# STELLOPT

State-of-the-art stellarator optimization code

# TERPSICHORE

The TERPSICHORE code calculates ideal kink stability from VMEC equilibria.

### Theory

The TERPSICHORE code utilizes an ideal MHD model to determine the stability of the equilibria produced by VMEC. The code performs a transformation to Boozer coordinates internally (does not make use of BOOZ_XFORM at this time). The code assumes a perturbation to the VMEC equilibria of the form $$\vec \xi = \vec \xi \left( {\xi ^s ,\eta ,\mu } \right) = \sqrt g \xi ^s \nabla \theta \times \nabla \varphi + \eta \frac{\vec B \times \nabla s}{B^2} + \left[ {\frac{J\left( s \right)}{\Phi '\left( s \right)B^2 }\eta - \mu } \right]\vec B$$ where the first component is normal to the flux tube. The perturbation is chosen to be divergence free eliminating the µ component. Stability is noted by an increase in potential energy for a given perturbation. Thus negative eigenvalues indicate unstable modes.

The TERPSICHORE code is compiled using a set of makefiles. The primary makefile will create an executable for running TERPSICHORE. There is also a makefile for producing an equilibrium interpretation code which converts VMEC wout text files into a text file TERPSICHORE can read (fort.18). The code must be recompiled if equilibrium variables change (this is not true if STELLOPT is used to call TERPSICHORE).

The modules.f file has a few variables worth checking.

• NI = NS-1
• MLMNV>= LMNV = (2*MPOL-1)*NTOR + MPOL (i.e. the number of VMEC modes)
• NJ >= 3*MM (MM is defined in the ft5tpr file, Max Boozer M mode)
• NK >= 3*max(N_boozer)
• MLMNB >= LMNB = MM *(NMAX-NMIN+1) (MM, NMIN, and NMAX are in the ft5tpr)
• MLMNS >= MMS*(NSMAX-NSMIN+1) (MMS is max(M) in the stability table, and NSMAX and NSMIN are the min and max n of the stability table)

### Input Data Format

The TERPSICHORE code is controlled by an input file which is passed to it via unit 15 (STELLOPT requires this file to be named terpsichore_input):

               ARIE3n1
C
C        MM  NMIN  NMAX   MMS NSMIN NSMAX NPROCS INSOL
17    -6    +8    30    -7    11     1     0
C
C     TABLE OF FOURIER COEFFIENTS FOR BOOZER COORDINATES
C     EQUILIBRIUM SETTINGS ARE COMPUTED FROM FIT/VMEC
C
C M=  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6  N
0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 -6
0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 -5
0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 -4
0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 -3
0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 -2
0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 -1
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1  0
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1  1
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1  2
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1  3
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1  4
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1  5
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1  6
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1  7
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1  8
C
LLAMPR      LVMTPR      LMETPR      LFOUPR
0           0           0           0
LLHSPR      LRHSPR      LEIGPR      LEFCPR
9           9           1           1
LXYZPR      LIOTPL      LDW2PL      LEFCPL
0           1           1           1
LCURRF      LMESHP      LMESHV      LITERS
1           1           2           1
LXYZPL      LEFPLS      LEQVPL      LPRESS
1           1           0           2
C
C    PVAC        PARFAC      QONAX        QN         DSVAC       QVAC    NOWALL
1.2500e+00  0.0000e-00  0.6500e-00  0.0000e-00  1.0000e-00  1.2500e+00     -1

C    AWALL       EWALL       DWALL       GWALL       DRWAL       DZWAL   NPWALL
2.8000e+00  1.8000e+00  5.0000e-01  0.0000e-00  0.0000e-00  0.0000e-00      0
C
C    RPLMIN       XPLO      DELTAJP      WCT         CURFAC
1.0000e-05  1.0000e-06  1.0000e-02  0.0000e-00  1.0000e-00
C
C     ANISOTROPIC PRESSURE MODEL APPLIED :                    MODELK =      1
C
C     NUMBER OF EQUILIBRIUM FIELD PERIODS PER STABILITY PERIOD: NSTA =      1
C
C     TABLE OF FOURIER COEFFIENTS FOR STABILITY DISPLACEMENTS
C
C M=  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6  N
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 -7
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 -6
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 -5
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 -4
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 -3
0 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 -2
0 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 -1
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0  0
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0  1
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0  2
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0  3
0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0  4
0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0  5
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0  6
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0  7
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0  8
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0  9
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 10
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 11
C
C   NEV NITMAX         AL0     EPSPAM IGREEN MPINIT
1   1500  -5.0 E-03  1.0  E-04      0      0
C


The following table explains each of these variables Wiki_TERPS.pdf.

