Experiments are conducted on JET to assess the potential for plasma diagnostic spectroscopy and mode stability control, of Alfven eigenmodes (AEs), excited by external antennas and by energetic particles. Information on the time evolution of the nonmonotonic current profile in advanced scenarios can be obtained from the frequency and mode number. of AEs driven by energetic particles. The appearance of a rational surface associated with a minimum value in the safety factor profile can be inferred with a time resolution of 10-20 ins. An indication on the presence of a region with negative current in the plasma core can be obtained from the simultaneous appearance of AEs with positive and negative helicity. By synchronously detecting the antenna-driven perturbations, the damping rate of low-n AEs can be measured in real-time every 10-50 ms. The dependence of the AE damping rate on the edge plasma shape may lead to the development of feedback control schemes for the modes. (C) 2003 American Institute of Physics.