  We present a permanent magnet arrangement that can achieve a tuneable axial
magnetic field from 1.80(8)-2.67(2) kG along the z-axis, where (x,y) = (0,0),
with a rms field variation of less than 1% over a length of 25 mm. The
instrument consists of an arrangement of off-the-shelf N42 neodymium-iron-boron
(NdFeB) axially magnetized ring magnets, of varying outer and inner diameters.
The magnets are organized into four cylindrical brass holders, whose relative
separation can be manipulated to achieve the desired magnetic field strength
and homogeneity over the region of interest. We present magnetic field
computations and Marquardt-Levenberg fits to experimental data and demonstrate
excellent agreement between theory and experiment. The apparatus has been
designed to accommodate a cylindrical atomic vapor cell of length 25 mm and
diameter 25 mm to lie within the bore of the ring magnets. The design has a
clear aperture of 20 mm in the x-y plane along the z-axis, which opens up new
avenues for imaging exploration in atomic spectroscopy.
