaly explained the key design issues for target coating and hohlraum materials and noted the organizations working on analyses and solutions. She listed the candidate materials being considered for both the laser and the HIB targets. She quantified the main parameters for the waste disposal rating (WDR) analysis she has been doing. She noted the WDR strongly depends on the end of life fluence (nominally 21 MWy/m2for SiC/SiC.) The WDR should be less than 1.0 to be acceptable. Considerations other than WDR will determine the preferred armor material. It is likely that the target coating and/or hohlraum material will coat out on the wall to a nominal thickness (8 mm for the laser and 1 mm for the HIB.)
Laila presented her activation results of the coating and hohlraum materials. Waste disposal ratings were presented for both laser and HIB cases. The activity levels of these materials drop rapidly within minutes. Laila recommended that Ag and Gd be excluded, and then select the best materials based on considerations other than WDR and take a small decrement in the COE if necessary.
Final Optics (Laser)
Effects of Mirror Defect and Damage on Beam Quality - TK Mau described the typical final beam optics as typified by the Prometheus layout. The final grazing incidence mirror is located approximately 20 meters from the center of the chamber and 15 meters from the first wall opening. TK discussed the concerns and the modeling tools for dimensional defects (gross deformations and surface morphology) and compositional defects (gross surface contamination and local contamination.) Surface contaminants can alter reflectivity, depending on layer thickness and incident angle. Neutron irradiation of the mirror can cause transmutations in the protective coating and substrate, thus altering the optical properties.
TK discussed the methodology used to model wave scattering from random rough surfaces by applying Kirchhoff theory and assuming Gaussian surface statistics. In the future, he plans to use optical design software to assess gross deformation effects and a wave-scattering technique to assess the effects of particles on mirror surfaces.
Target and Chamber Physics (Dynamics)
Indirect Drive Options for Laser IFE - Max Tabak described notions for a future indirect drive (ID) illumination geometry for a laser system as being similar to that of a HIB system, i.e., the laser light couples to distributed radiators much like those in HIB fusion. Indirect drive relaxes the beam quality. This is important for DPSSL systems because they have narrower bandwidth than KrF systems and hence more difficulty in beam smoothing. This two-sided illumination geometry may be more compatible with thick-liquid wall chambers (because beams can be concentrated in smaller regions.) The final optics also may be moved further away from the target because the beam tubes are concentrated along the symmetry axis as opposed to being distributed over 4 pi steradians.
The target, using a conventional NIF geometry, currently yields 110 MJ (175 MJ with optimistic symmetry assumptions) with blue or maybe green laser light. The target is 1.6 times the size of the NIF target. The coupling efficiency is around 26%. The point design has a larger case-to-capsule ratio than the HI-ID target. He is investigating improvements such as an X-ray blocking shield to improve burn symmetry. He also has a concern how to retain the distributor radiator in place.
Max also discussed a fast ignition, cone focus, one-sided target design using a 100-200 kJ laser for ignition.
Advanced Fuel Target Designs and Related Energy Cycles - John Perkins informed the group of the concept of an advanced fuel IFE target and the advantages of this approach. The principal advantage of the advanced fuel target (D-D or D-3He) is that most of the fusion energy appears in charged particles, the output spectrum of which can be tailored to the particular application, e.g. advanced, in-situ energy conversion. This target would also be useful in the application to space propulsion, achieving a very high specific impulse, Isp. NASA is investigating space propulsion systems based on fusion for deep space missions. This is a very unique niche market that is quite appropriate for fusion.
The reactivities of the D-D and D-3He reactions only become comparable to that of D-T at significantly higher plasma temperatures. Thus, such targets will require typical areal densities on the order of 10g/cm2 adequate fuel burn up and gain. Accordingly, "fast ignition" techniques will be required to make these concepts viable at (compression) driver energies of greater than or equal to 10 MJ. Adequate performance may be achievable by depositing the fast ignition energy in a small, pre-compressed ignitor region of D-T fuel that acts as a sparkplug for the main D-D or D-3He fuel. A conventional driver (HI, laser or Z-pinch) illuminates the surrounding hohlraum to compress the capsule for burn conditions on the 10's of nsec time scale, and once the fuel has stagnated at maximum density, a lower energy laser ignites the fast ignition D-T sparkplug on the time scale of a few 10ís of ps. The overall tritium inventory in the capsule can be less than 1%, and with appropriate design, these targets can be self-sustaining in tritium through the D(d,p)T branch of the D-D reaction, thus obviating the need for any tritium breeding in the external plant. In the design of these targets, a key optimization task is to match the output charged particle kinetic energy spectrum to the requirements of the energy conversion system.
Target Fabrication, Injection, and Tracking
Update on Target Fabrication, Injection, and Tracking -Dan Goodin stressed that target fabrication, injection, and tracking issues are being addressed in an integrated fashion. The critical fabrication issues are:
Dan summarized the communityís progress to fabricate both DD and ID targets with appropriate materials. Methods are being evaluated to add high-Z overcoats with necessary surface and permeation properties. Fluidized bed process is being investigated as a possible fabrication approach. The chemical process model is being assessed to determine feasibility and production costs.
