State-of-the-art stellarator optimization code

The BEAMS3D (Matthew McMillan and Samuel A Lazerson 2014 *Plasma Phys.
Control. Fusion* **56** 095019)
code is a guiding center particle code
capable of following both user defined ensembles of particles and
modeling neutral beam injection in parallel. The magnetic field is
represented by a three dimensional splines over a cylindrical grid. It
is currently interfaced to the MAKEGRID coils file, MAKEGRID output
file, and VMEC equilibria. Ionization and recombination models are
provided by ADAS.

Table of Contents

The BEAMS3D code follows the guiding center orbit equations on a cylindrical grid \(\frac{d\vec{R}}{dt}=\frac{\hat{b}}{qB}\left(\mu\nabla B +\frac{mv_{ll}^2}{2B}\left(\hat{b}\cdot\nabla\right)\vec{B}\right)+v_{ll}\hat{b}\), \(\frac{dv_{ll}}{dt}=-\frac{\mu}{m}\hat{b}\cdot\left(\nabla B\right)\). These ODE's can be solved via a NAG routine, LSODE, or Runge-Kutta algorithm. The magnetic field is splined over the cylindrical grid (R,phi,Z). The initial position and velocity of the particles can either be specified or modeled using a neutral beam model. The neutral beam model relies on ADAS for ionization and recombination physics. \(\mu = \frac{mv_\perp^2}{2B}\)

BEAMS3D is distributed as part of the STELLOPT package of codes through Git.

