# B-fields

### Adding a 1d-field

You can use addnewdipo (from lattice.py) to add a constant b-field in a area from z_start to z_end. By using a for loop you can therefore add a 1d b-field profile as in the following example:

N = 40 # number of grid cells for magnetic field

mag_grid = fzeros([3,3,N],'d') # also save information for later plotting

z_start = 0.0

length = 1.0

delta = length/N

for i in range(N):

zstart = z_start+i*delta

zstop = z_start+(i+1)*delta

zmid = (zstop+zstart)/2.0

field = MagField(zmid) # MagField returns the field in Tesla

mag_grid[:,:,i] = field

...

# plot the b-field

ppgeneric(grid=transpose(mag_grid[:,2,:]),filled=1,contours=20,

ymin=w3d.xmmin,ymax=w3d.xmmax,xmin=z_start,xmax=z_stop)

### Adding a 2d/3d-field

Similar to the above you can add a 2d grid as follows (the example shows a constant By field):

N=40

mag_grid = fzeros([N,3,N],'d')

for i in range(N):

zstart = z_start+i*z_delta

zstop = z_start+(i+1)*z_delta

zmid = (zstop+zstart)/2.0

for j in range(N):

xstart = x_start+j*x_delta

xstop = x_start+(j+1)*x_delta

xmid = (xstop+xstart)/2.0

field = calcBfield( zmid,xmid)

mag_grid[j,:,i] = field

print >> f, " "

### Interpolating data from a measured field

If you have a data in a text file (say r[mm], B[T]) you can use this to generate a calcBfield function that case be used as in the 2d example:

#include to be able to read data from file and to interpolate

import numpy as np

fielddata = np.genfromtxt("b-field.txt", delimiter=" ")

import scipy.interpolate as sp

# masage date from file into usable format

rfield = fielddata[:,0]

#scale from mm to m

rfield = rfield*mm

Bfield = fielddata[:,1]

calcBfield = sp.interpolate.interp1d(rfield, Bfield,fill_value=0.0,bounds_error=false)

...

field = calcBfield( sqrt( (zmid-zc)*(zmid-zc)+xmid*xmid)) # could be used like this in the 2d example