• You can directly edit the files to make small changes.
• Scripts may be modified by control programs (e.g., Perl, Python,…) to set up control loops for complex runs.
• It is easy to exchange setups with colleagues.
• The scripts provide an archive of your work — old solutions may be reconstructed quickly.

Regarding the last item, we have made an effort to maintain format consistency over program versions. One exception is a change that occurred several years ago between Version 5.0 and 6.0 of the TriComp programs EMP, EStat, Nelson, PerMag, Pulse, RFE2, TDiff and WaveSim. The old scripts had commands of the form

SET QuantName = Value
REGION(RegNo) QuantName = Value

All commands either began with SET (define a program parameter) or REGION (define a physical characteristic of the solution volume region). This form was redundant and inconsistent with the AMaze script format. The new form introduced in Version 6.0 is

QuantName = Value
QuantName(RegNo) = Value

Version 6.0 programs could read both forms. We eliminated the old commands in Version 7.0 to simplify the codes and to make it more efficient to introduce new command forms. At the same time, we created a utility so that code users can update an entire directory of input scripts automatically. The operation of format_correct is simple:

• Use the Set working directory button to navigate to the folder than contains the input scripts.
• Click on the Select input file button and specify a file with one of the standard TriComp suffixes (*.EIN, *.RIN, *.NIN, *.PIN, *.TIN , *.WIN). The program automatically corrects all files in the directory with the same suffix.

Format correct utility

Note that format_correct does any make changes to input files in the new format, so you can use it in a folder with a mixture of input types. Use this link to download the program: format_correct.

We introduced two other utilities in previous articles:

1. Check how much memory is available in your computer to allocate to a single task. This was an issue in 32 bit bit Windows, particularly XP. Download memtest32 (for 32-bit operating systems). Download memtest64 (for 64-bit operating systems).
2. Check the efficiency of parallel programs on your computer. Download openmptest (for 64-bit operating systems)

The STL capabilities of MetaMesh (for importing parts from SolidWorks, ProE,….) are quite reliable as long as the input files are valid. The nodes and sides of triangular facets must be logically connected and cover a contiguous three-dimensional surface. It’s important to realize that CAD programs don’t always generate valid STL output.

As an example, in a recent consulting project I received a faulty STL file as a part of a large assembly. Several hours evaporated while I narrowed down the cause of mesh distortions. Ironically, the part in question was simple: a cylindrical cathode with a cylindrical concavity in the emission surface (first figure below). The problem is apparent in a wireframe view of the cathode near the emission surface (second figure). The object was not originally constructed as a single part in the CAD program. Instead, a dished cap was added to a cylinder in a manner that did not preserve the logical continuity of facets. In fact, the cap doesn’t even fit correctly. Such a combination should not be represented in a single STL file. Usually MetaMesh will soldier through and create something acceptable, even with a corrupt file. In this application, the cathode surface was adjacent to a thin gap separating it from a focus electrode. Perfect fitting was essential. The obvious way to resolve the issue would be to ask the mechanical engineer to regenerate the STL file. Unfortunately, the design had evolved from multiple levels of multiple organizations through multiple CAD programs.  It would have taken a historian to find the original designer.

The time had arrived to stop pushing buttons and attack the problem the old fashioned way. An inspection of standard mechanical drawings of the part showed that it could be constructed from native MetaMesh solid models. I started with a cylinder and then machined the emission surface using an extrusion with a cylindrical side and the physical properties of vacuum. Fabricating the part did involve some work:

1. I had to do some trigonometry to find the vector outline of the extrusion.
2. I had to do some head-scratching and sketching to figure how to rotate the extrusion into position.
3. I had to think about the order for adding parts and the fitting specifications so that the trimming extrusion did not affect the focus electrode. (Note that the focus electrode was successfully represented by an STL model).

Although building the cathode from basic parts was harder, it certainly was quicker. It took less than half an hour to build the part from scratch. I cite this episode in support of some anachronistic views about computer simulations:

1. It’s often better to sit in a quiet place with a paper and pencil that to pound on the computer.
2. A simple approach is usually better than a sophisticated approach.
3. The best way to waste time is to let the computer do all the thinking.