The BEAMS3D code is controlled through command line inputs and an input namelist which should be placed in the input.ext file. While the entire VMEC input name is not required some parts will be read. The name lists should look like:

```
&INDATA
! VMEC input namelist (only need coil currents for runs with vacuum fields)
EXTCUR(1) = 10000.00
EXTCUR(2) = 10000.00
EXTCUR(3) = 12000.00
EXTCUR(4) = 12000.00
EXTCUR(5) = 6000.00
/
&BEAMS3D_INPUT
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!! GRID PARAMETERS !!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
NR = 201 ! Number of radial gridpoints, overridden if using mgrid
NPHI = 36 ! Number of toroidal gridpoints, overridden if using mgrid
NZ = 201 ! Number of vertical gridpoints, overridden if using mgrid
RMIN = 2.5 ! Minimum extent of radial grid, overridden if using mgrid
RMAX = 5.0 ! Maximum extent of radial grid, overridden if using mgrid
ZMIN = -1.5 ! Minimum extent of vertical grid, overridden if using mgrid
ZMAX = 1.5 ! Maximum extent of radial grid, overridden if using mgrid
PHIMIN = 0.0 ! Minimum extent of toroidal grid, overridden if using mgrid
PHIMAX = 1.2566370614 ! Maximum extent of toroidal grid, overridden if using mgrid
VC_ADAPT_TOL = 1.0E-3 ! Virtual casing tolerance (for plasma field outside)
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!! PLASMA PARAMETERS !!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
PLASMA_MASS = 1.6726231E-27 ! Mean plasma mass [kg]
PLASMA_ZAVG = 1.0 ! <Z>
PLASMA_ZMEAN = 1.0 ! [Z]
TE_SCALE = 1.0 ! Electron Temperature Scaling factor
TE_AUX_S = 0.0 0.5 1.0 ! Electron Temperature Knots [0,1]
TE_AUX_F = 0.0 1.0 2.0 ! Electron Temperature [eV]
NE_SCALE = 1.0 ! Electron Density Scaling factor
NE_AUX_S = 0.0 0.5 1.0 ! Electron Density Knots [0,1]
NE_AUX_F = 0.0 1.0 2.0 ! Electron Density [m^-3]
TI_SCALE = 1.0 ! Ion Temperature Scaling factor
TI_AUX_S = 0.0 0.5 1.0 ! Ion Temperature Knots [0,1]
TI_AUX_F = 0.0 1.0 2.0 ! Ion Temperature [eV]
POT_AUX_S = 0.0 0.5 1.0 ! Electrostatic Potential Knots [0,1]
POT_AUX_F = 0.0 1.0 2.0 ! Electrostatic Potential [V] (Phi, not dPhi/dr)
THERM_FACTOR = 1.5 ! Factor at which to thermalize (Vtherm*THERM_FACTOR)
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!! PLASMA PARAMETERS (MULTI-ION) !!
!! Note: This overrides the above profile deffintion !!
!! of PLASMA_MASS, PLASMA_ZAVG, PLASMA_ZMEAN !!
!! UNITS OF M [kg]; UNITS OF NI [m^-3] !!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
NI_AUX_Z = 1 1 2 6 ! D T He C
NI_AUX_M = 3.3435837724E-27 5.008267217094E-27 6.6464764063E-27 1.994423482E-26
NI_AUX_S = 0.0 0.2 0.4 0.6 0.8 1.0
NI_AUX_F(1,:) = 9.60E+18 8.00E+18 6.40E+18 4.80E+18 3.20E+18 1.60E+18
NI_AUX_F(2,:) = 9.60E+18 8.00E+18 6.40E+18 4.80E+18 3.20E+18 1.60E+18
NI_AUX_F(3,:) = 4.56E+18 3.80E+18 3.04E+18 2.28E+18 1.52E+18 7.60E+17
NI_AUX_F(4,:) = 2.40E+17 2.00E+17 1.60E+17 1.20E+17 8.00E+16 4.00E+16
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!! DISTRIBUTION FUNCTION !!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
NRHO_DIST = 64 ! Radial bins (0,1)
NTHETA_DIST = 4 ! Poloidal bins (0,2*pi)
NZETA_DIST = 4 ! Toroidal bins (0,2*pi), 4*NFP is a good option
NVPARA_DIST = 32 ! Parallel velocity bins (-vmin,vmax)
NVPERP_DIST = 64 ! Perpendicular velocity bins (0,vmax)
PARTVMAX = 3.0E6 ! Maximum velocity in dist. (vmax)
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!! PARTICLE INTEGRATION PARAMETERS !!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
INT_TYPE = 'LSODE' ! Particle trajectory integration method (NAG, RKH68, LSODE)
FOLLOW_TOL = 1.0E-12 ! Trajectory following tolerance (NAG, LSODE)
NPOINC = 100 ! Number of trajector points to save per particle
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!! PARTICLE INITIAL CONDITION (INDIVIDUAL) !!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
R_START_IN = 3.6 3.7 3.8 ! Radial starting locations of particles [m]
Z_START_IN = 0.0 0.0 0.0 ! Vertical starting locations of particles [m]
PHI_START_IN = 0.0 0.0 0.0 ! Toroidal starting locations of particles (radians)
VLL_START_IN = 1.0E6 1.0E6 1.0E6 ! Initial parallel velocity of particles [m/s]
MU_START_IN = 3*1.0E-15 ! Particle magnetic moment [J/T] (0.5*m*v^2/B)
CHARGE_IN = 3*1.60217733E-19 ! Particle charge [C]
MASS_IN = 3*1.6726231E-27 ! Particle mass [kg]
ZATOM_IN = 3*1.0 ! Particle charge number
T_END_IN = 3*0.001 ! How long to follow particles [s]
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!! PARTICLE INITIAL CONDITION (Neutral Beam) !!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
NPARTICLES_START = 1024 ! Number of particles per beamline
R_BEAMS(1,1:2) = 1.673 1.400 ! R chord deffinition (start,end) BEAM#1
Z_BEAMS(1,1:2) = 0.000 0.000 ! Z chord deffinition (start,end) BEAM#1
PHI_BEAMS(1,1:2) = 0.000 0.000 ! PHI chord deffinition (start,end) BEAM#1
ASIZE_BEAMS(1) = 0.15 ! Aperature Size [m] BEAM#1
ADIST_BEAMS(1) = 2.00 ! Aperature Distance [m] BEAM#1
DIV_BEAMS(1) = 0.1 ! Beam divergence [rad] BEAM#1
E_BEAMS(1) = 0.1 ! Beam Energy [J] BEAM#1
MASS_BEAMS(1) = 1.6726231E-27 ! Beam species mass [kg] BEAM#1
CHARGE_BEAMS(1) = 1.0 ! Beam species charge [C] BEAM#1
ZATOM_BEAMS(1) = 1.0 ! Beam species Z [norm] BEAM#1
T_END_IN(1) = 0.05 ! How long to follow particles [s]
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!! PARTICLE INITIAL CONDITION (Fusion Reactions) !!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
FUSION_SCALE = 1.0 ! Scaleing Factor to apply to energy
/
&END
```

It is important to note that neutral beam lines are defined by two points. The first index of the array is the beam number, the second defines the two points. The first point (X,1) is the origin on the beam. The second points (X,2) defines the beamline from that origin.

