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**
**  COLLISIONAL INPUT
**
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**
**  ncoef is the frequency of computation of collisional coefficients.
**    We use "n" as a time step (or cycle) designator.
**    If mod(n,ncoef)=1 or n=1, then compute the coefficients.
**    default: ncoef=1
**
**  mx is the highest order polynomial used in
**    Legendre expansion of distribution and potentials for
**    purposes of computing the collisional coefficients.
**    default: mx=3 (in aindflt.f)
**
**  colmodl selects the collision model:
**  ===> NOTE:  The default value is colmodl=1 <===
**              Be careful to specify the value you want in cqlinput:
**              colmodl=3 is the common setting.
**  colmodl=0 means fully nonlinear self-collisional coefficients for
**    the general species and with the background species specified
**    in the namelist input).  Applied power can give continual energy
**    growth (in absence of collisional losses with other species,
**    or other losses such as radiation or radial transport).
**    The self-collisions conserve energy perfectly, 
**    in the limit of small grid spacing.
**    Only include background species for which it is desired to
**    include general species collsions on the background Maxwellians.
**    Thus for colmodl=0, normally, do not include a background 
**    species for the (possible multiple) general species.
**  colmodl=1 means only contributions from background included
**    in the Fokker Planck collision term.
**  colmodl=2 means only contributions from the general species are
**    included (background ignored).
**  colmodl=3 is like colmodl=0 but the p0 component (0'th order
**    Legendre polynomial) from the general species is ignored.
**    Instead, a background Maxwellian of the same species is set up
**    at a fixed density and temperature which effectively
**    supplies the p0 component for the general species. The effect
**    of this "standard" treatment is that momentum and particles
**    are conserved by the like-particle collision operator, but
**    the background Maxwellian of specified density and temperature
**    act like a heat sink, and stabilizes the Fokker-Planck'd 
**    distribution against thermal runaway.
**        This option (colmodl=3) can be useful for runaway electron 
**    and resistivity calculations.  Unlike the colmodl=0 case, 
**    which keeps accurate track of momentum
**    transfer but would allow the distribution to run away in a tau_e
**    time or so, this option keeps the bulk at the background
**    electron temperature, but still calculates momentum transfer
**    accurately. 
**        Use of colmodl=1 keeps the bulk from running away but
**    the momentum computed from the solution f will be wrong. 
**    colmodl=1 is the technique employed in the past by many codes
**    to compute DC electric field driven electron runaway rates.
**        Clearly, the colmodl=3 option is a kluge, and it does not
**    guarantee a positive definite operator. Thus if the bulk stays
**    cold but the distribution tail gets excited, the p1 component
**    might dominate the p0 component, and the velocity diffusion
**    coefficient cbl could become negative. Caveat emptor!
**        Note that the p0 component referred to above applies
**    to the contribution from a Maxwellian species at a fixed
**    temperature of the same type. For electron runaway calculations
**    or other calculations with colmodl=3,
**    one would have both general species electrons and background
**    electrons. The colmodl=3 option would "see" the contribution
**    from the background electrons as the p0'th component and the
**    other components would come from the general species electrons.
**  colmodl=4.....(undocumented and unsupported option)
**    default: colmodl=1
**
**  kfield(k)="enabled" means that species "k" is included as a field
**    (background) species in the calculation of the collision integral
**    coefficients.  kfield(k)="disabled" means it is excluded.
**    Usually, the background species are in the namelist data, in order
**    to be used as field particles in the collision operator, and 
**    thus will want to be included.
**    default: kfield(k)="enabled"
**
**  cfp_integrals='enabled' [Option added in 2020-07]
**    This option specifies how certain integrals are computed
**    which are used in subr. cfpcoefn.
**    These integrals describe contribution to BA coll. coeffs
**    from collisions of general species with the background 
**    Maxwellian species; the integrals only depend on mass
**    and local temperature of Maxwellian species.