One lesson from the experience is that you should check the STL models when mesh generation problems occur. We have added a new STL viewer to Geometer to help in this process.

For reference, here’s the full MetaMesh specification that I created for the cathode:

PART
* Make the cylinder
Region: CATHODE
Name: Cathode
Type: Cylinder
Fab: 3.1600 0.6000
Shift: 0.0000 0.0000 -23.4500
Surface Region Vacuum
END

PART
* Machine the surface
Region: EmitDef
Name: SurfaceShape
Type: Extrusion
L    3.1600  -23.0000  3.1600  -23.3000
A    3.1600  -23.3000  0.0000  -23.6607   0.0000   -9.6387  S
A    0.0000  -23.6607 -3.1600  -23.3000   0.0000   -9.6387  S
L   -3.1600  -23.3000 -3.1600  -23.0000
L   -3.1600  -23.0000  3.1600  -23.0000
End
Fab: 6.50
Rotate: 90.000  0.000  0.000
Surface Region Cathode
Coat Cathode Emit
END

• Reduction of the incident electron energy by an order of magnitude.
• Initiation of electron trajectories outside the material in an adjacent  void.

The original example was set up energies in the range ?1 MeV. At 0.1 MeV, the user observed reported a backscatter ratio of 0.32 instead of the experimental value of 0.51.

The figure illustrates the source of the problem. At 0.1 MeV, the penetration depth for electrons in gold is less than 10 ?m. The particle in the Void region moves in virtual interaction steps determined by the parameter DsMax. If the particle crosses a material boundary in a step, GamBet interpolates the particle position to the entrance boundary and then makes a small adjustment (DElemG) to ensure that the particle is inside the material. In the past, GamBet set DElemG equal to a small fraction of the average element size. A problem could occur if the scale length of the solution volume was much larger than that of the interaction region. In this case, particles could be placed a significant depth inside the material, reducing the backscatter ratio.

We modified GamBet to alleviate the issue. The program now calculates DElemG by determining the minimum element size and then reducing it by a larger factor. Users should encounter no problems as long as the element width at the surface is inordinately large (i.e., >50 times the interaction depth).

At present, we are using EMP to test electromagnetic pulse capabilities of Aether. The command language of EMP 6.5 will be modified to reduce user effort and the possibility of errors. The command set will include single statements to set the properties of common material types. For example, the statement to set Region 5 as a perfectly-conducting metal is:

Metal(5)

In response, the code sets values of relative dielectric constant and magnetic permeability to give very low characteristic impedance while maintaining the speed of light. Similarly, the following commands set a region as an ideal absorbing layer:

AbsLayer(6) 0.025

AbsLayer(3) 0.10 2.7 1.0

The first parameter is the thickness of the layer in the current units. The code calculates and sets the required electrical conductivity. The second form includes optional parameters to set relative dielectric constant or magnetic permeability if the layer is adjacent to a non-vacuum medium. We are also creating new tutorials and reorganizing the instruction manual to make it easier to learn the code and to find information.

There are two ways to set the spatial variation of currents to drive electromagnetic pulses in Aether:

• Define shaped regions and assign components of current density [Jx,Jy,Jz] from mathematical functions.
• Create filamentary coils of any shape with MagWinder.

In both cases, you can apply modulation functions to set the time-dependence of up to 10 independent sources. The interactive, graphical MagWinder program was developed to fabricate coils for Magnum. We anticipate several improvements as we progress with Aether. To date, we have added a new command to set the working directory for coil building and have corrected errors in the ROD model.

Use these links if you want more information about EMP or WaveSim:
http://www.fieldp.com/emp.html
http://www.fieldp.com/wavesim.html

We supply the utility FTabTool (Field Table Tool) with OmniTrak. You can use the program to manipulate field tables (scaling, superposition, …) or to create tables of field values from mathematical functions. This week we made a general overhaul of FTabTool, correcting several bugs. We also standardized the format for writing tables (E13.5) and added the capability to add coordinate translations.