A PASER is a device that accelerates a coherent beam of electrons. This process was demonstrated for the first time in 2006 at the Brookhaven National Lab by a team of physicists from the Technion-Israel Institute of Technology.
Relativistic electrons from a conventional particle accelerator

Particle accelerator

A particle accelerator is a device that uses electromagnetic fields to propel charged particles to high speeds and to contain them in well-defined beams. An ordinary CRT television set is a simple form of accelerator. There are two basic types: electrostatic and oscillating field accelerators.In...

 pass through a vibrationally excited carbon dioxide

Carbon dioxide

Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

 medium in which the electrons undergo millions of collisions with excited carbon dioxide molecules and are accelerated in a coherent fashion. No heat is generated in this quantum energy transfer, thus all the energy transferred to the electrons is used in accelerating the electrons. The electron beam created from this process may result in electrons that are highly collimated in velocity in comparison to other acceleration methods.

The vibrationally excited carbon dioxide is the same medium used in a carbon dioxide laser

Carbon dioxide laser

The carbon dioxide laser was one of the earliest gas lasers to be developed , and is still one of the most useful. Carbon dioxide lasers are the highest-power continuous wave lasers that are currently available...

. This medium resonantly amplifies light with a wavelength near 10.6 or 9.4 micrometers, corresponding to a frequency of approximately 30 terahertz. In order to be accelerated, incident electrons must be microbunched at this frequency. An appropriately bunched electron beam strikes excited carbon dioxide molecules resonantly in order to efficiently stimulate energy emission.