NCSX Memorandum

To: T. Brown, A. Brooks, H-M. Fan, B. Nelson, D. Williamson, M. Cole, P. Goranson

CC: J. Schmidt, H. Neilson, P. Heitzenroeder, J. Lyon, S. Hirshman, M. Zarnstorff, A. Reiman. L. Berry, D. Strickler, R. Simmons, J. Graham, C-H. Jun

From: Wayne Reiersen

Date: 11/6/2000

Re: Minutes of 8/30 Engineering telecon

Presentations: [1] BN_saddle coil options_web.htm [2]MC_access.htm [3]TGB_saddle_configuration.htm [4]DW_0830.htm [5]PG_cooling options.doc

Our weekly telecon was conducted on Wednesday, 8/30. The main topic was progress toward selecting between the saddle and modular coil options. In addition, we discussed several topics raised at the Project Status meeting on Tuesday, 8/29:

Engineering participation in the Boundary Plasma Working Group - Peter Mioduszewski (ORNL) heads this group. Engineering will be represented by Brad Nelson (responsible for stellarator core), Paul Goranson (cognizant engineer for VV and PFCs), and Mike Cole (design integration).

Coil designs for plasma configuration 3351 – Ku has developed a configuration variant that features iota greater than 0.5 everywhere in the plasma cross-section. Action: Brooks to assess the coil implications of this new configuration.

Surface quality may be a key differentiator between the saddle and modular coil designs. Action: Jun to present the PIES results generated to date at next week’s engineering telecon.

The initial topic of discussion was progress in developing the saddle coil option. Tom Brown raised the issue earlier in the week of interferences between the saddle coils. Art Brooks indicated prior to the meeting the he had checked the separations in the coil set (which was generated by Miner and Valanju using a genetic algorithm). Nelson showed a spreadsheet that provided some clarification [1]. The crux of the matter seems to be what is assumed for each coil – is it constant J? constant cross-section? constant current per turn? The bottom line is that we want to minimize the maximum current density while preserving adequate space for the structural ligament and keeping the conductor current low (below ~10kA).

Action: Nelson to propose saddle coil cross-sections and turns consistent with desire to minimize J_max, preserve adequate space for structural ligament, and keep conductor current below ~10kA.

Later, Nelson provided an update on the revised test fixture to test the hypothesis that the gap size between the conductor and test fixture might be important for determining the effective stiffness of compacted cable conductor. Nelson expected the fixture back in two weeks. The first results might be available as early as the September 24 project meeting.

Paul Goranson reviewed several options for cooling the conductor [5]. Internal cooling with liquid (e.g., water) at room temperature looked promising. However, this would be applicable only to the modular option with J_max in the neighborhood of 10kA/cm². Internal gas cooling (single loop) did not look promising due to high pressure and long cooldown times. Conduction cooling to a heat sink (e.g., the bronze shell) looked OK (but assumed good thermal contact). The thermal resistance of the insulation appears to be adequately low to allow cooldown in less than 5 minutes. In reality, it is difficult to assure good thermal contact everywhere although thermal expansion does tend to make gaps disappear. Additional work appears in order to determine:

If conduction cooling looks OK with reasonable allowances for incomplete surface contact;

If, in the event that conduction cooling does NOT look OK, the trapped gas (N₂) or a high viscosity grease would provide sufficiently low thermal resistance.

Goranson also showed that a thin copper septum between coil layers would be very effective in conducting the heat to a heat sink or cooling tube.

More work is required to define how the heat should be removed from the bronze shell, which acts as a heat sink (with a large mass). One option would be to install tubes running vertically on the inside of the shell with pool boiling liquid nitrogen, which is then sent up a stack. Cooldown time should be an exponential function in which the temperature of the shell asymptotically approaches the temperature of the coolant. In order to achieve cooldown in a reasonably short time, some temperature difference between the shell and the coolant needs to be imposed. If we boil liquid nitrogen at 1.4atm (80K), it would be unreasonable to assume a starting temperature of 80K for the coils. If we boil nitrogen at 0.4atm (70K), then maybe an 80K starting temperature would not be unreasonable.

Action: Nelson to propose a cooling scheme for the conductor and shell and a starting temperature for the coils.

David Williamson reported on progress in twisting the conductor in the modular coils relative to the surface normal for the purpose of increasing the cross-sectional area and reducing interferences [4]. This technique might also be used to our advantage to reduce the maximum current density in the saddle coils.

Action: Williamson to explore reducing J_max by making saddle coils more parallel in regions of highest J.

Mike Cole reported on progress integrating the saddle coil assembly into the TF and PF coil set developed by Tom Brown [2]. The saddle coils for the LI383 plasma allow a large opening on the outboard side centered around the v=0 (banana-shaped) cross-section. The present model features a rectangular cutout, which is quite a bit smaller than the saddle coils would allow if the cutout was not constrained to be rectangular. A larger, non-rectangular cutout would reduce the mass of the bronze shell assembly that reportedly weighed upwards of 44 tons and simplify access. Similarly, the thickness of the bronze shell was kept uniform (11cm?), even away from the saddle coils. Structurally, the thickness of the shell could be substantially reduced (to 4cm?) away from the saddle coils. This too should have the positive benefit in reducing the mass of the bronze shell.

Action: Cole to optimize geometry of shell cutout and shell thickness away from saddle coils.

Cole showed that the PBX neutral beams could be readily accommodated in the saddle coil option with 18 TF coils. However, some additional refinement is in order. The beamlines at 7 and 11 o’clock should be relocated further out and fitted with bi-directional ports like the one at 3 o’clock (if they could be advantageously used for diagnostics). In addition, there is a concern about whether the co- and counter-injected beams would actually be pointing at each other.

Action: Cole to adjust beamline locations and port geometry. Show footprint of beams in plan view. Determine if beams ‘shooting at each other’ is indeed a problem.

Brown reported on progress in developing a TF and PF coil design for the saddle coil option [3]. The current option features an internal PF coil set. The TF coil set features 18 TF coils with a joint at the top and bottom. The core assembly is supported at the midplane. One alternative would be to put the PF coils outside the TF bore. It was agreed that this should be investigated but only after the downselection between modulars and saddles was made.

Hutch Neilson asked about the forecast for making a decision between the modulars and saddles. Reiersen voiced what appeared to be the consensus opinion that there was insufficient data on which to make an informed decision at this time. The hope was that, by the time of the September 24 project meeting, we would be in a good position to forecast a decision date. In the meantime, we need to firm up a basis for making that decision. Reiersen and Nelson agreed to draft a criteria list for review at the next weekly engineering meeting.

Action: Reiersen and Nelson to present draft criteria at next engineering meeting.