ARIES Documents -- Meetings ArchiveARIES Project Meeting, 10-11 January 2002
University of California, San Diego
|(GAT)||Abdel-Khalik, Durbin, Yoda|
|(LLNL)||Latkowski, McGovern, Meier, Tobin|
|(UCSD)||Baker, Dragojlovic, Gaeris, Mau, Miller, Najmabadi, Pulsifer, Raffray, Sze, Tillack, Wang, Zaghloul|
Budget and DOE Direction - Bill Dove informed the group he would be retiring around 1 March 2002. He has enjoyed working with this distinguished and talented group. He felt ARIES has accomplished a lot to help direct and advance the development of fusion.
Bill explained that the FY02 budget is in the implementation phase. The FY03 budget is being formulated and it might be slightly improved for the Systems Studies funding. It is likely the emphasis will be away from the inertial confinement toward the magnetic confinement concepts. The final direction will depend on his replacement.
Program Status - Farrokh Najmabadi stressed that there remains only 8 to 9 months to finish the IFE study. We must document the Dry Wall concept findings, complete the Wetted Wall (Sacrificial) concept and document it, and commence, complete, and document the Thick Fluid Wall. A lot must be done in a short time to complete the planned effort.
Snowmass Update ń Charlie Baker explained the goal of the July 2002 Snowmass Conference is to assess the candidate next step devices in both MFE and IFE. In MFE, the devices are the ITER, IGNITOR, and FIRE. There are numerous working groups collecting and assessing data for presentation at the Snowmass Conference. This data will allow in-depth discussions at the Conference that will provide critical fusion community input to the decision process of FESAC and DOE in 2002-2003. You can visit the Snowmass Web Site for more detailed information.
Next Meeting/Conference Call ń Several candidate meeting dates in the latter part of April 2002 were selected and Les Waganer was to poll the group as to their preferences and conflicts. The location of the meeting suggested was the University of Wisconsin, Madison as UW was scheduled to host the cancelled September 2001 meeting. The next conference call will be on Tuesday, 5 February 2002. Les will provide a call-in number.
ARIES-IFE Study Presentations
Systems Analysis and Integration
System Uncertainties - Ron Miller continues to model the power balance on the direct drive (DD) and indirect drive (ID) cases using the iDecide software. The ID targets are further subdivided either as a close-coupled target or a conventional, full-sized, distributed radiator case. For all cases, Ron presented gain, gross power, and recirculating power as a function of driver energy. Common assumptions were auxiliary power fraction, thermal conversion efficiency, and repetition rate. Repetition rate could be modified, within certain constraints, to obtain the desired net plant power output. One would like to determine the optimal driver energy for the lowest cost of electricity, but without a detailed plant definition, it cannot be modeled.
Ron discussed the target fabrication issues and possible analogous commercial products with similar issues. He summarized the issues and capabilities of incandescent light bulb blanks.
Heavy Ion Fusion Modeling Update - Wayne Meier summarized his progress since the last meeting as completing the driver code modifications, running the updated code for the distributed radiator target and proposing a new HYLIFE point design for consideration. A new point design will be the basis for HIF VNL and VLT work on interface design issues. The code now incorporates algorithms for final focus elliptical spot size accounting for emittance, space charge, aiming errors and chromatic and geometric aberrations over a range of driver energies. These models can calculate normalized transverse and longitudinal emittance growth. A focusing half-angle, qy, of 10 mrad is a good value for a broad range of driver energies and a nominal number of beams. Wayne provided a strawman point design for the 12/13/01 discussions with LBNL. For these sets of conditions, the COE minimizes around 5 MJ beam energy and a repetition rate of 6.31 Hz and a driver efficiency of 47%. The design uses an annular array of beams to provide access for a central target injection system.