The critical injection and tracking issues are:
Materials measurements are needed to verify ability to withstand required acceleration. Analyses are in progress to assess the capability of the DD and ID targets to survive the thermal chamber environment. Results for the ID target show minimal DT heating during injection and transit. It was determined that the initial SOMBRERO (chamber gas and wall) conditions resulted in excessive target heating. Removal of the attenuating chamber gas widens the window and may allow target injection within temperature window, providing substantiating test data is obtained. Dan outlined a test to determine DT response to a rapid heat flux similar to that seen in an IFE chamber. Dan also explained that a target transmissive to radiation might help to keep target from excessive heating.
To determine the ability to accurately track the target, Dan reported that GA is designing an experimental target injection and tracking system to obtain relevant data. The Conceptual Design Review was completed late in CY00. The preliminary and final designs are underway this year.
Direct Drive Target Protection During Injection - Neil Alexander reported the progress on the definition and analysis of DD target protection during injection and transit. Several ideas have been proposed and the more promising ones are being investigated.
A sabot has considered to protect the DD target during injection exo-chamber. Neal described a two-piece sabot that could continue to protect the target during transit across the chamber. Just before reaching the center of the chamber, the laser beams would illuminate rings at the end of the sabot to provide pressure to separate the two sabot halves, thus leaving the DD target. It is difficult to conceive this approach as being practical.
The UW suggestion of a sacrificial, cooled (internally cryogenic) tube continues to show promise. The tube must extend into the chamber a significant fraction of the chamber radius, and the erosion per shot is low enough to allow a steady-state insertion of the tube. The interior of the tube will be cooled at or near cryogenic temperatures, low friction, and low gas pressure.
A wake-shield approach has been proposed. The difficulty of this approach is to keep the wake-shield in close proximity ahead of the target to effectively shield the target. Then the shield should not block any beams or channels during illumination. Perhaps a thin film on the wake shield would be sufficient, yet allow beam illumination.
A DT frost coating is also being considered, but the fabrication of the frost would be difficult at best. This approach, along with most of the others, will probably require spinning of the target about an axis perpendicular to the velocity vector to uniformly heat the target.
Tritium Permeability in Different Target Materials - The tritium permeability for different target coating materials were reported. The candidate coating materials are sliver, gold, palladium and platinum. The permeability of these materials at 500K are shown in the table below.
Tritium Permeability in Different Materials (at 500K)
If the target coating is very thin, the tritium may migrate through small cracks. If that is the case, the requirement for a low permeability may not be critical for the target filling process.
Integrated Target Reflectivity Analysis - TK Mau explained the motivation to obtain the reflectivity of the target to provide input to target heating analysis being conducted by GA. TK assumed the incident radiation spectrum to be blackbody. He used a four-layer (vacuum/film/ polymer/DT) Fresnel model to calculate the intensity reflectivity for each wavelength and angle of incidence to obtain a total reflectivity. This was accomplished for Au, Ag, Pd, and Pt with results shown in graph form. Optical properties of DT are not available - instead properties are assumed to be similar to silicon. Gold and silver had the highest reflectivity. Maximum reflectivity (0.98) is obtained with film thicknesses equal to or greater than 0.07 mm. Target reflectivity is insensitive to target plastic shell thickness.
Status of ARIES_IFE Safety and Environmental Activities - Dave Petti gave an overview of the safety and environmental activities being conducted for ARIES-IFE. The first area involved radiological confinement and contamination control to minimize the spread of contamination within the facility. Dave pointed out concern about the definition of the strong second barrier. Buildings are typically used for confinement in IFE studies, but this might not be the best approach. The second barrier should be located as close as possible to the source. A probabilistic risk assessment will be conducted to consider these levels:
Dave identified methods to identify initiating events and explained the process used in the ARIES study. One technique involves the master logic diagram (MLD), which is being constructed for ARIES-IFE. Hydrogen gas is one safety concern that might be an initiating event (deflagration/detonation). Rogue shots could be considered as initiating events, thus Dave is working with Don Steiner on the Off-normal Shots Working Group. Dave also discussed the safety issues with the carbon whisker (fibrous) wall material. There has been a concern that the large surface area provided by the whiskers would result in rapid oxidation of the material under accident conditions. Analysis has shown, however, that the reaction rate is limited by oxygen availability, and thus, the whisker material does not pose a significantly different safety issue than the baseline solid carbon material.
Report from the Off-normal Shots Working Group - Dave Petti presented Don Steinerís charts about the working groups progress. Dan Goodin had provided the specifications and requirements for the radiation-preheat direct drive target to provide the group with the target physical and performance parameters. John Sethian provided informed estimates of the probability of breakdowns within the KrF laser system. Components were assumed to be the front end, pre-amplifier, driver amplifier, and main amplifier. Four levels of consequences were assigned to these components and then annual probabilities of occurrence were estimated for this matrix. Even if these breakdowns occur, the intermixing of beams only decrease the illumination intensity at the target. The intent of the analysis is to determine the consequence to the chamber and power plant of reduced yield shots arising from these reduced power shots.
ARIES-IFE March Meeting Action ItemsChamber Nuclear Analysis - El-Guebaly
Final focus mirrors