THE DIRECT DETERMINATION OF THE CURRENT-DENSITY AND MU-PROFILES OF A SPHEROMAK

We have used a probe containing both a Rogowski coil and a magnetic pick-up coil to measure both the current density and the mu value in the plasma of the SPHEX spheromak. The results are broadly in agreement with previous measurements but give much better spatial resolution; they reveal a discontinuity at the interface between the 'central column' and 'annulus' regions of the plasma. In the annulus, the range of values of mu spans the spheromak eigenvalue as expected from theory. When the spheromak current drive is terminated by shorting the Marshall gun, the current density at the magnetic axis initially rises to a maximum before decaying, and mu continues to rise towards the eigenvalue. These results are interpreted as showing that the relaxation process continues to operate in the decaying plasma.