ARIES Team Conference Call

Administrative

Mark Tillack has arranged for a splinter meeting in San Diego to commence on the morning of 4 May to address the design assumptions associated with the centerpost (CP). Attendance at the centerpost meeting will include key ARIES engineering members plus Ed Cheng, Igor Sviatoslavsky, and Gary Voss (of Culham). The purpose of the meeting is to try to reach agreement or understanding of the decisions by the ARIES and Culham teams to use a shielded CP, TF coil conducting material (Cu or subcooled Al), straight or flared CP, and the need/provisions for an inboard divertor slot.

To fully utilize the presence of G. Voss et al. from Culham, Farrokh Najmabadi is planning a Spherical Tokamak (ST) Physics town meeting commencing at noon on Wednesday, 6 April and running through noon on Thursday. Attendees are expected to be Farrokh Najmabadi, Steve Jardin, Gary Voss, Ron Stambaugh, Ron Miller, Rob Goldston, Ray Fonck, Bill Dove, and Martin Peng. The topic of discussion is similar to the Monday engineering meeting to reach some agreement and consensus as to the common and generic ST physics assumptions, modeling, and impact on the engineering systems and performance of the ST commercial power core.

Farrokh summarized the key points from the March IAEA committee meeting in Culham. Many of the papers reported results and findings from the Culham and ARIES ST commercial power plants. There were many similarities in their design approaches and overall findings. But the differences were most intriguing: subcooled aluminum TF centerpost with a high efficiency cryoplant, higher packing fraction in the optional Culham copper centerpost, lower plasma elongation with higher beta value, no divertor, high thermal conversion efficiency (~45%) with water cooled blankets, and a COE value in the range of 70 mills/kWh (equivalent). The EU presenters discussed the development needs for SiC structural material for fusion. The Japanese presenters reported benefits of the DREAM machine and discussed tokamak design approaches for us as a neutron source. Both EU and JA are comparing the cost of fusion electrical power production with that of advanced fission plants at levels equivalent to 60-100 mills/kWh (US$). Several papers dealt with inertial confinement experimental and power reactors. Two nearly identical papers discussed the CO₂ impact and energy payback ratios associated with all major current and potential electrical power generation sources - fusion and solar looked the best on this basis.

Plasma Physics

The PPPL analyses were done with a bootstrap fraction (BS) set to less than or equal to 100%. Again it was suggested that beta might be increased if this BS requirement were lessened.

Plasma Stability (GA) - Ron Stambaugh suggested that bn should be held fixed, while bp and bt were varied. He also noted that as the stability analyses were evaluated, solutions for higher mode numbers moved the conducting wall closer to the first wall. GA has provided plasma data to Ron Miller for use in the ASC code. Ron Miller thought he might have sufficient data for the ASC code without the related EQDISK files.

Heating and Current Drive) - TK Mau asked if R. Stambaugh intended to analyze the startup requirements for the ST configuration. Ron intends to do the calculations after the final strawman is adopted.

TK is redesigning the 8.5 MHz, low frequency, fast wave (LFFW) current drive antenna configuration. It now has 16 straps, with no Faraday shield. The plasma facing enclosure is 5 m long (toroidally), 2 m high, and 0.35 m deep, which accounts for 6% of the outboard first wall area. Laila and Mark asked TK to work with Xueren to incorporate the existing 2-D sketches into 3-D drawings of the power core sector. TK is concerned that the coax leads may not be compatible with the vertical maintenance scheme. The higher efficiency, neutral beam current drive system remains as an alternate approach.

TK is also defining the High Harmonic Fast Wave (HHFW) system to provide profile control for the plasma. This smaller system uses a folded wave-guide antenna which is approximately 1.47 m wide by 1.1 m high, and 0.4 m thick.

Systems Studies

Engineering

Mark summarized the results of the Engineering Conference Call, along with new inputs from the team:

• Mark Tillack noted that the general configuration remains similar to that discussed on the last call. The TF coil set (not an individual coil) is roughly spherical, with the outer coil thickness proportional to a 1/R relationship. W. Reiersen reported that the inner surface of the TF coil legs can be used as the primary vacuum vessel with any ports fitted with bellows. Current flow should be symmetric around the torus except for local openings. More detailed analyses may be required to verify ripple levels.

