Measurement of the instability threshold for toroidal Alfven eigenmodes in JET plasmas with forward and reversed magnetic field

In this paper we have investigated the effect of reversing the direction of the magnetic field and plasma current, hence the ion del B-drift direction, on the instability threshold of toroidal Alfven eigenmodes with toroidal mode number (n) in the range vertical bar n vertical bar = 3-10. These modes are driven by MeV-energy protons produced by minority H(D) ion cyclotron resonance frequency heating. A larger fast ion drive is found to be required to destabilize the modes when the magnetic field and plasma current are reversed with respect to the usual JET configuration (i.e. when the ion del B-drift is directed away from the divertor), with the difference decreasing for increasing n.