Welcome to the DEGAS 2 home page. DEGAS 2 is a Monte Carlo code for
studying neutral transport in plasmas, with emphasis on fusion
applications.
(Degas' The Rehearsal appears courtesy of
Nicolas Pioch.
To learn more about Degas, the artist, check out
his entry in the
WebMuseum.)
DEGAS 2 User's Manual
- The DEGAS 2 User's Manual as a PDF file.
- The manual as a postscipt file.
- The manual in html.
NOTE: The reader is strongly encouraged to read the manual
online as a PDF file.
In general, however, the PDF file will be more up-to-date than the
other versions. The HTML version is generated from the TeX source
using TTH. If the equations in the HTML file are unintelligible,
refer to the "Browser Problems" section of the
TTH manual. Also, check that the browser
is using "Western (ISO-8859-1)" encoding.
Source Code
The DEGAS 2 source distribution (which includes the User's Manual and all
other documentation) is available in the formats listed below. Older
versions of the code (labeled by their version number) can be found
in the anonymous FTP repository. The generic, version-less names will
be linked to the most reliable (usually, the most recent) version of the
code.
Atomic Physics Data
Some of the atomic physics data files incorporated into DEGAS 2 have in the
past been used elsewhere, either in other codes or for experimental and
theoretical analyses. They are available directly via the links below.
However, in the interest of simplifying maintenance,
the detailed documentation on these files is contained in the
User's Manual; the links here point to the corresponding page of the
Manual.
Collisional Radiative Model for Hydrogen
This data file describes the "multi-step" ionization and
recombination of hydrogen. Namely, at typical fusion plasma densities,
the collisional and radiative processes that excite and de-excite
the electron in the hydrogen atom occur at similar rates. A collisional
radiative model provides effective ionization and recombination rates
that account for the contributions of the excited states without them having
to be considered explicitly in the transport code (i.e., DEGAS 2).
For more on collisional radiative models in general, see
Sec. 2.8 of the User's
Manual.
The data file consists of tables of these
rates as functions of the
electron density and temperature; the file contains only minimal
documentation. A more comprehensive description can be found in
Sec. 2.8.1 of the
User's Manual.
This data file was recently updated to correct a problem with Lyman-alpha transitions and is incorporated into DEGAS 2 (as of V. 4.2). The older data file is still available here (and in the DEGAS 2 distribution) for testing or comparison purposes. The differences between the rates in the two files may be noticeable for quantitative, spectroscopic applications, so please use the newer one in those cases.
Collisional Radiative Model for Helium
The analogous data for the helium atom come from a code developed
by M. Goto [see: M. Goto, J. Quant. Spectros. Radiat.
Transfer 76, 331 (2003).]. For convenience,
this data file
has been formatted to largely resemble the one for hydrogen. The
actual file that comes with the DEGAS 2 distribution is in
a different format. For more detail on the model and the
contents of the data file, see Sec. 2.8.2 of the
User's Manual.
Elastic Scattering of Deuterium Atoms and Molecules on
Deuterium Ions
(Since these data do not currently exist in a readable
text format, this is just a placeholder.)
These data describe both the elastic scattering and
charge exchange processes and are based on detailed quantal calculations by
the Controlled
Fusion Atomic Data Center. More information on the
calculations and the model used to treat these processes
in DEGAS 2 can be found in Sec. 2.9.1 of the
User's Manual.
Verification and Validation
Code verification and validation are now being actively
discussed by fusion plasma code developers. The objectives
of V&V are to demonstrate, with some rigor, that a code
works the way its author claims and that it is a physically
reasonable description of reality. Public documentation
of this effort allows readers
of papers based on the code's results to evaluate these
claims for themselves. Since journal publication of detailed V&V
is problematic, an alternative approach, such as
a presentation on the code's web site seems
necessary.
As an initial step towards that objective, we have a
brief summary
of the verification and validation of DEGAS 2 that was presented
in a talk
at the 2005 Transport Task Force Meeting (Napa, California).
The other posters and talks listed elsewhere on this web site
describe these V&V examples in more detail. Hopefully,
this documentation will eventually be replaced with something
more specific.
A first example of a specific verification test has been prepared for presentation at the 2006 Transport Task Force Meeting (Myrtle Beach, South Carolina). More precisely, the presentation made there was of the proposed format for "reference problems" and of the fluid neutral momentum transport reference problem; see the associated talk in the presentations section. The resulting text document may be included in a centralized database of such reference problems for others to use. Regardless, it also serves as part of the DEGAS 2 documentation; hence, its inclusion here. Even more pertinent is the actual "verification test" in which DEGAS 2 is compared with solutions from this reference problem. Although this is strictly speaking a work in progress, the text document describing its current state is still sufficiently useful to be posted here as well.
The comparison of DEGAS 2 with NSTX Gas Puff Imaging Experiments described in the paper presented at the 2006 PSI Conference in Hefei was, in the end, sufficiently quantitative and detailed that a description of it in terms of "validation" was warranted. This notion was explored further in a poster at the 2006 APS-DPP Meeting in Philadelphia. The upshot of this discussion is that a quantitative measure of validation is required and that corresponding metrics need to be established. Moreover, well characterized, repeatable (and repeated) validation experiments will be needed to achieve the best possible values for those metrics.
The subsequent talk presented at the 2007 Transport Task Force Meeting (San Diego, California) discusses validation metrics in more detail and contains an attempt at evaluating a couple of them using these data. However, the metrics used are not well suited to this situation, these evaluations are for demonstration purposes only.
Presentations and Papers
This list has grown enough that it belongs on a
separate page.
Links
Readers of this web page may find these other sites interesting
and useful:
- EIRENE
- Is another Monte Carlo neutral transport code
for simulating fusion plasmas. The two codes have some similarities
and some differences. The web site also contains a large
amount of the atomic and surface physics data used in EIRENE.
- AMDIS
- This is the home page of the IAEA Atomic and Molecular Data
Unit. You can find not only data there, but also bibliographic
information. The GENIE search tool allows you to search other
available databases for data. There are also some codes
available online.
- CFADC
- The Controlled Fusion Atomic Data Center is one of the
other databases connected with the IAEA Atomic and Molecular Data
Unit. Their web site contains some important data compiled for
the U.S. fusion program, including neutral-ion and neutral-neutral
elastic scattering data.
- PPPL
- Readers needing additional background on the need for DEGAS 2
and on magnetic fusion in general should consult the
primary web site of the Princeton Plasma Physics
Lab.
- Stewart Zweben's
Home Page
- Stewart has led the development and application of the
gas puff imaging (GPI) diagnostic. His web site contains
additional details and presentations on the diagnostic as well
as countless movies of edge turbulence in NSTX and Alcator C-Mod.
- UTIAS
- The University of Toronto Institute for Aerospace Studies Fusion
Research Group focuses on edge physics (led by Peter Stangeby)
and plasma-surface interactions (led by Tony Haasz). See especially
their publications list.
- UIUC PMI Group
- David Ruzic's Plasma Material Interaction Group at the University of
Illinois Urbana-Champaign has worked with and
contributed to both DEGAS and DEGAS 2 over the years.
Artwork
Effective visualization is essential to interpreting the results of
a complex code like DEGAS 2. Along the way, some
pretty pictures
are bound to crop up.
Questions, comments, suggestions, etc. to
Daren Stotler
Princeton Plasma Physics Laboratory
P.O. Box 451, MS 27
Princeton, NJ 08543