The BEASM3D code is controlled through a combination of command-line inputs and an input namelist. The input namelist must be in the equilibrium input file. That file must also contain the VMEC INDATA namelist (although only the EXTCUR array will be used). The BEASM3D code is run from the command line taking an equilibrium input file as a necessary argument. This input file must have the INDATA (for EXTCUR) and BEASM3D_INPUT namelists in it.

```
BEASM3D -vmec <VMEC FILE> -coil <COIL FILE> -mgrid <MGRID FILE> -vessel <VESSEL FILE> -vac -full -noverb -help
```

Argument | Default | Description |
---|---|---|

-vmec | NONE | VMEC input extension |

-coil | NONE | Coils File |

-mgrid | NONE | Makegrid style vacuum grid file |

-vessel | NONE | First wall file |

-beamlet | NONE | Beamlet deffintion HDF5 file. |

-restart | NONE | Restart run from particles in previous run (HDF5 file) |

-vac | NONE | Only compute the vacuum field |

-beam_simple | NONE | Assume monoenergetic beams (normally 1% variance around injection energy) |

-collisions | NONE | Force use of slowing down/scattering operator. |

-depo | NONE | Calculate deposition only |

-field | NONE | Outputs the B-Field on the cylindrical grid only. |

-ascot4 | NONE | Creates input HDF5 file for ASCOT4 (BBNBI, no particles) |

-ascot5 | NONE | Creates input HDF5 file for ASCOT5. |

-hitonly | NONE | Only save vessel strike points. |

-plasma | NONE | Only compute fields inside the plasma domain (places wall at LCFS) |

-raw | NONE | Treats EXTCUR array as raw values (EXTCUR is a scale factor applied to what's in the coils file). |

-suzuki | NONE | Use Suzuki beam deposition model (default if no ADAS/PREACT). |

-w7x | NONE | Use W7-X beam shape model. |

-fusion | NONE | Use nuclear fusion thermal birth model. |

-fusion_alpha | NONE | Use nuclear fusion thermal birth model (alphas only). |

-noverb | NONE | Suppresses screen output |

-help | NONE | Print help message. |

In it's simplest invokation the code requires a VMEC input file.

```
>~/bin/xbeams3d -vmec ncsx_c09r00_free -mgrid mgrid_c09r00.nc -vac
BEAMS3D Version 1.00
----- Particle Initialization -----
S = [ 0.24490, 0.24490]; NS: 1
U = [ 0.00000, 5.96903]; NU: 20
V = [ 0.00000, 5.96903]; NV: 20
V_||= [*********, 2.00E+05]; NP: 10
Mu = [ 0.00000, 2.00E-15]; NP: 10
----- Profile Initialization -----
Ne = [ 0.00, 5.00] 10^19 [m^-3]; Nne: 50
Te = [ 0.00, 4.58] [keV]; Nte: 50
Ti = [ 0.00, 4.58] [keV]; Nti: 50
----- Input Parameters -----
R = [ 0.95223, 1.82142]; NR: 201
PHI = [ 0.00000, 2.09440]; NPHI: 60
Z = [-0.66634, 0.66634]; NZ: 201
# of Particles to Start: 4000
----- Constructing Splines -----
R = [ 0.95223, 1.82142]; NR: 201
PHI = [ 0.00000, 2.09440]; NPHI: 60
Z = [-0.66634, 0.66634]; NZ: 201
HERMITE FORM: 1
----- FOLLOWING PARTICLE TRAJECTORIES -----
Method: NAG
Particles: 4000
Steps: 320 Delta-t: 0.3125E-06
NPOINC: 320 dt_out: 0.1000E-06
Tol: 0.1000E-08 Type: M
----- WRITING DATA TO FILE -----
FILE: beams3d_ncsx_c09r00_free.h5
----- BEAMS3D DONE -----
```

The BENCHMARKS directory contains a set of tests for BEAMS3D based on an axisymmetric VMEC tokamak equilibrium. The input files are located in the BEAMS3D_TEST subdirectory. They can all be invoked by calling make beams3d_test from the BENCHMARKS directory. The comparrision scripts require Python.

The data from the run is output in two fashions. The first is a text file which contains counts of the number of particles lost from the simulation domain (hit a structure or the simulation domain). The second is and HDF5 file containing the simulation domain (magnetic fields on a grid) and the particle trajectories as a set of points. Each particle has it's trajectory saved NPOINC times.

The data from the HDF5 file may be easily plotted in many plotting packages. The R , PHI, Z, and V_PARALLEL coordinates of each particle are save NPOINC times along the trajectory. To aid in plotting the evolution of the distribution function the code outputs a textfile which bins by VLL the particles at each NPOINC time step.

NCSX Neutral Beam Injection Example Benchmarking and Validation

- McMillan, M. and Lazerson, S.A. "BEAMS3D: Neutral beam injection model." Plasma Phys. and Control. Fusion 56, 095019 (2014)
- Lazerson, S.A. et al. "Validation of the BEASM3D neutral beam deposition model on Wendelstein 7-X" Nuclear Fusion 60, 706020 (2020)
- Lazerson, S.A. et al. "Modeling and measurement of energetic particle slowing down on Wendelstein 7-X" Nuclear Fusion 61, 096006 (2021)