Chamber/Beam Physics and Chamber Clearing
Ballistic Neutralized Transport Simulation Results - Dale Welch, of Mission Research, described the physics of ballistic neutralized transport and his modeling efforts for the systems codes. One approach for ballistic neutralized transport uses a converging ion beam passing through a plasma plug at the chamber entrance. The plasma plug neutralizes the ion beam, which continues ballistically to converge on the target. The beam spot size is a function of the beam neutralization (f), emittance, and the evolution of the beam charge state. Mission Research Corp has an LSP code that simulates the beam transport behavior.
Dale showed an example of a 3-cm beam being transported 3 meters through a 3-mtorr Flibe atmosphere. Results were shown with and without photo-ionized plasma effects. The results showed plasma neutralization were crucial to a good spot quality. Foot-pulse beams, however, will not have the benefit of photo-ionization. For simulations with no initial plasma in the chamber, the higher charge state ions expand in the head and are neutralized by beam-impact ionization only. Without a local plasma, the beam transport efficiency is < 50 % within 2 mm for ěfootî pulse.
Simulations with the plasma plug were more encouraging. A 1-kA beam picked up neutralizing electrons from the plasma and carried them to the target region. This method can produce an acceptable spot size for the Distributed Radiator Target (90% within 2-mm).
Lower chamber pressure should help beam transport for both foot and main pulses for a given plasma plug at the chamber wall entrance. Neutralized ballistic transport should be feasible for a 6-meter diameter dry wall chamber with good vacuum.
Towards an Analytic Model for a Beam Spot Size in Ballistic Neutralized Transport - Simon Yu discussed the envelope equation, d2R/dz2 = e 2/R3 + U/R, which relates beam radius (R), beam emittance (e ), net perveance (U), and distance (z). If total neutralization is assumed, the equation simplifies to Rt = e /(RoL), where Rt is beam radius at target, Ro is beam radius at beam port, and L is the chamber radius. However, there is usually incomplete neutralization, which complicates the final emittance, et and net perveance, Ut, near the target. Depending on the size of the beam radius, Rt, or net perveance, Ut, near the target, space charge or pinch forces will dominate. The neck radius depends primarily on the final emittance, not the evolution of the emittance. Simon described how to calculate the final emittance at the target as a function of beam radial charge distribution factors.
Self-Pinch Transport Design Considerations - David Rose, of Mission Research Corp, summarized the transport schemes (vacuum ballistic, neutralized ballistic, preformed channel (assisted pinch) and self-pinch) recommended for different wall concepts, including some prior IFE design studies. All the pinch transport schemes appear to be favorable for all chamber concepts. David has been examining the self-pinch scheme in more detail with the MRC LSP code. A 3-cm meter radius beam is compressed in the final focus magnet section to 3-mm and is directed through the chamber blanket and wall. In the chamber that is filled with 10-150 m torr Xe gas, many ionized beams will be partially neutralized by the chamber gas (probably a dry-wall chamber) to converge on the ID target. Another scheme uses many beams, adiabatically focused down to ~0.5 mm outside the chamber and enter as two diametrically-opposed beams. These two beams have a charge of 80-150 kA each. A trumpet-shaped chamber entrance opening is used to control the beam electron density rise as the beam passes through the wall. The average net beam current peaks over a range of chamber pressures.
Chamber Clearing with Electrostatic Fields - Leslie Bromberg observed that clearance of charged aerosols in the plasma chamber might be accomplished with electric fields in a manner similar to particulate removal with electrostatic precipitators. The electronic charge on the aerosol is proportional to the electron temperature and the size of the aerosol droplets. Leslie assumed the density and temperature exponentially decay with respect to time, which results in an average electronic charge number decaying down to a steady-state value around 1000. After examining the aerosol viscosity, Leslie concluded the velocity of aerosols is small (several m/s) when compared to the chamber size of several meters and the repetition rate (several per second). Relatively large electrical fields (100 V/cm) are needed and difficult to inductively generate with the chamber geometry and likely materials. Accomplishing the electrical fields with moving fluid walls would be even more difficult.
The more promising application would probably to shield aerosol deposition on optical or magnetic optics, which would have smaller geometrical scale factors.
