1. Nion R&D , 11511 NE 118th St, Kirkland, WA , USA
2. Department of Mechanical Engineering, Stanford University , Stanford, CA , USA
3. Institute of Scientific Instruments of CAS , Královopolská 147, Brno , Czech Republic
4. Department of Applied Physics, Stanford University , Stanford, CA , USA
5. Department of Physics, Arizona State University , Tempe, AZ , USA