1. Department of Physics, Umeå University, SE-901 87 Umeå, Sweden
2. Department of Applied Physics and Electronics, Umeå University, SE-901 87 Umeå, Sweden
3. Center for Combustion Energy, Department of Energy and Power Engineering, Tsinghua University, Beijing 100084, China
4. Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Tsinghua University, Beijing 100084, China
5. Center for Innovation in Gas Research and Utilization (CIGRU), Department of Mechanical and Aerospace Engineering, West Virginia University, 1306 Evansdale Drive, Morgantown, West Virginia 26506, USA
6. Department of Applied Mechanics, Chalmers University of Technology, 412 96 Gothenburg, Sweden