Afterglow Plasma Processes in Dry-Wall IFE Chambers - Mark Tillack presented the results of Sergi Krasheninnikov regarding the afterglow phase, after a fireball expansion, that is characterized by cooling of the plasma-gas mixture and plasma recombination-neutralization processes. Typically it is thought that the plasma would be extinguished between shots on the time scale of 0.1 s, but this might not be the case. The recombination of the plasma species may result in a higher heat flux to the pellet than that predicted by the kinetic energy of colliding particles. The collective interactions of the fireball fast ions with the residual plasma can alter the fast-ion stopping processes.
The particle density in the chamber is determined from the pellet particle source term to be approximately 1018 to 1020 particles per cubic meter. With a mean free path of cold neutrals around 30 cm, this implies a short mean free path (fluid) regime of transport and a weak impact of diffusive effects. Plasma density reduction in the afterglow phase can be due to volumetric recombination effects (e.g., 3-body recombination) and plasma neutralization at the wall. The 3-body recombination is inefficient for plasma densities < 1-10 x 1018/m3. The rate of plasma neutralization on the chamber wall is determined by the plasma transport to the wall. The diffusive plasma transport for a residual neutral gas density ~1020m-3 is on the order of 0.1 s. Thus, it is difficult to decrease the residual plasma density below 1018 -1019 m10-3. It is expected there will be convective cells within the gas/plasma mixture with a scale length of ~ R and these effects will apply within these cells.
The residual gas/plasma temperature and cooling is determined by radiation, conduction, and convection. During the initial stages of the afterglow phase (T > a few eV), line radiation is very effective and the plasma cooling time scale is on the order of 10-3 s. At temperatures less than 1 eV, the line radiation is negligibly small.
At temperatures of T ~ 1 eV, the time scale of electron heat conduction to the walls is around (1-10) x 10-3 s, whereas at T~ 0.1 eV, the time scale is around 0.3 s to 3 s. For temperature reduction due to heavy particle (ions and neutrals), the heat conduction time scale is comparable to the diffusion time of 0.1 s.
It is expected the short scale-length convective modes will be quickly damped leaving only large cells with the scale of R. Conduction will determine the temperature of these large convective cells. The large-scale convective cells can be initiated and/or sustained by positioning the target explosion off center or by employing a slight asymmetry of the chamber.
Target heating by recombination can be much larger than heating by particle kinetic energy (16 W/cm2 as compared to 1 W/cm2 for radiation + convection). The residual plasma will be quickly be heated up by electron heat conduction to temperatures exceeding the ion temperature. Thus the stopping distance of the fast ions will be on the order of 10 cm. Concerning the residual gas/plasma interactions with the fireball, the initial stages of the fireball expansion will be in the fluid regime, but further expansion will occur in the collisionless regimes where plasma beam/stream instabilities will be crucial. Thermal electron emission from the wall can be important for power loading and formation of hot spots.
Target Debris Interaction with the Liquid Wall Chamber - Ahmed Hassanein discussed various issues of the interaction of target debris with liquid-film-protected first walls. He presented a comprehensive model on the interaction and deposition of first x-rays with thin liquid film, thermal response and evaporation of the film, hydrodynamic blow-off/transport of vapor, ion deposition in vapor/liquid-film/underlying structure, vapor re-radiation, and material erosion. These models are being implemented in HEIGHTS-IFE package. HEIGHTS can also study thick liquid wall response to various target debris including neutrons energy deposition. Models of liquid wall fragmentation due to generated shock waves are also being developed in HEIGHTS.
Thin Liquid Lead Wall Protection for IFE Chambers - Don Haynes presented results from his vaporization and preliminary condensation calculations. BUCKY simulations are presented for the ~ 400 MJ closely coupled HIB target and the laser direct-drive NRL target (160 MJ) in a thin liquid wall (1 mm Pb) chamber. For chambers with radii of 4.5 m and 6.5 m, and a starting chamber pressure of 1 mtorr, these preliminary results indicate that the chambers recover before the next shot, assuming a 5 Hz rep. rate.
