About
Recycling sources
There are several ways of prescribing the recycling source in TRANSP, which have been implemented over 15 years of use following specific requests from users.
The main control is to define either the particle confinement time or directly the recycling source. Note that code internally uses the recycling time, thus in the latter case the source is converted into an equivalent confinement time.
- NLTAUP
- Set this variable to .TRUE. to prescribe the particle confinement time from a 1D UFILE PRETPI/EXTTPI.
In this case the magnitude of the recycling source is set in TRANSP such that the balancing ion outflux reaches the level required to match the specified τ=N/[outflux rate].
-
The confinement time can also be prescribed as a time-independent quantity, by specifying
TAUPH(ig) for each thermal species and TAUP0 for impurities.
- NLRCYC
- Set this variable to .TRUE. to specify the recycling source from a 1D UFILE. In this case the particle confinement time is an output of TRANSP. Note that, when the recycling source is specified, the settings for τ are over-ridden.
How to specify the sources
When providing a UFILE for the recycling source (NLRCYC=.T.), this is interpreted in TRANSP as the total recycling source vs time, which applies to all species for which NDEFINE=0, as a group. The default is that the recycling source is hydrogenic, thus - unless otherwise specified - the summed source applies to all species ig that satisfy BACKZ(jg)=XZRCYC, with XZRCYC=1.
If NLRCYC=.F. or BACKZ(jg).ne.XZRCYC, the recycling source can be specified on a species by species basis.
For each thermal ion species ig, RFRAC(ig) specifies the distribution of the recycling source.
Unless the recycling source is provided for each thermal ion species individually, it is recommended to specify the value of RFRAC. In fact, if this is not specified, the recycling fraction is inferred from quantities related to the edge density fraction at the previous timestep. In this case, addtional restrictions apply in specifying the recycling source data.
When the recycling source is provided with a UFILE, the value of the particle confinement time - as specified in the namelist - is over-ridden. If the source is provided for any thermal species, τP is over-ridden for all species. If the source data is missing for some species (for example when a mix of hydrogenic and non-hydrogeninc species is set in teh namelist), RFRAC is used to scale the missing source information from the information provided.
For a species ig with NDEFINE(ig)=2, one may use the switch NLRCYI=.T. to have TRANSP give this (density profile input) species the recycling source rate needed to obtain the same global particle confinement time as the NDEFINE(jg)=0 ion species.
For impurities, set NLRCYX=.T. to enforce impurities having the same global particle confinement time as thermal ions.
Generally, if RFRAC is specifed, then, defining
r(ig)=RFRAC(ig)/[appropriate sum over jg of RFRAC(jg)]
the recycling source for species ig is
r(ig)*[the total recycling source]
where the total recycling source is as specified by input data
(e.g. NLRCYC=.T summed recycling source data) or is as required
to match a global Tau(p) specification.
The set of species summed over depends on the context:
NLRCYC=.T: summed source applies to species ig for which
BACKZ(ig)=XZRCYC.
Tau(p): the NDEFINE(ig)=0 ions considered as a group.
RFRAC can also be used to scale the recycling rate for species
for which no data is provided. For example, if a plasma
contains H,D,T and Lithium, and the H,D,T summed recycling
is controlled by NLRCYC=.T data, then the Lithium recycling
is scaled using the formula
RFRAC(Li)*[H+D+T recycling data]
[Li recycling] = --------------------------------
RFRAC(H)+RFRAC(D)+RFRAC(T)
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Particle confinement controls are relevant when using NRCYOPT=1
or when no other recycling source data is provided. When
source data is provided, Tau(p) controls are overridden.
Note that for various NDEFINE(ig)=0 species NMODEL options, the
mixing model may preclude matching the Tau(p) for all the
individual species; instead an "average" Tau(p) is matched.
The general practice when using Tau(p) has been to specify
the same Tau(p) for all species. With NLTAUP=.T this is
enforced.
RECYCH
For some recycling models, RECYCH(ig) specifies the "prompt
recycling coefficient" for species ig ion outflux.
In particular:
RECYCH is specified for all species when the NRCYOPT=1 model
is used;
For a species ig modeled using predictive transport
(NDEFINE(ig)=1) and for which no other recycling source
data is provided, the recycling source for this species
is RECYCH(ig)*[ion outflux]. The ion outflux is
determined by the transport model for the species.
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NRCYOPT_EQ_1
[New DMC 26 Jan 1995]
A new recycling model is now available. The model is based on
suggestions by Tim Jones at JET. The model is activated by
setting
NRCYOPT=1
in the namelist. (Default is NRCYOPT=0).
The model has the following features:
o Input global Tau(p) by namelist or using NLTAUP=.T
(Tau(p) data must be used; recycling sources cannot
be specified).
o For each ion species ig the recycling neutral flux is
given by
F0i = Fi(ig) * RECYCH(ig) + F0lim * RFRAC(ig)/Rnorm
where the namelist input RECYCH(ig) specifies the
fraction of species ig ion outflux Fi(ig) that is
"promptly" recycled, and RFRAC(ig) specifies the
fraction of some "limiter outgas" flux F0lim that
is assigned to species ig. F0lim is adjusted to
match the input Tau(p). Since a change in F0lim
affects the Fi(ig) terms also, this is an iterative
process, but it converges rapidly. Rnorm denotes
the sum over all species jg of RFRAC(jg).
RFRAC and RECYCH must be specified. All RFRAC(ig) must
be non-negative; all RECYCH must be between zero and 0.95.
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NRCYOPT_EQ_2
[New DMC 30 Apr 2004]
Another new recycling model is available. In this model, recycling
of neutral atoms "as neutrals" is suppressed. Unlike in the other
models, charge exchange neutrals exiting the plasma are not reflected
back in at low energy, and so, *all* particle recycling is controlled
by a particle confinement time specification, with RFRAC(ig) giving
the species-by-species fractions of the recycling neutral flux.
For NDEFINE(ig)=1 and NDEFINE(ig)=2 species, separate recycling sources
may be specified. If they are not specified, RFRAC(ig) applies to these
species as well as the NDEFINE(ig)=0 species.
The particle confinement time used is an edge-density-weighted average
of the namelist particle time TAUPH(ig) over all ion species. Exception:
for NDEFINE(ig)=1 species the transport is specified, and particle
confinement is taken from the species transport model instead of TAUPH.
Unlike the other modeling options, explicitly specified gas flow and
recycling neutral sources result in ion sources of reduced magnitude,
because, lost charge exchange neutrals are not reflected back into the
plasma.
To get this model, set
NRCYOPT=2
in the namelist.
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