Pulsed beam of extremely large helium droplets

Abstract We generated a pulsed helium droplet beam with average droplet diameters of up to 2  μ m using a solenoid pulsed valve operated at temperatures as low as 7 K. The droplet diameter was controllable over two orders of magnitude, or six orders of the number of atoms per droplet, by lowering the valve temperature from 21 to 7 K. A sudden droplet size change attributed to the so-called “supercritical expansion” was firstly observed in pulsed mode, which is necessary to obtain the micrometer-scale droplets. This beam source is beneficial for experiments that require extremely large helium droplets in intense, pulsed form.