[1] Swiss Plasma Ctr SPC, Ecole Polytech Fed Lausanne EPFL, CH-1015 Lausanne, Switzerland
[2] Max Planck Inst Plasma Phys, Boltzmannstr 2, D-85748 Garching, Germany
[3] CAS, Inst Plasma Phys, Slovankou 3, Prague 18200 8, Czech Republic
[4] Gen Atom, San Diego, CA 92186 USA