As you certainly know, a Helmholtz coil (consisting of two ring coils) makes a fairly homogeneous magnetic field in a rather large part of space. By measuring the coil current one gets the field by computation without major uncertainties. Constancy (time-wise) of the field translates into constancy of the coil current.
Now you ask for replacement options. It is not clear to me what you have in mind here precisely.
If you think of generating the field by permanent magnets (instead of an electromagnet such as a Helmholtz coil) then a ring geometry is not what you need. Much better would be two adjacent rectanguar blocks (bricks). For such a configuration the price depends strongly on the volume in which you need the magnetic field. Further, computing the field from data-sheet values for the magnetic parameters of the PM material can only be a first orientation. For precise experments you had to measure your field with Hall devices (or whatever).
I once headed an industrial project in which homogeneous magnetic fields were generated by permanent magnets. We computed the fields by our own formulas but also got offers to do these calculations from most suppliers of PM material. Unless you have particular knowledge on fields of permanent magnets such an supplier provided service would probably be advantageous for you.
thank you Ulrich I am focusing on the development the SPR. but sometimes we need to pass the time as possible. the calculation of magnetic field and what related to, it i don't have a good knowledge in.
I will send you soon about my research..
Many thanks.
In my group we use permanent magnets for the laser spectroscopy quite often.
There are magnets available in the cylindrical / ring shape that allow sending the laser beam through a small hole. Fields we achieve are up to 0.5 T for approx. 1 cm distance between the two largest magnets, which is way above your needs. Smaller fields will be easier to achieve, and lower in cost.
The volume in which you demand the field to be homogenious might be crucial.
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