• The location of the upper and lower PF coils has not been determined yet (i.e., inside or outside the TF coils). This analysis is being delayed until the strawman is "finalized" (see above).

• Maintenance is still to be accomplished from below. X. Wang reported that product literature from hydraulic lift manufacturers quoted capabilities such that the entire power core (6000 tonnes) could be supported with approximately 6 lift cylinders. Mark wanted an extra cylinder to be provided as a spare. Mark also noted that UCSD is planning to replace the cantilevered support beams with swing-in compression beams as suggested by Les Waganer.

• Mark suggested that Dai-Kai modify his blanket design to be consistent with an integrated sector approach, not modular.

• Mike Billone obtained new tritium solubility data for SiC from Rion Causey. This new data indicates a factor of 4 orders of magnitude lower tritium inventory in SiC, which would suggest that the use of SiC as a thermal insulator inside the high temperature blanket region would be acceptable. Mike will distribute a memo to the Engineering Team with more detailed information on this subject.

• Mark affirmed that the ARIES-ST design needs the inboard divertor slot capability; hence, the straight CP is required.

• Mark also discussed other aspects of the divertor: Slotted ducts are needed on both inboard and outboard X-point regions. (Ron Stambaugh noted that GA has confirmed that a double null plasma is the preferred configuration for an ST.) The surface of the divertor may employ tungsten, cooled with helium flowing in separate substrate material. Porous or sintered metal material is another possible configuration.

• TK is ready to commit to an LFFW CD system decision. He has given X. Wang the sizing requirements and Xueren has incorporated them into his 2-D sketches. There will also be a requirement for an RF profile control system (probably the HHFW systems) to be defined ASAP.

• Wayne Reiersen reported on the thermal-hydraulics analysis of the cool water versus warm water cooling of the copper TF centerpost for the Case J strawman configuration. For the cool water case, the outlet water was around 65°C plus a DT of 60°C for the combination of the interface film conductance and the bulk copper conductivity yields an average copper temperature of 125°C which results in an overall resistivity of 2.9 to 3.0 x 10⁸ ohm-cm. With the warm water and an inlet temperature of approximately 140° C, the centerpost could not be adequately cooled without very high flow rates on the order of 25 m/s. The desired velocity should be around 10 m/s. Wayne recommended the use of the cool water option.

• Wayne also mentioned that he is planning to use aluminum as the conducting material in the outer TF coil legs. The resistivity is twice that of copper but the volume (did you mean mass?) is reduced by a factor of three; hence, there is an advantage to using aluminum. (The cost factor would also probably favor aluminum, but the strength would favor copper.) Wayne had consulted with other PPPL personnel and they felt the spherical TF shell with discrete bus bar connections would yield sufficiently uniform fields for the plasma if the penetrations were kept small.

• Laila El-Guebaly is also working with the Case J strawman results. With a 20-cm inboard steel shield and a 80-cm-radius copper centerpost, the waste disposal rating (WDR) was 0.85; whereas with a 60-cm-radius copper centerpost (and the same shield thickness), the WDR is 1.0. Alternatively, with the thicker CP, the shield could be reduced to 18 cm and still retain a WDR of 1.0. Another option is to consider thinning the shield near the top and bottom, which would still maintain acceptable WDR values.

  Shield Thickness, cm	CP Copper Radius, cm	Waste Disposal Rating
  20	60	1.0
  20	80	0.85
  18	80	1.0

  Hesham Khater mentioned specifying copper alloys with low Nb concentrations to help achieve the desired WDRs. With this grade of copper, Nickel⁶³ is the limiting radioisotope to achieve Class C waste criteria.

• One of the reasons for shielding the centerpost was achievement of Class C waste capability. Farrokh Najmabadi stressed that it is not the only reason - others are:
  1. the recovery of the dissipated and deposited power in the inboard components, :

  2. damage to the TF center conductor, :

  3. reduced breeding, and:

  4. TBD:

  These reasons all should be documented when discussing the decision to shield the centerpost. It was noted that Clement Wong is now adding a shield on his ST reactor design.