Equilibrium Parameters

Back

TRANSP supports several equilibrium solvers. At JET it is common to use TEQ which is the equilibrium module from the CORSICA transport code developed at LLNL and consequently this is the only equilibrium solver discussed in detail here. TEQ is selected by setting LEVGEO=11. The default configuration uses TEQ to solve for the MHD equilibrium taking the pressure and q profiles as inputs as well as a fixed plasma boundary. The vacuum value of ${RB}_{Z}$ is used as a boundary condition. TEQ alters the edge q profile to match the total plasma current. The parameters below can be used to change this behaviour but it is common at JET to use default values for most if not all of them.

NSCRUNCH_OPT : If set to a non-zero integer then the flux surfaces will be scrunched so poloidal flux lines are separated by equal arc lengths. The number of moments used internally (NMOM) will be reset to the maximal permissible value. This is usually turned on in order to improve the numerical stability of the equilibrium solver.

Comment from R. Andre:
When converting the flux surface representation from R,Z points to a moment representation the default scrunching method is to use the VMEC method of calculating reduced moments. This can sometimes fail (I think the problems were seen near the X point) so this option was introduced to allow the use of the more robust equal arc method. This method also requires more moments so it resets NMOM to use the maximum number of moments.

NTEQ_STRETCH : This parameter sets the distribution of radial points in TEQ. Setting this to 0 gives the default.

11 :	Uniform
22 :	Quadratic axis and edge
-22 :	${sin(\frac{i π}{2})}^{2}$ ; $i=1,mpsi$
-21 :	Stretch quadratic on axis
-12 :	Stretch quadratic at edge
-32 :	Cubic on axis, quadratic at edge
-23 :	Quadratic on axis, cubic at edge
-33 :	Cubic on axis and at edge

NTEQ_NRHO : The number of radial points to use internally in TEQ. By default this is set to the larger of 71 and 1.5*(number of TRANSP boundaries). LLNL suggests this should be in the 61-81 range. NTEQ_NTHETA : The number of poloidal points to use in TEQ. The default is 127. NTEQ_MODE : This switch selects the free parameters matched in the Grad-Shafranov solution. The default is NTEQ_MODE=5.

1 : < J.B > , plasma current
2 : Q , plasma current
3 : F , plasma current
4 : Q , edge F = vacuum R*Btor
5 : Q , edge F = vacuum R*Btor with a loop to match plasma current
6 : < J.B > , edge F = vacuum R*Btor

TEQ_SMOOTH : This variable specifies the half-width in

\sqrt{Ψ_{Tor} (Normalised)}

to use for smoothing the input profiles prior to use in TEQ. If TEQ_SMOOTH < 0 then a half-width of mod(TEQ_SMOOTH)/max(NZONES,30) is used. The default is -1.5. A common value for use on JET is 0.025.

TEQ_AXSMOOTH : This variable is used to apply smoothing near the axis using the formula:

Half-width = mod(TEQ_AXSMOOTH)*min(1, (χ_max-χ) / (χ_max-χ_cut) )

where χ = $\sqrt{Ψ_{Tor} (Normalised)}$ , χ_max = 2*mod(TEQ_AXSMOOTH) and χ_cut=mod(TEQ_AXSMOOTH). TEQ_AXSMOOTH=0.15 was found to be needed for a D3D regression test. If TEQ_AXSMOOTH < 0. then the pressure profile will be dehollowed before smoothing by forcing the pressure profile to have the peak value all the way to the axis. The default is TEQ_AXSMOOTH=.05

SOFTTEQ : The average Grad-Shafranov error is checked after the TEQ execution and if it is larger then this value or HARD_GSERROR then it is considered a failure. (default 0.3)

Back