A 1-mm thick coating of liquid Pb protects permanent target chamber structures from x-ray and ion loads with a 1-mtorr initial chamber gas pressure. The effects of higher pressures will be investigated before the next ARIES meeting.
Chamber conditions are re-established by 0.2 s after homogenization, but exact time for homogenization is not known. The transit time for the shock waves from the target and the flash vaporization is on the order of a few tenths of a millisecond. Experiment or higher dimensional simulations may be required to determine if a few or a thousand transit times are required.
A self-consistent chamber pressure for both pre- and post-shot conditions will be attempted by varying Tcool.
Future work would try to affirm these preliminary data by benchmarking the condensation part of the code with experiment, or at least with CONRAD, and also take into account the effects any non-condensable target material left in the chamber, and the thermal properties of the equilibrium liquid composition
Chamber Wall Engineering and Chamber Clearing
Design Considerations for the Beam Insulator Port Rings - Simon Yu described the progress to design the beam insulator port rings to sustain a high voltage standoff (10-15 kV) while preventing beam breakdown in a solid and thin liquid walls chamber environment. He showed several prior design approaches and pointed out some of the deficiencies. To conceive an improved port design, he examined the desired electric field profile. From this arrangement, a new design was derived with the insulator positioned in an area not in the direct line of sight of the neutrons. It was proposed to place the insulator behind the shield and the design be examined by the ARIES neutronic experts to help refine the design to improve the insulator design lifetime.
ěOverlapî Design Regions for the IFE Dry Wall Concept - For the past several months, RenČ Raffray has been trying to define a set of conditions, design assumptions, parameters, and analysis models to arrive at a self-consistent design window for an IFE dry-wall concept. RenČ showed a simple analysis at the maximum temperatures (< 1000C) for tungsten and SiC/SiC wall temperatures for IFE energy deposition for two direct drive (laser) and two indirect drive (heavy ion) target cases (one low energy drivers and one high energy drivers). RenČ evaluated different sets of parameters to maintain W and SiC/SiC temperatures below design limits. He concluded that there was no clear advantage to either a small chamber with a low repetition rate or a large chamber with a higher repetition rate. However, a larger sized chamber with higher cycle efficiency (and electric power) seems to be preferable. These results applied to both types of targets (and drivers). RenČ provided a set of strawman parameters of the target/driver combinations for the team.
Scoping Analysis of Condensation for Wetted Wall (Chamber Designs) - RenČ Raffray first discussed the major issues associated with the evaluation and design optimization for wetted wall chambers. These issues involve wall protection and chamber clearing.
RenČ felt the best approach was to establish the fundamentals of condensation on the wall and then to proceed toward an integrated analysis. The film condensation is based on kinetic theory. Droplet condensation equations are based on droplet/environment equilibrium and nucleation theory. The condensation rate and characteristic time to clear the chamber will be assessed for the chamber vapor conditions, film temperature, vapor velocity, and amount of non-condensable gas. Liquid lead is the assumed film material on a 5-meter chamber radius.
Above a certain threshold vapor pressure, the condensation characteristic time to clear the chamber does not decrease, which is shorter than the time between shots. The vaporized thickness is dependent upon the energy deposition depth. The vapor motion (velocity) toward the chamber enhanced the condensation rate. The presence of a non-condensable gas can slow down the lead condensation. However, the effect seems negligible in the range of vapor and gas pressure anticipated. Based on condensation, it seems better to have a shorter energy penetration depth (softer spectrum) that results in less vapor at higher pressure. A film thickness correction via preferential condensation can occur for a given scenario producing the required local film delta T.
However this approach may not be reliable and other methods should be used. Aerosol formation could be a problem although it is not clear that IFE conditions would result in droplet growth.
