Welcome to the DEGAS 2 home page. DEGAS 2 is a Monte Carlo code for
studying neutral transport in plasmas, with emphasis on fusion
(Degas' The Rehearsal appears courtesy of
To learn more about Degas, the artist, check out
his entry in the
DEGAS 2 User's Manual
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 this section of the TTH
web page. Also, check that the browser
is using "Western (ISO-8859-1)" encoding.
- The DEGAS 2 User's Manual as a PDF file.
- The manual as a postscipt file.
- The manual in html.
DEGAS 2 usage is now governed by a licensing procedure. Once the licensing form has been submitted and approval given, the user will be able to download the DEGAS 2 source distribution (which includes the User's Manual and all other documentation) as a compressed tar file. Older versions of the code (labeled by their version number) can also be obtained.
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
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. 2.9 of the User's
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. 2.9.1 of the
One can also find the version history of this file.
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. 2.9.2 of the
Elastic Scattering of Deuterium Atoms and Molecules on
(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
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. 2.10.1 of the
Bateman Format for Plasma-Material Interaction Data
This format is referred to in the User's Manual (Sec. 3.9.2) and is used by other codes, including EIRENE. Since the original report is not readily available to the fusion community, a digitized verion has been posted here.
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
As an initial step towards that objective, we have a
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
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.
The work presented at the 2010 Plasma Surface
Interactions conference documents what is essentially
a lower level, on the validation hierarchy, validation
exercise. Since no plasma was involved in these
lithium evaporation experiments, the prospects for
being able to accumulate comprehensive diagnostic data
from them were initially deemed good. Moreover, the
components of the relatively simple model were thought
to be well characterized. Both of these assumptions
turned out to be inaccurate in the end, with
signficant uncertainties arising from both
experimental measurements and the input to the
simulations. Moreover, comments from the PSI
reviewers led us to conclude that our baseline model
was perhaps too simple; the resulting changes were
incorporated into the final
version of the paper and the contemporaneous poster
presentation at the 2010 APS-DPP meeting. The
validation exercise was successful, however, in that
it resulted in ideas for more discriminating
Analysis of 2010 NSTX GPI experiments by Chinese visitor Bin Cao
resulted in comparisons of DEGAS 2 results even more
detailed than those presented in 2006. Improved
experimental methods and calibrations, as well as a
simplified simulation procedure allowed four shots
to be examined. The finite
spatial resolution of the Thomson scattering electron
density and temperature profiles remained a significant
source of uncertainty, as did the effect of turbulent
structures on the time averaged images. The peaks and
widths of the radial
profiles of the observed and simulated light emission
matched to within the estimated errors (+/- 0.3 cm). Absolute
calibrations of the GPI camera and gas puffing system
allowed for the first time a quantitative comparison
of the light emission rate. Because only the total
number of deuterium molecules was known, the number of
photons recorded by the camera per puffed deuterium
atom was used as the basis for the comparison. The
experimental result was 1/89 photons per atom (+/-
34%), while DEGAS 2 yielded 1/75 +/-18%; again, the
results are within the estimated uncertainties. The
paper describing this work can be obtained as a PPPL
report or as a journal article (Fusion Science and
Technology, to appear July 2013).
Presentations and Papers
This list has grown enough that it belongs on a
Readers of this web page may find these other sites interesting
- 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.
- 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
- 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.
- 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
- Stewart Zweben's
- 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.
- 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.
- Center for Edge Physics Simulation (EPSI)
- A SciDAC-3 collaboration targeted at developing first principles
codes to simulate the edge and scrape-off layer of tokamak plasmas. DEGAS 2 is
being coupled to kinetic plasma codes as part of this project.
Effective visualization is essential to interpreting the results of
a complex code like DEGAS 2. Along the way, some
are bound to crop up.
Questions, comments, suggestions, etc. to
Princeton Plasma Physics Laboratory
P.O. Box 451, MS 27
Princeton, NJ 08543