Parameterization and measurements of helical magnetic fields

Magnetic fields with helical symmetry can be parameterized using multipole coefficients (a¯_n,b¯_n). We present a parameterization that gives the familiar multipole coefficients (a_n,b_n) for straight magnets when the helical wavelength tends to infinity. To measure helical fields all methods used for straight magnets can be employed. We show how to convert the results of those measurements to obtain the desired helical multipole coefficients (a¯_n,b¯_n)