HI Beams and Vacuum System
Magnet Considerations for ARIES IFE - HTS/LTS Algorithms and Design Options - Leslie Bromberg summarized the near-term magnet developments that are ongoing for IFE for both high temperature and low temperature superconducting magnets. Present IFE work is ongoing in High Current Transport Experiment (HCX) and integrated Research Experiment (IRE). For the HI driver system, cost minimization of the magnet system is the paramount design criteria. This is accomplished by using epitaxial techniques, high Tc to minimize quench requirements and cost of the cryogenics systems, and optimize structural requirements. A different winding approach may simplify the winding pack. Leslie also addressed the superconductor implications for both the high temperature superconductor and the low temperature superconductor. Design options for unshielded plate magnets for IFE quad final optics have been investigated and design algorithms have been developed. Leslie noted that the irradiation effects need to be determined for the low temperature superconductor.
Plasma Window Options and Opportunities for IFE - Leslie Bromberg summarized the vacuum needs for the heavy ion beam (10-6 to 10-9 torr in the beamline while the chamber operates at 10-3 to 10 torr). Because of the large openings for beam propagation and large gas throughput across the HIB final focus and the chamber ports, sizable vacuum pumping speeds are required. It is not clear if it is possible to maintain a large pressure differential with the available space for pumps. A plasma window establishes a barrier to gas flow by creating a hot plasma discharge that results in a higher effective viscosity and a lower number density. Plasma windows can separate high pressure and atmospheric pressure as well as high and low vacuum conditions. Several example plasma window installations were shown along with operating parameters. Particle and photon transport have been demonstrated. Leslie showed plasma window scaling parameters and how it might be applied to heavy ion beams.
Chamber and Final Optics Nuclear Analysis
Liquid Wall Radiological Assessment and Feasibility of Target Recycling - Laila El-Guebaly informed the group that her objectives was to identify the radiological issues for candidate liquid wall materials (Pb, LiPb, Sn, and Flibe (to be added in the future)) and to address the feasibility of recycling candidate hohlraum wall materials.
Laila described the parameters to be assessed (activity, decay heat, and waste disposal rating (WDR)) and the parameters/assumptions being used in the analysis. She is specifically analyzing only the liquid used as the first wall and does not mix this fluid with that contained in the thicker blanket and shield regions. The assumption is that the first wall fluid passes through the chamber and it remains only a short time (~ 3 min) outside the chamber while being processed for return to the chamber. Because the in-chamber residence time of the liquid is unknown, Laila parameterized the residence time to cover a wide range from a fraction of a second to an hour.
The Pb and Sn activity is very low for a single shot, but activity increases with residence time and saturates after 10,000 pulses (~ 40 minutes in-chamber residence time for 4 Hz). Decay heat also saturates after 10,000 pulses. The main WDR contributors were shown for both Pb and Sn. The Class C waste requirement could be met by filtering out the Bi and Ag elements on-line and disposing of it as HLW or, preferably, limit the reuse of Pb to 1 year and Sn to 8 years. Tin generates higher activity and decay heat than lead, but at a lower WDR. LiPb (without T) exhibits similar behavior to that of Pb.
Laila also outlined the objectives of her target recycling study, which concentrated on identifying the target waste elements and which materials should be recycled. She noted the hohlraum wall materials represent a small waste stream as compared to the power core. She listed the candidate target materials, concentrating on the indirect targets as they pose the more severe recycling problem. Laila pointed out that the generic ARIES goal to recycle all materials. Ron Petzoldt told the group that it is generally accepted that the target materials would not be recycled. All spent target materials (~1 m3/y) would be disposed of and new target materials would be supplied anew. This sparked a discussion on reserves of key materials (e.g., gold and other materials).
Laila discussed the recycling process along with a detailed flow diagram and description of process elements. Cooling periods of < 2 years can reduce hohlraum inventories by factors of 10 or more. She also noted several factors may terminate the recycling process to introduce new target materials. Laila also discussed the effects of differing levels of buffer gas pressure.