Variable Name Description
MM Maximum poloidal mode number (m=0…MM) in Boozer Spectrum
NMIN Minimum toroidal mode number (n) in Boozer Spectrum
NMAX Maximum toroidal mode number (n) in Boozer Spectrum
MMS Maximum poloidal mode number (m=0…MMS) in Displacement Spectrum
NSMIN Minimum toroidal mode number (n) in Displacement Spectrum
NSMAX Maximum toroidal mode number (n) in Displacement Spectrum
NPROCS Number of processors (not used, should be defaulted to 1)
INSOL 0: VMEC Equilibrium, 1: Solov’ev Equilibrium (radius as radial variable), 2: Solov’ev Equilibrium (volume as radial variable)
Boozer Table This table should match the above spectrum definitions. 0: off, 1:on.
LLAMPR Prints flux surface index i, mode pair index l,m,n, and lambda (16 file 0/1)
LVMTPR Prints the VMEC toroidal angle Boozer Fourier Amplitudes on inner 4 and outer 5 suraces and the Boozer Jacobian amplitudes from 2 alternative reconstructions (16 file, 0/1)
LMETPR Prints the Boozer Fourier amplitudes of R, Z, and VMEC toroidal angle (16 file, 0/1)
LFOUPR NOT USED
LLHSPR Prints the submatrix blocks of the LHS stability matrix and the double Fourier flux tube integrals (16 file, 0/9)
LRHSPR Prints the submatrix blocks of the RHS stability matrix (16 file, 0/9)
LEIGPR NOT USED
LEFCPR NOT USED
LXYZPR NOT USED
LIOTPL NOT USED
LDW2PL NOT USED
LEFCPL Write Xsi and Eta vectors (16 file, 0/1)
LCURRF Controls parallel current density, 1: Reconstructs from charge conservation / MHD force balance, 2: Uses VMEC parallel current density, 9: Construction from metric elements.
LMESHP NOT USED
LMESHV Radial mesh accumulation in the vacuum region, 0 : Exponential, 1 : Equidistant, 2 : Quadratic, 3 : Cubic (recommended), 4 : Quartic towards PVI
LITERS NOT USED
LXYZPL NOT USED
LEFPLS NOT USED
LEQVPL NOT USED
LPRESS Default=0. Otherwise 2 uses VMEC pressure gradient. 9 skips Mercier criterion evaluation
PVAC Exponent governing transition away from PVI to conducting wall (>1)
PARFAC Controls period breaking modes. 0 for periodicity breaking modes. For stllarator symmetry breaking modes (mode number n divisible by number of periods), two modes parities exist 0 and 0.5.
QONAX Q on Axis (for Solov’ev equilibrium)
QN Set ot 0 due to VMEC flux zoning, also applies to TERPSICHORE.
DSVAC Value of radial coordinate s at conducting wall
QVAC Exponent governing transition towards the conducing wall from the PVI (>1)
NOWALL -2: Determine normal at each point of the PVI and rescale by AWALL to obtain conducting wall, (recommended)Turnbull A D, Cooper W A, Lao L L, and Ku L-P 2011 Ideal MHD spectrum calculations for the ARIES-CS configuration Nucl. Fusion 51 123011, -1 : Conducting wall obtained by multiplying (m/=0) Fourier components by AWALL0 : Conducting wall extrapolated from PVI., 1 : Prescribed conducting wall, Drozdov Formula (GWALL, AWALL, EWALL, DWALL, DRWAL, DZWAL, NPWALL)
AWALL Minor radius of conducting wall.
EWALL Elongation of conducting wall
GWALL Major Radius of conducting wall
DRWAL Horizontal helical modulation of wall.
DZWAL Vertical helical modulation of conducting wall.
NPWALL Number of toroidal field periods of conducting wall (ignored for NOWALL<1)
RPLMIN Minimum absolute value of R, Z to reprint the active Boozer mode table (6 and 16 file, 1E-5)
DELTAJP Resonance de-tuning parameter for magnetic differential equation (recomend 1E-4 to 0.04)
WCT Horizontal modulation of n=1 m=0 component of wall (nowall=-1).
CURFAC Factor to multiply average parallel curren density in noninteracting fast particle stability model (1.0)
MODELK 0: Noninteracting anisotropic fast particle stability model with reduced kinetic energy, 1: Kruskal-Oberman anisotropic energy principle model with reduced kinetic energy (recommended), 2: Noninteracting anisotropic fast particle stability with physical kinetic energy, 3: Kruskal-Oberman anisotropic energy principle model with physical kinetic energy
NSTA Number of equilibrium periods per stability period (usually equal to equilibrium periods)
Displacement Table This table should match the above spectrum definitions. 0: off, 1: on.
NEV Number of eigenvalue compuations (usually 1, when > 1 it resets AL0 to 95% of previous guess)
NITMAX Number of iterations to converge eigenvalue to that closest to AL0.
AL0 Initial guess for eigenvalue
EPSPAM Relative errof ro eigenvalue convergence.
IGREEN Intended for Green’s function solution in vacuum (not implemented)
MPINT The stability mode table is shifted in m by MPINIT. The table usually goes from 0 to 55, with MPIINIT=20 it goes from 20 to 75.

### Execution

The TERPSICHORE code is executed by calling the tpr_ap.x executable from the command line. TERPSICHORE requires that the fort.18 contain the equilibirum data as calculated by the conversion routine. Here is an example call to TERPSICHORE:

> ./tpr_ap.x < terpsichore_input


### Output Data Format

The data is output into four files by unit number. The fort.16 file contains the primary output of the code. The fort.17 file contains the equilibrium coefficients. The fort.22 file contains information about the perturbation modes. The fort.23 file is a binary file containing the growth rate information.

### Visualization

Explain how to visualize the data.

### Tutorials

TERPSICHORE NCSX Tutorial