Recently, I read one paper on Libbrecht-Hall-Style circuit. As shown in the following picture. 
(arxiv, AIP journal)
I found that the circuit uses one LM399 as a voltage reference fed to the non-inverting input pin of Op amp(AD8671) to set the current of the circuit.
I refer to the datasheet of LM399, I found that the voltage across the Rset resistor would be 6.95V, and the voltage fed to the non-inverting pin of AD8671 could be adjusted by the value of Rset. But I do not know how to calculate the maximum current value that the circuit could deal with. It seems that the AD8671 works as a comparator, so how to maintain a stable current?
Could any one give some suggestion?
Any comments are highly appreciated.
Edit
According to Neil_UK's answer, I draw a simple circuit to see if I grasp the working principle of the circuit. In the picture, I use a zener diode to represent the LM399.
The different value of R7 and R8 would set the non-inverting voltage, so the inverting voltage would change correspondingly. Thus the voltage across the Rsense voltage change to maintain a balance.
Best Answer
The FET IRF9Z14 is a Pchannel, used as a follower (the source at the top follows the gate) thus is noninverting. The OpAmp thus is wired, properly, to provide the inverting loop polarity.