Text Box: NCSX Memorandum

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

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

From:    W. Reiersen

Date:     10/4/00

Re:        Minutes of 10/4 Engineering telecon

Attachments:       [1] NCSX_PS_System_Web.htm         

 

 

 

An engineering telecon was held on 10/4 to discuss plans in the wake of the project meeting last week.  The direction given by H. Neilson was to focus on making the modular option work for the PVR.  Work on the saddle option should be limited to improving the surface quality.

At the project meeting, there were a number of critical issues associated with the modular option, including:

*   Access

*   Adaptability

*   Power

*   Maturity

 

We need to work on these if we are to present a plausible design at the PVR.  A primary access requirement is tangential NBI.  Presently, access is blocked by a modular coil located on the symmetry plane where the beamlines cross (as shown in the figure below).

Dennis Strickler is working on several options for resolving the interference.  One option is to move the outboard leg of the modular coil further in or out.  Moving it in reportedly did not look promising.  Looking at the figure above, moving it out does not look promising either.  More promising options entail moving the modular coil off the symmetry plane where the beamlines cross.  This could be done in several ways – rotating the modular coil positions over by half a position or going to 18 or 24 coils without any coils on symmetry planes.  These options would avoid the interference with the beams on the symmetry plane but some adjustment of the coils closest to the symmetry plane would likely be required.  Strickler indicated that a revised coil design might be available as early as next week.  Engineering needs to support Strickler in assuring that the revised coil designs do indeed meet the neutral beam requirements for co- and counter-tangential access.  Once we have a coil design that accommodates tangential NBI, we need to assess whether and how HHFW, diagnostics, and personnel access can also be accommodated.  Based on the discussion below, it seems prudent to incorporate a cryostat in the design.  Action: Cole

Adaptability is a fuzzy term describing the ability of a design to accommodate revisions springing from new understanding and ideas.  In the saddle coil design, the coils are low profile coils closely coupled to the plasma and embedded in a monolithic shell.  This is both a curse and a blessing.  The coils need to be close to do their job.  This does not allow a whole lot of space for shape flexibility outboard of the plasma (the curse), although our biggest problem seems to be too much space between the last closed flux surface and the first wall.  Additional coils can be located virtually anywhere outside the saddle coils.  These coils would still be close to the plasma (the blessing).  Candidate coil sets that have been considered include L=3 coils for shear control and field error correction coils for island suppression (or excitation).  These supplemental coil sets can be mounted to the completed shell without impacting machine assembly.

Modular coils are fundamentally different.  Because of their high current (they carry the 17 MAT required to provide 2T at 1.7m), they want to be further away outboard although still close in on the inboard side.  In principle, this allows more shape flexibility outboard (and perhaps top and bottom).  However, supplemental coils outside the modular coils would be quite far away, perhaps too far away in most cases to be practical.  There is very little room inboard to add coils and adding coils inside the bore of modular coils could have a major impact on assembly.  As a consequence, we probably have to wait until we have a plausible modular coil set and assembly scheme before testing the adaptability of the modular coil option.

Power was the third issue raised with the modular coils.  Prior to the engineering telecon, Phil Heitzenroeder and I met with Ron Hatcher and Charlie Neumeyer to discuss power supply issues.  The previous look at power supply issues was done by Ron Hatcher in 1999.  The basic approach advocated by Hatcher was to bring over TFTR power supplies to C-site.  At the time, he looked at bringing over 12 power supplies.  Each power supply has two 24kA, 1kV rectifier sections.  These units would be located in the room presently occupied by the PBX OH and EF supplies that would be removed.  The TFTR units would basically fill the room.  Transformers for the power supplies would be located outside in the space currently used for the capacitor bank.  The low current (5kA) PBX power supplies could be used for trim coils.

Hatcher and Neumeyer raised several alternatives that Neumeyer nicely documented [1].  It is not clear right now which is the best way to go but the constraints are clear.  There are 16 TFTR power supplies that are not being used by NSTX.  There would probably be space to house these in the room where the PBX EF and OH supplies presently are if we constructed a mezzanine.  Using more TFTR power supplies would require sharing run time with NSTX.  This is not very palatable from an experimental or cost standpoint.  The incremental cost of new power supplies (if we could find a place to put them) would be significant.  Neumeyer estimated $12/kVA pulsed, $94/kVA continuous.

I took a look at the power supply requirements for the reference scenario for three options: the saddle option, the modular option running at 10C, and the modular option running at 85K.  Ramp times were adjusted to suit the different coil designs.  The results are shown in the table below.

 


 


 


 


The modular option (even if refrigerated to 10C) is problematic.  It will require 75% more power supplies than are available from TFTR.  Just the simple cost of the power supplies alone will be $7M.  Resistive power will exceed 400MW.  Peak temperatures will be approximately 100C, too hot for room temperature cured epoxy by a long shot.  Cooling it down to LN2 temperatures relieves a lot of these miseries.  It reduces the number of new power supplies required by 2/3, primarily in the modular coil circuits.  Current density and peak temperature are no longer problems.  The downside is that a cryostat must be introduced.

One curious thing in the modular option is that the PF coil currents determined by the optimizer are LARGE.  Physics should determine if this is fundamental or if this would easily go away by targeting PF coil currents in the optimizer. Action: Zarnstorff

Hatcher and Neumeyer will come back with a recommendation on which way we should go to bring additional power to C-site.  In the meantime, I think we need to adopt a cryostat in the design.  This may significantly complicate assembly and access but the alternative looks pretty dismal unless we can substantially reduce the current density. Action: Nelson

The fourth item was maturity.  Several aspects of the modular coil design had not been worked out to the degree that they had on the saddle coil design.  Perhaps the most significant is the structural design.  The integral structure concept proposed for the modular coil design at the time of the project meeting (at which time it was barely a couple of days old) did not show any lateral support for the ‘I-beam’ sections.  The ‘I-beam’ sections were supported by membranes that extended back to an axisymmetric shell.  Also, the windings within the I-beam sections did not have a substantial cover plate to keep them in the winding cavity in the event that the EM loads would push them that way.  Initial analysis of the self loads indicated that the over most of the perimeter, the self loads were towards the web of the I-beam.  However, the optimized waveforms show large, dynamic currents in the PF coils.  Intuitively, these coils should impose loads that would tend to push the winding out of the cavity.  We should look at the EM loads on the windings from the PF coils and see where substantial support to keep the winding pack in the winding cavity may be required.  In short, we need to look at improving the lateral support for the I-beam sections and for keeping the winding packs in the winding cavity.  Tom Brown suggested we look at a ‘2-coil module’ concept such as the one used on FIRE.  Action: Williamson

Because we appear to be going forward with the modular option for the PVR, we will limit our attention on the saddle option to those activities related to improving surface quality.  In our group, that involves primarily Art Brooks.  If additional engineering effort is required to address a critical issue, we will do so.

The next engineering telecon will be next Wednesday, October 11 at 1:30pm.  See you there!