1. Univ. Grenoble Alpes , CNRS, IPAG, 38000 Grenoble , France
2. Center for Integrated Plasma Studies, Physics Department , University of Colorado, 390 UCB, Boulder, CO 80309 , USA
3. JILA, University of Colorado and National Institute of Standards and Technology , 440 UCB, Boulder, CO 80309 , USA
4. Department of Astrophysical and Planetary Sciences , University of Colorado, 391 UCB, Boulder, CO 80309 , USA