---
title: MAGNETICDATA.TXT
section: 5
---

# NAME
magneticdata.txt --- MHD equilibrium data for **gray**(1)

# FORMAT
The MHD equilibrium data can be specified in two ways: as the free parameters
of a simple analytical model, or as numerical data in the G-EQDSK format.

## analytical equilibrium
The format for the analytical magneticdata.txt file is as follows:

    R₀ z₀ a
    B₀
    q₀ q₁ α
    npoints
    R₁ z₁
    R₂ z₂
    ...

where:

- *R₀, z₀* are the coordinates of the magnetic axis (m)
- *a* is the plasma boundary minor radius (m)
- *B₀* is the value of toroidal field on the magnetic axis (T)
- *q₀, q₁, α* are the parameters of the safety factors (see below)
- *npoints* is the number of points in the plasma limiter contour
   that followsAa
- *R_i, z_i* are the coordinates of the points in contour (m)

Note: When viewing the torus from above, a positive B₀ results in a
      counterclockwise toroidal field and the plasma current is
      clockwise if q and B₀ have opposite signs.

### analytical model notes
The model used in GRAY describes a plsma with circular cross section centered
on the magnetic axis at a major radius *R₀* and extending up to the minor
radius *a*, where the density is exactly zero.
There is no X point nor separatrix.

The flux surfaces are circular and the normalised toroidal radius is exactly:

    ρ_t(R, z) = √[(R - R₀)² + (z - z₀)²]/a

The toroidal field is given explicitly by *B_t(R) = B₀ R/R₀*; the poloidal
field is determined implicitly by the safety factor profile.
The latter is parametrised as:

    q(ρ_p) = q₀ + (q₁ - q₀)ρ_p^α,

where *ρ_p* is the normalised poloidal flux.

## numerical equilibrium
The format of the numerical equilibrium is the G-EQDSK informal standard
(See the GRAY manual for the specification).

The data strictly necessary to perform a GRAY simulation are limited to:
poloidal flux, safety factor, poloidal current function, magnetic axis and
plasma boundary.
If a limiter contour is provided it will be used to compute reflections.

# EXAMPLES
1. An analytical magneticdata.txt

        2.96 0.0 1.25  ! R₀ z₀ a
        6              ! B₀
        3.5 10 2       ! q₀ q₁ α
        4              ! npoints
        1.61  -1.35    ! a simple square limiter
        1.61  +1.35
        4.31  +1.35
        4.31  -1.35


2. An analytical magneticdata.txt without limiter

        2.96 0.0 1.25  ! R₀ z₀ a
        6              ! B₀
        3.5 10 2       ! q₀ q₁ α
        0              ! no limiter contour

# SEE ALSO
**gray**(1)