1. Blackett Laboratory, Imperial College, London SW7 2AZ, United Kingdom
2. Plasma Science and Fusion Center, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
3. Sorbonne Universités, UPMC Univ Paris 06, Observatoire de Paris, PSL Research University, CNRS, UMR 8112, LERMA, F-75005 Paris, France