May 15, 2001

A meeting was held on May 15.  Agenda items included:

1. Modifications to CoilOpt to incorporate improved engineering metrics and constraints (Williamson, Strickler)

2. Progress in assessing design impacts of an inboard RF launcher (Cole)

3. Progress in assessing disruption loads on the vacuum vessel (Brooks)

4. Re-calculation of coil inductances (Jun)

Dave Williamson described changes being implemented in CoilOpt to improve the engineering metrics and constraints.  An algorithm is being incorporated to assure adequate space for tangential NB access.  The algorithm places points along the centerline of the NB and restricts the minimum distance from a coil to the CL to be >27cm. [DW - Please provide definition of build elements to demonstrate that 27cm is the appropriate constraint.]  It was noted that the distance needs to be measured to both co- and counter-directed beam centerlines.

A minimum coil separation of 12cm is imposed.  This implies that adjacent winding packs are nearly parallel and that the clamps (brackets) on adjacent winding packs have to be staggered.

Previously, the length of a coil was constrained.  This constraint has been turned off and replaced with a maximum coil-to-plasma separation constraint.  The purpose of this constraint is to suppress the tendency of the code to jockey adjacent coils in and out to increase coil-to-coil separation.  Jockeying the coils in and out makes supporting the coils off a smooth shell more difficult to achieve.  Implementation of this constraint needs to be restricted to the inboard side so tangential NB access can be accommodated.

The minimum radius of curvature constraint is 3x the width of the conductor.  Since we are measuring the radius of curvature at the coil centerline, we need to add the half width of the coil too.  The minimum radius of curvature in the lateral and radial directions would be just under 10cm.  [DW - Are we going to use the radius of curvature measured by looking at the CL as a space curve?  If so, please indicate what the minimum radius of curvature should be.  Also, is this squared-up cable conductor more susceptible to keystoning that solid conductor?  What allowances are appropriate to accommodate keystoning in determining the overall dimensions of the winding pack?]

The minimum coil-to-plasma distance is set at 18cm.  There was considerable discussion about the implementation of this constraint.  There is nothing sacred about the plasma boundary. The purpose of this constraint is to assure that there is enough space for everything that needs to go between the plasma and coils.  A uniform 18cm stand-off does not seem appropriate.  There is a scrape-off layer of non-uniform thickness surrounding the plasma.  (This could be calculated in the VMEC version modified by Zarnstorff to include a "bloat" parameter.)  Locally, expanded envelopes are required for an inboard RF launcher and a pumped divertor.  Furthermore, the coils have to be far enough away that they can be slid over the vacuum vessel for assembly.  The question is this - how do we go forward?  I propose the following steps:  

1. Lay out the magnetic surface that intercepts the plasma facing surface in the divertor region.  The layer between this surface (the SOL boundary) and the last closed magnetic surface (the nominal plasma boundary) is called the scrape-off layer (SOL).  Physics (Mioduszewski) should define the appropriate SOL thickness (presumably at the v=0 cross-section on the inboard midplane).  He should also provide the locus of points constituting the SOL boundary from Grossman's calculations.  The surface defined by these points should be compared to the surface calculated by VMEC using an appropriate bloat parameter.  If there is reasonable correspondence, then we can use VMEC directly to define this SOL boundary directly.  The SOL boundary defines a "stay out zone" for laying out the PFCs.

2. Determine the minimum envelope between the SOL boundary and the inside of the vacuum vessel.  This is the minimum envelope for PFCs.  Locally, we must also  be able to accommodate (if required) expanded envelopes for an inboard RF launcher, a pumped divertor, and perhaps even the bolted closure joint for the vacuum vessel.  Cole is working with Majeski to define the incremental RF envelope requirements.  ORNL should work with Mioduszewski to define an envelope for a pumped divertor.  ORNL should define any additional envelope requirements for the bolted joint.  This information should allow us to define a surface that the vacuum vessel has to stay outside.

3. The envelope between this surface and the coil centroid must accommodate  the vacuum vessel and its cooling lines, thermal insulation, a gap, and the maximum distance to the winding centroid (which may be large than the half- thickness or half-width of the coil).  When these envelope requirements are added to the surface that defines a stay-out zone for the vacuum vessel, we have a stay-out zone for the winding surface.  From this perspective, this stay-out zone is not nearly as simple as a fixed 18cm offset from the nominal plasma surface.

[BN-Please review these steps and provide feedback.]

One of the undesirable features of the 1017 coil set is that in at least one of the coil types, the coil folds back on itself so it cannot be simply machined from the top, flipped over, and machined from the bottom.  This "double back" feature may be inherent to modular coils built around the li383 plasma configuration.  No penalty functions have been incorporated into CoilOpt to avoid this feature.

Nelson discussed the purchase of conductor for early R&D.  2000' of conductor would cost about $13K.  Smaller lengths would not save any money.  The discussion centered around whether the conductor should be bought now or to wait until the coil design was finalized.  If the conductor was bought now, we could do tests on it to establish the orthotropic properties that should be used in FEA modeling.  We could test how easy (or hard) it is to wind and whether keystoning is something we need to accommodate in laying out the winding pack.  The major reservation was finding the money and perhaps having to repeat the tests if the conductor dimensions changed significantly.  [BN-Please provide an update of your plans in this regard.]

Mike Cole discussed several options for configuring an inboard RF launcher.  The most appealing was the last option, which had a gap between Faraday shields at the bolted joint. [MC-Continue to work with Majeski to develop satisfactory concept and define envelope requirements as input to the coil design effort.  Also, determine required launcher-to-plasma distance and tolerance and whether the plasma can ride on the launcher or if it needs to be surrounded by (set back from) a bumper limiter array.]

Art Brooks reported on progress developing disruption loads on the vacuum vessel.  The purpose of these calculations is to determine if disruption loads will result in stresses that are small, of the same magnitude, or large relative to stresses from vacuum pressure loads.  Brooks' initial results indicated that the stresses due to disruptions loads (with a 175kA plasma and a 1.7T field on axis) are slightly larger than the stresses due to vacuum pressure loads and that thicknesses greater than 0.25" will likely be required.  (The nominal thickness is 0.375".) [AB/FD-Calculate stresses due to combined loads with a thickness of 0.375" and a 2T field (1.7T from modular coils and 0.3T from TF).  Re-calculate stresses with a 350kA plasma current.]

Chang-Hoon Jun reported on progress in re-calculating the coil inductance matrix.  Previously, the inductances were calculated using a German code that appeared to have a primitive algorithm.  At a minimum, we wanted a check against those numbers.  Jun has developed a stand-alone Fortran code for calculating the coil inductance based on a finite cross-section.  The results are sensitive to the coil segmentation.  Further code development and benchmarking is proceeding.

The next Project telecon will be Wednesday, May23, at 1:30pm.  At PPPL, we will be convening in the Engineering Conference Room.  We will be using the PPPL bridge. We will be connecting using the PPPL telephone bridge (x2822).  When you get in, dial 6789 to get connected to the bridge.  Note there may not be an announcement prompting you to enter the 6789 code.  If not, just do it after the beep.  

Please forward any comments to reiersen@pppl.gov

(last edited on 09/25/2001 03:47 PM )