1. Centre of Plasma Physics, Institute for Plasma Research, Nazirakhat, Sonapur 782402, Assam, India
2. Institute for Plasma Research, HBNI, Bhat, Gandhinagar 382428, Gujarat, India
3. Department of Physics, University of Oslo, Blindern, PO Box 1048, NO-0316 Oslo, Norway
4. Princeton Plasma Physics Laboratory, Princeton, New Jersey 08540, USA