**    Instead of calculating them over and over again
**    [as it was done originally; now accessed 
**    with cfp_integrals='disabled'], 
**    calculate them as a table over temperature grid 
**    and then reuse them by matching a local T
**    with the nearest values in the T-grid
**    [accessed with cfp_integrals='enabled'].
**    These integrals are updated if the temp() of 
**    the Maxwellian species is changed in time. 
**    Maxwellian species also include impurity ions 
**    with all ionization states [see description for 
**    namelist variable imp_depos_method].
**    This option can give some speedup in case of high-Z impurity.
**    Default is 'disabled' for backward compatibility. 
**
**  gamaset .gt. 0. means that all Coulomb logarithms are set to
**    gamaset. gamaset=0. means that the log is computed internally.
**    default: gamaset=0. **However, need to use gamaset.ne.0 for ion
**                        **general species.  This needs attention.
**  !YuP[2020-06-23] New option:
**  gamaset=-1.  means: use NRL definitions for gama(kk,k). 
**
**  gamafac = "enabled", then for electrons as (only) general species, 
**    multiply the Coulomb logarithms (e-e and e-i) in the
**    collision FP coefficients by and energy dependence as follows:
**     The energy dependent Coulomb log will be
**       alog(flamcql + min(sqrt(gamma-1),1)*(flamrp-flamcql)),
**       where flamcql is the argument of cql e-e Coulomb log,
**       and flamrp is the argument of the Rosenbluth-Putvinski
**       (Nucl. Fus. 1997) high energy electron Coulomb log.
**       The sqrt(gamma-1)-factor is chosen in accord with
**       the energy factor in the the CQL Coulomb log
**       (cf., Killeen et al. book.  The CQL Coulomb log
**       reduces to the NRL, Te.gt.10eV expression).  
**       This factor becomes significant at electron energies 
**       of order of greater than me*c**2.
**
**  qsineut = "enabled" or "maxwel" forces background electron density to
**    evolve to maintain quasi-neutrality in time. The midplane line
**    density of the electrons will equal the sum of the midplane line
**    densities of all the ionic species for qsineut="enabled".
**    For qsineut="maxwel" the sum is over background ions only.
**    Note that this option will not
**    apply if general species electrons are present. Use of this option
**    precludes use of locquas="enabled" below.
**    default: qsinuet="disabled"
**
**  locquas="enabled" means that if (1) electrons are not a general species
**    and (2) if qsineut="disabled" (above) then the background electrons are
**    not treated exactly as Maxwellians for purposes of computing their
**    contribution to the collision integral. Rather, they are assumed
**    to be local Maxwellians (at z along the orbit) with a density that
**    maintains local quasineutrality at z with all the ionic
**    species. A true Maxwellian is constant in density as a
**    function of z.
**    If locquas="disabled", this option does not apply.
**    default: "disabled"
**
**  trapmod="enabled", modifies the magnetic well on each flux surface
**    by transforming B/B_min ==>    B/B_min - trapredc*(B/B_min-1.),
**    where B is the magnetic field as a function of poloidal angle,
**    B_min is the minimum value on the flux surface.
**  trapredc is the fraction by which the magnetic well is reduced.
**    Thus, trapredc=0. gives no reduction.
**    trapredc=1. gives complete reduction, i.e., constant B (no well).
**    This option is to help in a heuristic evaluation of the
**    effect of trapping.
**    Have had problems with the theta mesh for trapredc.gt.0.99
**                                       (BobH, 981018). 
**    default: trapmod="disabled"
**  trapredc= fractional reduction in magnetic well on each flux surface.
**    default: trapredc=0.
**
**  scatmod="enabled" modifies the collisional "pitch angle scattering"
**    F coefficient, according to scatfrac, below. 
**    Setting mx=0 gives zero value to the 
**    collisional C, D, and E coefficients (the Rosenbluth g and h
**    functions become independent of pitch angle).  F is the only
**    remaining coefficient of a pitch angle derivative term.
**    F is multiplied by scatfrac, thus scatfrac=0. shuts off
**    the collisional scattering.
**    (default: scatmod="disabled").
**  scatfrac= multiplier of collisional C,E,F coeff, for testing purposes.
*     Also applied to rf coefficients.
**    (default: scatfrac=1.  Also, reset to 1. if scatmod.eq."disabled:).
**
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