Target Fabrication, Injection, and Tracking
Target Injection in Sacrificial Wall, Aerosol-Filled Chambers - Ron Petzoldt noted that aerosols are likely to form in the wetted wall chambers, which will coat the target, slow the target, interfere with laser penetration through target window, and interfere with in-chamber tracking. He said that 1 g/m3 in the chamber environment could cause a 0.3 mg/cm2 accumulation on target over a 3-meter distance to the center of the chamber. This is roughly 1% of the ion beam range for a 3.5 GeV Pb beam, so the energy loss is acceptable for heavy ion indirect drive targets. Direct drive targets have much tighter surface finish and material thickness requirements; therefore they require much lower aerosol density in the reaction chamber.
The aerosols will absorb the beam tracking light and coat beam windows on the ID targets. To assess these effects, more data will be needed on beam absorption and beam scattering on windows. Ron concluded with an incomplete table outlining the particle size and density limits for the combination of targets and drivers.
Updated on IFE Target Fabrication and Injection Activities - Dan Goodin outlined the critical issues for target fabrication and injection for both laser and heavy ion targets. To fabricate the targets, the ability to fabricate and fabricate economically the target capsules and hohlraums must be demonstrated. Then the complete process to fabricate, assemble, fill, and layer the targets must be demonstrated at the required rates. Target economic goals have been established at $0.25 per target. For injection, the target must withstand the acceleration during injection, survive the thermal environment within the chamber, and demonstrate the accuracy and repeatability of the target tracking. A target injection and tracking experimental plan has been prepared. The injection system procurement is underway.
A US/Japan Workshop on target fabrication and injection was held December 3-4, 2001 at General Atomics to allow specialists in these areas to exchange detailed technical information.
Overview of Radioactivity vs. Chemical Toxicity Issues for Potential Target Materials - Jeff Latkowski presented Susana Reyesí presentation on this subject for the thick liquid wall concept. It was indicated that indirect drive (ID) targets would require high-Z materials to produce the necessary x-rays. Material selection will be determined by target performance, cost, extraction from the waste stream, material compatibility with other materials and safety and environmental issues. Previous safety work has shown both Hg and Pb are likely materials. Economics would favor these materials over the current use of gold and gadolinium materials. Both materials are appropriate for target production and coolant cleanup systems. However, other important aspects must be considered before final selection.
The use of these materials should be compatible with structural and piping materials, such as SS-304 and SS-304L. Key issues involve creep fatigue and corrosion. With lower melting temperature materials, designs prevent precipitation within the piping.
The accident dose limit has been re-evaluated and Hg must be limited to 4.2 kg and Pb to 20 kg release. Chemical toxicity issues have only been examined to a preliminary extent. The TEEL-2 criteria has been adopted as a goal standard. Air dispersion calculations have been done to assess chemical exposures. When releases of these two materials were postulated along with the evaporative model, the maximum allowed release rates with evaporation at normal temperature would result in a 13-m radius for Hg and 440-km Pb pool. The HOTSPOT code was used to analyze the releases for the radiological assessment. For a 4.2 kg of Hg gas or 20 kg of Pb gas release, a concentration of 6.33 mg/m3 for Hg or a 30 mg/m3 for Pb is postulated. These values exceed the TEEL-2 limits by an order of magnitude, however this was a conservative analysis assuming that all the mass is directly released as a gas.
In summary, there are two types of toxicological hazards: radioactivity of the activated materials and the chemical toxicity. Both Hg and Pb can be classified as adequate when analyzing the contact dose rates and WDR. Hg is the most hazardous from a radioactivity release standpoint; however, segregation of the inventory in the plant would achieve the 1-rem goal. In accidents involving the primary coolant loop, Pb poses the greater radiological hazard due to its higher inventory.
National Ignition Facility
Experimental Opportunities on NIF - Brian MacGowan, NIF Diagnostics Program Manager (acting), LLNL, discussed the opportunities to install experiments on the NIF before NIF achieves full operational status. He described in detail the experimental windows and testing capabilities during the incremental buildup of the beam lines. He encouraged anybody interested to contact him for further information.